6" TS Imaging newtonian - stars not round

[sunka]

Hello

I own 2 tubes, first was Newton Skywatcher 200P and recently I purchased 6" newton reflector (http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4762_GSO-6--Imaging-Newton---150mm-Oeffnung-f-4---2--MONORAIL.html). My mount is HEQ5 Pro and I am working via ASCOM. Guider camera is QHY5-II Mono and capture camera is Canon EOS 50D. Also, I am using f4-f6 coma corrector (http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3822_TS-Optics-2--Newtonian-Telescope-Coma-Corrector-from-f-4.html). For focusing I am using Bahtinov mask.

This is image taken with 200P and stars are nice and round to the corners: https://dl.dropboxusercontent.com/u/6013601/8_inch_photo.jpg.

This image is taken with 6" and stars are strangely shaped: https://dl.dropboxusercontent.com/u/6013601/6_inch_photo.jpg

I've taken several photos and resuls are always similar. I have tried recollimating it, tweaking guiding parameters so errors are smaller, refocusing but nothing seems to help - even short exposure photos do not have round stars but effect is worse with long exposures.

Any ideas, I am about to give up here?

Thanks!

[daiwelly]

Hi and welcome to SGL

[michael.h.f.wilkinson]

With coma correctors, the distance between chip and corrector is quite critical, I am told. Maybe that needs adjustment?

[sunka]

Daiwelly, hello and thanks?

Hello Michael, I have thought of that but since it works ok with 200P I presumed that is not the problem. Specified optimal distance is 55mm, do you maybe know how I could measure that for DSLR?

Thanks

[sunka]

OK I found what I needed to measure the distance: http://en.wikipedia.org/wiki/Flange_focal_distance

I'll measure and report back.

[michael.h.f.wilkinson]

The difference might be caused by the fact that the spacing is much more critical on an F/4 than on an F/6, due to the huge increase in the amount of coma.

[sunka]

So I checked; flenge focal distance for EOS DSLR is 44mm. M48 ring thickness adds 10.9mm which adds up to 54.9mm. Specified optimal distance from M48 thread to chip for my coma corrector is 55mm so I think distance is pretty much optimal.

Additionally I untightened collimation screws since they could cause mirror flexing and recollimated.

Tonight I took photos of Vega with and without coma corrector. I shall post them later for comparison.

[sunka]

I attached two images of Vega, with and without coma corrector. Since distance from coma corrector to chip is OK I recollimated laser collimator and telescope again for sanity check - my conclusion is coma corrector is not appropriate for f/4 system.

Does anyone else have any suggestions? Maybe I should try better coma corrector for further testing...

Regards!

[sunka]

Another test, Messier 27:

Coma corrector:

No coma correector:

Stars are obviously smudgy even in center of field when coma corrector is used.

[sunka]

I cant edit post so, first m27 is without coma corrector and second is with.