QHY8, Coma, Reflections, and the MPCC

[toryglen-boy]

So, i get bad coma on the F5 scope when taking pictures, and i use an MPCC, although i found it tricky to get the spacing right, so i ordered the QHY8 MPCC back focus matched adaptor from Peter Tan, and although its machined very nicely and fits, but i still get coma, even though i am using the right spacing (55mm)

Also, i get really bad reflections of the colimation marker in the primary when i use the MPCC, and i mean REALLY bad, any brightish stars in the image, produce a red doughnut, right next to it.

Have any of our QHY8 and MPCC users managed to do anything about getting the right spacing, and getting rid of reflections?

thanks

[multiweb]

Hey Duncan, I have the exact same configuration as you. F/5, MPCC,QHY8. Here's what I do.

1_ I use a custom telescopic adapter that Peter Tan made for me. He's go the blue prints. 55mm was too short of a distance in my case.

2_ I got a NULL adapter (no glass) to replace the standard nosepiece. No more reflections from the MPCC glass onto the nosepiece glass.

3_ I flocked my scope. No stray light and reflections.

4_ I tried to collimate it real real real good. Well as good as I could but you get the idea.

All in one it made a huge difference. I'll still get halos if I push exp time on alnitak or M45 but that's a given with pretty much any scope. HTH. :thumbsup:

[pmrid]

Duncan, it sounds as though it is mainly a collimation issue but in order to at least isolate the source of the problem, see if you can borrow a DSLR from someone and take some shots with that (leaving everything else in place). It may show you that the reflections either are, or are ot within the QHY8. That's a start.
Peter

[toryglen-boy]

1. I hear ya, ok i will ask about that, although it seems strange that its sold as back focus matched, and when i enquired about it before ordering i was told it would put it to the correct distance.

2. i dont use the nosepiece anyway, its MPCC adaptor, then MPCC thats it!

3. Thats a good idea

4. Thats another good idea

:)

[pmrid]

Duncan, I see that you said you don't use the nosepiece anyway. You seem to be saying you have removed it from the QHY8. I take it that you screw the MPCC spacer straight into the body of the QHY8 and the MPCC into the other end. Is that right? If it is, it probably means you are not achieving the 55mm separation the MPCC is designed for. You have probably taken the width of the nosepiece out of the equation. You may need to find a way to compensate for that distance.
Peter

[toryglen-boy]

I hear you Peter, i'll tell you .. this is getting ridiculous !!

[multiweb]

Correct. Duncan the nosepiece (12.5mm) has to stay. If you remove it you have to make up for the spacing. There's your problem ;) . If it helps my total spacing MPCC flange to QHY8 face is 46.51mm.

[pmrid]

Cheers Duncan. If there is any consolation it is in the sure and certain knowledge that others have struggled with the same things and climbed the same slippery J-curve of learning. Count me among them and I'm still well down on the curve too.

Peter

[Terry B]

Peter.
I'm not sure how the donut on the primary could be the cause of the halos.
The donus should not get any light on it unless you have off axis light hitting the mirror. The secondary should shaddow the centre of the mirror completely.

[pmrid]

I have assumed he's talking about a Newtonian and you're right, assuming the thing is reasonably well set up. But Murphy's law is alive and well and I prefer to solve problems 'bottom-up' so to speak and collimation seems to me the thing you would check first.
Peter

[Manav]

I've read that skywatcher OTA and MPCC have an issue due to the 2" E.P. adapter supplied by Skywatcher. Source: http://stargazerslounge.com/equipment-reviews/69211-baader-multipurpose-coma-corrector-mpcc.html

Any ideas?

[bmitchell82]

Duncan what you are seeing is a reflection from either the sensor upto the MPCC and back down to the sensor or the front window to the back of the MPCC and back down to the sensor there is a formula for figuring out the distance it is.

It is not the primary mirror spot more a image of your secondary holder. if you where to use a refractor it would do exactly the same thing bar the doghnut would be a solid circle. In my shot of M42 i have the same problem its just that i used some careful cloning and healing brushes in photoshop because they annoyed me! bright stars highly focused are just something we cannot avoid!

[RobF]

Check out this thread Yugant, specifically pics in post #35:
http://www.iceinspace.com.au/forum/showthread.php?t=33290

I've had my 2" adaptor cut down in this way and has been working great with Canon 450D DSLR all year. Other solution is to cut length off the main OTA tube.