GSO Coma Corrector correct spacing

[astrosathya]

I have a GSO Coma Corrector which I purchased for my GSO 6" f4. When I try to use spacers equalling 75mm as advertised online (including 17mm CCD to flange), I get elongated stars. Last night I tried 55mm and it works brilliant. What could be happening here?

[Thalestris24]

2 hours ago, astrosathya said:

I have a GSO Coma Corrector which I purchased for my GSO 6" f4. When I try to use spacers equalling 75mm as advertised online (including 17mm CCD to flange), I get elongated stars. Last night I tried 55mm and it works brilliant. What could be happening here?

Hi
Is it this one or this one or another? 

Louise

[astrosathya]

2 minutes ago, Thalestris24 said:

Hi
Is it this one or this one or another? 

Louise

Its the latter one, shown on Agena Astro website.

[Thalestris24]

1 minute ago, astrosathya said:

Its the latter one, shown on Agena Astro website.

Hi

I have one of those which I bought from Teleskop Express in 2015 though they no longer seem to sell it. That did indeed have a 75mm backfocus. I suspect the one you have isn't what you think it is! The one I have also increases focal length by about 10% or so. Can I ask where you got yours from?

Louise

[astrosathya]

12 minutes ago, Thalestris24 said:

Hi

I have one of those which I bought from Teleskop Express in 2015 though they no longer seem to sell it. That did indeed have a 75mm backfocus. I suspect the one you have isn't what you think it is! The one I have also increases focal length by about 10% or so. Can I ask where you got yours from?

Louise

Hi Louise,

I have purchased mine directly from the Indian Distributor of GSO. It looks exactly like the one in the image with the same markings etc, as well as the removable eyepiece adapter (conical shape) as well as the M48 to M42 reducer ring.

[Thalestris24]

Hmm…. have you platesolved a sub taken with it? I know when I used mine with a f5 130pds it increased the focal length by 14%

It could be that your f4 scope prefers a shorter backfocus though I've no idea really. I think the one I bought was quoted as 75 +/- 5mm. Or... the one you have is a different version. I don't think the one I bought is generally available anymore though a couple of people still advertise it. It may be that GSO have changed the design but not their advert. Or it could be they've inadvertently supplied a different one. The actual coma corrector isn't marked so could be mixed up with another one. I think I'd first check the pixel scale you have and see if it corresponds to an expected and increased focal length. Or contact the distributor and check with them.

Louise

[benzomobile]

Hi Satya,

what you report is very interesting because I have the same corrector and with the recommended backfocus I see very obvious coma and chromatic aberration!

So, when you have tested the distance of the c.c. to 75 mm, have you also seen a form of radial chromatic aberration, ie in direction of stellar elongation?           

If your answer is affermative, does this chromaticism disappear together with the coma, if the backfocus is reduced to 55 mm?

Thank you,

Beppe

[SkyGibbon]

Try this link. It may be of help.

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

[benzomobile]

I know it, thank you. That is a great discussion about visual use only.

Sathya pointed out the perfect backfocus - for imaging task, would be different as commonly described (55 vs 75 mm).

As former owner of GSO coma corrector, I finded big issues, in terms of coma bad correction and radial chromatic aberration.

I find GSO CC - at 75 mm backfocus, definitevely inusable with my 8" f/4 Newt!

Speaking about  CC  phographical use ...

Beppe

[benzomobile]

That's an design/assembly issue. 

It seems many people received faulty GSO CC in last times!

Beppe

[Thalestris24]

On ‎19‎/‎02‎/‎2019 at 10:36, benzomobile said:

Hi Satya,

what you report is very interesting because I have the same corrector and with the recommended backfocus I see very obvious coma and chromatic aberration!

So, when you have tested the distance of the c.c. to 75 mm, have you also seen a form of radial chromatic aberration, ie in direction of stellar elongation?           

If your answer is affermative, does this chromaticism disappear together with the coma, if the backfocus is reduced to 55 mm?

Thank you,

Beppe

Hi

My one worked fine at f5, no aberrations. I only did imaging, no visual. I can't remember what the specifications were now. It may not have been specified to work at f4 - not sure. It's been a few years since I last used my newts...

Louise

[benzomobile]

Hi,

at f5 nobody had problems, as far as I know  ?

Beppe

[benzomobile]

PS:

https://agenaastro.com/gso-2-coma-corrector.html

Beppe