Coma?

[MattJenko]

Morning All,

I set up my 10" dob for some imaging last night and things went pretty well. I found that the light gathering ability of this tube and mirror is something else and I have had to drop exposure times drastically for stellar objects. Here is 40 x 1 min L exposures of M15. My question however relates to the CCDInspector image below. It appears that there is some serious curvature distortion in my image, which I can only assume is off axis coma? I am only using a small chipped camera (an Atik 414) so this means the coma is pretty severe, with this being a F4.9 system. It seems that I am going to benefit from a coma corrector?

 

 

[Moonshane]

Depends how fussy you are. I can see only minor coma in the corners of your excellent image.  If you want to remove this then a cc will be needed.

[laudropb]

Lovely image. As Moonshane says it all depends on what you want.

[Louis D]

Remember, there will also be some field curvature, even for a Newt.  Most 4 element coma correctors also correct for field curvature as well.  That is, they act as field flatteners.  You could start with the GSO CC at the low end cost-wise and see if it improves things enough to want to try the more expensive solutions.  It's only a $120 here in the states.

[MattJenko]

Cheers guys. Given I actually have a coma corrector already, I will give it a go. Nothing like trying to eek that last bit of performance from one's setups!

[jsmoraes]

Attention: coma corrector needs backfocus. Perhaps you will need spacer to get the correct distance to surfance of sensor. Few milmeters  plus or minus can cause more distortion than you are seeing without it.

And that is the truth: all newtonian with parabolic mirror HAS coma distortion. Mainly if it is f/5 or more fast.

[MattJenko]

11 hours ago, jsmoraes said:

Attention: coma corrector needs backfocus. Perhaps you will need spacer to get the correct distance to surfance of sensor. Few milmeters  plus or minus can cause more distortion than you are seeing without it.

Thanks. I am familiar with back focus requirements for the flattener for my little refractor, so all prepared to spend a few nights adding in tiny spacers to my imaging train to try and get this right

[Louis D]

4 hours ago, MattJenko said:

Thanks. I am familiar with back focus requirements for the flattener for my little refractor, so all prepared to spend a few nights adding in tiny spacers to my imaging train to try and get this right

Keep us updated with your progress.  It's always interesting to follow others to avoid the same pitfalls.