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Classic Telescope Testing

beginner classic collimation optics refractor
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#101 DMala

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Posted 07 January 2019 - 02:34 PM

It sounds interesting but I am not clear on one aspect. I assume Allan's telescope lenses were well collimated. Now the collimations screws have been slightly twisted to center the interference rings. What happened to the initial collimation? Wouldn't a star test to verify collimation be warranted now?

#102 DAVIDG

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Posted 07 January 2019 - 02:58 PM

 No,  it is not the collimation screws he turned but the screws that hold the retainer ring on.  To do the test, the lens in it's cell is removed from the telescope and your adjusting the retainer ring of the cell. Adjusting the air gap is independent of collimating the objective.

 

              - Dave 


Edited by DAVIDG, 07 January 2019 - 03:01 PM.

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#103 AllanDystrup

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Posted 07 January 2019 - 03:00 PM

DMala,

 

     The 3 screws I’m adjusting for centering the interference pattern are the small screws in the lens retaining ring (not the 3 screws in the “ears” of the lens cell for collimating the cell to the tube/focuser). The screws in the retaining ring presses down in 3 points on the front lens, thus adjusting the tilt/spacing between the front and rear lens.

 

     — Allan


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#104 DMala

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Posted 07 January 2019 - 03:36 PM

Got it thanks

#105 Bomber Bob

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Posted 27 January 2019 - 10:01 AM

BB's DPAC Rig v3...

 

BBs DPAC Gear v3 S01.jpg BBs DPAC Gear v3 S02.jpg BBs DPAC Gear v3 S03.jpg BBs DPAC Gear v3 S04.jpg

 

Much simpler construction that puts my eye closer to the grating, and gently presses the LEDs to it.  Loaded with a green and a red, and takes maybe 2 minutes to swap wires between them.  I turn the intensity to MAX to check alignment, then down to MIN for testing, and it's much easier on my eyes.  The barrel rotates so I can get the bars vertical.

 

No surprise that the Takahashi FC-50 had thin black straight bars -- another reference lens!


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#106 AllanDystrup

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Posted 27 January 2019 - 11:11 AM

Nice setup BB!

 

Here's my test setup;  -- somewhat simpler, but works great for me:

 

DPAC-01.jpg
Flat on a table stand on the left;
Battery box with On/Off & Dimmer center;
Ronchi eyepiece and camera on the right

 

DPAC-02.jpg
Close-up of battery box & Ronchi Eyepiece
The lid with the LED can be flipped up and the Ronchi screen changed to another LPI.
Also the top of the EP can be changed to a lid with another color LED.
The barrel of the EP is an old 1.25" plastic film canister.

 

 

 

Here's my latest test run, a 2" Unitron with a very good objective:

 

DPAC-03.jpg

DPAC-04.jpg

 

     -- Allan

 

 


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#107 rcwolpert

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Posted 03 February 2019 - 01:45 PM

BB's DPAC Rig v3...

 

attachicon.gif BBs DPAC Gear v3 S01.jpgattachicon.gif BBs DPAC Gear v3 S02.jpgattachicon.gif BBs DPAC Gear v3 S03.jpgattachicon.gif BBs DPAC Gear v3 S04.jpg

 

Much simpler construction that puts my eye closer to the grating, and gently presses the LEDs to it.  Loaded with a green and a red, and takes maybe 2 minutes to swap wires between them.  I turn the intensity to MAX to check alignment, then down to MIN for testing, and it's much easier on my eyes.  The barrel rotates so I can get the bars vertical.

 

No surprise that the Takahashi FC-50 had thin black straight bars -- another reference lens!

 

Very nice setup!! waytogo.gif



#108 Bomber Bob

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Posted 03 February 2019 - 02:25 PM

Thanks, y'all.  Simpler works better.  The 3 "L's" are the same size - makes alignment easier & quicker - and they stack in a corner of the closet.  Pictures are better, but finding the "sweet spot" with the Nikon is still tricky.


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#109 tim53

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Posted 04 February 2019 - 04:20 PM

I think i'm going to do something similar for testing Newt mirrors, by simplifying what i last set up for the Schaefer scope's new mirror.  In that setup, I have my 12" Perkin Elmer flat and the mirror under test both on adjustable cells based on Gary Seronik's design.  But because my flat isn't perforated, I had to insert a diagonal and focuser into the train.  That worked fine when I was testing the mirror with it's degraded coating still on, because there was enough reflectivity that I could see the laser grid reflected back to the mirror under test, and could get the flat aligned perpendicular to the mirror.  But yesterday, after I'd cleaned the coatings off, I couldn't see the grid reflection anymore, so I couldn't get the flat perpendicular close enough to see anything in the ronchi eyepiece (with camera in place of eyeball).  So I think that, since the camera is smaller than the secondary mirror, I'll take the secondary out all together, and mount the camera between the mirror being tested and the flat.  I'll have to make a 1 1/4" holder for the camera that can be adjusted in height and angle, to enable collimating it with the mirror and the flat, but I think that should be a lot easier than it currently is - where I just have too many things to adjust and too faint reflections off the uncoated primary.

