Hi there folks,

I am an amatuer astronomer from Melbourne, Australia. I am wanting to drill my own planetarium sphere, but I am having some trouble with finding a gore chart for the purpose. Do you have any ideas where I may be able to locate one? What do you use?

I am hoping that you aren't going to say that you all plot the stars by hand... that would be a bit much.

The size is not too important, Larger would be better, as I can resize any image on my computer to fit the bill.

I looked over the forum and found a conspicuous absence of diagrams or directions to them. It would be nice for there to be plans of different styles of Planetariums on the forum, for those who want to build their own.

I hope you can help.

Also has anyone thought of using an optical fibre with a frosted glass ball on top (1 to 1.5 mm dia) as a pinpoint light source? I would think that it would work rather well for a small dome. A plastic optical fibre could always be melted on the end with a gas flame and rotated while melting, to make a ball on the end.

Also how about little filters of optical gel (plastic filters for spotlights) stuck over the pinholes to give the correct colour for some of the larger stars?

Clear skies and dark rooms,
Tim.

Hi Tim

Welcome aboard!

I've been thinking the same thing about an optical fiber with a ball on the end, but I don't know how well it would work. Apparently a micro fisheye lens at the end of an optical fiber is the preferred technique. I want to play around with that concept, but I'm kind of dead in the water right now due to mental overload, burnout, or whatever you want to call it. I've got too many issues that are competing for the few brain cells that I have left. I'm kind of hoping that someone else will pick up the ball and come up with something that we can all use.

Regards,
Ken

Hi Ken,

I just read the other thread about the fibre optics and it seems that that method would be best. I grabbed an old fibre that was laying around here and made a ball on the end, it seems that the light is concentrated at the end and not dispursed too much. I would say that the fisheye lens would be the only good way of going. The light rays have to go from air to glass and back again to refract the light and scatter it wide enough. It would take a half a ball to work properly and then you have a fish eye lens..... so I think I will go for the fisheye when I am ready.

Any thoughts on the star chart to lay over a sphere?

Regards,
Tim.

OK, if you really want me to activate my remaining brain cells this early in the morning, here goes. I have a book from Edmund Scientific called "The Edmund Scientific Mag 6 Star Atlas". It contains spherical charts of the sky, and it seems like you could manually transfer that to the spherical surface to make a drilling pattern. My other thought is using a Celestial Globe. I have a small one that I use as a reference to locate patterns on my star projectors. There should be larger ones available to make the process more accurate.

Hey Ken,
Are you talking about a spherical projection map like appears in the cambridge star atlas, as opposed to the Mercator projection? I am looking for the segmented map as in tapered segments of longditude from north pole to south pole on a world map that makes up a globe. Most celestial globes are made of cardboard and it would not be too easy to transfer the segments to a globe, hence trying to find a chart.

It may be a good idea to have a chart in printable resolution available on the forum for those who want to try their hand at making their own projector.

Sorry for getting you to activate those cells.... I guess it had to happen sooner or later.

Kind Regards,
Tim.

Tim

Yes, the charts are spherical rather than the distorted Mercator projections, so it should be the same as what's in the Cambridge Star Atlas. I suppose that there is some distortion getting a flat chart onto a spherical surface, but it shouldn't be too much trouble. If you drew the RA and Dec lines onto the sphere, and then eyeballed the stars into their respective sectors for a drilling pattern, you should come out pretty close. Maybe someone else can jump in here with some better suggestions.

Hi Tim and welcome to our little group.

I wish there was an easy way to plot stars on a globe but if there is my feeble mind has yet to find a way. I've even experimented with projecting a star chart onto a globe with disappointing results.

I'm adding a bunch of 5th magnitude stars to my A3P star globe and the plotting is a pain in the rear.

First I laid out grid lines on the outside of the globe. I thought I could do it by just plotting against the exsisting stars but did not trust it. Since your starting from scratch you will need the grid lines. The nice thing about an Earth globe is that these lines are already plotted thus saving a lot of time.

As far as a star chart, I found a great set free at:

http://www.mwvastronomy.com/charts.php

Just remember to take them into Photoshop (or some such program) and flip them as they need to be backwards since your doing your drilling from the outside in.

The size of your star ball will be the limiting factor to the magnitude limit of your projector. I would not go any smaller then 10 inches (25 centimeters). Another factor is the size of dome your planning.
Maximum dome size for a 10 inch star ball would be probably 16 feet (5 meters).

So a few questions come to mind.

How big will your projection dome be?

How many stars and to what magnitude do you wish to project?

As far as the entire projector, what other celestial objects do you wish to project?

What automated motions would you like to have?

Do you want to be able to project the entire celestial sphere or just basically the southern hemisphere?

How much time do you have to devote to this project?

Approximate budget? (There will probably be cost overruns )

Again welcome and I sincerely hope we can help you get your home planetarium up and running as quickly as possible.

Hi Tim:

I'll be in Melbourne the end of October - first of November. It would be fun to meet. I have yet to see the planetarium in the heart of the city. Is it a Zeiss? I did see the planetarium in Brisbane two years. They have a wonderful video projection system built into the dome's horizon that is amazing. I am sure you know about. I believe it was an Australian company that created it.

