Choosing a coma corrector for 8" f/4 telescope?

[stoffe]

Hello

I have a Telescope Service newtonian 8" f/4 telescope and I have a DSLR camera with a 36x24 mm sensor. And I need a coma corrector, I have looked at the Skywatcher f4 Aplanatic Coma Corrector and I think it is good with a M48 thread to the camera.

The sensors diagonal is 43,3 mm is a M48 thread to narrow for my camera? If I screw the corrector into the M48 thread, I guess the opening would be something about ~45 mm?

Any suggestions about which corrector that I should buy? Ofcourse I want the best optical correction. I have borrowed a MPCC Mk II from a friend but I don't want that with a small T-thread (M42) it does not work so good on my scope, it is made for f/4,5 and I have f/4 and it is also to narrow with that T-ring.

Any ideas?

[Moonshane]

I have not used mine yet but I recently bought a used altair coma corrector which is well built and quite cheap. I'll be comparing it to a televue paracorr shortly.

[stoffe]

The altar is the cheapest I have found but it use T2-thread and I want one with M48 thread, but publish your result. It could be of interest anyway.

[Moonshane]

I am pretty sure it comes with a M48 male thread on the actual CC. a baader fine tuning ring screws right onto it

https://www.altairastro.com/product.php?productid=16363

[stoffe]

Oh it was M48 thread, I saw it on another web shop and it was only a T2 thread there, maybe it was an older one or wrong text.

Why is the Aplanatic one much more expensive than this one? Is it better or the same?

[Moonshane]

no idea of the differences. I cannot really help with imaging applications as I am solely a visual astronomer. adding a single baader ring (28mm) between corrector and the visual top means that I have the correct spacing for eyepieces too. will be interesting to see the visual images cf the paracorr.

[stoffe]

I understand what you mean, the only thing I want to do with my 8" f/4 telescope is astroimaging with my DSLR (or a cooled CCD if I buy one in the future). I have a much better scope for visual and planeteray use, its a Cave Astrola Deluxe B 8" f/8 that is outstanding for everything except deep sky imaging with a DSLR.

[narrowbandpaul]

The full frame sensor will be more demanding of the corrector. I'm not sure if the Baader or skywatchers will correct over more than APS C. You may need to look at ASA to correct over the full frame

[stoffe]

Hello

It seems like the ASA 2 0,77 corrector only can 22 mm field.

http://shop.textalk.se/en/article.php?id=19067&art=18083982

In the text here I think the GPU Aplanatic corrector is the best one. It covers a full frame view, I know it could be some vignetting but maybe it could be corrected with flats and a little crop of the image.

http://shop.textalk.se/en/article.php?id=19067&art=20214199

I saw some very interesting comparision here! And I think it could help many people to choose the right correcotor.

http://www.teleskop-austria.at/shop/index.php?lng=de&m=2&kod=bar2-kom-kor-4

[stoffe]

I also found this bloggpost, it can be translated with google translate.

http://komakorrektor.blogspot.co.at/

But I am curious how it would perform with 36x24 mm sensors and also which T-ring I would get to optimize the distance between the sensor and corrector?

[glappkaeft]

Hello

It seems like the ASA 2 0,77 corrector only can 22 mm field.

http://shop.textalk.se/en/article.php?id=19067&art=18083982

In the text here I think the GPU Aplanatic corrector is the best one. It covers a full frame view, I know it could be some vignetting but maybe it could be corrected with flats and a little crop of the image.

http://shop.textalk.se/en/article.php?id=19067&art=20214199

Tjena Stoffe!

On the ASA homepage there are three 2" coma correctors. The one with the largest field is the Quattro x1.175 with a 30mm field (specs @ http://www.astrosysteme.at/images/Corr_2Zoll_ComaCorr.pdf) that can be extended to 40mm with some vignetting and a minor loss in performance. For optimal performance on 36x24 mm chips 2" is just a little to small but then again so is the secondary mirror on you scope.  

/Patrik

[stoffe]

Tack Patrik! (we are both Swedes and are IRL friends)

The ASA for 550€ looks really nice but of course, expensive. I think with a magnification of 1,175x it would reach focus on my scope without any problems. Actually it would cost more the my telescope I can change my secondary mirror as well, it is a 63 mm mirror.

[stoffe]

I just saw that 550€ is without VAT and shipping. To bad, it would be nice with that corrector. But I still wonder what the difference is between the ASA corr and the Skywatcher (GPU) corr. Both are 4-element designs and correct the field very good.

[stoffe]

I also saw this thread about the Aplanatic corrector, but I haven't found anyone that have acctually bought it and used it?

Btw, 55 mm backfocus, is that to tight if I want to upgrade to a dediceded CCD with filterwheel and offaxis guider?