Best Coma Corrector for 130 PDS

[RogerOfHeaton]

Hello Folks,

I run a 130 PDS and tube-ring converted 250PX ex-Dob newt on an AZ EQ6 GT mount, using the Skywatcher 0.9 coma corrector. This is with a modded DSLR

On the 250PX, the bright star shapes are good and the coma corrector is fine.

On the 130 PDS, no matter how I set up with the Skywatcher CC, the bright stars in the  image are flattened on one side. It has begun to cheese me off somewhat, and so I'm considering a second CC alternative for the 130 PDS. I've attached a cocoon nebula image to show the effect.

I've read this thread back to front, and I guess the Baader MPCC III and/or the GSO are the main candidates.

My first question is do either of these have any drawbacks at all?

I understand that some users have had spacer probs with the Baader but only with narrowband and filter drawer. I also understand that the GSO pushes the scope focal ratio out to 5.5. No big deal, although I'll admit I do like the 0.9 cc for bringing the focal ratio down to 4.7.

My second question is.....Is it possible to use a star mask and modify the flattened side?

In PI there are loads of erosion options but can't see anything to refill up on one side. In CS2 Astronomy Tools I can brighten or diminish the whole star but that's it AFAIK.

Any feedback much appreciated. Thanks folks.

[StamosP]

I have the same problem with the same setup, D shaped stars.

But i have the impression that this is caused by the focuser tube as it protrudes too much in the OTA...

(Of course the sw coma corrector's performance is unacceptable since it causes many bad reflections in the images but i like the fact that it acts as a reducer too...)

[RogerOfHeaton]

Thanks for responding, StamosP. My understanding was that if it was just the focuser protrusion, the Baader and GSO would give a similar effect.

My understanding from folks like Louise in Glasgow, who has the GSO, and the guys who started this thread on pages 1-3 like Uranium, who has had both Skywatcher and Baader types, that they don't have the same effect.

If I'm wrong, then OK. If so, then I'll give serious consideration to the MN 190 DS Pro, which is coma-free apparently, and needs no corrector.

[Uranium235]

The reason why my coma correctors dont give me the cutout star shapes is because they are internally mounted via a custom built focuser adaptor, which means the drawtube has to move out further for it to reach focus. Some have taken to hacksawing their drawtubes, but that is not for the faint of heart!

Personally, the cutouts dont bother me since stars take second place to nebulosity. As long as the majority of stars are round, thats all that matters.

Be wary of the MN190, its heavy and very tricky to collimate.

[Thalestris24]

Hi

I do indeed have the GSO CC - and the Baader. The main difference between them and the SW is that the former are non-reducing coma correctors. In fact I found that the GSO has a lengthening affect. The Baader is neutral. A possible advantage of the GSO is that it has a greater backfocus. Neither cause problems with focus tube ingress, unlike the SW...

Louise

[richyrich_one]

Just move the mirror up the tube...