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How DPAC your refractor

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#51 peleuba

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Posted 24 September 2018 - 12:52 PM

Well, with the gracious help from Paul (Peleuba), I assembled my first 2" DPAC Eyepiece.  It has a slot for my Ronchi Screens .Com gratings and removable LEDs.  

 

<SNIP>

 

So what camera settings do you all use as I found I need to put my eye right up to the screen?

 

 

 

Jeff - that is awesome!  I was glad to help but you were already a very good DPAC tester.  

 

I put the camera on a tripod and place it right up against the Ronchi eyepiece and start shooting.  I usually leave all the setting on automatic.  Out of 50 shots, I will get 10 that are usable.  But its very quick to do so with a 80% reject rate its not bad.    


Edited by peleuba, 24 September 2018 - 01:21 PM.


#52 peleuba

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Posted 24 September 2018 - 01:11 PM

For a simple DPAC test you need:

 

1.  An optical Flat

2.  A stand for the flat to rest on

3.  V Blocks to support the telescope under test

4.  A 2" eyepice with LED's and Ronchi Screen

5.  A power supply (3 Volts) to power the LED's

 

I test in Green, Blue, Red and White light allowing me to judge correction at each wavelength.  By far the most useful to me as a visual observer is green and white.

 

Pictures of my DPAC setup...

Attached Thumbnails

  • DPAC (1).JPG
  • DPAC (2).JPG
  • DPAC (3).JPG
  • DPAC (8).JPG
  • DPAC (21).JPG

Edited by peleuba, 24 September 2018 - 02:19 PM.

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#53 starman876

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Posted 24 September 2018 - 01:16 PM

How much of a difference have you seen with the blue LED versus the green LED.   So far I have found the blue LED test more challenging for most optics.  

 

You could place a web cam on the back of your 2" eyepiece and see the results on a laptop screen.   I have found the Microsoft life cam to be excellent for this application. 



#54 starman876

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Posted 24 September 2018 - 01:19 PM

For a simple DPAC test you need:

 

1.  An optical Flat

2.  A stand for the flat to rest on

3.  V Blocks to support the telescope under test

4.  A 2" eyepice with LED's and Ronchi Screen

 

Pictures of my DPAC setup...

I like the way you slide the RONCHI screen mounted on the slide into the 2" eyepiece.  That is really nice.  I cut mine in a circle to fit inside the 35mm film canister,   



#55 peleuba

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Posted 24 September 2018 - 01:20 PM

Additional pictures...

 

 

Attached Thumbnails

  • DPAC (10).JPG
  • DPAC (11).JPG
  • DPAC (12).JPG
  • DPAC (16).JPG
  • DPAC (17).JPG
  • DPAC (9).JPG

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#56 starman876

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Posted 24 September 2018 - 01:20 PM

http://www.ronchiscreens.com/

 

this is the source for RONCHI screens



#57 peleuba

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Posted 24 September 2018 - 01:41 PM

Some more...

 

 

Attached Thumbnails

  • DPAC (5).JPG
  • Test.JPG
  • IMG_1361.jpg
  • DPAC2.jpg

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#58 peleuba

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Posted 24 September 2018 - 01:55 PM

I like the way you slide the RONCHI screen mounted on the slide into the 2" eyepiece.  That is really nice.  I cut mine in a circle to fit inside the 35mm film canister,   

 

Thanks.  See here:

 

https://www.cloudyni... dpac eyepiece
 

 

It explains the evolution of the eyepiece.  Its 3D printed and very handy.  



#59 peleuba

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Posted 24 September 2018 - 02:06 PM

How much of a difference have you seen with the blue LED versus the green LED.   So far I have found the blue LED test more challenging for most optics.  

 

You could place a web cam on the back of your 2" eyepiece and see the results on a laptop screen.   I have found the Microsoft life cam to be excellent for this application. 

 

There is some difference in the bowing of the bands when testing achromats.  Also, surprisingly, a TEC160FL was really nice in green, good, too in Red, but poor in blue.  The bands had a definite/noticeable bow in Blue.  I need to find the exact wave-length that my LED's are...

 

Thanks for the advice on a Microsoft Life camera.  I use a both Nikon and Canon digicams and DSLR's and these are not the easiest to use to get good bench photos.  

 

Which MS Life camera have you had success with?  I see there are a ton of different models.


Edited by peleuba, 24 September 2018 - 02:50 PM.