 

-Tim.



#110 DAVIDG

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Posted 04 February 2019 - 06:06 PM

 Tim,

   With the lights off I can see the laser off the uncoated mirror onto the flat just fine.  The first thing I do is get the secondary and the mirror to tested aligned using the laser. So the beam off the secondary, hits the mirror to be tested and then back onto the secondary and back onto the laser collimator. You should be able to do that with the light on. Then I switch to the grid pattern in the laser collimator and turn the lights off. I can easily see the grid projected onto the flat and adjust the flat so the pattern reflected off of in lines up with the one on the mirror being tested. It take me about 3 minutes to get things lined up to test.  One thing that is critical is that your get the Ronchi screen or knife edge pretty close the focal plane of the optics being tested. If not that will make it hard to find the image. 

   A camera in replace of the secondary is going to make things more difficult to get aligned unless your going to be able to switch it out for the laser and have the camera go exactly back into place of the laser.

   Personally I don't like viewing test results only with a camera because it can add artifacts that you won't detect unless unless you  actually look with your eye. Remember when you posted a test image with straight lines and you thought is was good but I noticed no shadow of the secondary because somehow you focused on the Ronchi screen and not the image coming through it.  That is the kind of stuff you can miss when just using camera.

 

      

 

                  - Dave 


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#111 tim53

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Posted 04 February 2019 - 09:04 PM

My lasermax grid must be too dim for this.  I did everything you described before i stripped the coatings off the mirror, and had no problem getting the grid to overlap back on itself from the flat back on the mirror under test.  With the coating gone, I still do everything you describe with having the laser light come back on itself using the Hotek Blug (with barlow built in).  But when I replace the dot with the grid, I can't see the grid reflecting from the flat.  Maybe I need to dust the mirror so I can see the reflection better?

 

I like using the camera.  I could try making another ronchi eyepiece (I cannibalized the one i had to make this one), but I can't seem to remember where I put the other screen I had.  Anyway, the camera and software I have allows me to set the exposure time anywhere from a few milliseconds on up to several minutes, and adjust the gain and gamma to bring out faint glows to help find the return beam, but all I could see was a faint reflection off the walls of the film cannister, with no clue what direction to move the flat.  I might need to try again when I'm not frustrated, though.  Currently, I mount the ronchi eyepiece in the focuser after collimating everything (except the flat, this time) in the focuser.  If I remove the secondary and put the camera there, it will be in a 1 1/4" tube I can put the laser in first.  

 

-Tim.



#112 tim53

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Posted 04 February 2019 - 09:06 PM

As an aside, I used the same lasermax with the oil flat to get the optic axis of the telescope perpendicular to the oil.  I had no trouble seeing the second and third reflections of the lasermax grid on the oil, but all those tests were of coated optics (however degraded).  



#113 DAVIDG

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Posted 05 February 2019 - 09:53 AM

 Tim,

   I think what the problem might be is that you not close enough to the focal plane. So if your off, the   out of focus image will be dim and small so easy to miss with the small field of view of the camera. That is another reason to use your eye along with a camera. I move my head around and can find the image off axis and adjust it to bring it in.  Another thing that is critical is the light source needs to be very close to optical axis when your using a camera vs the eye. If your slightly off the optical axis the image is shifted by the same amount to the other side and the camera won't see it. 

   Using a camera to document the result is fine as long as they agree with the visual result but if you just use a camera there is always the possibility of issues that they  had when testing the Hubble and unknown error made the results wrong.

   I found that the my cell phone will distort the image badly when testing optical flats be interference. Visually I'll see  straight interference bands but a slight tilt of my cell phone makes them bow very strongly making a 1/10 wave surface look like one that is 2 waves !  Another possibility is if you have astigmatism, you can adjust the tilt of the  camera to cancel it so you won't see it in the test results. 

   So I strongly recommend that a camera be used as the secondary means to visualize the test results and be sure they match what you see with your eyes. 

 

                     - Dave 


Edited by DAVIDG, 05 February 2019 - 09:56 AM.

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#114 tim53

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Posted 05 February 2019 - 03:28 PM

Hi Dave:

 

What I did with planetary imaging with the machine vision cameras was to increase the exposure time and gain to make the out of focus planet image easy to see, even if it's outside the field of view when focused.  I could also often see the scattered light around the planet and adjust pointing to bring it into the field of the ccd.  So i was trying the same technique with the Firefly camera.  It should have worked, but maybe I didn't try a long enough exposure (like maybe 1 second with the gain all the way up).  Before that step, I made sure I was close to the focal plane.

 

I've always had trouble while using the oil flat setup, in finding the ronchigram if I haven't gotten the system bang on collimation and perpendicular to the oil surface before trying to find it with my eye.  So now, same thing.  

 

I probably won't have time until maybe Sunday to try again, as we need to meet the inspector at Cosmic Acres for our (hopefully) final inspection on Friday.

 

-Tim.




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