Am I a Coma-tose Idiot?!

[PhotoGav]

Have a look at this:

It is a single 1800s Ha sub of my recent 2 pane mosaic project of IC1805 - The Heart Nebula. Look at those corners... Yuk, hyperspace style stars. Whereas the central portion is lovely and flat.

The question is: are the stars elongated in the corners because I had the spacing wrong between flattener/reducer and camera. I am using a Skywatcher ED80 DS-Pro with the dedicated Skywatcher flattener/reducer. The camera is a QSI 683-WSG8. The back focus of the camera is 50.2mm, the reducer is set up to work with the 55mm DSLR back focus distance. I have been set up with a 5mm T2 adapter on the QSI and then a T2 to M48 adapter (3.5mm) to connect to the reducer. So, I was operating with a total back focus of 50.2 + 5 + 3.5mm = 58.7mm.

Would nearly 4mm over the recommended distance create that much coma?

If I remove the 5mm adapter I am down to 53.7mm which is only 1.3mm off the optimum distance. Would that make a significant difference to the results?

Or, is the ED80 and reducer never going to give a really flat field to the corners and the above image is the best that I can expect?

Thanks for you help and advice in advance.

[Epicycle]

Spacing is quite critical for flatteners/reducers and 4mm seems quite a lot to me, particularly given the big chip of the qsi 683,

E.

[PhotoGav]

Thanks Epicycle - that's potentially good news really as if I drop it to only 1mm out, hopefully the results will improve. Just a shame I didn't realise this four months ago when I started using the new camera!!!

[Epicycle]

For me the optimal distance is about 57: filters and the glass in front of the chip add a bit extra (e.g. Astrodon filters add an extra mm).

TS sells 3 mm spacers and some spacer rings of 1.5 and 1mm which I have found very useful,

E.

[PhotoGav]

For me the optimal distance is about 57: filters and the glass in front of the chip add a bit extra (e.g. Astrodon filters add an extra mm).

TS sells 3 mm spacers and some spacer rings of 1.5 and 1mm which I have found very useful,

E.

What kit are you using?

[Epicycle]

What kit are you using?

Megrez 72 with reducer 6 and 130pds with mpcciii, atik 460

E.

[PhotoGav]

Megrez 72 with reducer 6 and 130pds with mpcciii, atik 460

E.

Excellent - thank you very much for your help.

Well, I will try the reduced spacing next time there's a break in the clouds...

[ChrisLX200]

Seeing as the bottom-right corner is not as severe as the other 3 I would also suggest you also have a degree of tilt between camera and optical axis. It may well be a bit of focuser sag, or perhaps you're using a push-fit nosepiece?

ChrisH

[lensman57]

Have a look at this:

corners01.png

It is a single 1800s Ha sub of my recent 2 pane mosaic project of IC1805 - The Heart Nebula. Look at those corners... Yuk, hyperspace style stars. Whereas the central portion is lovely and flat.

The question is: are the stars elongated in the corners because I had the spacing wrong between flattener/reducer and camera. I am using a Skywatcher ED80 DS-Pro with the dedicated Skywatcher flattener/reducer. The camera is a QSI 683-WSG8. The back focus of the camera is 50.2mm, the reducer is set up to work with the 55mm DSLR back focus distance. I have been set up with a 5mm T2 adapter on the QSI and then a T2 to M48 adapter (3.5mm) to connect to the reducer. So, I was operating with a total back focus of 50.2 + 5 + 3.5mm = 58.7mm.

Would nearly 4mm over the recommended distance create that much coma?

If I remove the 5mm adapter I am down to 53.7mm which is only 1.3mm off the optimum distance. Would that make a significant difference to the results?

he spacing of the

Or, is the ED80 and reducer never going to give a really flat field to the corners and the above image is the best that I can expect?

Thanks for you help and advice in advance.

Yes the spacing of the FF/FR is critical and the published distance is usually quite correct so try and stick to 55mm if possible. can use the Baader Derlin spacers to achieve very accurate spacing. You also have quite a heavy camera so make sure that the focuser is not sagging under the weight.

A.G

[PhotoGav]

Seeing as the bottom-right corner is not as severe as the other 3 I would also suggest you also have a degree of tilt between camera and optical axis. It may well be a bit of focuser sag, or perhaps you're using a push-fit nosepiece?

Thanks Chris, yes, it does appear worse on the left hand side. This is probably due to the focuser not coping with the heavy camera. The ED80 may be nearing the end of its useful service for me!

Yes the spacing of the FF/FR is critical and the published distance is usually quite correct so try and stick to 55mm if possible. can use the Baader Derlin spacers to achieve very accurate spacing. You also have quite a heavy camera so make sure that the focuser is not sagging under the weight.

Thanks AG, I will definitely try with more accurate spacing and will check out the spacers you recommend. As above, the focuser may well be a culprit in this problem too... Upgrades are being considered!

[Twisted Lip]

Thanks Chris, yes, it does appear worse on the left hand side. This is probably due to the focuser not coping with the heavy camera. The ED80 may be nearing the end of its useful service for me!

http://www.iankingimaging.com/show_product.php?id=1320

You know you want to lol

[russellhq]

I have a similar set-up but different camera. I could check one of my subs for you but my diagonal is only 16mm vs your 22.2mm. I might have an old DSLR sub I could check, it's got a bigger diagonal, 26.7mm.

[PhotoGav]

Good idea to check a DSLR sub, I have plenty of them... Hoping that I'll be able to run experiments tonight with new spacing.

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[PhotoGav]

http://www.iankingimaging.com/show_product.php?id=1320

You know you want to lol

Oh yes indeedy, that's what is tempting me...!

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[Uranium235]

Firstly (in my experience), a spacing thats too long usually creates a radial distortion, rather than coma. When using the 383 with the 80ED, my spacing was about 57mm (which took into account filter thicknesses), which still didnt deliver proper corners - but.... there is a way to blag your focus.

Dont focus on a centrally placed star, move it out to about 2/3rds away from the centre - then focus. That will balance out the field and and eradicate most (if not all) of the coma.

It works - try it!

[swag72]

Gav, stop trying to get us to say that you need a Tak!!

I think that the 4mm spacing would be quite critical, but the only way to know will be to try it. Don't ask us, get  out there and do it :D Show us the results when it's done!

[PhotoGav]

Well, I thank the Cloud Gods for their kindness this evening. I have run the revised spacing experiment and the results are rather significant:

It is amazing what a difference 5mm makes.... So, it's a relief that the problem is solved. I thank you all for your input on this one. The only trouble is that I can now delete all the rubbish data of my recent Heart mosaic and start again...

Oh, and for the record Sara, I do still need a Tak anyway :grin:

[swag72]

Glad that it's all sorted now Gav

[lensman57]

Well, I thank the Cloud Gods for their kindness this evening. I have run the revised spacing experiment and the results are rather significant:

corners02.png

It is amazing what a difference 5mm makes.... So, it's a relief that the problem is solved. I thank you all for your input on this one. The only trouble is that I can now delete all the rubbish data of my recent Heart mosaic and start again...

Oh, and for the record Sara, I do still need a Tak anyway :grin:

You can correct the stars in PS.

A.G

[PhotoGav]

You can correct the stars in PS.

I have to ask.... How?!