Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Question about making DPAC eyepiece

  • Please log in to reply
26 replies to this topic

#1 sarastro

sarastro

    Viking 1

  • *****
  • topic starter
  • Posts: 865
  • Joined: 12 Aug 2004
  • Loc: Seattle

Posted 14 December 2021 - 04:43 PM

I've wanted to do DPAC on my vintage refractors for years now. My knowledge of electronics is virtually nil, so I've been intimidated by making the eyepiece.

 

One question I have is can you use a RGB LED?

 

I guess you would need a controller like this? 

https://www.superbri...dimmer/117/526/

 

Or should I just use a single green LED?



#2 Bomber Bob

Bomber Bob

    ISS

  • *****
  • Posts: 21,483
  • Joined: 09 Jul 2013
  • Loc: The Swamp, LA (Lower Alabama)

Posted 14 December 2021 - 04:52 PM

The base & prong assembly on the LEDs that I bought (clear, red, blue, & green) are too big for making an eyepiece, so I went with a 2" Screen & Frame design.

 

Now, I'm glad that I did:  All 4 LEDs are arranged around the frame, and I used a combo 4-position switch w/ digital dimmer control.  I can test in each color without moving anything, or swapping out parts...

 

 

I can also rotate the frame, and lock it straight vertical with a thumbscrew.


Edited by Bomber Bob, 14 December 2021 - 04:54 PM.

  • markb likes this

#3 Matty S

Matty S

    Messenger

  • *****
  • Posts: 488
  • Joined: 10 Sep 2017
  • Loc: The Bog, Nh

Posted 14 December 2021 - 06:56 PM

The base & prong assembly on the LEDs that I bought (clear, red, blue, & green) are too big for making an eyepiece, so I went with a 2" Screen & Frame design.

Now, I'm glad that I did: All 4 LEDs are arranged around the frame, and I used a combo 4-position switch w/ digital dimmer control. I can test in each color without moving anything, or swapping out parts...


I can also rotate the frame, and lock it straight vertical with a thumbscrew.


I've wondered about testing lenses too, but have no clue about how it works. Your setup sounds interesting and more useful. Do you have a pic of your rig you can share?
  • Bomber Bob likes this

#4 clamchip

clamchip

    Voyager 1

  • *****
  • Posts: 12,050
  • Joined: 09 Aug 2008
  • Loc: Seattle

Posted 14 December 2021 - 07:56 PM

I'd like to make a new one, mine is full of mistakes (my own) that I've fixed over time.

I would like to start fresh and I remember seeing someone glued a LED right to the eye side of a 133 LPI

Ronchi slide, it looked just like a 35mm film slide with the cardboard frame.

I wish I could remember where I saw it.

 

Robert



#5 markb

markb

    Vanguard

  • *****
  • Posts: 2,284
  • Joined: 16 Jun 2006
  • Loc: Arizona at last, goodbye NY. Light pollution still awful though

Posted 14 December 2021 - 08:49 PM

Mine is still pending assembly, clearing the needed area, and getting a 3D printer, but one thing I did during the accumulation process was to get tiny reasonable output leds in a multicolor pack. I had tons of 'standard' LEDs in my long term electronics device boxes, but bought a multicolor set of 1.8mm LEDs (eBay) for the project.

 

Bomber Bob's design sounds perfect for the build. Once life allows me to get back to the DPAC build and resuming flying.


Edited by markb, 15 December 2021 - 08:55 AM.


#6 Bomber Bob

Bomber Bob

    ISS

  • *****
  • Posts: 21,483
  • Joined: 09 Jul 2013
  • Loc: The Swamp, LA (Lower Alabama)

Posted 14 December 2021 - 10:05 PM

I've wondered about testing lenses too, but have no clue about how it works. Your setup sounds interesting and more useful. Do you have a pic of your rig you can share?

Here ya go:

 

BBs DPAC Gear v3 S02.jpg

 

This is an old photo (Version 3).  Version 4 has the same 3 identical platforms, but I made improvements to the Ronchi Platform (on the far right).


  • Matty S likes this

#7 DAVIDG

DAVIDG

    Voyager 1

  • *****
  • Posts: 10,881
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 15 December 2021 - 09:51 AM

 You guys are making this way to complex. 90% of the time you just need to test in green light. The reason is that almost all achromats and APO designed for visual work are designed to have the least amount of aberration in the green. Both spherical and chromatic correction go hand in hand. If one is off the other is off as well. So if you test in the green and see no problems the odds are very good all is good with the figure on the lens. If it off in green then it going to be off in both chromatic and spherical at other wavelengths as well. 

