Star Projector

[Steve Hamley]

It's a cold, gray morning about to rain here in Tennessee, and I'm not inclined to get out until the rain washes the salt off the roads which means I have too much time to think...

Bob Salomon has mentioned the Linhof and Sinar use a $500k Siemens star projector to test lenses, and I want one. Note that I did not say that I wanted to BUY one! Although when I Googled "Siemen's Star Projector" I did get a lot of hits to compare prices and write reviews for one.

So how does this thing work? I assume a lot of the high cost is producing an easy-to-use, rugged, high volume machine for Linhof and Sinar, so maybe I could make something that remotely resembles one. It seems to be something like projecting a pattern and looking for changes as the lens is rotated.

Questions:

How does this thing work in theory? It sounds sort of like an enlarger projecting a "resolution chart" with the lens to be tested used as a projector lens.

What's a "star"? Is it a 5-pointed thing like we're accustomed to, or is it something like a fine pattern of 3 or more intersecting lines, etc.

Are all the "stars" the same size or are there large ones and small ones? I assume they are very precisely cut as in laser cut?

What's the light source requirements?

Is there a "reference lens" that candidates are compared to, or do people just use their judgment about what's good enough?

So could I use an 8x10 enlarger head to project an image of a laser-cut pattern through a test lens mounted in a "lens chuck"?

Well, that should be enough to incite some comments.

Cheers, Steve

[Arne Croell]

This is a Siemens star, a specific pattern: http://en.wikipedia.org/wiki/Siemens_star

I have seen one of those machines about 19 years ago during a workshop at Linhof. Yes it is like a slide projector with several Siemens stars or other patterns as slide. What I seem to remember is that the lens to be tested is mounted on a rotating mount, driven by a remotely controlled motor - that way one could stand at the projection screen (actually a large wall) and check for centering problems driving the rotational lens position back and forth. Of course there was also a means to move the lens axially to focus.

[Paul Fitzgerald]

"So could I use an 8x10 enlarger head to project an image of a laser-cut pattern through a test lens mounted in a "lens chuck"?"

If you could find a decent 4x6" microfiche film and glass mount it, it should work very well for this. The printing is exactly even across the whole film and would show any defects of the lens. They were extremely sharp and high contrast with different sized print across the film.

Good luck with it.

[Arne Croell]

Not a Siemens star, but 2 or 3 of these babies mounted on a larger piece of glass would be perfect as "slides": http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productid=1790

[Bill_1856]

It's a cold, gray morning about to rain here in Tennessee,
Cheers, Steve

Welcome to East Tennessee between Christmas and Easter (roughly speaking), Steve. I spent my first 30 years in that part of the country, and until moving away I had no idea how F***ing depressing on the human spirit those miserable dark months were!
(Incidentally, I didn't start using this kind of language until after I'd moved away -- it just wasn't done in those days.)

[Peter K]

So could I use an 8x10 enlarger head to project an image of a laser-cut pattern through a test lens mounted in a "lens chuck"?
No, the test projector is a collimator (http://en.wikipedia.org/wiki/Collimator), so the test-pattern is for "normal" lenses optically located at infinity and for e. g. enlarging lenses in the distance the lens is made for.

Peter

[Steve Hamley]

Peter,

That's useful - so the projector has to be collimated light?

Cheers, Steve

[Peter K]

That's useful - so the projector has to be collimated light?
Yes, but over the whole image circle of a lens. This can be 500mm for a LF-lens. So this needs a highly corrected and focussable image not only in the center but also in the outer aereas.

Peter

[Arne Croell]

No, the test projector is a collimator (http://en.wikipedia.org/wiki/Collimator), so the test-pattern is for "normal" lenses optically located at infinity and for e. g. enlarging lenses in the distance the lens is made for.

Peter

Why would the test pattern be at infinity? It would be where the film normally is, whereas the projected image would be at "infinity" (in reality at the right distance for 20x or 50x or similar enlargement on the screen). A collimated light source to illuminate the pattern is a different story, though.
You use a pattern (slit or edge) at infinity for MTF measurements, but that is a different setup.

[Steve Hamley]

Peter,

But "collimated" light is basically light coming in from an object at optical infinity, right? So in theory I could use the sun as a collimated light source - direct sunlight that is, or light from a uniform sky? And the light coming out of the lens that's projected on a wall or screen is not collimated, so why would it make a difference if the source is collimated provided it didn't diverge highly over the star?

Cheers, Steve

[Nathan Potter]

I think Peter is saying that the projected image is at infinity or whatever the lens is designed for - 10X, 20X, 50X, etc. while the star is at the image plane. LF lenses can cover 500mm or more so that a single star at the focal plane needs to be translated in X and Y over the full range of coverage in order to evaluate the optical performance of the full field of view. Collimated light is desirable to maximize the contrast of the projected image. Peter will correct me if I jumbled his thoughts. :)

For similar tests I used a collimated point source with chrome Toppan resolution glass mask at the image plane.

Yes, the sun is at infinity but is an extended source so is not entirely collimated. :)

Nate Potter, Austin TX.

[Peter K]

It would be where the film normally is, whereas the projected image would be at "infinity" (in reality at the right distance for 20x or 50x or similar enlargement on the screen).
With such an equipment only lenses corrected for infinity can be tested. But as I know also enlarging lenses etc. corrected for a magnification factor of e. g. 4x can be tested with this device.

[Arne Croell]

With such an equipment only lenses corrected for infinity can be tested. But as I know also enlarging lenses etc. corrected for a magnification factor of e. g. 4x can be tested with this device.

Certainly - since the distance between lens mount and test pattern slide must be variable anyway to focus, it will be possible to evaluate other magnifications by changing the screen distance. In that case it will probably be necessary to have further enlargement of the projected (aerial) image through a microscope for a full evaluation.

When I saw the one at Linhof in 1991, it was set up for projecting a really large image around 50x and was demonstrated with regular taking lenses.