Coma Corrector

Hello good people.

So i just got myself a new SkyWatcher Newtonian 150/750 PDS along with the Baader multipurpose coma corrector. I’m using it with a DSLR with a crop sensor for now so it should work well but i still get coma in the corners. The manuals say the corrector has to be 55mm from the sensor and i measured it to be the right distance. Any ideas?? Is this normal, maybe i cannot eliminate all of the coma? Maybe i’m not collimated right? Or is there a better corrector i could use? 

Thank you

For starters, just to clarify: How did you measure the 55mm? and what Baader MPCC do you use (II or III)?

I got the MPCC III which according to Baader needs 55mm from the sensor. So, I researched my cameras distance from the flange to the sensor (44mm) and then added that to the T Ring and the corrector screwed on which I measured (roughly to be honest with a measuring tape) was about the right distance. is there a proper way to measure?

My guess is you basically go it right, in principle. 44mm from camera flange to sensor if you have a Canon camera then add the T-ring thickness (best measured with a caliper to be honest) but they usually come in the 10.5/11 mm thickness range. You best bet (other than buying a better CC) is to add spacers (from 0.2mm thickness to 0.5mm thickness with internal diameter of 42mm) to see whether the field coma improves by adding some but to be honest not sure it will do.

Hi,

As others have said here, the distance matters. It's best to get it within 1 mm.

Also, if there are filters in the optical path, these will contribute some optical distance - random guess would be 50% longer than their thickness, but perhaps don't worry about that first.

My own experiences are with the MPCC  III, an f/4 Newtonian and a CCD with a chip diagonal of 22.5 mm.  I had slight coma in corners depending on the camera rotation. Very slight, but I was obsessing. This led me to rigorously going over everything related to collimation and in the end it was still there (example).

In your case, how much does f/5 matter with collimation? Not sure, but you should check that too. You didn't say if the coma was about the same in all four corners; if it isn't I'd venture a collimation issue. Also, if the camera chip is large, you'll certainly be seeing a degradation the further away from center you get.

Here’s the manual just in case…

https://www.baader-planetarium.com/en/downloads/dl/file/id/483/product/3004/mpcc_mark_iii_instruction_manual.pdf

for back focus …




once you’ve sure collimation is correct , if your using Nina for taking images, there’s a handy app call Hocus Focus which can tell if you back focuser is correct, 

Here’s a video explaining 

https://youtu.be/M1-izvBlO44?si=NTDv5sI4Pj5v883q

But before you go down that rabbit hole , collimate 

look up Vic Menard on Cloudynights 
Jim Fly of Catseye collimation 
there’s also a guy named Jason who’s up there with the best on cloudynights,

once you’ve that down , with the correct measurement …
you’ll be teaching others 😉

I bought the TS-GPU back in February. It is definitely a step up. Check out this thread on Cloudy Nights near the bottom for my experiences with it.