   So here is a picture of my  testing "eyepiece". It is 35mm film canister. I drilled a hole in the cap and was able to squeeze a 133 LPI glass Ronchi grating into the back of cap. I filed the end of a  mini green LED flat and super glued it just slightly  off center.  You want the light source as close as possible to the optical axis. They have "chip" LEDs that are even smaller  The LED is power by 2 D cells with a on/off switch and a dimmer.

 You also need an optical flat the size of the optics your testing or larger or you can use a pool of oil.  There are tons of threads on doing double pass autocollimation here but happy to answer any questions.

 

                              Happy Holidays !

                                 - Dave 

                                 

   doublepassronchieyepiece.jpg


Edited by DAVIDG, 15 December 2021 - 08:07 PM.

  • tim53, markb, Terra Nova and 3 others like this

#8 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • Posts: 6,481
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 15 December 2021 - 10:26 AM

 You guys are making this way to complex. 90% of the time you just need to test in green light. The reason is that almost all achromats and APO designed for visual work are designed to have the least amount of aberration in the green. Both spherical and chromatic correction go hand in hand. If one is off the other is off as well. So if you test in the green and see no problems the odds are very good all is good with the figure on the lens. If it off in green then it going to be off in both chromatic and spherical at other wavelengths as well. 

   So here is a picture of my  testing "eyepiece". It is 35mm film canister. I drilled a hole in the cap and was able to squeeze a 133 LPI glass Ronchi grating into the back of cap. I filed the end of a  mini green LED flat and super glued it just slightly  off center.  You want the light source as close as possible to the optical axis. They have "chip" LEDs that are even smaller  The LED is power by 2 D cells with a on/off switch a dimmer.

 You also need an optical flat the size of the optics your testing or larger or you can use a pool of oil.  There are tons of threads on doing double pass autocollimation here but happy to answer any questions.

 

                              Happy Holidays !

                                 - Dave 

                                 

   attachicon.gifdoublepassronchieyepiece.jpg

So I have a question.  Is the led behind the ronchi screen, and do you look through the ronchi screen when looking at the optic under test?



#9 sarastro

sarastro

    Viking 1

  • *****
  • topic starter
  • Posts: 865
  • Joined: 12 Aug 2004
  • Loc: Seattle

Posted 15 December 2021 - 12:05 PM

OK, I'll go with a green LED.

I have a Ronchi screen of unknown origin. Looks to be 5 lines/mm.

Looks to be a bit ragged. Is it good enough?

 

 

Attached Thumbnails

  • 20211215_080520a.jpg


#10 DAVIDG

DAVIDG

    Voyager 1

  • *****
  • Posts: 10,881
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 15 December 2021 - 02:05 PM

So I have a question.  Is the led behind the ronchi screen, and do you look through the ronchi screen when looking at the optic under test?

 Yes the LED is behind the Ronchi screen and you look through the grating at the image returned through it. The Ronchi test is the most sensitive when the slit is the same width as the spacing between the lines. The slit also needs to be parallel to the lines on the grating. So by placing the LED directly behind the grating and shiny through it, the grating is making it's own slits, all of them  of the  perfect width and always perfectly parallel to the other lines on the grating. So it is a self aligning test ! 

 

                         - Dave 


  • markb, rcwolpert, Bomber Bob and 1 other like this

#11 DAVIDG

DAVIDG

    Voyager 1

  • *****
  • Posts: 10,881
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 15 December 2021 - 02:07 PM

OK, I'll go with a green LED.

I have a Ronchi screen of unknown origin. Looks to be 5 lines/mm.

Looks to be a bit ragged. Is it good enough?

 Good enough. It will show you all the typical errors.

 

                - Dave 



#12 RichA

RichA

    Fly Me to the Moon

  • *****
  • Posts: 5,486
  • Joined: 03 Jun 2010
  • Loc: Toronto, Canada

Posted 15 December 2021 - 02:20 PM

I've wanted to do DPAC on my vintage refractors for years now. My knowledge of electronics is virtually nil, so I've been intimidated by making the eyepiece.

 

One question I have is can you use a RGB LED?

 

I guess you would need a controller like this? 

https://www.superbri...dimmer/117/526/

 

Or should I just use a single green LED?

Attach a grating and a beamsplitter cube to the end of a small flashlight with a pin-hole for the light to go through.  Of course, you still need an optical flat.



#13 sarastro

sarastro

    Viking 1

  • *****
  • topic starter
  • Posts: 865
  • Joined: 12 Aug 2004
  • Loc: Seattle

Posted 15 December 2021 - 04:26 PM

OK, the Ronchi screen is adequate to do the job.

I guess ronchiscreens.com is no longer around so that's not an option.

 

So, if I use a potentiometer to control the brightness, will one of these do the job? Or do I need something else altogether?

