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

3D Printed Coma Corrector

  • Please log in to reply
18 replies to this topic

#1 Tucker512

Tucker512

    Ranger 4

  • *****
  • topic starter
  • Posts: 333
  • Joined: 30 Oct 2010
  • Loc: Arizona

Posted 28 August 2014 - 03:34 PM

I thought I would post some details on the 3D-printed 1.25" coma corrector I just made with some stock lenses for my little fast Newtonian.

 

I fully understand why there is no real market for a 1.25" coma corrector. There are very few fast telescopes that only have 1.25" focusers, and very few owners of such small telescopes are likely to put a several-hundred-dollar wide-field eyepiece on such a scope. But I had an old 4.5" f/8 mirror I acquired and turned into an f/3.4 mirror then mounted it in the OTA for an Orion Starblast (with the metal tube cut off to the correct length).

 

I primarily made the scope to fit around my 7mm Pentax XW eyepiece, but I also have a 15mm Celestron Luminos 82-degree eyepiece. On the 4.5" f/3.4 it would give a 3.3-degree FOV at 25x. Such a view would be great except that, as is, all you see is the most textbook example of coma I've ever beheld at the eyepiece! Yikes!

 

So I poked at some design ideas in Zemax using stock lenses and came up with something quite decent using just 3 elements: a plano-convex (PCX) and plano-concave (PCV) together acting as a meniscus lens, and a double-convex (DCX) lens.  Ideally all three lenses would be BK7 glass, but I found the PCX and DCX in BK7 and the PCV in SK18 at Anchor Optics (part of Edmund Optics), all for about $50. I tweaked the design and it works fine with only a slight enlargement of the on-axis spot, but it isn't noticeable at only 25x.

 

The PCV is uncoated as no coated version was available that would work. My only concern with that is the two plano surfaces of the PCX and PCV lenses mate up. The best solution would be to cement the elements together. The lazy solution that I went with is to space the plano surfaces with shims by 0.25mm = 0.010". Tests on bright targets show no noticeable internal reflections.

 

The coma corrector reduces the focal length by about 2%, in this case from 377mm to 369mm. The corrector actually pushes the focus back about 9mm, so no extra in-travel on the focuser is required, which is good, because I didn't have a lot left. However, this short outward travel precludes mounting the coma corrector in a barrel with a 1.25" female fitting and set screw, like a Barlow lens; there isn't enough room. Instead, the lens needs to screw into the filter threads on the bottom of the eyepiece.

 

I originally planned to get the coma corrector's three parts (body, lens spacer, and retaining ring) machined, but then we got a 3D printer at work and I've done nothing but play for 2 weeks... The only thing holding me back from printing the coma corrector parts was how to attach it to the eyepiece since you can't print or otherwise easily cut such fine threads. Then I thought to make slightly flexible tabs that would push out against the female threads of the eyepiece barrel, which worked great. The coma corrector presses into the eyepiece and is held securely, but it is pretty easy to wiggle it back out.  It doesn't seem to wear on the PLA type plastic, and it hasn't loosened up with repeated use.

 

The spots are significantly better off-axis, with no visible degredation on-axis.  The edge stars aren't perfect, but are rounder and easily 5x smaller!  It makes the 15mm EP completely useable, which was certainly not the case before.

 

The attached diagram shows the internal layout of the coma corrector and the placement of the lenses. The photos show the corrector by itself (you can see the tabs that hold it into the eyepiece barrel) and the corrector installed on the 15mm eyepiece. All in all a fun little project!

 

Scott

Attached Thumbnails

  • Coma Corrector Layout.jpg
  • Coma Corrector.jpg
  • Coma Corrector on EP.jpg

  • ccs_hello, rnabholz, BoriSpider and 9 others like this

#2 Chriske

Chriske

    Kijkerbouw Urania

  • *****
  • Posts: 3,120
  • Joined: 15 Aug 2004
  • Loc: Boechout, Belgium

Posted 28 August 2014 - 04:28 PM

:waytogo:



#3 BoriSpider

BoriSpider

    Soyuz

  • -----
  • Posts: 3,785
  • Joined: 10 Aug 2004
  • Loc: S.W.FLA

Posted 28 August 2014 - 10:17 PM

Nice.



