Coma, mirror issue or poor collimation ?

[Spaced Out]

Hi all

I’m a total newbie just starting to learn about telescopes/imaging.

Last night I attempted to set up my Baader MPCC MKIII with the 130PDS. I tried a number of different spacer options with the T2 ring and compared the test images. Eventually I decided that 1.6mm spacing gave me the best results. However, even with the spacing as good as I could get it there seems to be star streaking around the corners and edges. The distribution of this (coma ?) isn’t even and it seems worse in the bottom corners, I also noticed the stars look a little bit triangular, which I remembered reading somewhere indicated maybe a mirror pinching issue ?

With this in mind I did a quick star test for collimation and it seems the scope is slightly out and in need of collimation (a scary task that I need to learn how to tackle !).

SO...... my question is this....

Do you think those stars around the edge are still showing coma or does it look more like poor collimation or a pinched mirror issue ? Or maybe I am just expecting too much ?

I’m hoping to try collimating the scope today or tomorrow using Astro Babys guide, fingers crossed that may help. I’m still pretty pleased with the MPCC, it's so much better than without, just a bit frustrating I can’t get it perfect to the edges !

Thanks in advance for any advice you can offer.

 

[andrew s]

Poor collimation shows up as coma but it will be worse at one edge/corner of the field than the others. If the collimation is fine then any residual coma should be similar in all corners.

You can try CCDWare's CCDInspector for free. 

Regards Andrew

[alacant]

Hi. It looks like tilt from the bottom right. Collimate with the Cheshire inserted at the same draw-tube position that your cc-camera combination reach focus. Now, rack in fully then rack out fully to see if the collimation changes. Then, move the telescope to different angles and see if the collimation changes. That should tell you/us about the optical to mechanical relationship of focuser, tube and mirror cell to the optical axis. 

But hey, IMHO it's good enough and in a stack of similar images, you'll notice the issue less. HTH. 

[Charic]

I see some coma mainly along the bottom edge, zoomed right in,  but If were new to astrophotography, I'd be chuffed with an image like that
I'd just crop the bottom off........eek!

The right folk  here will see you right, but well done.

[bobro]

As someone whose f5 telescope has quite significant coma (with some flying saucer effects at one corner that may be due to mirror pinching perhaps), I'm mightily impressed by your single image. There doesn't seem too much to fix.

If you do wish to experiment more, perhaps the corrector could be removed and the star test repeated. That would show the underlying collimation of the scope - coma tails should point away from the centre of the image. The corrector could then be replaced for a re-test.

Wish I had similar results with my DIY coma corrector! 

[tooth_dr]

It does look pretty good Gary!

[Spaced Out]

Thanks everyone for your feedback and those positive comments too !

I hope I’m not far off with this then, I’m getting the feeling (and hoping) that the corners, particularly at the bottom, are more likely to do with poor collimation than the coma corrector. I did the star test without the CC fitted and that did suggest it was slightly out.

I will attempt to collimate the telescope for the first time today and then test it again on the next clear night.  

[Gerry Casa Christiana]

I have the 150pds some of my pictures are exactly the same in fact worse and it was the collimation. I rushed going out and didn't collimate! In yours I won't worry looks pretty good.