I made three different star globes when I was younger. All were made of clear (thin plastic). I had a celestial globe (12"), put thin globe over it and painted tiny specks over each star. I used different colors representing different drill sizes for different magnitudes. Then I painted the interior of the thin globe flat black and finally started drilling.

What I found was that the celestial globe was not accurate enough because the star magnitudes were represented by various size circles. That coupled with my dots which varied in size, I found I could be off an 1/6 - 1/8 of an inch when I drilled.

The next star globe I made was 16" diameter. I had a metal earth globe that I split in half and used as a template. The metal earth globe already had lines of latitude and longitude. I substituted those for RA and Dec. and began to plot the stars--about 800 or 900 as I recall. This time I drilled right on the mark.

Now it was easy to set this template over the thin plastic and drilled out the plastic globe. Plus I could duplicate it. But to answer the question: I really did plot each star to make it accurate.

I do think your idea of a star chart in 24 sections would be an easier way to do it if you find one.

One thing I remember. Although I set up different drill sizes for different magnitudes, I found I keep adjusting these slightly because sometimes there wasn't enough difference between them. I looked at the individual constellations by projecting light through and many times drilled a bright star a little bigger so get it to look proportionally right. What a pain!

Are planning on using a metal globe or plastic?

Charles

Ga'Day Tim:

It was forty years ago that I made my own planetarium. I forgot some details. When I "upgraded" to the 16" globe, I told you I plotted the stars on a metal earth globe. Now I remember what I did. I had some kind of "protractor" that I made from a thin metal strip that ran from the pole of the sky to the equator. I marked off degrees. I would guess I marked only every two degrees or so because I made this myself.

Then I had a circle marked off for 360 degrees. The metal hemisphere sat on the circle and the "protractor" swiveled around the hemisphere. The hemisphere, being an earth globe, still had latitude and longitude for reference. I had a catalog of stars listing them in order of brightness. What was pretty neat was the fact that I could go down the list for all the first magnitude stars and drill those without changing the drill sizes. I had to look up the magnitude for the drill size anyway, so it wasn't that much extra work to get the coordinates, swing the protractor around find the location. I think the whole operation went fairly fast because I didn't have to change the drill bits out often.

I also remember that using this "template" to drill the stars in a plastic globe had problems. I missed some stars, so I finally set up a little section of sky and drilled holes for the stars in that area. The problem was I was always changing drill sizes.

Looking back it may have been just as fast using the protractor device for plotting and drilling the stars in order of magnitude.

I know I used quite a variety of drill sizes. For example, one size for 1 - 1.3 mag. stars, another drill for 1.4 - 1.6 mag., etc. The smallest was the standard number 80 drill.

If you are using a metal sphere and make a mistake, auto body putty and a little sanding will fix the problem.

Ron, if you read this, I don't know if this scheme will work for your A3P globe you want to add stars to because the lens assemblies might get in the way of the "protractor." But you might figure a way to modify it.

I wonder how Spitz made their star globes? I can't imagine they plotted each globe separately, but as I found, the template idea had it's drawbacks.

I realize now that I should have noted that the star charts are in reverse compared to the star globe. The Celestial globe is, however the same as the star globe. As Ron pointed out, you need to reverse the star chart image before using it for a template.

I can tell you that Viewlex used a template for the Apollo globes. The guy I bought mine from was a distributer for the Apollo planetariums, and he toured the factory to see how the projectors were fabricated. He observed the use of the template. If I recall correctly, he said they used a hand drill with different sized bits for the various hole sizes.

Ed Albin is the guy who built the Starland Projector that I have. This is the one that uses a dodecahedron like the Spitz. I dont know for sure how he fabricates those, but there are holes that have been masked in the paint. Inside the masked hole he has punched out the pinhole. Those holes are in fact punched out rather than drilled. I wouldn't have chosen that technique, but it does seem to work well in spite of the somewhat curled-in ragged metal edges.

Hi Ron,

Well lets see, I am not planning to build it immediately, I am just getting the stuff together that I will need to have to do it. Probably in a year from now.

Now I have thought that a 20 food dome would be good, but it may end up a bit smaller. I want to be able to project both the northern and southern skies and also I want to be able to switch the constilation pictures on and off. Which means that it will have to be a form of dumbell with some wide angle projectors at the base of the star spheres.

It would be nice to show the precession of the equinoxes which can be done in a couple of ways, and naturally it has to have sun moon and planetary projectors too. I want stars up to at least 4th magnitude. (Yes I know ....... Lots and Lots of drilling)

So all in all I would say it is a small project

I am thinking of using a 20 or 22 inch ball or pair of balls for the star spheres. I would like to use metal as it is much easier to work with, but I would rather not have to have some spun.

By that time I will have a machine shop that is more complete, so the making it won't be a problem.

So as far as time goes, well I will be working on it in my spare time, I will be building a house sometime around then so that will be not much. I plan on making the northern hemisphere first and then adding everything else as I go. So starting simple which won't take long but also having it as an element of the final projector.

