That's interesting about the thickness of a lens effecting its focal length as once light is within a medium it shouldn't effect it at all. It is the change from one medium (air/glass) that bends light and allows lenses to operate in the first place. However, the fact the the Sun looks bigger at the horizon would support that because we are looking through more and more air.

I looked more closely at two different lenses from my star ball to see if I could see exactly how they were manufactured. A second magnitude lens system appears to be made from two lenses with the limiting star hole placed between them. These lens system appear to be pressed fit into a small metal tube with a flange about 1/4 inch in diameter for easy mounting in the star globe. It is extremely hard to tell but it appears that both surfaces facing outward appear to be flat. Thus I'm guessing that there are two plano convex lenses placed on either side of the limiting hole very much like a condenser set.

I was able to take apart the lens for Sirius as it is held together with a retaining ring very much like the lenses for the Milky Way and I found this one quite interesting. The size limiting hole is placed first facing the light source. The lens is then placed in the support. What is interesting is that this lens is a convex concave design. The convex side facing the light source, the concave facing out toward the screen. The amount of grind is very small on both sides and I wasn't sure until a put a straight edge across both sides to actually see the curvature. This curvature was less then that of the 18 inch star ball.

This led me to believe that the 2nd magnitude lens perhaps has a slight curve as well but it is so small that I couldn't say one way or another. It could be that there is just a small optical flat on one side of the star hole for support.

A few of the first magnitude star lenses have the star limiting hole on the outside facing the screen, but this could be because there metal and size could make them self supporting. They also hold a filter behind them for color if required.

In my mind it is obvious that the lenses are made by or for Spitz for this use only. Because of the very slight grind they are obviously a fairly long focal length. I'm concluding that the addition of a concave side to the lens that they no longer act as a focusing unit but more like a collimating one.

I don't think that any of these lenses will actually focus an image like a normal plano convex one will but I could be wrong.

It is interesting that they would use both ND filter as well as aperture to control star brightness. Perhaps this is because they are using a focusing lens. But in that case the aperture would control how bright the image was and you would not need the ND filter,

The only reason for both is if they only made a couple of aperture sizes, like for1st and 2nd magnitude. Then they would use ND to get the variations within each of the magnitudes.

In the later A3P globe there are many different size holes as well as apparent lens designs. Some are even numbered but the number doesn't appear to have any correlation with the star's magnitude. Like there are several with a 5.5 nomenclature for an obvious 2nd magnitude star.

Someplace in the Spitz archives this information must exists, it's just getting to it.

I think you hit the nail on the head.


Boy would that be great if we could get our hands on it.

from http://www.nfos.org/degree/opt12/Geometric...ay_tracing.html

The formulas for the power of the lens are:
Click to view attachment

where: F1=front surface power.
F2=back surface power.
t = thickness at the vertex in meters.
n = index of the material of the lens.

Click to view attachment
This is for "thick lenses", for which the Model A lenses probably qualify. The curvature of the lens is very small compared to the lens thickness. And my previous testing on these lenses did show a difference in focal length depending on orientation. I can't remember exactly how much, but I think it was a few inches (definitely inches and not feet).

This is interesting though I must say things are getting a bit over my head.

Looking at the diagrams they show parallel light beams (from infinity) entering the lens and then being focused to a point at the obvious focal length of the lens. What if the direction of light was reversed. If we placed a point source of light at the focal length of the lens, I would assume then that the light leaving the lens would then be a "shaft" or light beam the exact diameter of the lens. This would lend me to believe that this beam would be the same diameter at any distance from the lens. Placing an iris over the lens would then limit the diameter of the projected beam or size of the star projected.

With a pin hole we know the light beam emerging from the globe continues to expand in diameter the further out the projection goes. Thus a limit is placed on the size of hole to limit the diameter of the projected star. If indeed the lens works as outlined above the a hole one inch in diameter would remain one inch in diameter no matter how far from the projector the dome was. That projection would however contain all of the light projected from the one inch hole and thus look much brighter then a pin hole that would diverge to a spot one inch in diameter on that size dome.

Now we all understand this and it should work, but we still get actual focused images of the light source on the screen. Why??? Is it because the light source is not an exact point source but a filament that has a dimension?

I think it obvious that all the Spitz projector lenses are set up with the focal length of the lens the exact distance to the light source. This would (if we think of a camera in reverse and the light source as the film) project a focused image of the light source at infinity. The closer to the lens the fuzzier the image would become. This goes totally against the experiments you have made as you get a focus at around six feet. Why????? It doesn't make sense in my mind.

There is another diagram in that source you quoted that shows what appears to be an optical flat. Note the divergence from a point source on the left transitions into a parallel light beam out the other side. This does not compute in my mind as an optical flat would offset an image or point but not change what it looks like.

When an early A3P star ball is converted to an arc lamp, it is said that all of the star lenses have to be replaced with new ones. Why???? The light source is at exactly the same point, why should it matter???? It the exsisting lenses project out a focused image of the filament, the arc point should make for some great looking stars as they would be sharp points. Why change the lenses???

From experimentation, the smaller the light source (placing the Spitz wide angle lens over the stinger bulb for instance) the better the projection of all the stars. Progressing to the arc lamp makes for even better stars.

Now I have only had the actually arc bulb on once just to make sure it was all working properly and I must admit that I was so overwhelmed by the star field that I really did not scrutinise the projections from either the pin holes or the lenses. The beauty of it just blew my inquisitive nature away. I do need to make a more intelligent report on the differences of the star projector light sources.

Hopefully that will come soon as I will have a dome of some sort up and running as well as the support structure for the A3P which I have been told will be arriving in a week and a half.

I must admit, I feel like a five year old the week before Christmas.

Is the dome going to be ready that soon? Is this a temporary dome -- the one that you were making out of fabric? I'm pretty sure your permanent dome is not going to be up in the immediate future. It does sound, as you say, like Christmas. I wish I could be there to see it.

That lens and ray tracing stuff has my head spinning as well. The light source in my projectors is not at the focal point, or it wouldn't focus an image at a non-infinite distance (by the way, the image focuses at about 3 or 4 ft on the Model A). The magnification of the size of light source also increases as light approachs the focal point. I think the light source is actually about an inch or two outside the focal point.

One lens ray tracing tool I like to play with is at: http://webphysics.davidson.edu/Course_Mate...tics/lenses.htm

Yes this is the "temporary " dome. It will probably be a very long while before I can make a permanent one. I just want to be able to actually use some of this stuff and be able to get useful information out of the experiments. The walls of the room are not cutting it any more for me.

That tool is fun and I can get the output of the lens to be a shaft of light. Just not at all sure whet it means as I don't see any information about focal length etc..

I don't understand why Spitz would place the lenses so far from the focal point but I'm sure they had a good reason. Sometimes I think were trying to reinvent the wheel, but it's the only way to really understand how these lenses work.

Gare, on your copper cylinder, how do the stars look projected through both the pin holes and the lenses. Please be extremely detailed in your descriptions as to the size and appearance of the stars. I'm particularly curious as if you can see any resemblance of the filament of the bulb projected onto the dome.

When I run that applet, if I click on the lens, I see the focal length and position of the lens. You can grab the focal point and drag it to make it longer or shorter, and drag it to both sides of the lens to make it concave or convex. If you click on the object, you see its height and position displayed and you can pull it up and down to make it shorter or taller. You can place more than one lens, and you can place a beam, a point source, or an object. There's more, but you can experiment and discover for yourself.

Here is the updated url for those star charts http://mwvastronomy.com/mount-washington-v...my-star-charts/