M57 Focus/Drift/Coma Help

[deanen]

Hi All

 

I've tried photographing M57 (ISO 3200, Expo 1:45m) with my Nikon D3100 with some success (i think), however I would like some guidance on the stars at the edges of the picture. All of the stars seem to have a tail radiating out from the centre of the pic. I assume this is coma, but I'm not entire sure whether my drift alignment  or focus is spot on. I haven't stacking the 5 subs I managed to take last night as I didn't have time to take the darks or flats.

 

Any criticism/guidance will be greatly appreciated.

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[Wyvernp]

Hi,

It does seem to be some sort of aberration, what scope/correctors are you using? and also is your camera sensor at the correct backfocus distance? don't worry if you don't know the answe to the last one as we can probably help figure that out.

Wyvern

[deanen]

12 minutes ago, Wyvernp said:

Hi,

It does seem to be some sort of aberration, what scope/correctors are you using? and also is your camera sensor at the correct backfocus distance? don't worry if you don't know the answe to the last one as we can probably help figure that out.

Wyvern

Hi Wyvern

 

Rookie mistake not posted all the required info.

 

I was using my SW explorer 200PDS with a SW x2 barlow and MPCC III. I'm not entirely sure on the back focus, but I believe I'm at 54.8 so with the 1mm deviation window.

[MarkAR]

I think with stars radiating out at the edge it could be your back focus is a tad short.

[Wyvernp]

Could it possibly also be the barlow magnifying the effect of coma? or even affecting the backfocus distance? Do you have any pics taken without the barlow to compare with?

The only reason I ask is that when I use filters I have to account for their thickness in the backfocus calculations, I'm not sure if the same applies to a barlow?

edit, missed that you were usung the mpcc III corrector

Edited August 18, 2020 by Wyvernp

[deanen]

Unfortunately, I don't have any without the barlow.

 

The weather looks like it might be decent for a few hours this evening. I'll try a few snaps with and without the barlow, and adjusting the back focus and post the results. 

[michael8554]

The MPCC is suited to Canon DSLRs which are 44mm Back Focus + 11mm for the T Adapter = 55mm

Nikons Back Focus is 46.5mm I believe, have you accounted for that difference ?

The Barlow makes it difficult to judge  what distance to set the MPCC to.

But the charts often shown on SGL suggest the camera is too close, as Mark has suggested.

Michael

[deanen]

@white dwarf Thanks for posting that nugget of info, it helped me find another post confirming the bayonet to sensor distance and I learnt something new ( DSLRs have phi mark on the body inline with sensor allowing measurement of the sensor to the bayonet distance). The T adaptor I have is 8.5mm so puts me at 55mm on the dot. 

 

It would seem that the barlow does introduce the artefact as the picture below was taken with just the MPCC earlier this week and it isn't present. I'll need to investigate whether inserting the barlow deeper into the focuser will eliminate it but that tends to cause a back focus issue during visual observing and I would assume the same with a camera attached (I may be entirely wrong).

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