StellaLyra 2" Photo-Visual Coma Corrector?

[kev100]

Hiya.

Just wondering if anyone has experience of using the above (link) for visual use in an f 4.7, for example, Newt?

Kev.

[Don Pensack]

Look up posts and reviews for the GSO coma corrector, which is what this appears to be

(and for a good price, too!!).

Edited September 30, 2022 by Don Pensack

[cajen2]

@Mike Q always uses one in his 10" dob. I haven't felt the need in my otherwise similar 8". Both are around f/6.

Edited September 30, 2022 by cajen2

[Don Pensack]

One thing to bear in mind:

The top is too short to put the eyepieces at the correct distance from the lens, though it would work for imaging as is.

An additional spacer needs to be added between the upper barrel and the lens barrel.  How much spacer will be determined by the position of the focal planes of your eyepieces

that require the most in-focus of all your eyepieces.  It is often around 19mm but will vary according to your eyepieces.

This thread should make it clear:

https://www.cloudynights.com/topic/462985-setting-up-the-gso-coma-corrector/

[Guest]

I run one with my Q70s.  I run it as is with no spacers at all.  It clears up the bit of fuzziness that is in those eyepieces very well.  For 100 bucks and change its a steal. 

Edited October 1, 2022 by Mike Q

[Louis D]

I've been using one with a 25mm spacer ring in my f/6 Dob.  It corrects about 99% of the coma in the outer field for eyepieces focusing with 5mm of their shoulder in this configuration.  Perhaps in a faster scope you'd need to be more precise with the spacing.  Just replace the thumbscrews that came with it.  One of them sheared off in the holder tube on mine, and I can't get the remains out.  The screws are made of pot metal.

[kev100]

Thanks everyone for the info. I was particularly interested in perhaps using one with my 20mm myriad (having recently seen the positive effects of a televue paracor on a 20mm nagler in a fast scope. However, I’m not sure it would be a good idea for me. I use a low profile focuser plus extension tube on my scope, then a 2in EP adapter, then the eyepiece. Adding further weight to the focuser sounds iffy… unless the coma corrector sits in place of theEP adapter…

cheers,

kev

[Don Pensack]

22 hours ago, kev100 said:

Thanks everyone for the info. I was particularly interested in perhaps using one with my 20mm myriad (having recently seen the positive effects of a televue paracor on a 20mm nagler in a fast scope. However, I’m not sure it would be a good idea for me. I use a low profile focuser plus extension tube on my scope, then a 2in EP adapter, then the eyepiece. Adding further weight to the focuser sounds iffy… unless the coma corrector sits in place of theEP adapter…

cheers,

kev

The coma corrector will require some in focus.  Whether it will substitute for any of your extensions, I can't say.

 

That eyepiece has its focal plane 9mm above the 2" shoulder it sits on, so add 9mm to the height of the top of the proposed coma corrector to get the distance from the lens to the focal plane of the eyepiece.

Subtract that figure from 73.5-75mm to get the length of the spacer you need to add in between the lower section of the CC and the upper.

Once you've added that spacer, you place translucent scotch tape across the 2" opening of the CC in an X pattern.

Put the CC in the scope, point the scope at the moon and focus the moon on the tape.

Remove spacers, adapters, or extensions in your focuser as needed until you can achieve focus on the tape across the CC.

Then, freeze or lock the focuser at that point.

If you remove the tape, the 20mm should be close to exact focus as you slide it in.

 

All your other eyepieces will need to be pulled out of the CC to get them to focus (remember, NOT moving the focuser).

Where each eyepiece comes to focus, sliding it out, is its correct position relative to the CC lens.

For repeatability, you can add parfocalizing rings to the eyepieces so they automatically stop at that point when you slide them in.

Eyepieces that require an unsafe amount of pulling out of the CC should have a barrel extender added so the position is not unsafe for the eyepiece.

You will essentially parfocalize your eyepiece set so the CC is at the correct position in the light cone from the primary mirror and so each eyepiece is at its correct

distance from the CC lens.

 

It is unlikely you'll get everything EXACT, so after you parfocalize everything, you'll still need +/- 1mm of fine focus at the focuser, but that small a difference won't matter.

Any new eyepiece you buy will be parfocalized to your other eyepieces so it, too, will be at the correct distance from the CC lens.

 

The need to parfocalize your eyepieces and find a spacer of the correct length is one of the pains of using this coma corrector, but you only need do it once, so it's not that big a chore.

The Explore Scientific and TeleVue coma correctors have adjustable tops to do the parfocalizing for you without rings added to the eyepieces, but they are a lot more expensive.

 

And, you can parfocalize all your eyepieces first to save time later.  You parfocalize all your eyepieces to the eyepiece that needs the most in-focus of all your eyepieces.

In this case, that is likely to be the 20mm Myriad.

 

[kev100]

Wow Don, thanks for the information. I'll need to give it some thought before committing. However, given that I already have a low profile focuser, with an adjustable extension tube, which, on any given night is adjusted depending on the EP being used, it may be the case that I scribe a mark into it for the myriad, and just use my current 2in/1.25 adapter for the other EPs. It's only really the wide fov that needs the CC ...

 

[Elp]

My gso one is available having used it in a F5 130pds. For visual you have to be really nitpicky to have to use a CC I find, it's really better for photography though only flattens the edges so much, the distortion is still there. I bought it as it allowed it's use without modifying the tube length of the focus draw tube, for imaging I had to add around 20mm to the camera for backfocus requirement.

[Louis D]

4 hours ago, Elp said:

For visual you have to be really nitpicky to have to use a CC

I guess that's me.  Sometimes I forget to put it in the focuser, and put one of my ES-92s in without it and instantly think, "What is wrong with this eyepiece tonight, it's usually sharp to the edge?".  Then I realize I forgot the CC, put it in the focuser, and all is well with the world again. 😁

[Elp]

Eyepiece subjectivity is a forum debate upon itself...