#60 starman876

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Posted 24 September 2018 - 02:35 PM

http://www.microcent...o-usb-20-webcam

 

this one works very well.  With a 2" nosepiece you could make it fit inside.  It focuses really well for the DPAC application.   I have used the blue LED in the webcam as my light source.  Only problem is you need to cut down on the light output because it is to bright.  I opened up the camera and took out the blue LED an replaced it with a green LED and a control to adjust brightness.   If you do not want to use the BLUE led all you need to do is mask it off.   I have been told there is another model which will zoom in 2X.  I will have to find it, but that sounds like it would help when taking pictures for better resolution.  



#61 starman876

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Posted 24 September 2018 - 02:43 PM

Some more...

really nice DPAC on that lens. That lens should perform really well.



#62 starman876

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Posted 24 September 2018 - 02:47 PM

There is some difference in the bowing of the bands when testing achromats.  Also, surprisingly, a TEC160FL was really nice in green, good, too in Red, by poor in blue.  The bands had a definite/noticeable bow in Blue.  I need to find the exact wave-length that my LED's are...

 

Thanks for the advice on a Microsoft Life camera.  I use a both Nikon and Canon digicams and DSLR's and these are not the easiest to use to get good bench photos.  

 

Which MS Life camera have you had success with?  I see there are a ton of different models.

It would be nice if they provided the wavelength of the LED.   

 

My TMB 100/800 has nice straight lines in blue.   

 

Is the TEC 160 a doublet or a triplet.    My limited knowledge of optics tells me that it might be hard to control the blue spectrum unless you have a triplet.  



#63 peleuba

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Posted 24 September 2018 - 02:49 PM

One last thing...    Below is a device that was invented/assembled by a good friend who in the past, managed the operations (as opposed to science) team that supported WFC-3 and WFPC-2 on the HST.  He recently left STScI and is doing his own research.  He is slated to be on the Ops team for the James Webb instrument whenever that telescope goes into orbit. 

 

The gadget below is a star test device that is used in double pass.  If you like to star test, this device is the bomb.  It generates a parallel beam of light which presents itself as an Airey disk using an LED a beam splitter, 5 micron pinhole and Baader Fluorite Flat Field Corrector.  You place this into the focuser of the telescope and you rack it in/out of focus as you would a conventional  star test.  The beam splitter allows you to view the results.  Since the light traverses the beam splitter and Baader FFC it needs to be of VERY high quality as the aberrations in each are cumulative where as the aberrations in the flat are negligible as long as the flat is smooth and regular.  This is not an inexpensive device.  Parts were sourced from Edmunds and other optical companies serving the pro market.  But, Scopestuff supplied some of the plumbing to make it all work.       

 

Now, I just need more telescopes to test!

 

In closing, I have found bench testing to be of great help to me in understanding a telescope's relative performance.  I never have to wonder if its bad seeing or a bad lens.  The answer always presents itself. 

Attached Thumbnails

  • DPAC Star Tester.jpg

Edited by peleuba, 24 September 2018 - 06:46 PM.

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#64 peleuba

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Posted 24 September 2018 - 02:55 PM

http://www.microcent...o-usb-20-webcam

 

this one works very well.  With a 2" nosepiece you could make it fit inside.  It focuses really well for the DPAC application.   I have used the blue LED in the webcam as my light source.  Only problem is you need to cut down on the light output because it is to bright.  I opened up the camera and took out the blue LED an replaced it with a green LED and a control to adjust brightness.   If you do not want to use the BLUE led all you need to do is mask it off.   I have been told there is another model which will zoom in 2X.  I will have to find it, but that sounds like it would help when taking pictures for better resolution.  

Thanks!   Regarding brightness, I have a Potentiometer installed in-line of my power source for my LED eyepieces.  See below.   The pot is the orange and black knob and allows me to dial down the brightness of the LED.  

Attached Thumbnails

  • DPAC (20).JPG

Edited by peleuba, 24 September 2018 - 02:59 PM.

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#65 peleuba

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Posted 24 September 2018 - 02:59 PM

Is the TEC 160 a doublet or a triplet.    My limited knowledge of optics tells me that it might be hard to control the blue spectrum unless you have a triplet.  

 

It was a triplet and outstanding visually.  I don't take photographs (unless bench images!), strictly visual here.



#66 starman876

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Posted 24 September 2018 - 03:15 PM

DPAC takes all the surprises out of wondering is it my scope or is it the seeing.  If you see nice straight lines and then when you take it outside to view and nothing seems right you know it is not the scope.  On the other hand if you see a lot of bowing in DPAC you might as well not even take it outside.