#4 cheapersleeper

cheapersleeper

    Skylab

  • *****
  • Posts: 4,450
  • Joined: 22 Jan 2010
  • Loc: Sachse TX

Posted 28 August 2014 - 10:28 PM

That is fantastic.  Just built a 6" f5 and it would be nice to be able to just use a 32mm plossl.

 

B



#5 starcanoe

starcanoe

    Vanguard

  • *****
  • Posts: 2,493
  • Joined: 07 Nov 2013
  • Loc: Gulf Coast, Panhandle of Florida

Posted 28 August 2014 - 10:54 PM

Nice indeed.

 

And much cheaper than a paracorr!

 

You could put a drop of thick oil between the two lenses to increase light throughput and decrease an ghosting.



#6 coinboy1

coinboy1

    Apollo

  • *****
  • Posts: 1,466
  • Joined: 03 May 2011
  • Loc: Tulsa, OK

Posted 29 August 2014 - 10:34 PM

Super awesome! I love this type of ATMing!!



#7 Scott in NC

Scott in NC

    Refractor Fanatic

  • *****
  • Administrators
  • Posts: 32,999
  • Joined: 05 Mar 2005
  • Loc: NC

Posted 29 August 2014 - 10:40 PM

Very interesting.  Thanks for sharing the details of your creation with us! :waytogo:



#8 ds-in-wa

ds-in-wa

    Explorer 1

  • -----
  • Posts: 62
  • Joined: 08 Feb 2012
  • Loc: Vancouver, WA

Posted 30 August 2014 - 01:22 PM

Very interesting.  Thanks for sharing the details of your creation with us! :waytogo:

 

 

What details?  This is a very interesting topic and it would be nice to see spot diagrams and design details (curvatures, thicknesses, spacing, and diameters).

 

Douglas 



#9 Chriske

Chriske

    Kijkerbouw Urania

  • *****
  • Posts: 3,120
  • Joined: 15 Aug 2004
  • Loc: Boechout, Belgium

Posted 31 August 2014 - 02:48 AM

 

Very interesting.  Thanks for sharing the details of your creation with us! :waytogo:

 

 

What details?  This is a very interesting topic and it would be nice to see spot diagrams and design details (curvatures, thicknesses, spacing, and diameters).

 

Douglas 

 

agree...!



#10 Tucker512

Tucker512

    Ranger 4

  • *****
  • topic starter
  • Posts: 333
  • Joined: 30 Oct 2010
  • Loc: Arizona

Posted 31 August 2014 - 04:31 PM

Thanks for the feedback.  Here are some more details.

 

These are the three Anchor Optics lenses I used. Cost about $50 total, but I noticed they are 50% off right now:

 

AX27253:  25mm x 100mm PCX
AX27982:  25mm x -50mm PCV
AX27744:  25mm x 75mm DCX

 

Attached are images showing the prescription, spots sizes with and without at the same scale, and a layout of the lenses/mirror.

Attached Thumbnails

  • Coma Corrector Prescription.jpg
  • Coma Corrector Spots.jpg
  • Coma Corrector Layout.jpg

  • Oregon-raybender, coinboy1, stargazer193857 and 1 other like this

#11 starcanoe

starcanoe

    Vanguard

  • *****
  • Posts: 2,493
  • Joined: 07 Nov 2013
  • Loc: Gulf Coast, Panhandle of Florida

Posted 01 September 2014 - 09:47 AM

Surplus Shed has some coated cemented achromatic lenses that look like the planoconvex and planoconcave ones you have mated together.

 

And I'd bet they have some coated singlets and doublets that would be in the ballpark of the other lens.

 

With some research and some fiddling you could probably come up with something pretty nice using those.

 

They also have bigger diameter lenses. And the best news is the lenses are more like $5 to $7. Wait for one of the major discount sales and get em even cheaper!



#12 Relativist

Relativist

    Cosmos

  • *****
  • Posts: 8,152
  • Joined: 11 Oct 2003
  • Loc: OC, CA, USA

Posted 04 September 2014 - 04:02 PM

are you going to publish the stl file as well?



