Greetings,
I am curious about a line of discussion I heard at a regional meeting of planetarians (sic?) and it regarded what some seasoned hands view as a compression of the brightness dynamic range as produced by planetarium projectors. I'm hoping that someone more knowledgeable than I will jump in.

The thought occurred: IF a pin-hole style projector used drilled star holes whose areas were proportional to apparent magnitude and IF such a projector used condensor lenses on individual brighter stars (say, the brightest 100 or 200 stars), WOULDN'T that projector have an instrinsic advantage over the classic Zeiss (or old Morrison) planetarium projector? I recall that the classic projectors were essential "slide projectors" that represented brighter stars by using larger transparent disks for each brighter star. But then, the very brightest stars would be assigned individual projectors to get around what I'm considering a limitation of the "full optical" design.

I'm thinking pinhole projectors -- aided by condenser lenses -- could be better!

So.... IF the pinhole design, aided by individual condenser lenses essentially projects the bulb's glowing filament, would this mean that the pinhole projectors (assuming the assignment of magnitude by the hole area, where mag 2 holes had 2.51x the area of mag 3 holes) have an intrinsic advantage in terms of creating a more realistic star field? I just can't seem to get past the notion that the "fully optical" projectors were, by design, projecting larger disks for brighter stars.... a different basic priniciple that what's behind pinhole projection (again, assuming bigger "pinholes" had condenser lenses).

Any thoughts for me on this?

Hi Charlie,

My first assumption, since both the pinhole and optical both could use separate projectors for the brighter stars, I guess we are confining our discussion to the lesser magnitude stars?

First, you are correct in saying the older optical projectors did use a larger disk for the brighter stars. However the stars being optically replicated were smaller, better defined circles of light versus the fuzzy images from pinhole projection. Having seen and played with both, I liken it to looking at the night sky through a thin haze (pinhole), versus a perfectly clear night (optical).

The new fiber-optic projectors are ideal since they can optically project a small size disk of varing intensity and color for an extreamly realistic sky.

Just my thoughts,

"O"

Museums' Optical Goto Venus Projector -
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With the fiber optic feature on the latest projectors, I think they can add another level of control on light intensity of individual stars. I have to say that I have never seen a lensed pinhole projector sky display that looked as good as the "field" projector sky. With the "field" projectors they still use separate lenses for the brighter stars. I can actually use binoculars to see stars and sky objects that are invisible to the naked eye in domes using the latest generation of projectors. That suggests a very wide dynamic range.

Thanks for these thoughts, which I gather are based on real-life comparisons. I guess what I'm taking home is that even with the older generation all-optical projectors, the sizes of projected disks were, overall, smaller than those projected via pinhole. What about comments I've read that the "dynamic range" of some projectored star fields seemed compressed? Was this due to some compromise in the scaling of the star disks, some other limitation, or...?

Hmm, interesting about fiber optics... so that's how it's implemented? That is an impressive step. Does anyone have any technical blurbs, schematics, about that approach? I'd like to learn more.

Thanks.

c.m.

Here is a link to learn more: http://www.zeiss.de/c12567b00038cd75/Conte...12569430054f471

"O"


Click to view attachment

They don't directly mention controlling light intensity to individual fibers, so I'm not absolutely sure about that. They do simulate scintillation, and that effect is pretty cool! Thanks for the link, Owen. I have a lot of questions about how the fiber optic technology is actually implemented. If I had more time I would be closely studying the Konica Minolta projector at the the Fujitsu Planetarium. Right now I'm concentrating on getting the video projection system set up at the Hopkins planetarium. I have a lot of projects in limbo until that gets finished. I'm doing the Hopkins stuff in my spare time, and that time has been limited lately due to the demands of my job, and family obligations.

While my memories of the Adler Zeiss are just that, I can make comment on the projectors that I have access to. This is basically a shootout between the Spitz A3P pinhole star ball and the Minolta/Viewlex Series IIB. For those looking foe a quick answer, the optical projector produces a star field (that to my old eyes) looks orders of magnitude better.

Now my thoughts. Any pinhole projection, unaided or through a condenser lens, will project on the screen an image of the light source. The smaller the hole the dimmer, but sharper, the image will be. Thus the larger the hole, the image of the light source will not only be brighter, but more defuse. This is a positive as the projection light source is not as detailed and thus not as noticeable defined image. The use of a condenser lens just intensifies (by collimation) a larger and more defuse image, thus producing the most star like look. Compare this to an optical projector which we know is just basically a slide projector. It projects an image of small holes in a slide at the focal point of the lens, not an image of the light source. An optical planetarium projector is just a collection of 100 or so slide projectors attached together. Since we are projecting an image, we can now work with extremely small holes. Basically as small as the resolving power of the projecting lens which in virtually all cases is smaller then the resolving power of our eyes. So small in fact, that some of the largest projectors require the viewer to use binoculars to even see many of the stars projected.

