1x CAT (Schwarzschild design) lens help

kds315* · Post by kds315* » Fri Oct 05, 2018 5:46 am

Got a no-name industrial 1x CAT inspection lens (seems to be just two mirrors plus compensating lens(es) ) with RMS mount but no further info on it. Guessed it is Schwarzschild design (reverse Cassegrain) from what I can see. It looks similar to these lens: https://www.newport.com/f/reflective-mi ... objectives

Now when I mount it onto a camera using a suitable RMS adapter plus helicoid, I still see the dark four spikes, which hold the front secondary mirror in place, in the resulting image. If I use longer extensions, the spikes disappear but I get a cylindric dark ring in the center.

What am I doing wrong?

RobertOToole · Post by **RobertOToole** » Fri Oct 05, 2018 7:29 am

I can't help but thanks for putting in the effort, glad you are trying one of these Klaus, not me.

Robert

ray_parkhurst · Post by **ray_parkhurst** » Fri Oct 05, 2018 7:29 am

I guess it's rated at 1x mag? When the extension is set for 1x, does it show the 4 supports, or a dark center, or something in between? It may be very picky about being at the rated mag.

kds315* · Post by kds315* » Fri Oct 05, 2018 8:53 am

ray_parkhurst wrote:I guess it's rated at 1x mag? When the extension is set for 1x, does it show the 4 supports, or a dark center, or something in between? It may be very picky about being at the rated mag.

I don't have any data of that lens, except the 1x printed on it, so as it has RMS mount, I assumed it either has 160/210mm or infinity. At 160mm and 210mm it shows the 4 spikes and a sharp image between them (??)

g4lab · Post by **g4lab** » Fri Oct 05, 2018 11:43 am

This type of optic are often used on FTIR and Raman spectrographs. The imaging performance may not be the parameter that they optimized.

kds315* · Post by kds315* » Fri Oct 05, 2018 11:51 am

g4lab wrote:This type of optic are often used on FTIR and Raman spectrographs. The imaging performance may not be the parameter that they optimized.

Not this one, it was used for deep UV inspection, as far as I know

rjlittlefield · Post by **rjlittlefield** » Fri Oct 05, 2018 2:42 pm

The one time that I tried to use a reflective objective, I found that it was very difficult to mask against stray light that would contaminate the image.

I'm wondering if perhaps the spikes that you're seeing in the image are actually shadows cast by light coming from outside the nominal field.

Are you getting a high contrast image with spikes, or is there some veiling glare?

Can we see the image you're getting?

--Rik

Smokedaddy · Post by **Smokedaddy** » Fri Oct 05, 2018 5:11 pm

... aren't the spikes coming from the mask?

rjlittlefield · Post by **rjlittlefield** » Fri Oct 05, 2018 5:46 pm

Smokedaddy wrote:... aren't the spikes coming from the mask?

The relevant theory is that in-focus parts of the image are not affected by the shape of the limiting aperture, except for diffraction and aberrations. This is why lenses can get away with using diaphragms that are quite far from circular, for example even triangular in some cases.

For this sort of objective, the limiting aperture should be only that four-paneled hole formed by the spikes and the central mirror. In that case the spikes should not be visible in the image, since they're just part of an oddly shaped aperture.

The spikes could become visible if there is another limiting aperture someplace in the system, for example if one of the mirror surfaces has a big glob of crud on it. Or if there is veiling glare caused by light getting through the lens without bouncing off the mirrors as expected, then the spikes could simply cast shadows.

--Rik

kds315* · Post by kds315* » Sat Oct 06, 2018 1:03 am

Smokedaddy wrote:... aren't the spikes coming from the mask?

Yes, EXACTLY!

kds315* · Post by kds315* » Sat Oct 06, 2018 1:06 am

rjlittlefield wrote:
Smokedaddy wrote:... aren't the spikes coming from the mask?
The relevant theory is that in-focus parts of the image are not affected by the shape of the limiting aperture, except for diffraction and aberrations. This is why lenses can get away with using diaphragms that are quite far from circular, for example even triangular in some cases.

For this sort of objective, the limiting aperture should be only that four-paneled hole formed by the spikes and the central mirror. In that case the spikes should not be visible in the image, since they're just part of an oddly shaped aperture.

The spikes could become visible if there is another limiting aperture someplace in the system, for example if one of the mirror surfaces has a big glob of crud on it. Or if there is veiling glare caused by light getting through the lens without bouncing off the mirrors as expected, then the spikes could simply cast shadows.

--Rik

Yes eyxactly Rik, this is why I am so wondering about what's happening here. I also have mirror lenses for my cameras and some have also this construction and those do not get visible except in OOF areas. I'll have a look and see if there are some other "apertures" at work and also try it out with a tube lens to see if it might have been designed for an infinity system.