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#67 moshen

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Posted 24 September 2018 - 10:55 PM

Here's my self designed 3D printed Ronchi eyepiece testing my Tak FC-100DF. I have RGB LEDs I've now ordered from digikey that have specs with peak wavelength. 

 

Results in RGB. 

 

Screenshot 2018-04-25 11.30.25.jpg

Attached Thumbnails

  • DSCF5278 (1).jpg

Edited by moshen, 24 September 2018 - 11:06 PM.

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#68 starman876

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Posted 25 September 2018 - 07:21 AM

Here's my self designed 3D printed Ronchi eyepiece testing my Tak FC-100DF. I have RGB LEDs I've now ordered from digikey that have specs with peak wavelength. 

 

Results in RGB. 

 

Very nice

 

What is the circuit board that looks like it has a readout on it do?



#69 Jeff B

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Posted 25 September 2018 - 10:45 AM

I just love the idea of 3D printed DPAC eyepieces.  What a perfect application.  Sure, you have to program it but once that's done, just crank them out.  I did mine the old fashioned way with wood, glue, scroll saw, drill, paint and tweezers to remove the splinters, no bandages......yet.

 

I'll post a picture of my eyepiece and set up.  Not nearly as nicely crafted as Paul's but it works just fine.  

 

I've tested a bunch of refractors and have learned a great deal.  None, so far, have been dogs, but I've pretty much weeded those out already. 

 

I have begun retesting with the red and blue LEDS today and the results are really cool and informative.  I'm going to start a log and take notes of results including subjective evaluations of SA in R,G, B, "smoothness" and zones at focus", and astigmatism. 

 

I have also evaluated to a limited degree the effects of diagonals in the light path, especially my Baader/Zeiss T2 prism diagonal.  

 

One very nice surprise so far (but not really as I new it was a good lens) has been my good old 6" F10 Istar achromat.  Truly excellent SA in G&R with a very small amount in B, a very smooth figure and no astigmatism.  Excellent.

 

I'm still trying to figure out what's the best way to use my Canon 6D to take a few photos.  With mine, I think I will need to do it manually, but I have to crack the menu code again.  Funny, my simple 15 year old early Olympus digital camera seems to work just fine.

 

A few observations and lessons learned so far.

 

1. This is easy to do really, once you are all set up. I cycled through at least 6 refractors in a couple of hours.

2. I will need the pot to reduce the LED brightness, especially the R&B.  Right now I'm using a neutral density filter and R&B filters.

3. The green LED has a fair amount of yellow in its spectrum so I also use a medium green filter too.

4. Using filters not only reduces the brightness a bit but also narrows the pass band, giving sharper bars.  I can't detect any distortions from the filters.

5. I need to clean my flat.

6. With the brightness of the un-filtered LEDs, aligning everything is really easy as I can shift the scope around a little so that the return spot on the grating is right above the LED source.

7. As some have mentioned, the camera can introduce its own SA.  Easy to see when comparing the shot to the live eye view.  The cheap Olympus seems very neutral in that regard.

8. I need to clean my flat.

 

More to come.

 

Jeff


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#70 starman876

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Posted 25 September 2018 - 03:12 PM

Glad to see we have more people doing DPAC.  I am sure between all of us we can come up with some really nice setups that will make DPAC enjoyable and easy to do  for many.  



#71 TG

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Posted 25 September 2018 - 03:32 PM

One last thing...    Below is a device that was invented/assembled by a good friend who in the past, managed the operations (as opposed to science) team that supported WFC-3 and WFPC-2 on the HST.  He recently left STScI and is doing his own research.  He is slated to be on the Ops team for the James Webb instrument whenever that telescope goes into orbit. 

 

The gadget below is a star test device that is used in double pass.  If you like to star test, this device is the bomb.  It generates a parallel beam of light which presents itself as an Airey disk using an LED a beam splitter, 5 micron pinhole and Baader Fluorite Flat Field Corrector.  You place this into the focuser of the telescope and you rack it in/out of focus as you would a conventional  star test.  The beam splitter allows you to view the results.  Since the light traverses the beam splitter and Baader FFC it needs to be of VERY high quality as the aberrations in each are cumulative where as the aberrations in the flat are negligible as long as the flat is smooth and regular.  This is not an inexpensive device.  Parts were sourced from Edmunds and other optical companies serving the pro market.  But, Scopestuff supplied some of the plumbing to make it all work.       

 

Now, I just need more telescopes to test!

 

In closing, I have found bench testing to be of great help to me in understanding a telescope's relative performance.  I never have to wonder if its bad seeing or a bad lens.  The answer always presents itself. 

