DIY 1.25" Coma Corrector

[bobro]

Those comet shaped stars towards the edge of images taken with my Newtonian f5 scope were starting to annoy me, so it was time to look for a coma corrector. Probably due to lack of demand, there didn't seem to be much available in the way of CCs for 1.25" focusers. So, after a bit of internet searching, a magazine article dating back to 1991 with an optical design for CCs was found. After a couple of minor design changes to suit stock lenses I could find, lenses were ordered plus the other metal parts to construct a housing with.

The optical design is a Ross type two element corrector, producing no change to the focal length nor focusing of the telescope. It's not the best type of corrector design, but stock lenses are easy to obtain for it and it's not a critical design.

As the lenses are 25mm diameter, they fit nicely into a 1.25" filter housing - one for each lens as the lens spacing isn't supposed to be critical. The remainder of the CC is made from a 1.25" eyepiece barrel with a filter extension tube between the barrel and the two filter housings.

Sitting inside the telescope focuser tube, eyepiece screws hold the CC in place, allowing for a bit of fine tuning of the distance between the camera focal plane and the CC lens.

It's strange to look through the CC - seems like looking through a piece of plain glass as the two lenses cancel each other out. Hopefully it will do more than plain glass for imaging!

Next step, of course, is testing. I'll post results when clouds allow..........

If anyone has experience or thoughts on this type of design, please comment.

 

Edited March 8, 2017 by bobro

[Andy Mitchell]

Could you combine this with an OTA from one of the mini dobsonians with fast mirrors for a super wide angle newtonian for astro photography, at least for sub APS sized CCD sensors? Something like the mead lightbridge 114 (4.5" 450mm f3.9). Suspect trying to do this with my camera would overload the focuser's weight bearing ability.

[bobro]

55 minutes ago, Andy Mitchell said:

Could you combine this with an OTA from one of the mini dobsonians with fast mirrors for a super wide angle newtonian for astro photography, at least for sub APS sized CCD sensors?

Possibly not a good idea for use with a small sensor. This type of corrector, whilst improving coma as distance from the image centre increases, actually makes spot sources (such as stars) larger near the image centre. If a small sensor is used with a fast Newtonian, it shouldn't suffer too much coma as coma increases linearly with distance from the image centre in an uncorrected Newtonian (parabolic mirror).

[PeterW]

More details please... what sort of focal rationwilk this correct, which lens parts do we need and what sort of spacing is needed?

 

PeterW

[bobro]

13 minutes ago, PeterW said:

More details please... what sort of focal rationwilk this correct, which lens parts do we need and what sort of spacing is needed?

 

PeterW

The design came from a 1991 magazine - I can't advise on specifics as I'm not an optical designer, though it was described with an f4.5 scope. The spacing between the two lenses was described as not critical. The approx distance to the camera focal plane and the lens specs are in the diagram at the start of this topic. I purchased the lenses from Eksma Optics.

So far there hasn't been an opportunity to test the CC - waiting for a gap in the clouds.

[bobro]

A gap in the clouds appeared - long enough for a with/without CC test. Almost full moon too.

Images below show the results. There is some additional vignetting and loss of light when the CC is used. The camera needs to be moved away from the scope by about an additional 10mm to achieve focus- unexpected. Possibly the CC position in the focus tube needs to be adjusted. The CC  seems to be doing what it's meant to do though.

[Yearofthegoat]

Very interesting. That second image looks good and the lenses aren't especially dear at about €20. Seems a good way to save some money with a bit of effort. Will you test them further e.g. playing with spacings etc? Thanks

[bobro]

Certainly I do wish to test out the CC more, but clouds are doing their best to prevent this! In particular bringing the lenses closer together. I'm also testing out a guiding setup of an Arduino Nano driving simple DC RA/DEC Economy Motors (not steppers), with PHD2 controlling this. Seems to work, but perhaps I need to move to where there are clearer skies - living on a small island means sea mist can scupper efforts even if there are no clouds!

[Yearofthegoat]

Ah yes, H20. Can't live with it, can't live without it.

I looked at the Eksma site the other day, but today I can't seem to find the 25mm lenses, only 25.4mm. Is it me, am sure they were there... will have another look.

 

[bobro]

I checked on the lenses I ordered from Eskma - they are 25.4mm dia. Part numbers 111-0222E (bi-convex) and 112-0227E (plano-concave).

