SW coma corrector spacing

[nekkar]

Hello friends,

I'm a newbie from Majorca.

Recently I read several posts about the Skywatcher coma corrector (CC) in this lounge, and as opinions are good, I bought one for my Newtonian 8" SW. Baader CC users says that they cannot focuse with a DSLR and some work has to be done to get it in focus.

Shooting with my Canon 40D, SW CC focus well, but stars are elongated, also in the center. I put some spacing and is better but problems continue.

I see that the space between the focal plane, say, the place where the chip is installed, and the CC glass, is 60 mm.

My question to SW CC users: Did you find same problems? Did you put some spacing rings to shoot with your DSLR?

Sorry for my bad english

Regards from our island, and thank you very much in advanced.

[lukebl]

Can you post an image showing the problem?

I have a an SW Coma Corrector which works fine on my 200mm SW Newtonian and Canon camera, so it should work for you.

[Digz]

Not sure if you have this, but the attached file may shed some light.

I am able to directly attached my camera via the correct t adapter to the MPCC and slot this into my scope, with no spacers being used.

HTH

mpcc_e.pdf

[lukebl]

Here's how my SW Coma Corrector fits to my camera. It works fine in the 2" focusers of both my 200mm & 250mm Newts.

[nekkar]

Thank you very much for responding. Well, there are no problems installing the hardware, but as requested, I attach a picture shot tonight, driven with an autoguider (MaximDL + Orion Starshoot mono) and it worked very fine, with errors of 0,06 or so. The problem is better seen in the picture at 2X, hope you can see it, if not, I'll post a picture with more resolution. Thank you for your help.

Uploaded with ImageShack.us

[nekkar]

Sorry I put the picture without processing, there's no flats and no bias.

[brantuk]

I think the elongation is due to either tracking error, or more likely, inaccurate polar alignment. I wouldn't think it was anything to do with coma correction.

[nekkar]

Thank you for your commets Brantuk.

Elongations in this (and other) pictures go in different directions, so the explanation can' be tracking. I shot a couple of pictures last night with no coma correction, and stars weren't elongated, tracking performed very well.

About polar alignment, well, mine is not perfect, but it isn't bad. The mount is put on a fix column in my observatory at home. And also, tracking was made with an autoguide system.

I'm thinking in coming back to the SW ED80: it is light and easy to use. :-(

[brantuk]

Hmmm..... different directions.... that's wierd and certainly thrown the cat in my pidgeon coup lol. And it sounds like your tracking and guiding is good so I don't know what to suggest now. If I come across any other info on this I'll be sure and let you know

[ollypenrice]

I agree that it will probably come from the coma corrector spacing. Do you have any instructions with the coma corrector about the distance from corrector to camera chip? Usually with field flatteners it is about 55mm. I would begin by trying to get close to that figure unless the coma corrector instructions say otherwise.

I have not actually used any of your equipment so I am just guessing.

Using refractors is always the easiest way to go imaging! However I think you will solve this problem.

Olly

[nekkar]

Thank you very much Olly. SW instruction book says nothing about spacing, and as I investigated the problem in this forum, I could watch several pictures that were posted here, that were taking with same equipment and no spacing. Baader CC needs it but SW not.

I put by myself several spacing rings in the CC but I couldn't solve the problem. A few minutes ago I called the shop that sold it to me, and they have no information about it, but offered to change the CC. By the moment I prefer to listen your advices, that have a very better information than the shop.

Best regards.

[ollypenrice]

The other thing to check is 'orthogonality' which means, is the camera chip at right angles to the light coming from the telescope. It has to be accurate. If the draw tube in the focuser is not a firm fit or the focuser is loose or the tube of the telescope is too flexible you can have this problem.

Olly

[nekkar]

Yes Olly, very good advice. I wonder now if the light comes very flat to the chip. I do not if I say this in a correct English, I'm sorry: I'll try to shoot rotating the CC and the camera to know if the image is the same in every picture or there are differences.

However, I have to say that the system fits very well and there are no flexures, neither the CC and camera nor the autoguide system (Orion Starshoot and a Canon 200 mm f/ 2,8 lens).