Budget will be scattered over the several years that it takes to make it, so I think that it won't be too noticeable, a few hundred here and there will surely build up. I plan on making it with as many scrounged parts as possible, we will see how that plan goes.

I think that an accurate chart glued to the globe would be a real help and after each hole is drilled it can be marked off. A coloured chart would be best with different colours for each magnitude of star.

Thanks very much for your suggestions. I appreciate them.

Unfortunately those charts are in PDF format and I can't extract the graphics from them. I have e-mailed the guy who made them in 2005 and asked him if he is willing to make a mercator projection one and then pass it through a gore map making utility that is on the web. We will see what he has to say. It would be nice to have one even if it is showing less stars. It would be nice to have some good resources available on this forum, plans or diagrams etc. and some good how-to information. I will pass on what ever I come up with. It is good to share information.

Regards,
Tim.

G'day Charles,
I may be here during your visit, I alternate between Melbourne and Sydney. Drop me a line a few weeks before and we may be able to organize a visit to the Planetarium. I think that I went to it once a loooong time ago. Say 17 to 20 years ago. It may be a Zeiss but I can't be sure. I went to the one in Nashville several times while I was living over there. I think that one was a Zeiss it sure looked like it. I have just started to get interested in Planetariums and I have only been back here in Australia for 2 years so I don't know anything about the one up in Brisbane, sorry.

Thanks for your tips about your projector making experience, I am wondering if you still have one of the projectors that you can measure the hole size for the different magnitudes. That would make things a bit less hit and miss when I get around to doing the drilling. I know that it changes with the diameter of sphere but I can calculate it from that to whatever the size is that I end up using. Or perhaps one of the others here can check the hole size on their projectors and make a small list for that diameter of sphere.

With the clear plastic spheres can you paint the outside black and then scribe the constellation lines on it and have them project onto the dome? It is a long shot and I don't think that it would work but I am just curious.

Finding a metal sphere or two would be a bit difficult, I may be able to find one, we will have to see.

Thankyou for sharing your experiences.

Kind Regards,
Tim.

G'Day Tim:

I think if you use a "thick" acraylic plastic globe (these are easy to find on the internet), paint the INSIDE flat black and scribe the the outlines of the constellations INSIDE it, it will work fine. This of course is in conjunction to your main star globe.

The star globe must be of thin material (metal or plastic) and may have to be made custom for your purpose. Spun metal or vacu formed plastic plastic material. This would cost a few dollars, but be worth the expense. If you are lucky, you may find a "metal" earth globe in an antique store which would be purfect to use as I did for my template globe. Remember, for the star globe the material should be as thin as possible. And it will be easily to work with if you can split it in half (two hemispheres).

Yes, I still have one of the two projectors I built. I bring it down from the attic occasionally. I do not have the "template" or the codes for the drill sizes any longer. But the deal with the drill sizes is simply this: You start with the smallest drill easily obtainable - a number 80 drill. I am told it is only 4 times the size of a human hair. For promotional promotional purposes, this sounds great! Find out the diameter. Look at the largest size hole you want to drill. Maybe it is a number 50 or 55. (After a point, the larger holes drilled become to large and do not look like pinpoint stars anymore.) Figure what magnitude stars you want to show. 3rd or 4th or more. Then make a chart of the magnitudes from 1 to 4 using the 30 or so drill sizes (50 to 80). This is an example.

Remember, from the first magnitude to the sixth, there is a hundred times difference in brightness. You will not be able to demonstrate this with 30 drill sizes. To do this accurately, the largest drill size would be so large it would probably look like the sun or moon in the sky. So you have to cheat. That's why Spitz, 50 years ago, came up with separate lens to increase the brightness of the "bright stars" without increasing the diamater of the hole drilled. And Zeiss, even early on, had separate projectors for the really bright stars. That's hard for us to do in a home built planetarium. So you need to cheat a bit. When you look at a constellation and want to show a brighter star in the area, you may want to drill a larger hole to constrast with the other stars in the area, even though your "chart" may determine a smaller diameter hole for that star.

But you really can come up with a chart of your own determining drill sizes to magnitudes. This is half the fun!!! Ha Ha!

And thinking back on my history now and how I did it, the "protractor" idea of plotting is simple and really was very fast considering. It saved all the time of changing drill sizes. Theoretocally, you don't even need to mark a line on the star globe if you do this. i know there are catalogs available listing the stars by magnitudes and their coordinates.

If you like, I can photograph the mechanics of my simple projector (I called it a Classroom Planetarium) for you to evaluate.

Charles

Hi Tim again -

Thinking more about the constellation outlines:

Painting the interior and then scribing the outlines may not work because you may cause the light to be diffused when it projects through the scribed lines. But try it. Try it on a flat sheet of plastic, painted black, and scribe a line. Put a concentrated incandescant lamp near it and see what happens!

