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Help with Coma corrector distance


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I have recently acquired a Baader CC and have attached it at the prescribed distance of 55mm for my ASI533. I am still getting coma and was wondering if there is a simple way to figure out if I need to add or remove spacers to fix the issue. The scope is the 130PDS. (650mm FL)

I am attaching a defocused image to show the extent of coma. Collimation is not perfect too but thats a separate issue ๐Ÿ˜žย 

2023-01-04T09_25_55.png.c3e50009b5937093ec8c0cfa2eff3aee.png

Edited by AstroMuni
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Do you have an image with the stars in focus, also is this a crop of the full image or is it just shrunk down, it is showing as 653 pixels square to me?ย  It's a bit hard to tell what is going on with the defocused image.ย  It doesn't look like the central obstruction is heading towards a common focal point or if they are it's somewhere to the left of the centre of the image.ย  It could be you've got a flavour of tilt.

Is it the MPCC Mk3 coma corrector and how are you attaching as there is 2 ways, one of which needs 55mm and the other needs 57.5mm of back focus.

Capture.JPG

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You will need to look at a short exposure image which is in focus at the centre. If the back focus distance needs increasing then the stars at the edges/corners of the image will be elongated towards the centre & if the stars at the edges are radially elongated then the back focus needs to be decreased. There's a good illustration of this in this old thread:-

I had a problem getting decent backfocus with my ZWO 2600 & Baader coma corrector with the M48 connection. As per @alacant's suggestion increasing this from 57.5 to approx 60mm helped:-

Cheers
Ivor

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6 hours ago, Ratlet said:

Is it the MPCC Mk3 coma corrector and how are you attaching as there is 2 ways, one of which needs 55mm and the other needs 57.5mm of back focus.

Its attached to the left of this train. This gives a total of 55mm (48.5mm of spacers + 6.5mm distance to sensor)


image.png.2509de1fbeddc3017b2fc234bbcde6d4.png

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On 04/01/2023 at 10:30, AstroMuni said:

a defocused image to show the extent of coma

Hi

That isn't coma, it's astigmatism; some Baader cc examples are better than others

You can see that the corrector is doing its job on the coma as it distorts the coma-stretched stars back into shape.ย  Going to 60mm may help but also fatten the stars centre field as may ensuring the front element of the cc isn't pinched.

Assuming the secondary checks out OK (substitute one of known performance) and you have good perfect collimation,ย  the only definitive method of curing both aberrations is to go with one of the 4 element ccs. Or just accept it and correct in software. Over a 533 sensor, it should be relatively slight anyway..

Cheers and HTH.

ย 

Edited by alacant
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4 minutes ago, alacant said:

Assuming the secondary checks out OK (substitute one of known performance)

Its a brand new 130pds from FLO. I dont have other similar secondary mirrors that I could substitute ๐Ÿ˜ž

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18 minutes ago, AstroMuni said:

dont have other similar secondary mirrors

Don't know where you are, but there must be an astronomy club/gathering/meeting somewhere close.

When getting low end gear up to ap standard, they're a god send!

Cheers and good luck.

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1 minute ago, alacant said:

Don't know where you are, but there must be an astronomy club/gathering/meeting somewhere close.

When getting low end gear up to ap standard, they're a god send!

I have been taking help from my local astro club to get this in a better shape. Let me check if anyone has a secondary to spare ๐Ÿ™‚

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On 04/01/2023 at 18:00, Ratlet said:

If you're attached using the m48 on the baader then you'll need closer to 58mm back focus

Thank you. I added a few spacers and the image has improved. Yet to find the perfect spacing but getting there....

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I finally got a chance to do some imaging last night and experiment with spacers, tilt and collimation. Here is an image of the Perseus Double cluster and coma correction seems to be a lot better. Comments welcome.

1995286400_Perseusdblcluster.thumb.jpeg.1cc0ca5b6d1fef3f5ab044c3024c5854.jpeg

Edited by AstroMuni
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