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Optical bench design

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#1 ToolMayNARD

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Posted 14 June 2018 - 05:01 AM

Hello to all,

 

I kindy ask you help to retrieve correct parts I would need to purchase in order to prepare an optical bench for complete telescope testing (not single optics, only complete telescopes).

 

I already have a 400 mm optical flat (built by Zen in Italy) and now I need to retrieve other parts such as the beamsplitter and a light source. I tried to find some info surfing over the Internet but there are lot of contraddictory info and honestly I am very confused now. I need to purchase stuff in EU so Edmund Optics is an option, Surplusshed is not.

 

Thanks

 

Mauro

 

 


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#2 BGRE

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Posted 14 June 2018 - 05:12 AM

What tests are you planning?
Edmund do have Ronchi gratings and a selection of various types of beamsplitter for example.
There is also a European (Germany) source of PDI plates.

Edited by BGRE, 14 June 2018 - 05:14 AM.

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#3 Pinbout

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Posted 14 June 2018 - 05:14 AM

https://youtu.be/OYqttRps4Bk

 

heres a gimble  you can use for the flat

 

ronchiscreens.com sell $7 gratings


Edited by Pinbout, 14 June 2018 - 05:28 AM.

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#4 ToolMayNARD

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Posted 14 June 2018 - 05:45 AM

Hello,

 

long term project is a Shack-Hartmann sensor, but for now I will just do some in-house roddier test in auto-collimation, just to understand what can be done with this software.

 

I will buy also a Ronchi grating, it could be useful but not mandatory for type of test I have to do.

 

Thank you pibout, your video shows an interesting solution!

 

Regards

 

Mauro



#5 ToolMayNARD

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Posted 14 June 2018 - 05:47 AM

With reagards to Edmund Optics... I have no idea what beamsplitter I have toi purchase. There are dozens of type and models!



#6 TOMDEY

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Posted 14 June 2018 - 05:52 AM

Autocollimation testing is Wonderful! Depending on how fancy you may want to get... Since you are starting from scratch, might want to provide for both horizontal and vertical testing! Many (most?) large scopes manifest gravity-induced aberrations, when disposed horizontally, that go away when pointing upward. The AC flat, when pointing down: I use a "Silly Putty Ring" around the edge that rests in a groove in a ring. The ring is on 6-point whiffle-tree and rotates so that the plano residuals can be backed out of the test. I have some pictures someplace of a 24-inch flat that I mounted that way. To get test accuracy better than 1/20-wave... vertical with backouts is the way to go.

 

For just a good warm feeling that the scope is decent, horizontal with a beamsplitter, Ronchi, and knife edge will work fine.  Tom


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#7 BGRE

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Posted 14 June 2018 - 06:23 AM

On axis autocollimation testing isn't a complete test of telescope optics in that its blind to a turned up or turned down edge of a Newtonian, Cassegrain or Gregorian secondary.
The test is blind to errors in unilluminated portions of the various optical surfaces.

#8 Mike I. Jones

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Posted 14 June 2018 - 06:39 AM

Not really, just rotate the telescope to different field angles.  DPAC works off axis as well, thus allowing performance assessment right to the edge of components.

 

Mauro - have you considered interferometry in your thinking?  That would produce quantitative assessment that Ronchi etc. will not provide.  Only drawback is being monochromatic, but using both green and red lasers would help with that.

 

S-H works in white light, but requires a well-corrected collimator eyepiece to form the exit pupil that the lenslet arrays are located at.  I've designed, built and used several of those, they're basically eyepieces with a very narrow FOV.  Off-the-shelf achromats usually do just fine.  I'd be glad to design one for you.  You'd need different S-H collimator eyepieces to accomodate different test telescope focal ratios, unless you have lenslet arrays of different sizes.

 

I really like Tom's idea of testing vertically, he's right about that.  How thick is your 400mm flat?  Even if your flat sags under gravity, it ends up only being a very slight focus shift.  

 

Mike


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#9 ToolMayNARD

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Posted 14 June 2018 - 07:55 AM

Hello,

 

thank you very much for given answers.

 

Mike, I decided to go with the Shack-Hartmann and leave out interpherometere because it is too much demanding. Shack-Hartmann test should be less sensitive to vibrations and can be done in monochromatic light as well using proper filters (just to mjeasure spherochromatism in refractors or compund optics). It should be the SH by Alcoor System (http://www.alcor-sys...ope100x100.html), not decided yet. 

 

The optical flat is 45 mm thick and we will prepare an adequate cage to hold it. It is made of Suprax. Not the best material, I know, but it was offered at good price and it has a smooth surface. Zen is very good at doing this.

 

I tried to figure out how to prepare a "state-of-the-art" optical bech for testing complete telescopes (not single lenses or mirrors, I don't need this since I do not build them myself) but I wasn't able to find a complete practical guide, only some info around.

