First M83 for me this year

[kosh]

Hi all,

I managed to get some decent subs of M83 the other night. There appears to be issues with either, coma, focuser sag, collimation, i don't know. My CCD chip is only small so the coma is rather surprising. I'm at a bit of a loss really.

Any feedback appreciated.

10x 6min Lum, 10x 3min R,B,G binned 2x2, 8" newt F4 on a neq6 mount, guided.

Thanks for looking.

[Alchemy (Clive)]

Coma should be easy to qualify, take a short exposure of a very bright field of stars, one would be sufficient. Usually chip to coma corrector distance is the culprit.

Focuser sag, that's one I had to deal with, in my case it was more the tube deformed due to the weight of camera as everything rotated, tried reinforcing around the focuser, but couldn't extend my exposures much beyond 5 mins, Ultimately I gave up on it and bought a refractor, 20 min exposures no problem, just wish it was f5 like the newt... Never mind

Pic looks good, as with all things more exposures will beat down noise, boost signal. With deconvolution stars tend to get black halos and or have crispy edges, I found that if you duplicate image before decon, then do as many iterations as you want ignoring the stars, then select the stars from the undeconvolved image and paste them back over the top of the deconvolved image, you may have to tinker with background brightness of the original and use some feathering to make it look natural, but then you get the best of both worlds.

Not trying to be critical, just passing on things I found worked for me.

[Flugel88 (Michael)]

Gordan some nice detail in the core of M83

Stars are odd shaped just like my schmidt Newtonian though i don't yet own an MPCC your stars should be looking better than that as Clive mentioned could be Focuser sag or something to do with where your MPCC is placed.

I see what looks like Vignetting in the Right bottom side also. Have you done any flats? you could try tidying up with some kind of gradient tool like the one used in startools.

Im still learning things myself but seems we have similar problems.

[kosh]

Originally Posted by Alchemy

Coma should be easy to qualify, take a short exposure of a very bright field of stars, one would be sufficient. Usually chip to coma corrector distance is the culprit.

Focuser sag, that's one I had to deal with, in my case it was more the tube deformed due to the weight of camera as everything rotated, tried reinforcing around the focuser, but couldn't extend my exposures much beyond 5 mins, Ultimately I gave up on it and bought a refractor, 20 min exposures no problem, just wish it was f5 like the newt... Never mind

Pic looks good, as with all things more exposures will beat down noise, boost signal. With deconvolution stars tend to get black halos and or have crispy edges, I found that if you duplicate image before decon, then do as many iterations as you want ignoring the stars, then select the stars from the undeconvolved image and paste them back over the top of the deconvolved image, you may have to tinker with background brightness of the original and use some feathering to make it look natural, but then you get the best of both worlds.

Not trying to be critical, just passing on things I found worked for me.

Appreciate the comments clive.

I agree that the background is too dark. When using the colour calibration in PI it really helped the Galaxy, but no matter what I did the background would go too dark.

Perhaps my tube is flexing too. I'm only using an 8" dovetail that came with the thing, and there is heaps of steel either side of the tube rings.

Originally Posted by Flugel88

Gordan some nice detail in the core of M83

Stars are odd shaped just like my schmidt Newtonian though i don't yet own an MPCC your stars should be looking better than that as Clive mentioned could be Focuser sag or something to do with where your MPCC is placed.

I see what looks like Vignetting in the Right bottom side also. Have you done any flats? you could try tidying up with some kind of gradient tool like the one used in startools.

Im still learning things myself but seems we have similar problems.

Thanks for your thoughts Michael,

The placement of my MPCC is definitely an issue (it was still in the cupboard )
Seriously, i haven't really used it other than on my DSLR as the chip is only 11.5mm in diameter, perhaps i should reconsider. Not sure about the vignetting though, for same reason as above and I can't really see it, even stretched with the original before PI *******ised the background with the colour calibration.

[raymo]

I think, at f/4, that tube flexure is vey unlikely; I'd look at everything
else first.
raymo