What's causing my weird star shapes

[ste7e]

Hi all

I bought a new scope this summer and I'm a very frustrated with it.  I'm desperately hoping someone can help me. 

My problem is with the stars around the edges of the image.

I've so far been concentrating on the collimation and I'm confident that I now have it very well collimated but I still have the problem.

My equipment is:

    ONTC 8" f/4
    QHY268M
    QHY CFW3-M-US
    QHY OAG
    Baader MPCC MKIII

    Antlia filters

The below image was a 2 min sub of Andromeda.  I took this the first night I used the new telescope and the collimation was a little out at this stage.

This is a close up of the corners of the above image (from AberationInspector in PixInsight):

You can clearly see the problem especially in the bottom-right sector. 

I have tried removing the OAG to rule that out.  I have measured and re-measured and re-measured again the MPCC to sensor distance.  I have even varied the coma corrector to sensor distance (despite measuring that it's already at the proscribed distance) and it has not helped.

As I said earlier I have improved the collimation since these first shots but the problem remains.  Here is an image from my collimation camera showing the view down the scope with a series of concentric circles showing that everything is collimated.  

The circles are:

• yellow circle is the bottom of the focusser tube; 
• blue circle is outlining the secondary mirror
• green circle is outlining the primary
• and the red circle is surrounding the central ring on the primary (you'll need to zoom in to see that one):

 

Do you have any idea what could be causing this aberration?  So far I have wondered about:

• Reflections
• Collimation
• Bad coma corrector to sensor distance
• Something in the light path
• The stars really are that shape and no-one else has noticed?  (I did say I was desperate!)

Thanks for any help.

Steve

[Elp]

I'm no expert but that collimation looks wrong to me, everything should be concentric to the centre. Is your primary mirror centre dotted, it makes collimation easier, I used to do it simply with a collimation cap.

As for the stars it is coma on the RHS so possibly due to the collimation or there's some tilt somewhere causing the CC not to do its job properly.

Edited November 22, 2023 by Elp

[bosun21]

1 hour ago, Elp said:

I'm no expert but that collimation looks wrong to me, everything should be concentric to the centre. Is your primary mirror centre dotted, it makes collimation easier, I used to do it simply with a collimation cap.

As for the stars it is coma on the RHS so possibly due to the collimation or there's some tilt somewhere causing the CC not to do its job properly.

Fast reflectors have an offset in the secondary mirror I believe.

[ste7e]

55 minutes ago, bosun21 said:

Fast reflectors have an offset in the secondary mirror I believe.

That's right.  The shadow of the secondary should be offset towards the primary (i.e. the right in that photo).  That's what I understand, anyway

[michael8554]

Hi Steve

"I have measured and re-measured and re-measured again the MPCC to sensor distance. "

And what was that distance ?

And there's nothing stopping you from adding spacers until you get best corner star shape.

The corner images look to me like the effects of some of the mirror clips pinching the mirror, and a bit of coma due to FR spacing too close.

Michael

Edited November 24, 2023 by michael8554

[ste7e]

2 hours ago, michael8554 said:

Hi Steve

"I have measured and re-measured and re-measured again the MPCC to sensor distance. "

And what was that distance ?

And there's nothing stopping you from adding spacers until you get best corner star shape.

The corner images look to me like the effects of some of the mirror clips pinching the mirror, and a bit of coma due to FR spacing too close.

Michael

I'm using the T2 thread onto the QHY spacers (between CC and filter wheel) and it measures at 55mm between the end of T2 thread (as in the left-hand picture) and the sensor. 

I have tried adding spacers and it hasn't made a difference.  My next test (when I get chance) will be to reduce the distance a bit and see if that improves things.  Or, does something in the aberration indicate that the FR distance is too small rather than too large?

I suspect I may have multiple problems at the same time - maybe tilt (the aberration is always worse on one side and that side is fixed to the starfield regardless of camera orientation indicating a tile somewhere in the OTA), mirror pinch and not-perfect backfocus distance?

 

[michael8554]

The coma shape suggests "too close".

But sort the Tilt and possible Pinching first.

Michael