 

TOMDEY, your idea is very interesting and for sure we can design something that allows for vertical testing.

 

Mauro


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#10 Pinbout

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Posted 14 June 2018 - 09:02 AM

What's going to be a typical test with the flat?



#11 ToolMayNARD

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Posted 14 June 2018 - 09:31 AM

On axis check of a point source through a sensor. 



#12 TOMDEY

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Posted 14 June 2018 - 02:19 PM

I really like Tom's idea of testing vertically, he's right about that.  How thick is your 400mm flat?  Even if your flat sags under gravity, it ends up only being a very slight focus shift.  

 

Mike

Thanx for that affirmation, Mike. I realize most people won't want to go to that extent... but it does eliminate one of the greatest unknowns. Here are a few pictures of that test tower plano mount. I built that in the barn, and surrounded it with a big foamboard shell with fans. Technique is to blow it out and then fans off for testing. A tower accommodates ANY up-pointing telescope or mirror. If the plano is perforated, can put the mirror focus right at that hole and conveniently test from right behind the plano!

PS: Advantage of AC also is that that double-pass config is doubly-sensitized to the scope or mirror, but only singly-sensitized to any residual imperfections in the AC flat.  Tom

Attached Thumbnails

  • 02.0 test tower mirror 60.jpg
  • 02.1 plano mount 1.jpg
  • 02.2 roto-ring and edge-support hose 60.jpg

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#13 TOMDEY

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Posted 14 June 2018 - 02:20 PM

one more tom

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  • 02.3 plano mounted.jpg

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#14 tim53

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Posted 14 June 2018 - 03:37 PM

I like that.  I was thinking about doing something similar, but opposite, for testing scopes using an oil flat.  For the mirror support, a simple cross of thin plywood would allow assessment of the edge of the mirror, but probably would cause distortion if the mirror is large or thin.



#15 Pinbout

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Posted 14 June 2018 - 03:41 PM

https://www.youtube....fqQcW8rBwA&t=0s

 

https://www.youtube....21IUi_tyFmSieQr


Edited by Pinbout, 14 June 2018 - 03:42 PM.

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#16 tim53

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Posted 14 June 2018 - 04:54 PM

I thought Dick Parker's optical flat was 16", that he figure himself?



#17 Pinbout

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Posted 14 June 2018 - 05:15 PM

I thought Dick Parker's optical flat was 16", that he figure himself?

That’s the one he keeps at his shop, the 12.5 is what he travels with.



#18 BGRE

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Posted 14 June 2018 - 05:25 PM

Not really, just rotate the telescope to different field angles.  DPAC works off axis as well, thus allowing performance assessment right to the edge of components.
 
Mauro - have you considered interferometry in your thinking?  That would produce quantitative assessment that Ronchi etc. will not provide.  Only drawback is being monochromatic, but using both green and red lasers would help with that.
 
S-H works in white light, but requires a well-corrected collimator eyepiece to form the exit pupil that the lenslet arrays are located at.  I've designed, built and used several of those, they're basically eyepieces with a very narrow FOV.  Off-the-shelf achromats usually do just fine.  I'd be glad to design one for you.  You'd need different S-H collimator eyepieces to accomodate different test telescope focal ratios, unless you have lenslet arrays of different sizes.
 
I really like Tom's idea of testing vertically, he's right about that.  How thick is your 400mm flat?  Even if your flat sags under gravity, it ends up only being a very slight focus shift.  
 
Mike

Yes, but how many actually do that.
Exactly the same considerations apply to star testing, yet some use the results of an on axis test to guide figuring.

Tilting either the return flat or the OTA is required to do a true off axis test using DPAC.
Few DPAC setups appear to make provision for this.
With a liquid flat the OTA has to be tilted for off axis testing as the flat cannot be.

#19 ToolMayNARD

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Posted 15 June 2018 - 04:27 AM

Guys, you are giving very informative answer, thank you. However no one gave me some specific info I need, for instance what beamsplitter and what light source should I choose. Let's narrow choice to Edmund Scientific catalogue.

 

Are standard cube beamsplitters ok? Or better the broadband non-polarizing cube beamsplitters? Which size? Transmission ratio 50/50 I suppose.

 

 

https://www.edmundop...-Beamsplitters/

 

https://www.edmundop...-Beamsplitters/

 

https://www.edmundop...-Beamsplitters/

 

Any suggestion about light source? Fiber? Laser? ... completely ignorant about this!

