are these comas?

[devilgas]

had a pop at M42 last night. all images suffered the same problem with the stars. i'm presuming it's coma and can be fixed by a coma corrector or have i just not set the OTA up properly / something else wrong? i get similar when doing white light sun images or moon - the image edges are very iffy.

scope was skywatcher 200 pds, captured on a canon 40d using ep projection through a 26mm ep.

thanks!

yes, i know the image isn't M42, but this shot shows the problem a lot better

[kookoo_gr]

if your scope is a newton then it is heavily uncollimated, you need to collimate your scope, eiter that or you are out of focus. Coma at scopes shows the stars elongated at the edges of a picture at a circular patern and surely without the donut hole at the center as your picture shown here

[ollypenrice]

I'm wondering why you're using an EP at all on M42? Prime focus would surely be ideal? This is coma, yes, but it is extreme in the extreme and may be to do with the EP.

Very rarely does anyone want to slow down the F ratio of their scope in DS imaging. The whole charm of a fast Newt is that it is fast. I'd run it at prime and with a coma corrector.

Olly

[devilgas]

if your scope is a newton then it is heavily uncollimated, you need to collimate your scope, eiter that or you are out of focus. Coma at scopes shows the stars elongated at the edges of a picture at a circular patern and surely without the donut hole at the center as your picture shown here

Edit.... the scope is a skywatcher explorer 200pds reflector.

thanks. guess i'd better check the OTA again. the focus was set using a bahtinov mask, so should've been just about spot on. the only other thing i can think of to explain the donut is that the central spider holding the secondary isn't central. sound plausible?

olly - i'll try prime and see if this helps. i can't remember if i can get the dslr to focus at prime though. the ep was a 1.25" meade 4000

so,

1 - see if camera can focus at prime

2 - check centrality of spider / secondary

3 - collimate

4 - get a coma corrector

[Merlin66]

Devilgas,

Before considering a coma corrector you really need to be getting better results than the one posted.

Dump any eyepieces and concentrate on mounting the camera body directly to the focuser...use the mask/ live view to get best focus then try some sample exposures.

The distortions of the stars you currently show are out of the ordinary and suggest something else may be a problem.

[devilgas]

thanks merlin

i think half the problem is that when the scope was delivered earlier in the year, one of the vanes holding the secondary was loose - the screw attaching it to the tube had detached. as a result, the spider was quite loose and distorted. i guess that in re-attaching the spider, it isn't quite central (i didn't want to over-tighten anything). as a start, i'll totally remove the spider, re-attach and re-centralise. thinking of the light path etc, i'm pretty certain that the secondary / spider not being absolutely central is causing the donut effect.

cheers, dave

[devilgas]

ok, OTA was seriously out of alignment. spider tightened, secondary now central and scope colimated. just need some clear skies (we're currently alternating between clear skies & rain, with the changover being minutes)

[devilgas]

clear skies earlier this evening. the effect has mostly gone, so, lesson learnt, if i see that again, i know the collimation needs badly doing

thanks guys.

[JimStan]

Did you get rid of the eyepiece ?