If it doesn't work, I would think that you could create a "transparency" of each constellation outline, and "paste" these in some way to the interior of a plastic globe. I am sure you understand what I mean. Take a drawing of the constellation outline, photograph it, have a photo shop make a negative for you which will be all black, except for the constellation outline. Cut this negative and fit it into the globe. the only trick will be getting it the correct size, but you can figure that by trial and error.

G'Day (again) Tim:

I wanted to add a couple of notes:

If you paint the interior of a plastic globe black and can remove bit of the paint (the constellation outline), it will project if you remove only the paint and do not scratch the plastic. If you scratch the plastic (with the scribbing procedure) it probably will diffuse the light and as a projector will not work. It would be interesting to look at as a globe. So, if you can figure a way to remove the paint without scratching the plastic interior, you can do it.

Also, I think I remember that each star's magnitudes is something like 2.5 times from 1 to 2 to 3 etc. This goes up exponentially making the difference from the brightest star to the 6th magnitude about 100.

Am I right??? Someone help here!!! I can't remember my basic 101 Astronomy classes.

If this is as I think I remember, you can see immediately how impossible it is to show a sky in a planetarium from 6th magnitude to 1 mag. by drilling holes 100 times difference in diameter. This is why the planetarium doesn't want to really show the sky from the sky all the way down to the 6th mag.-- the theoretical limit of what the human eye can see an a clear night in the middle of nowhere!!!

Finally, we all want to know if the stars rise and set the same way in Australia as they do here in America. Maybe they rise in the West and set in t he East!!

Come on, you drive on the opposite side of the road, the water goes down the drain in a different direction, and you don't have a "north" star to navigate with. But I guess we don't have a southern cross to look at in America.

Te He!

Charles

Ps: I am in Sydney about the third week of October.

The planetarium in Brisbane is a small Zeiss (1980's vintage). But they have an amazing video projector system that involves something like 8 + projectors around the dome that projects the sky, and a computerized storyboard that fills the dome. Talking with the folks there, I remember hearing that it was an Aussie company that developed the techniuqe and it is used in planetariums as well as domed theaters. It was amazing because each video projector avoided images that would be blocked by the Zeiss projector. Apparantly another video projector took over avoiding the projector, so the overall impression was a full dome, without any shadows of the planetarium projector.

Good information on star magnitude follows:

http://www.stargazing.net/David/constel/magnitude.html

http://library.thinkquest.org/2595/reference/mag.html

http://www.historyoftheuniverse.com/magnitude.html

http://www.stargazing.net/David/constel/howmanystars.html

Basically each magnitude is 2.5 times brighter or dimmer then the next.

Now onto the design of a home planetarium projector.

I'm thinking of making this a new thread.

wait i take 2 days off and TIM arrives! dudes, actually ive got another newbie emailing me, i'll try to get him over here

i made constellation templates out of tin, actually roof flashing, then transferred them to vinly for needling. im the worlds only 54 yearold man who recently purchased a pin cushion (its an apple!) g

This stuff on pinhole sizes and star magnitudes is great stuff. I was just now trying to find my way back to this discussion, and it took me a while. It would be nice if this kind of detail could be put together somehow in a way that we could easily reference it. I'm thinking along the lines of basic design principles for Do-It-Yourself planetariums. This would include pinhole principles, lenses, light sources, mounting, and motor drives.

The pinhole discussion helped me understand why the strictly pinhole projectors cannot do as nice a job of showing the sky as the projectors outfitted with lenses for the brighter stars. The sky projected by the pinhole-only projectors looks nice, but it's harder to pick out the constellations. Now I understand more about why that can happen. Without lenses you have to make compromises between the size of the projected star disk and the range of brightness that can be achieved. Star disks that are too large do not look realistic, and without the full range of brightness it is harder to pick out features in the sky.

Hi Ken -

Actually, optical planetarium projectors have the same problem reproducing the sky. Each lens, afterall, projects a section of the sky depicting brighter stars by reproducing them with larger diameters than fainter stars.

In the later Zeiss models, quite a number of the brighter stars ( 40 + I think) had separate projectors to compensate for this. The custom built Korkosz (not sure of the spelling), all optical projector, in Boston had, in effect, three separate star globes with different lamps, each depicting different magitudes stars. It had two hemispheres for one criteria of magnitude stars, then four quarter spheres for another magnitudes, and another set of quarter spheres for another set of magitude stars. All this to make the sky more realistic.

Actually with pinpoint projection, I have found that the star images seem to be about the same diameter when projected on the dome . Each pinhole crudely focusses the image of the lamp's filament or arc on the dome. The larger pin holes are brighter. But there is a point when the pinhole is large enough and does not act as a lens any longer and reacts more like a "shadow box" effect. That's where the pin hole appears more as a round disk on the dome.

In some ways, the early Spitz models, with lenses to create a brighter image for the bright stars , was a great innovation--something the optical projectors didn't do.

All in all, it is easier to depict a more realistic sky when only showing stars to the 3rd or 4th magnitude. Who needs to show 5th and 6th magnitude stars that the average audience will never see?