 

Thank you

 

Mauro



#20 BGRE

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Posted 15 June 2018 - 07:43 AM

If you intend to use a SH array in a wavefront sensor as is done on the VATT, NOT, MTT etc., first you need to use a lens to image the telescope (aperture) STOP (usually the primary mirror) onto the SH array.
Usually a pair of cemented doublet achromats in a Petzval style setup suffices to image the pupil onto the SH lenset array.The light incident on the SH array is collimated (ie the pupil image formed on the SH lenset array is telecentric). A laser source and a pinhole are used to create a diffraction limited source to calibrate the SH sensor and pupil imaging relay errors.
These SH wavefront sensors usually use a star as a source to measure the telescope errors. In your case either a diffraction limited pinhole source or a laser coupled to a single mode fiber together with a beamsplitter (cube is probably best) would be used.
However the wavefront error contribution of the beamsplitter needs to be measured.

Now such a setup wherein which the relay lens has to be tailored to suit the f# of each telescope is a lot more complex and expensive than using a simple point diffraction interferometer plate with an array of pinhole sizes so that the correct one can be selected to suit the f# of the telescope being tested.
One merely needs a diffraction limited source a beamsplitter cube and a camera with lens to image the pupil onto the camera image sensor.

Manual for F/5 MMT wavefront sensor:
https://www.cfa.harv.../wfs_manual.pdf
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#21 MKV

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Posted 15 June 2018 - 09:47 AM

Now such a setup wherein which the relay lens has to be tailored to suit the f# of each telescope is a lot more complex and expensive than using a simple point diffraction interferometer plate with an array of pinhole sizes so that the correct one can be selected to suit the f# of the telescope being tested. 

waytogo.gif



#22 BGRE

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Posted 15 June 2018 - 04:11 PM

The achilles heel of using a beamsplitter in most setups is there is no easy way to measure and correct the inevitable aberrations produced by the beamsplitter.
The only solution that I'm aware of is to reimage the pupil using an image space telecentric lens and place the beamsplitter in this space. In the case of a point diffraction plate an additional lens is used for form a real focus for the PDI plate. In the case of a SH wavefront sensor the lenset array is placed at the reimaged pupil.
The laser (or other) light source is collimated so that the pupil reimaging lens forms a real accessible focal spot.
Calibration is a 2 step process:
1) A diffraction limited point source of spherical waves is place at this focus with the test source blocked or turned off.
The resultant measured wavefront gives the combined error of the SH sensor (or the additional lens in the case of a PDI plate) and the pupil reimaging optics.

2) a plane mirror is used in the catseye position at this real focus with the test source on.
The resultant measured wavefront gives the error of the test source and associated optics plus the error of the pupil reimaging optics and the SH sensor (or the additional lens in the case of a PDI plate).

Both of these measurements can then be used to correct the measured wavefront for the telescope being tested.

#23 BGRE

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Posted 15 June 2018 - 04:34 PM

Another somewhat expensive option is to use a beamsplitter with very low wavefront error with collimated light so that its wavefront error in divergent or convergent light can be accurately calculated.
Suitable low wavefront error beamsplitter cubes are available for a few thousand USD each but not from Edmund optics.
A wedged fused silica beamsplitter plate is probably a cheaper option as its somewhat cheaper and readily available with low error surfaces.
Calculating its wavefront error can either be done by raytracing or deriving an exact analytical solution to arbitrary order is a fairly simple exercise.
The test source should be transmitted through the beamsplitter and the returning beam reflected from the beamsplitter should be used for wavefront measurements so that the aberrations incured when imaging the pupil onto the detector in the case of using a PDI plate (or onto the SH lenset array) are small.

#24 BGRE

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Posted 15 June 2018 - 06:23 PM

In principle a suitable (ROC ~ telescope exit pupil distance from wavefront sensor, R/D ~ equal to telescope f#) accurate concave spherical mirror could be used to measure wavefront sensor (or beamsplitter) errors.

Edited by BGRE, 15 June 2018 - 07:00 PM.


#25 Mike I. Jones

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Posted 15 June 2018 - 07:45 PM

Guys, you are giving very informative answer, thank you. However no one gave me some specific info I need, for instance what beamsplitter and what light source should I choose. Let's narrow choice to Edmund Scientific catalogue.

 

Are standard cube beamsplitters ok? Or better the broadband non-polarizing cube beamsplitters? Which size? Transmission ratio 50/50 I suppose.

 

 

https://www.edmundop...-Beamsplitters/

 

https://www.edmundop...-Beamsplitters/

 

https://www.edmundop...-Beamsplitters/

 

Any suggestion about light source? Fiber? Laser? ... completely ignorant about this!

 

Thank you

 

Mauro

Hi Mauro - I guess I'm confused about what you want to do.  Are you going to buy the Alcor SHWFS for quantitative testing that gives real wavefront error values, but also perform non-quantitative assessment with Ronchi, null KE, etc.?  Helping you select components would be much easier if you describe the goals for your test setup.  I for one do not follow what all you really want to do.

 

Also, McAfee virus checking software is saying the Alcor website is very dangerous to connect to.  I don't know why, but I'm not going to visit that site.




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