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Help with foucault test results

Optics Mirror Making ATM
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#1 alexkoenig

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Posted 06 April 2022 - 02:30 PM

I've attached two images from testing my 12.5" f/4.75 mirror. (I would've added a third for the other side of the offset but I hit the image size limit).

 

Qualitatively, does my mirror suck? Is it OK? Or is more information needed (and if so, what information)? This is my first time running a foucault test and although I got it working, I have no idea how to interpret the results!

 

 

Knife edge centered:

 

Offset:

Attached Thumbnails

  • Screenshot from 2022-04-06 15-23-35.png
  • Screenshot from 2022-04-06 15-21-34.png


#2 Jim45157

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Posted 06 April 2022 - 02:46 PM

have you read at least 4 zones 



#3 Steve Dodds

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Posted 06 April 2022 - 02:55 PM

Those images are so poor it's really hard to tell anything.



#4 dave brock

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Posted 06 April 2022 - 03:47 PM

Are the images rotated 90 degrees?
You can put the 3rd image in a different post.

#5 ccaissie

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Posted 06 April 2022 - 04:23 PM

OK, your mirror is not a sphere,  but without more info can't tell if it's Oblate Spheroid (undercorrected) or Prolate/Parabaloid/Hyperboloid.  The first image shows a gray flattish center so your knife edge is roughly at that radius.

The second shows a donut shadow and the knife edge is at the radius of the approx 60-75% zone...the crest of the donut curve.

 

Is this a commercial mirror in your telescope?  How does it perform?  Why are you testing it?   I'll ask you.....does it suck?  If it's not up to a certain standard that you have in mind, what will you do with it?

 

Testing....now all you have to do is figure out how far the knife edge has to travel from the point where the center zone is gray, to the next zones out.  

 

There are dozens of instructive videos and articles on doing all kinds of testing.  You have accomplished showing the shadows with the Foucault setup, now you need to take careful measurements on more zones and plug these numbers into a formula to compare your readings with the ideal.

 

What software will you use for data analysis?   Have you tried using a Ronchi test?  Have you made a zonal mask?  How about the star test?  What does it show?



#6 Dale Eason

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Posted 06 April 2022 - 04:29 PM

As said by others those images are pretty much useless.  So first you need to learn how to take useful Foucault images.  We can help you learn how to do that.  First what is the camera you are using.  Does it have user controls that let you set focus and exposure?  Can you take a picture of your Foucault apparatus for us?  

 

If you need to alter the size of the image to CN's requirements I use IrFanView on a windows PC.  With it you can change the size of the image and also when saving a .jpg specify a file size limit that it will use to compress the image down to.  I usually set it to 300K.

 

Then here is an example of a good and useful Foucault image that you should try to for.   Then the info you need to give is the mirror diameter and F number and the distance of the knife from the mirror or at least from the radius of curvature of the mirror.

 

It is actually hard to judge the mirror quality from just one Foucault image.  In particular it is next to impossible to analyze if the correction is proper.  Instead we use a technique called Foucault data reduction.  Usually using a mask and several knife offsets to measure were each zone focuses.

 

Here is a good link that explains it.  https://atm-workshop.com/foucault.html

 

Dale

Attached Thumbnails

  • zoc_foucault.jpg


#7 davidc135

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Posted 06 April 2022 - 04:30 PM

I take it that the knife edge is from below? It looks moderately smooth with some tde and under-corrected?

 

Show or describe your setup. Does the source move as one with the knife edge?

 

To get from the top photo with the mirror centre zone nulled to the .71r zone nulled (approx as above) should be 2.1mm and to the edge nulled 4.2mm movement of the source/knife edge. For a very rough idea.

 

David


Edited by davidc135, 06 April 2022 - 04:32 PM.


#8 Dale Eason

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Posted 06 April 2022 - 04:32 PM

The first problem with your images is that they are out of focus.  The camera should be manually focused on the mirror.



#9 alexkoenig

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Posted 06 April 2022 - 06:01 PM

Thanks for the tips all, it's clear I don't know what I'm doing so I appreciate the help, haha. I'll do my best to answer most of these questions and see what I can make of this.

 

I want to test this mirror because I'm considering using it for a compact travel dob -- it would be nice to see whether the mirror is up to the task first. It seems to be a DIY mirror that was a leftover from a friend who bought it on ebay from an anonymous seller a long while ago, so I don't have strong reasons to trust that it will work well before I go and build a scope around it.

 

I've since taken the test rig down, but I can describe it. In this setup the knife edge is placed horizontally from below. The light source was placed at 2x the focal length of the telescope such that the reflected point converged at the same distance as the source. The light source and camera were kept stationary, and I varied the position of the knife edge. I used a canon T7i for imaging, and I found I had to take the lens off in order to get an image -- which means I wasn't able to focus on the mirror, since I had no lens to focus with. I just moved the camera sensor back until the image filled most of the frame. I tried 35 mm and 100 mm lenses but those didn't seem to work for whatever reason -- it seemed like the where the point source re-converged was 'inside' the camera lens, and so I couldn't place the knife edge on it.

 

My images also had trouble with glare / a double image -- especially visible in the first image -- which may both be a result of improper focus, but I wasn't sure how to resolve that. The contrast in my images is obviously much more poor than that of the one that Dale provided.

 

I think I'll need a more robust testing setup. Mine is simply on my floor, which is the sturdiest surface I have that is sufficiently long, but the floor is quite shaky. I also have no good method to precisely position the knife edge; I was moving it by hand.

 

So my understanding is that I'll need to be able to take cleaner images, do so for several knife edge locations, and then use the data reduction software to process results quantitatively. I think that will be doable, though my images are probably far from sufficient quality as they are, and it's not too clear why.



#10 Steve Dodds

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Posted 06 April 2022 - 06:07 PM

You need to get a good image visually, then use a camera with the lens on and focused at infinity.



#11 ccaissie

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Posted 06 April 2022 - 06:45 PM

If your main objective is to learn testing, then you'll have lots of help here.

If you just want an evaluation of the mirror, there are a lot of folks here who can do that for you, too.

Where are you located?



#12 ccaissie

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Posted 06 April 2022 - 06:52 PM

So my understanding is that I'll need to be able to take cleaner images, do so for several knife edge locations, and then use the data reduction software to process results quantitatively. I think that will be doable, though my images are probably far from sufficient quality as they are, and it's not too clear why.

Pre-internet/digital images, ATM's just learned to measure visually and record their test data for reduction by calculator...or paper and pencil.  

 

Building a better tester and spending time working with it, and reading up on optical test principles/ ideas is key.  



#13 alexkoenig

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Posted 07 April 2022 - 11:43 AM

If your main objective is to learn testing, then you'll have lots of help here.

If you just want an evaluation of the mirror, there are a lot of folks here who can do that for you, too.

Where are you located?

You know, this might be the better option, though I do want to work more on the test setup too. How much does it usually cost for a service like this? Online I only found AstroReflect, which is free (!), but I'm based in the Boston area.




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