What should one expect from a star projector. That depends on what one plans to use the projector for. That might seem like a rather senile statement, but think about it for a moment. What do you want from a star projector? I can only speak for myself, and self wants a realistic sky. Others might want an actor (in a spotlight) to wax eloquent about sky stories, or a native american to sit about a camp fire and talk sky lore. All well and good if that is what you want to use your projector for. A Zeiss IX with a separate optical fiber for each star can produce a field so bright that the stars can be seen with the house lights on. Thus easily seen with a spotlight on an actor. But is it real, no, I have sat around a campfire and the stars all went away. Does it need to be real, no, not for the intended purpose.

The thing that made the Zeiss "The Wonder Of Jena" was how accurate the sky looked. The Zeiss II did a better job and improvements continued to about the Zeiss VI which, in this writers warped opinion, was about the best there ever was.

When making an accurate looking sky, one needs to compare the projected image to the real thing. The Zeiss II projected some 9K stars to a limiting magnitude of about 6.5. The number of stars that are visible from the best viewing sites on Earth. No, not typical, but very accurate. What can the average common star projector maker/collector be happy with? Lets start first with a simple projector like the Spitz Jr.. Four hundred or so stars, enough to introduce anyone to the constellations, and more then most people are used to seeing. Why not be happy with that? No accuracy at all. One need go no further then the little dipper to see this. All of the stars in this group are clearly visible, even the one close to the sixth magnitude. Not so in the real sky. Even for me at a dark sky site, I find it hard to see all of the stars when looking directly at them. This is reality for me, and it is what I am trying to achieve in projection.

So what is so wrong with seeing all of the stars in the little dipper when projecting only 400 stars? Nothing I suppose, but it is a lie, and no one with a 400 star sky will ever see or even find the little dipper. Even those few who do venture out at night will not see what they thought they might and that spark of interest could be lost forever.

So, where does one begin, and what is right for a star projector? Actually I think Zeiss was dead on with the original type I projector. It reproduced approximately 4500 stars down to the 6th magnitude. Basically what one can see at an average good dark sky site after dark adaption. With such a site available to me, I started in earnest detailing in my mind just what I could and couldn't see. Basically I found that one must begin with the very faintest things one can see. For me, they are the 6th magnitude stars that are hard to make out looking directly at them. I will not go into cones and rods, but suffice it to say that at our sharpest vision we have the least sensitivity. It also takes me about 10 to 15 minutes to become dark adapted enough to see all I'm going to see. If I have a campfire (or candles to keep the bugs away) I louse the fainter stars very quickly. This sky is the one I want to reproduce.

So we can now see that we need to plan for our faintest stars first and let the brighter ones grow from these. At 6th magnitude (and probably many of the 5th magnitude ones) I must find them hard to see looking directly at them. For whatever size hole, for whatever type of projection, my light source must be set so I see these stars this way. For the brighter stars, the size of the holes can be easily calculated. The math can be found elsewhere in these pages.

The big problem with all projection systems is that one can accept approximately 4 magnitudes before one can either no longer see a star image (it is too dim) or the star image is so large that it looks like a circle of light rather then a star like point. Optical projectors can project very fine dots of light better then a pin hole projector and thus can probably add another magnitude before other optical means are required.


so what is right and what works. I guess if it works for you, then it works for you. My comments.

My view of the Spitz Jr. is mentioned above.

The Homestar projects a lot of stars and looks great, but the first and second magnitude stars look more like moons then stars. Also, the Milky Way, though spectacular, looks more like a time exposure then a real naked eye view.

A Spitz A3P (or equivalent) has about 1500 stars and looks very realistic with a small lamp. The arc lamp is almost too bright when compared to the real sky. The 1500 stars takes one to about the 5th magnitude and is more stars then most people will ever see. Also the relationship in star brightness is very accurate.

I was at first a bit disappointed with my Minolta projector. I didn't think the stars were bright enough nor plentiful enough. As I have done my studies, I have found it to be an extremely accurate machine indeed. The reproduction of the stars from the faintest ones on is dead on based on my views of the real night sky. What I first thought of as very few stars becomes folly after my eyes become dark adapted, after 10 to 15 minutes, just as in nature. The brightest stars, with additional lens systems, are bright indeed and limited to about 10mm in diameter at the projection distance. I am very happy.