@peleuba, do you have details on the device? I'm planning to build something similar but since it involves using a prismatic beam splitter, that's going to affect the wavefront especially at faster f/ratios.

 

Tanveer



#72 peleuba

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Posted 25 September 2018 - 07:18 PM

@peleuba, do you have details on the device? I'm planning to build something similar but since it involves using a prismatic beam splitter, that's going to affect the wavefront especially at faster f/ratios.

 

Tanveer

 

Yes.  All the details.  I helped put it together and currently have possession of it.  Yes, the beam splitter must be uber-high quality.  The Baader FFC helps mitigate some of the the aberrations in the beamsplitter, but its (beam splitter) surface flatness is better then 1/8 wave. 

 

BTW, I came across an artificial star device you created using an LED, an eyepiece and some plumbing.  I am about to make it for myself for quick use indoors.  Thanks for publishing it.


Edited by peleuba, 25 September 2018 - 07:21 PM.


#73 Sasa

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Posted 26 September 2018 - 06:53 AM

I have built DPAC setup up when testing my lens 82/1670mm for slightly different reason than to check optical quality. The primary goal was to measure difference in focal points in red and blue. I wanted to see how far away they were. The precision on refractive indexes provided with the glass discs was not enough. The foci could differ by up to 0.4mm. Not  much  compared to  ~1 mm defocus of traditional achromat but relevant for apochromatic performance as the overall defocus in those two lines with respect to the green light should be 0.5 mm. At the end I found blue focus to be closer to green by 0.10+-0.02mm and decided that it was not worth to retouch the first surface of the lens, also because my T2 prism introduces almost exactly opposite shift and it brings blue (F-line)  red (C-line) foci back together.

 

I was also lucky that I could use one room for this in our work. Also I could borrow some professional holders from my colleagues:

 

https://www.fzu.cz/~...0451991_HDR.jpg

 

https://www.fzu.cz/~...0612148_HDR.jpg

 

Of course I replaced CCD camera with Ronchi eyepiece as well and checked the quality of the lens. This is the only lens that I tested with DPAC. For others, I just rely on judging their performance on planets and stars, and on star testing them.


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#74 SandyHouTex

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Posted 26 September 2018 - 09:34 AM

For a simple DPAC test you need:

 

1.  An optical Flat

2.  A stand for the flat to rest on

3.  V Blocks to support the telescope under test

4.  A 2" eyepice with LED's and Ronchi Screen

5.  A power supply (3 Volts) to power the LED's

 

I test in Green, Blue, Red and White light allowing me to judge correction at each wavelength.  By far the most useful to me as a visual observer is green and white.

 

Pictures of my DPAC setup...

So I'm trying to figure out the configuration of your eyepiece.  It looks like there's a half moon cutout with an LED taped to the other side.  I'm assuming there's a hole for the LED to shine through toward the scope.  Where is the ronchi grating?  Is it screwed in the bottom of the eyepiece where the filter threads are?  Does it cover the whole field of view?

 

Thanks,



#75 SandyHouTex

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Posted 26 September 2018 - 09:36 AM

One last thing...    Below is a device that was invented/assembled by a good friend who in the past, managed the operations (as opposed to science) team that supported WFC-3 and WFPC-2 on the HST.  He recently left STScI and is doing his own research.  He is slated to be on the Ops team for the James Webb instrument whenever that telescope goes into orbit. 

 

The gadget below is a star test device that is used in double pass.  If you like to star test, this device is the bomb.  It generates a parallel beam of light which presents itself as an Airey disk using an LED a beam splitter, 5 micron pinhole and Baader Fluorite Flat Field Corrector.  You place this into the focuser of the telescope and you rack it in/out of focus as you would a conventional  star test.  The beam splitter allows you to view the results.  Since the light traverses the beam splitter and Baader FFC it needs to be of VERY high quality as the aberrations in each are cumulative where as the aberrations in the flat are negligible as long as the flat is smooth and regular.  This is not an inexpensive device.  Parts were sourced from Edmunds and other optical companies serving the pro market.  But, Scopestuff supplied some of the plumbing to make it all work.       

 

Now, I just need more telescopes to test!

 

In closing, I have found bench testing to be of great help to me in understanding a telescope's relative performance.  I never have to wonder if its bad seeing or a bad lens.  The answer always presents itself. 

What is the beam splitter for in this post.  In your test set-up in other posts it doesn't look like you use it.  If you do, what is it's configuration?




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