Well Hi Guys,

I have been offline for a few days and just got back on a few hours ago. I have been working on a star chart with gores, it isn't perfect, but it should work for the purpose of putting holes in a globe. I used the mercator projection chart that Ron pointed me to for the star positions, and it worked pretty good, but there were a few errors which I had to correct, I think that I have got them all, but if you find any more extra stars let me know.

I have sized the stars according to the drill size that should be used, all except the small ones of course. I also put smaller dots in the centers of the larger circles to aid in drilling. This chart is already reversed so is perfect for the making of a projector, it includes stars up to the sixth magnitude, if you need a chart with only one magnitude of stars then I can make one for you and e-mail it across.

These are the colors,
Yellow/red-0 mag,
orange/red-1st and second-mag,
green-3rd mag,
purple-4th mag,
orange - 5th mag,
blue - 6th mag

The dots are sized according to their respective hole diameters within their range of magnitude. I calculated the dots with seventh magnitude in mind as a possible addition, on a 19" sphere (the chart does not have 7th mag on it yet). The scale can be slid up and down if needed. The 19" globe chart is better for precision when drilling, however the file is a bit larger than the 10" sphere one.

Click on the links to download the JPEGs.
10" Star Map with Gores 1.6mb
19" Star Map with Gores 4.4mb


Magnitudes with respective drill sizes.
7th mag. #80
6th mag. #75
5th mag.#66
4th mag. #55
3rd mag. #44
2nd mag. #29
1st mag. #3
0 mag. 21/64th of an inch.
-1 mag. 33/64th of an inch.

Just size it for your globe and then print it out and cut the blue background out.

I hope someone finds it of use,
Cheers,
Tim.

33/64 inch sounds way too big. I've been concerned about the "cartoonish" or unrealistic look of stars that are projected with holes as large as 3/16 inch or slightly smaller. A lot does depend on the size of the dome, the diameter of the star globe and even the size of the lamp filament, and the thickness of the material that the pinhole is drilled through.

We need a good technical presentation on the details of pinhole projection. I don't feel qualified myself.

Hey Ken,

The size was calculated from the area of the hole and therefore the amount of light that is allowed through, I forgot to mention that the larger holes have lenses on them to focus the light, like the spitz.

Tim.

OK Tim. That explains it. What is your strategy for selecting and obtaining lenses?

I think Ron Walker pointed out a relationship between the size of the lamp filament and the size of the pinhole. Is this the transition point between lens and shadowbox? Did Ron say that the pinhole needed to be smaller than the size of the lamp filament? This is why I would like to see all these details pulled together in one place.

By the way, every time I see that Ron hasn't posted anything for several days, and he doesn't jump into a discussion like this, I start worrying about him. I hate to be a busybody, but where are you Ron?

Alive and well, thanks!

Just trying to get my info straight in my head before I post.

Also want to actually measure some of the star holes on the A3P star globe and see how they correspond with Tim's calculations.

Still am back at getting a picture of a Greenly punch. Also had a chance to see one of those strange laser planetariums that Edmond Scientific was selling. More later.

Hey Ken -

A pinhole can act as a lens. We know there are pinhole cameras, even eye glasses fitted with pin holes instead of a lens. Can't say if they were any good!!

From my experimentation, I found that the small pinholes crudely focussed the image of the lamp's filament on the dome. That's why I noticed "crescent" shaped stars at certain angles in my star globe using the GE 605 bulb (the bulb the early Spitz projectors used). And, as I recall, the closer the screen is to the pinhole (the relative dome size), the more distinct the image of the filament is.

The fact that we dim the stars down to faint images, erases the picture of the filament. What I remember discovering is that the larger size of the pinhole would let more light through, but the focussed image (the filament) on the dome remained the same size as the fainter stars. This must be when pinpoint projection works to it's advantage for planetariums. The image size is the same, but the brightness varies. Perhaps, someone else can verify this.

Also, the fact that a pinhole doesn't have the clarity of a true lens, the Spitz planetariums used this to their advantage. A projected lamp filament looked so fuzzy, it was mostly hard to distinguish it from a fuzzy dot of light.

An arc lamp eliminates any image of a filament, so the effect is a dot (the arc), is projected on the dome. The actual diameter of the stars projected should remain the same, but the brightness varies depending on the pinhole size.

As you say, you reach some transposition point when the light projected is no longer through a "pinhole" sized opening. Then you'll notice the stars appearing as round images on the dome (even if you don't have a lens system like the Spitz projectors). I guess we need a physicist to explain just when the pinhole doesn't act as a lens anymore and why.

Oddly, this is not true in optical projectors--the brighter stars are really depicted as larger circles on the dome. And the stars projected through the lens systems of the Spitz units are circles as well.

Either way, we show the difference in magnitude by the size of the opening the light is projected through including the digital planetariums.

another name can be born here, i've experienced it often, 'the weakest link' or the 'dullest star'.. as you add sharpness to your sky, the next level of fuzzy stars begins to bug you. say you add lenses to the 1st mag stars (slog through 'the quest for cheap lenses' board for some of our attempts to find something REALLY cheap, which is whats holding me back personally), then the 2nd maggers begin to bug you. its like the wallpaper hanger who alone probably notices the one crooked piece above the door, but if our skies dont please us we will drift somewhat .

gare with a philsophical sidebar

Having an assortment of projectors, I've been able to do comparisons of pinhole projections of various sizes along with lensed projections. My limitation is not having a dome to get the full effect (I would like to see Owen tell us more about the effectiveness of using a large tent for his shows -- I do have an inflatable dome, but no place right now to set it up). The Spitz projectors with lenses for the brightest stars do a significantly better job of representing the sky and making the brightest stars and constellations "pop out".