It would have been interesting to see the STP (whatever the name) projector used at San Diego. It was a pinhole projector which had lenses for about 300 or so of the brightest stars. It would have been interesting to see the sky it produced. Unfortunately the computer control system went bad without any inexpensive replacement parts. Thank God for dumbbells.

Ron- On your series Minolta IIB (which took the place of the MS-10) does it produce a scintillation (twinkle) effect? Did you invetigate how it does this? [In the older MS-10/ MS-15 is was rotating "chicken wire" inside the star globes!]

Also when you got the machine did it come with a special lamp alignment device? In the older versions it was necessary to properly align the bulbs fillament when installing a replacement lamp.

From what I've been able to find out, the scintillation device was an option which could be ordered with the projector or added later. Kind of like the large plate just below the star balls for mounting additional projectors. My projector does not have this option. I remember Mr. Snow, who ran the planetarium, saying something about running the projector very slowly in diurnal motion and the very faintest stars would actually almost disappear as they projected over the small holes punched in the dome for sound control. The "chicken wire" device would be fairly easy to implement as the bearing for rotation could be easily placed around the lamp support stalk.

Yes the projector did come with a special jig for aligning the star projector lamp. It looks like a miniature of the Zeiss one I've seen pictures of.

What I do find interesting are the cooling fans that are on the star globes. There are two which both appear to blow air into the unit. They are not the original size. They are smaller. The only reason I can think of for this type of replacement is to hold the noise down. I did find a full size replacement which are somewhat noisier. Since the bulbs have been lasting something around 2K hours, I'm sure there is enough cooling. The light baffling beyond the fan tends to knock down the air flow which I think is more to cool the bulb socket more then anything else. I'm thinking of reversing one of the fans so that one pushes air in and one removes it. This would help keep the entire star ball cooler as it does get very hot to the touch after about 15 minutes of running. I'm also thinking of adding a small extra fan, like the kind used to cool a computer processor chip. It would direct the input air flow directly toward the bulb socket.

Ron,
As usual, I appreciate your thought along with the other wise owls here.... from your post and earlier ones of other contributors, it looks like the "slide projector" method beats the "pin hole + condenser on bright stars" because one cannot resolve the projected star-disk sizes? While I don't want to argue myself into an itty bitty corner (I'd take an old Zeiss or Goto over a Spitz), i have a bit of a reservation about that one detail... if, with the "slide projection" approach star brightness is coded by disk size, then if the eye fails to resolve increasingly smaller disks, then wouldn't the eye/brain then fail to code smaller intensity?

other little comments: I've recently seen an authentic (old) A3P representation of the night sky.... using the old GE incandescent bulb and can attest that it's (to my preference) inferior to the arc-lamp set up. Also, the folks at Ash Enterprise confirm your thoughts on "over doing it" with too bright an arc lamp... a rep there has advised me to steer clear of the 75W arc lamp, as his view is that it is clearly inferior... perhaps because 2ndary defects (diffraction?) become apparent with too bright a bulb.

These bits aside, I strongly agree with your concerns about the super power of the most modern opticals. I"m bothered by the over-detailed look of the Milky Way. Even tho the Zeiss (and now others) do a helluva job reproducing MW details, it isn't what the eye sees, but what a time-averaged photograph can see. Your comment about seeing all this with some room lighting on is also, at least from my POV, right on. It is as if the grand masters of star projection are throwing in the towel, demanding that the firmament be even better than it is! Bah Humbug! Why are we buying into this "hyper reality" crap? It seems a fad, tied into computer graphics and going with the "state of the art" rather than the actual state of things. Having said that, I guess one could argue for lower limits to dim-star resolution... .if you were an astronaut free from the limits of our atmosphere. So, I can see having that capacity... but it's a bit of a lie to routinely show such over-the-top views of the "night sky". Funny... we are now living in a Golden Era of planetary research and cosmology.... but those entrusted with the "delivery" of this information are using over-powered skies!

(SIGH)

For me, astronomy was always as much about what you didnt see, or almost saw, for a fleeting moment, or might see someday, than what you actually saw. Camping trips are long forgotten except for .. was that omega centauri on the horizon in the pine trees? did i really think i just got a glimpse of that companion? i mean, even the dim star in the bowl of the little dipper .. sometimes i swear its not there! what other science has something like 'averted vision' .. see more by looking a little away - its totally part of the charm and romance .. partly cloudy nights ... moonlight .. the milky way is so elusive most places . i used wonder why i wasnt that impressed for some reason with go to scopes, huge dobs, ccd cams .. it was almost as if i didnt want to see that much