#13 Chriske

Chriske

    Kijkerbouw Urania

  • *****
  • Posts: 3,120
  • Joined: 15 Aug 2004
  • Loc: Boechout, Belgium

Posted 05 September 2014 - 12:06 AM

And if so, not just the stl file but also the source code file please. If you publish that original file, corrections can be made in case of other eyepiece brand or type.

Best should that file be converted to STP or STEP format first. A STEP/STP format file can be opened in all 3D drawing software.

 

Thanks



#14 Heph333

Heph333

    Lift Off

  • -----
  • Posts: 14
  • Joined: 14 Mar 2017

Posted 15 April 2017 - 02:00 PM

Bump: would also love an STL file. The lenses are still available... still $50, but I have no idea the dimensions for the spacers. 



#15 Tucker512

Tucker512

    Ranger 4

  • *****
  • topic starter
  • Posts: 333
  • Joined: 30 Oct 2010
  • Loc: Arizona

Posted 15 April 2017 - 03:56 PM

Here is the current iteration of the 1.25" coma corrector in STL format:

 

http://darkskyimages...rrector_STL.zip

 

It is optimized for a Delos eyepiece.  If the field stop position in your eyepiece is different, the design would probably have to change slightly to be ideal.  If you know what eyepiece you plan to use, I don't mind modifying the file.

 

The hole in the side of the body is made to be tapped for a small 4-40 set screw to hold the retaining ring.

 

Scott


  • stargazer193857, Heph333 and cloudypatio like this

#16 Heph333

Heph333

    Lift Off

  • -----
  • Posts: 14
  • Joined: 14 Mar 2017

Posted 15 April 2017 - 08:04 PM

Thanks so much. I actually don't even have any eyepieces yet (pretty sure I have some old Kellners somewhere). I'm currently printing new primary & secondary mounts for the mirrors out of an old 6' f/5 I built 20 years ago.

#17 Vla

Vla

    Viking 1

  • *****
  • Posts: 611
  • Joined: 08 Mar 2014

Posted 17 November 2018 - 07:49 AM

When I raytrace this prescription, it does correct coma, and flattens the field, but there is a load of spherical aberration instead (3/4 of a wave p-v). Also, primary spectrum is over 0.2mm, i.e. over 3 waves of defocus in the blue F-line, and 1.1 wave in the red C-line. It agrees with raytrace spots presented here, which are about 1/10 of the 400 micron square, or 40 microns in diameter. Airy disc diameter at f/3.4 is 4.5 microns (e-line). Error in prescription?


  • stargazer193857 likes this

#18 ed_turco

ed_turco

    Mercury-Atlas

  • *****
  • Posts: 2,753
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 17 November 2018 - 12:14 PM

Thanks so much. I actually don't even have any eyepieces yet (pretty sure I have some old Kellners somewhere). I'm currently printing new primary & secondary mounts for the mirrors out of an old 6' f/5 I built 20 years ago.

Kellners are too poorly corrected to show any improvement with a coma corrector.   That $1K Paracorr requires premium eyepieces to work well and thrill the observer.  I learned this the hard way by pairing a Paracorr with, of all things, a Kellner eyepiece!  smile.gif 

 

Twenty years ago.


Edited by ed_turco, 17 November 2018 - 12:14 PM.


#19 stargazer193857

stargazer193857

    Cosmos

  • *****
  • Posts: 9,945
  • Joined: 01 Dec 2013
  • Loc: Southern Idaho

Posted 24 November 2018 - 07:03 PM

Surplus Shed has some coated cemented achromatic lenses that look like the planoconvex and planoconcave ones you have mated together.

And I'd bet they have some coated singlets and doublets that would be in the ballpark of the other lens.

With some research and some fiddling you could probably come up with something pretty nice using those.

They also have bigger diameter lenses. And the best news is the lenses are more like $5 to $7. Wait for one of the major discount sales and get em even cheaper!


You would get some coma correction, but without being able to select the glass type or radii, you could be in for plenty of chromatic aberation and spherical.


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