As I have pointed out before, the pinholes and lenses both tend to project images of the lamp filament. There is a distance at which the pinholes do focus a sharp image, and that depends on the size of the starball and possibly the size of the pinhole. At longer distances the image defocuses and looks more star-like, so the size of the dome is an important factor.

If I remember my physics correctly a hole smaller then the light source size will project the image of the light source more clearly then when the hole becomes larger then the light source. Once the hole becomes larger the the light source the projection becomes softer. Thus for star projection you would always want the light source to be smaller then the holes your projecting through.

Also the need for lenses is dependent on the faintest star that one wishes to reproduce. There comes a time when further reducing the size of a pinhole will actually produce a larger projected star because of refraction. Because of this a number 80 drill is usually the smallest size used. If we only reproduce a sky of the 4th magnitude and assign the #80 drill for them, then you can probably get away without any lenses even on the first magnitude stars as their holes will not be terribly large. However, if you want to reproduce to the 6th magnitude and assign the #80 drill to them, then the relative size of the first and second magnitude stars becomes so large that they no longer look star like on projection. The only way to get the full brightness of the brightest stars without them looking unnaturally big is to use a lens to "collimate" the light into a beam that doesn't do the normal expanding but rather concentrates all the light from a larger hole to a smaller dot on the screen.

Also, the larger the star globe, the smaller the star projected at any given distance. There is no way to get away from optical refraction however so your kind of limited to that #80 drill.

Most small projectors with star globes up to about 12 inches are usually limited to about 750 stars or 4th magnitude and will project in domes up to about 20 feet in diameter.

Medium size projectors with star globes of about 18 inches will project well in domes as large as 40 feet in diameter and will reproduce up to 3500 stars to about 6th magnitude.

Large size projectors with star globes of 36 inches will project well in domes as large as 60 feet in diameter and will project as many as 9000 stars to about 6.5 magnitude.

In all these cases the smallest magnitude projected is a hole produced with a #80 drill. They project smaller because of their distance from the light source.

That's an interesting point. But as a matter of practicality, aren't the incandescent (as opposed to arc lamp) bulb filaments larger than the holes you find in the typical pinhole planetarium projector?

Is it true that the images produced by the smaller holes are also smaller images of the filament, and thus harder to recognize as not being starlike?

Is it true that smaller holes may refract better, but allow less light through because of the smaller aperture?

And that is the problem with most bulbs (used because they are cheap) with large filaments. The larger holes project more of a elongated "blob" while the small holes project a very clear image of the filament. Neither project very good stars.

When the holes get to small not only will the stars be dimmer, but the projected image will also get larger, There is a limit as to how small a hole one can project through and still get a smaller projection.

Think about the pin hole camera. the smaller the hole, the sharper the image but the more light needed to do an exposure. The image is sharper because smaller detail is projected through the system.

I kind of feel in retrospect that some of my questions and comments can be interpreted as being snide or derisive. It's hard to tell what the poster intends in this kind of format. Please assume that I'm just trying to learn more, and there is never ever any intent to convey any other message. Also don't be put off if I ask seemingly stupid and uninformed questions. I'm just trying to put it all together in my head.

And here all along I just thought you hated my guts!!!

No, seriously, I'm so used to snide and derisive comments from my clients that I just don't notice them at all anymore!!!

No, really, and I am serious now. I, for one, have never thought that at all of you. I'm personally glad that all of you out there have found this little corner of the world and are all contributing to its continued growth.

Hopefully any of my answers are useful and clear to all. If not, please ask again. I have a habit (learned [that I had the habit that is] during my teaching years) of assuming that those that I'm talking with have the same basic understanding of the subject that I have and I need to be reminded of this from time to time. If I'm confusing and unclear, ask.

Remember a lot of the time my explanations are based on "my understanding" of the science behind them as well as results of "my experiments" and then all tempered by "my level of acceptance" of any given result. What this all means is that my posts might not always be 100% correct to current scientific fact. (But then I remember that the Earth was flat was at one time scientific fact.) However, I believe them to be close enough to be very useful in the building of planetaria. The only real variable is "my level of acceptance" like I might consider a projection of a star "acceptable" while others may not. I will temper that last comment with the admission of my "perfectionist" attitude!

My biggest problem is when my left brain fights with my right brain. I want the best most accurate sky as possible, but then just seeing stars projected makes me melt, even if some of them do look a little like filaments.

Amen to that!

This is my 200th post. I am no longer a bunch of galactic dust. I am now an asteroid.

