Uh oh.....Coma..I think. What now?

[Rodd]

Well, I was floored when I inspected the edges of this image of sh2-129 taken with the FSQ 106 and .6x reducer during a n almost full Moon.  The image contains 63 5min Ha subs.  I have never been afflicted by coma before, or serious curvature, or what ever this is.  I am distressed and do not know what to do.  Should I add a 1mm spacer to the Optical train between the reducer and camera?  Or should I remove 1mm of space.  I did not notice this problem with my other images taken with this configuration (Sh2-119 and the Veil).  Then again I did not inspect them closely.  I have included the processed image as well as a corner/center comparison image generated using the aberration spotter script in Pixinsight.  Anybody have any advise?   

[pipnina]

In honesty, those star shapes look slightly deformed. Although it does seem minimal (the stars are the same size in the middle and the periphery, merely misshapen slightly).

I presume this was taken with the 1600MM? If so take a note of the ideal distance the reducer asks for, the back-focus distance of the camera (1600mm claims 6.5mm "back-focus") and the additional optical spacing between those components. See if your total distance is above or below the ideal spacing suggested by the reducer.

[Ruud]

If you hadn't pointed it out I would not have spotted it, though I might see eventually it on a print of say, a square metre.

But really, if it's any consolation, it's a very high quality image!

[MarsG76]

Personally I wouldn't worry about such a tiny amount... If you didn't say that its there I would have noticed it.

If its bothering you, crop the frame slightly.

 

[Stub Mandrel]

2 hours ago, Rodd said:

I have never been afflicted by coma before, or serious curvature, or what ever this is.  I am distressed and do not know what to do. 

Isn't the answer obvious? ?

Look at your other images with the same setup, if they don't show  the same effect, then it was something about that evening.

If they do and you haven't noticed for months, then it can't be that serious...

(Looks to irregular to be an optical fault if you ask me).

[wornish]

Are you serious? the image is great.

If this distresses you I dare not share any of mine?

[carastro]

Same here, I can't even see any problem.  If it is coma and it is bothering you then just make sure the spacing is spot on.

Carole 

[Rodd]

1 hour ago, pipnina said:

In honesty, those star shapes look slightly deformed. Although it does seem minimal (the stars are the same size in the middle and the periphery, merely misshapen slightly).

I presume this was taken with the 1600MM? If so take a note of the ideal distance the reducer asks for, the back-focus distance of the camera (1600mm claims 6.5mm "back-focus") and the additional optical spacing between those components. See if your total distance is above or below the ideal spacing suggested by the reducer.

see below

1 hour ago, Ruud said:

If you hadn't pointed it out I would not have spotted it, though I might see eventually it on a print of say, a square metre.

But really, if it's any consolation, it's a very high quality image!

see below

1 hour ago, MarsG76 said:

Personally I wouldn't worry about such a tiny amount... If you didn't say that its there I would have noticed it.

If its bothering you, crop the frame slightly.

 

see below

16 minutes ago, Stub Mandrel said:

Isn't the answer obvious? ?

Look at your other images with the same setup, if they don't show  the same effect, then it was something about that evening.

If they do and you haven't noticed for months, then it can't be that serious...

(Looks to irregular to be an optical fault if you ask me).

Well--I thank everyone for the response.  Just spend about 4 hours on the phone with OPT and TAK.   It turns out that I had the spacing off by about 2.5mm.  Its because when I tried to connect the camera (filter wheel) to the reducer, I could not make the parts that my supplier gave me work--wouldn't come to focus and/or did not have the right threads.   So, having a bunch of Takahashi odds and ends, I found a part that would thread in and I could achieve focus.  I thought that was good enough.  Almost.  I am getting correct connectors sent to me.  One of the most frustrating things about this endeavor is the connections between the camera system and the scope system.  

Rodd

[James]

If it’s worrying you, I’d add just a little spacer in your optical train and see if that makes a difference. Maybe just half a mm one, something like a Delrin spacer if you have a T thread somewhere you can put it on. 

To me the ‘problem’ looks consistent around your corners and I think you just need to adjust your spacing.

But... this is only visible when you zoom way in on your image - that image when viewed at the size it should be looks great

[James]

Aha. I posted just as you were - try the spacing and see

[Rodd]

On 26/06/2018 at 16:39, James said:

If it’s worrying you, I’d add just a little spacer in your optical train and see if that makes a difference. Maybe just half a mm one, something like a Delrin spacer if you have a T thread somewhere you can put it on. 

To me the ‘problem’ looks consistent around your corners and I think you just need to adjust your spacing.

But... this is only visible when you zoom way in on your image - that image when viewed at the size it should be looks great

Thanks James--yes--I have ordered the appropraite spacers--a 2.5mm difference.  Right now I am 2.5mm too long.  What does that mean as far as my effective focal ratio?  Instead of teh F3 I am supposed to be at, does that mean I am at F4 or some number higher than 3?

Rodd