 

 

Attached Thumbnails

  • 20211215_130316a1.jpg


#14 DAVIDG

DAVIDG

    Voyager 1

  • *****
  • Posts: 10,881
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 15 December 2021 - 05:00 PM

OK, the Ronchi screen is adequate to do the job.

I guess ronchiscreens.com is no longer around so that's not an option.

 

So, if I use a potentiometer to control the brightness, will one of these do the job? Or do I need something else altogether?

 All those could work it depends on the value of the voltage you power the LED with.   A typical LED needs to draws no more the 20ma.  So you need a fixed value resistor in series with the pot to make sure that when  the pot is turned to it's  have the LED at it's brightest ( ie the least resistance of the pot) that is still enough resistance so no more then 20 ma is drawn by the LED or it will burn out.

   So to calculate  the value of the fixed resistor  ( Supply voltage -  LED voltage drop) /0.020 amps   A type LED has about 2 volt drop 

 So if you power it with two 1.5 volt battery which would be 3 volts  we have  3 -2/0.02 =  50 ohm. I standard 47 ohm resistor would be fine.  I would use the 1K pot with the 47 ohm resistor. You will  find that for visual testing having  the LED at it's brightest is the best. I only turn down the brightness when I try to take pictures of the test results or my phone will over expose the image.

 

                    - Dave 


  • rcwolpert and Bomber Bob like this

#15 sarastro

sarastro

    Viking 1

  • *****
  • topic starter
  • Posts: 865
  • Joined: 12 Aug 2004
  • Loc: Seattle

Posted 15 December 2021 - 05:45 PM

 All those could work it depends on the value of the voltage you power the LED with.   A typical LED needs to draws no more the 20ma.  So you need a fixed value resistor in series with the pot to make sure that when  the pot is turned to it's  have the LED at it's brightest ( ie the least resistance of the pot) that is still enough resistance so no more then 20 ma is drawn by the LED or it will burn out.

   So to calculate  the value of the fixed resistor  ( Supply voltage -  LED voltage drop) /0.020 amps   A type LED has about 2 volt drop 

 So if you power it with two 1.5 volt battery which would be 3 volts  we have  3 -2/0.02 =  50 ohm. I standard 47 ohm resistor would be fine.  I would use the 1K pot with the 47 ohm resistor. You will  find that for visual testing having  the LED at it's brightest is the best. I only turn down the brightness when I try to take pictures of the test results or my phone will over expose the image.

 

                    - Dave 

 

This is exactly the explanation I needed.

 

Thanks Dave!



#16 rcwolpert

rcwolpert

    Mercury-Atlas

  • *****
  • Posts: 2,912
  • Joined: 13 Aug 2012
  • Loc: South Hutchinson Island, Florida

Posted 16 December 2021 - 02:37 PM

 You guys are making this way to complex. 90% of the time you just need to test in green light. The reason is that almost all achromats and APO designed for visual work are designed to have the least amount of aberration in the green. Both spherical and chromatic correction go hand in hand. If one is off the other is off as well. So if you test in the green and see no problems the odds are very good all is good with the figure on the lens. If it off in green then it going to be off in both chromatic and spherical at other wavelengths as well. 

   So here is a picture of my  testing "eyepiece". It is 35mm film canister. I drilled a hole in the cap and was able to squeeze a 133 LPI glass Ronchi grating into the back of cap. I filed the end of a  mini green LED flat and super glued it just slightly  off center.  You want the light source as close as possible to the optical axis. They have "chip" LEDs that are even smaller  The LED is power by 2 D cells with a on/off switch and a dimmer.

 You also need an optical flat the size of the optics your testing or larger or you can use a pool of oil.  There are tons of threads on doing double pass autocollimation here but happy to answer any questions.

 

                              Happy Holidays !

                                 - Dave 

                                 

   attachicon.gifdoublepassronchieyepiece.jpg

Exactly! It’s simple and except for the Ronchi grating, I got all parts on Amazon for a few bucks.


  • PawPaw and Bomber Bob like this

#17 Ben Bajorek

Ben Bajorek

    Messenger

  • *****
  • Posts: 466
  • Joined: 11 Sep 2016

Posted 20 December 2021 - 08:52 PM

Here's a link to my 3D printed DPAC eyepiece design from a few years back.  It uses a button type battery which won't burn out an LED and requires no resistor and really no wiring.  

 

https://www.thingive...m/thing:3553363


Edited by Ben Bajorek, 20 December 2021 - 09:04 PM.