The Abbatantuono thesis on Spitz offers the following about pinhole sizes:

"Based on trial and error experiments, Armand Spitz discovered that the smallest pinhole which also produced an acceptable star image was 0.004 inch (Norton, The Planetarium and Atmospherium, p. 68). For the planetaria in his various Model A series, therefore, pinholes of this diameter produced the star images for the unit's minimum stellar magnitude. In the Model A, the dimmest of the roughly 1,200 stars projected were generally of the fourth magnitude, with variances made for completing certain constellations as Spitz saw fit; for the Model A-1 Spitz used stars as dim as the fifth magnitude (Schran, "The Age of the Spitz Dodecahedron", Phenomena, Fall 1993/Winter 1944, p 3.) With the extension in the magnitude limit, the .004 inch hole size was simply transferred to the dimmer stars."

Note that this was based on a Dodecahedron diameter of appriximately 18 inches.

For some reason the new designation of Asteroid makes me want to reach for the Preperation H.

Somehow that takes something away from the thrill of being honered with the new title.

I should get a copy of the Abbatantuono thesis to add to my collection but I will elaborate on what I've based my findings on. Abbatantuono is essentially correct but he doesn't take his conclusions far enough (at least based on the above quote and the information he bases his findings on.)

When building any man made device we must consider not only the physics involved but also the actual mechanical limitations.

Abbatantuono notes page 68 of the Nortin book which I will quote now:

"The smallest pinhole for a point source is about 0.004 inch. For a smaller pinhole a curious phenomenon known as diffraction becomes the influential factor. Diffraction is the bending or spreading of light around an object, in this case the circular aperture, causing an increase in the emergent cone angle which results in a larger image. The linear size of the light source is another limiting factor. No light source is a geometrical point (with the exception of a distant star), and a source larger then this will produce a larger projected image."

Interpretation would suggest that, while a theoretical limit of 0.004 inch is possible, producing a light source small enough (and drilling the hole for that matter) would not be.

Various articles on the arc bulb used in the Spitz A3P projector put the actual arc size at anywhere between 0.015 and 0.008 inch. From actual examination of an arc bulb, I would tend to believe the 0.015 more. Since we would probably want to keep the hole larger then light source I would probably try and limit my hole size around this size as well. One point to remember is that actual projection of the arc would not at all appear as a filament would on the dome and thus this "rule" could be relaxed a bit. Now, I would not at all dismiss the 0.008 figure as this might be the apparent size of the light source after transmission through the Spitz lens.

Other collected articles also back up my conclusions. When the Flint planetarium installed the Spitz model "B" the following information was provided. The star globes at either end of the projector were 36 inches in diameter (twice that of the A3P and A1 at 18 inches). The 54 brightest stars of magnitude 2.0 and brighter are done with lenses and the remaining of the 3,083 stars are done with pin holes. A 5.8 magnitude star (the faintest shown) is projected through a hole 0.0135 inch in diameter. This is a number 80 drill.

In an article about the A3P the following information was obtained. Of the 1500 stars projected, 71 of the brightest are reproduced with lenses. The limiting magnitude is 5.0, however fainter ones are shown through pin holes as small as .0145. This is a number 79 drill. This information is for an A3P installed in 1963.

Improvements in the star globe apparently culminated in about 1968 when about 110 stars were projected with lenses (2.5 magnitude and above). At that time you could (and still can on the 1024) get a star globe with the basic 1500 stars or as many as 4000 stars. My guess is that they drilled that extra 2500 stars with a #80 drill. Basically (and Abbatantuono backs this up), the more of the brightest stars that are limited in size by lens projection, the more fainter stars can be reproduced by the smallest hole attainable.

I have on my own A3P star globe noted no hole smaller then a #80 (.0135 inch) but then I'm sure it is a standard globe with the basic 1500 stars. Of the fainter ones (drilled to fill in familiar constellations) again no hole is smaller then a #80 drill.

I do not have an A1 but I would bet that you will find no hole smaller then the #80 drill. I would be very surprised to find anything approaching that 0.004 theoretical limit.

Basically what Abbatantuono has said is technically correct but I believe one could be led astray in assuming certain conclusions. The availability of materials in the real world limits drill size to a #80 at 0.0135 and I'm assuming this is the limit Spitz used on all of his planetaria.

Keep posting and you'll soon be a planet. At least they don't have sharp edges.

Some of the experiments that Spitz went through when designing the A3P was to limit the size of the stars with lenses. Since the threshold of the average eye's resolution is about 0.05 degrees or three minutes of arc, they designed the star globe so that no star would be larger then 3/16 of an inch on a 30 foot diameter dome. On a smaller dome the projections would be proportionately smaller and vise versa on a larger dome. To a viewer looking up would see stars the same size. The only place this would not hold true was someone sitting directly at the edge of the dome looking at projections of stars right next to him. Under these conditions thew would look like bigger circles no matter what projector was used.

From my limited experiments I will tell you that the stars look great from three feet to 12 feet away.

Thanks for all the clarifications Ron. I thought .004 inch sounded awfully small for the pin holes. I'm assuming that you have a used arc lamp, so the gap may have increased in length from use, and that may account for the light source being at the long end of the range. I'm guessing that those lamps don't actually "burn out", but that the gap gets too long at some point.