HTH

[Yearofthegoat]

Thanks @bobro

Will give this a whirl once I get some filter holders.

[bobro]

Carried out another test with the CC - this time with the space between the 2 lenses reduced to only the ring that is normally used to hold a filter in its holder. Results in the attached sample images (processed quickly so a bit rough) with 300 second subs. Lens to camera focal plane still at 88mm. My scope tends to make little flying saucer stars towards one corner. Perhaps down to the simple focusser.

I will try and test with a different lens to focal plane distance.

 

[SilverAstro]

On 08/03/2017 at 13:10, bobro said:

 So, after a bit of internet searching, a magazine article dating back to 1991 with an optical design for CCs was found.

sounds like an interesting read, have you a link ?

[bobro]

8 hours ago, SilverAstro said:

sounds like an interesting read, have you a link ?

Here is the link - a download at US$3.95.

https://www.myscienceshop.com/product/digital-download/tmkpdf045

Edited April 24, 2017 by bobro

[Stub Mandrel]

I looked up the cost for 38mm to and the price rockets.

I assume you went uncoated? Still I can't see any reflection issues in your images.

[Chriske]

On 24-4-2017 at 10:39, bobro said:

Here is the link - a download at US$3.95.

https://www.myscienceshop.com/product/digital-download/tmkpdf045

What..!!! Are they still charging for magazines that old...??

[bobro]

54 minutes ago, Stub Mandrel said:

I looked up the cost for 38mm to and the price rockets.

I assume you went uncoated?

The EKSMA lenses I used were uncoated BK7, part#s 112-0227E and 111-0222E (17 Euro each plus charges). Putting together a 1.25" CC isn't going to be low cost, but is certainly lower cost than swopping out a scope or focuser in order to be able to purchase a 2" CC if the original scope has a 1.25" focuser.

Bob

[Stub Mandrel]

10 minutes ago, bobro said:

The EKSMA lenses I used were uncoated BK7, part#s 112-0227E and 111-0222E (17 Euro each plus charges). Putting together a 1.25" CC isn't going to be low cost, but is certainly lower cost than swopping out a scope or focuser in order to be able to purchase a 2" CC if the original scope has a 1.25" focuser.

Bob

Seems a fair deal to me. Might do it for the 76mm newt I rescued from a mate's skip, just because I can!

[AdamTheSkunk]

I know this thread is old but Andy Mitchell mentioned the lightbridge 114 mini.

I have toyed with that, i might build one of these. 

I actually use a lightbridge mini 114 and asi290mc. 

It gives me decent shots.

I have been toying with the idea of modding it and throwing a big metal crayford focuser on it, and maybe trying the dslr. 

 

 

[Stub Mandrel]

good to revive this thread a s more people should be aware of this option!

[AdamTheSkunk]

Indeed, I just got a 6 inch astrograph, and I use a 1.25 nosepiece adapter for my DSLR camera. 
 

I wonder if this will work for f4, if I adjust spacing... if anyone here is still listening in on this thread, that would be awesome. 
 

I can also share my experience testing this on my lightbridge mini. If it works, I’ll surely get a better metal focuser for it. 

[bobro]

Hi,

The original article describes how the design works with an f#4.5 reflector used as an example. Results will also depend on the frame size of the camera as coma gets worse as the distance from the image centre increases.

If you go ahead it will be interesting to see the results. For testing an aperture mask (a piece of card with a circular hole across the front of the scope) could be used, for example, to reduce the scope from f4 to f4.5 to see what difference this makes. 

Bob

[GeoffWaldo]

Is see the EKSMA lenses are 25.4 mm OD. All the filter carriers I find require a minimum substrate diameter of 26mm or so…the 25.4 mm OD lenses just fall through the filter holder! Do you have a part number or supplier for the filter jolders you used?

thanks!

Edited July 9, 2022 by GeoffWaldo

[bobro]

My CC used filter holders with an internal filter locking ring that screws inside the holder. I used 2 of these, by SVBONY, to sandwich each lens, with careful positioning to centralise the lens.

[PeterW]

You could 3D print some bits to jam things together… short prints, so tweak till they fit right. What are the current best parts for making one of these, I assume smaller lenses will vignette somewhat?

Peter