  • markb, Bomber Bob and Kasmos like this

#18 markb

markb

    Vanguard

  • *****
  • Posts: 2,284
  • Joined: 16 Jun 2006
  • Loc: Arizona at last, goodbye NY. Light pollution still awful though

Posted 20 December 2021 - 09:08 PM

Thanks Ben!


  • Ben Bajorek likes this

#19 Star Shooter

Star Shooter

    Vostok 1

  • *****
  • Posts: 180
  • Joined: 20 Jul 2019

Posted 20 December 2021 - 10:24 PM

Edmund Optics has Ronchi screens in stock.



#20 Ben Bajorek

Ben Bajorek

    Messenger

  • *****
  • Posts: 466
  • Joined: 11 Sep 2016

Posted 21 December 2021 - 06:04 AM

Edmund Optics has Ronchi screens in stock.

You can also print out Ronchi screens on an ink jet printer with transparent film you can buy at most office supply stores.  There were some free files available online of different Ronchi screens, or you can make your own files in some sort of graphics program.  I have a lifetime supply of empty 35mm slide holders somewhere around here from my experiments  


Edited by Ben Bajorek, 21 December 2021 - 06:05 AM.


#21 RichA

RichA

    Fly Me to the Moon

  • *****
  • Posts: 5,486
  • Joined: 03 Jun 2010
  • Loc: Toronto, Canada

Posted 21 December 2021 - 07:45 AM

I've wanted to do DPAC on my vintage refractors for years now. My knowledge of electronics is virtually nil, so I've been intimidated by making the eyepiece.

 

One question I have is can you use a RGB LED?

 

I guess you would need a controller like this? 

https://www.superbri...dimmer/117/526/

 

Or should I just use a single green LED?

This kind of eyepiece can be used for double-pass and at the scope when viewing a star single-pass without the of course.  The beam and view are collimated.

 

dpac eyepiece.jpg



#22 markb

markb

    Vanguard

  • *****
  • Posts: 2,284
  • Joined: 16 Jun 2006
  • Loc: Arizona at last, goodbye NY. Light pollution still awful though

Posted 21 December 2021 - 03:39 PM

Ronchiscreens.com is, sadly, gome and no longer has valid page content, but the owners were kind enough to post the PDF file for three screen sizes in the past.

 

It is, thankfully, still downloadable through the web archive, the Astro fan resource for most of the missing, now dead, content we need.

 

I just made an additional copy while searching out the webarchive link date that worked. 

 

https://web.archive....nThreeSizes.pdf

 

I got a odd not saved content page display, but the pdf downloaded just fine.

 

EDIT: I clicked on the link in the box just below the 'Wayback Machine' logo, see next post. The Archive may not have directly saved the asset.

 

EDIT EDIT: As mentioned below, I used a webarchive tool to add the pdf to the saved assets, so the webarchive should work sans clicking the link to ronchiscreens- I expect that direct link to expire eventually.

 

I would have this printed at a FedEx Kinkos, etc on transparent acetate on a high res laser printer. But I'm sure a home laser and possibly ink jet (ugh, a $75 canon laser blows ink jets away) might make a usable grating.

 

I find it very strange that Ronchi gratings, possibly the best affordable Astro tool I've ever used, are so hard to get now. I had to order my metal film Gerd Neumann directly from Germany.

 

Start printing those screens!

 

Let others know if they work well for you.


Edited by markb, 22 December 2021 - 09:53 AM.

  • PawPaw likes this

#23 markb

markb

    Vanguard

  • *****
  • Posts: 2,284
  • Joined: 16 Jun 2006
  • Loc: Arizona at last, goodbye NY. Light pollution still awful though

Posted 21 December 2021 - 03:43 PM

I had clicked on this link on the webarchive search response page; still working as of this posting date.

 

http://www.ronchiscr...nThreeSizes.pdf

 

If necessary, right click or hold and click to "download link".


Edited by markb, 21 December 2021 - 03:46 PM.

  • PawPaw likes this

#24 markb

markb

    Vanguard

  • *****
  • Posts: 2,284
  • Joined: 16 Jun 2006
  • Loc: Arizona at last, goodbye NY. Light pollution still awful though

Posted 21 December 2021 - 03:52 PM

I believe I fixed the link using a webarchive save function.

 

If so, it should remain available for the foreseeable future.



#25 SandyHouTex

SandyHouTex

    Fly Me to the Moon

  • *****
  • Posts: 6,481
  • Joined: 02 Jun 2009
  • Loc: Houston, Texas, USA

Posted 22 December 2021 - 04:43 PM

This kind of eyepiece can be used for double-pass and at the scope when viewing a star single-pass without the of course.  The beam and view are collimated.

 

attachicon.gifdpac eyepiece.jpg

Doesn't the light have to go through the Ronchi screen to get to the optic under test?




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics