QUOTE(Charlie Miller @ Sep 1 2010, 09:35 PM)
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?
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.