Since that projector of yours probably came from a dome that was on the order of 30 ft size, I'm guessing that the stars look ok at even more than 12 ft away. Did you notice any focal length on the lenses?

Basically an arc lamp reaches the end of its useful life when the gap between the anode and cathode have reached a point when the high voltage starting current can no longer jump the gap. This is usually well past the useful (bright) life of the lamp. I have heard stories of planetarium projection from an A3P that looked very dim and the stars look like long dashes even with arc projection. This is probably because the arc bulb is being used long beyond its useful life. I've been told that the life of a Spitz arc lamp is around 1000 hours and my guess is that is its useful life in a planetarium projector. I'm guessing that such a bulb will actually burn more like 2000 hours before it can no longer be lit. New bulbs cost the better part of a grand, so I would expect most institutions to use the lamp until it will no longer light. I also have read that only the 20 watt arc lamp uses the Spitz lens and the 75 watt lamp does not. I'm guessing that the extra heat generated by the 75 watt lamp causes problems when used with the lens. Also the 75 watt lamp is used in the larger domes (probably 30 to 40 feet) and the stars should look fairly small in a dome of that size. While I do need to get a spare bulb at some time, I was told the bulb I received with my Star Globe has only 200 hours on it and I would believe that from looking at it. I work with a lot of arc lamps, both in projection films and in light sourced for the production of films and I can tell you from first hand knowledge that it is easy to see use on a bulb. It can show up as physical deterioration of the anode and cathode of the bulb and/or deterioration of the interior of the bulb glass envelope itself. The history of the Star Ball that I have states that it came from a school with a dome of 20 to 24 feet. This I would believe as the star images at 12 feet look very good indeed. I have actually only used the arc lamp for about a half hour (to make sure everything worked well) and the projection was just beautiful.

Again the lenses on my particular generation of Star Globe where of the collimation type which were introduced in the later 60's. Rather then actually projecting and focusing an image of the arc or filament onto the screen, they take a rather large hole and limit the normal expansion of the light rays emanating from the larger hole. Thus the lenses restrict the normal expansion of the light cone from a large hole and "concentrate" the light to a brighter small star. They really don't have a focal length per say. If the star was projected by just a 1/4 inch hole say, the image on the dome would be three inches or larger in diameter. With the lens, the hole image is kept to a design limit of 3/8 of an inch or so on the screen.

What is interesting is that the projection from a lens-ed projector weather formed from an arc lamp or filament lamp appear the same. It is the projection from the smaller pin holes that make a difference on the dome.

Ron, thanks.

I keep picking up bits and pieces of knowledge, little by little.

I think you said before that these lenses were "slightly negative", which I assume means that they were slightly concave rather than convex like the lenses that I have.

The lenses for the Milky Way are very obviously concave. I'm thinking that the star lenses are actually slightly convex but the convex side is facing into the projector. Some may have a very slightly concave surface facing the outside of the globe but it is very hard to tell without taking the entire lens apart. I have not removed one of these lenses to find this out, but to do what they need to do optically would indicate this. They basically need to take the normal expansion of the light beam that goes through the larger hole and bend the light beam in such a way that there is little or no normal expansion. This is pretty much the case as the larger holes project a star about the same size from three or four feet away out to twelve feet. There is probably a slight increase in size and it would probably be worth doing some measurements at various distances to get some very accurate information.

I hadn't considered ramifications of turning the plano-convex lenses around. I'm going to try some experiments with that (assuming that the flat side is facing inward now). Thats easy to check on the Model A projector. Those lenses disassemble very easily. Also, it's easy to turn the stalks inward on the A3 to see what difference it makes.

Looking forward to your results.

Ron

I don't know if I thanked you sufficiently for doing that test on the fisheye lens. It is appreciated.

I did the quick experiment on the plano-convex lens flip, and if there is any difference it is very subtle. The sharp focus point is about the same, and at 6 ft, I can't tell any difference in the spot size. I also noticed something that I had forgotten (but probably mentioned before). There are not only differences in aperture on the lensed Model A projector, but there are filters as well.

I went back to basic optics 101 (searched the internet), and discovered that a plano-convex lens does basically the same thing if you flip it around, BUT the focal length will be different. The focal length will be longer if you present the flat surface to the light source. The difference in focal lengths increases as the thickness of the lens increases, and it increases as the power of the lens increases. As I think about it, I remember observing this effect before with plano convex lenses. If I had observed more closely, I would have noticed that the focal length had changed when I did my experiment, but it was not shifted by a lot.

The filters are for star color and can be used with a lens because they don't really effect the optics much. A filter over a pin hole would be a different story and could very easily shift the position of a star kind of like how a fish swimming under water is not really where we see it but is diffracted by the water itself.

Wow! I didn't know that about pinholes and filters. I have another fact to store in my "wish I could remember" memory.

The filters that I looked at seemed to be neutral density filters (shades of grey). It looks like the star brightness is being determined both by aperture and neutral density filtering.