M3 Globular Cluster

[MarkS]

Preview of M3 last night (i.e. Sunday night)

28 x 5min : modified Canon EOS 350D on Celestron C11 with F6.3 focal reducer and CLS filter.

http://gallery.orpington-astronomy.org.uk/albums/userpics/10046/m3_290309.jpg

Guiding was bad - I used the ED80 for guiding for the first time (mounted atop the C11) but it was a too heavy for the C11 dovetail and made the whole assembly a bit "springy".  Seeing was pretty bad as well.  I also experimented with yet another reducer to CCD spacing (to reduce coma) but used some flats from a different session.  You can see that I still haven't completely eliminated the coma.

It looks as if I may have captured one or two faint fuzzies in the background.

Mark

[RobertM]

That's a wonderful result, the colour contrast between the blue and red giants shows up really well - I'd certainly be more than chuffed with that result.

There's noticably less coma than you've had before so that spacing can't be far off.  I think you really are pushing the Vixen dovetail to it's limit.  It could well be that instability limiting your guiding, from experience it certainly doesn't take much to ruin an image at that f/l.

btw, how do you do your flats ? I've been getting strange gradients towards the middle when I use flats taken with a white sheet over the lens.  Last time I used a starfield with all the stars removed and that seemed to work really well though it does eat into precious imaging time.

Robert

[Fay]

Really nice colours Mark, that must have been a real heavyweight setup. You did well as it was not that good last night

[MarkS]

Thanks Robert,

The whole image is a little soft for my liking - a combination of poor seeing and not so good guiding.  But I'm reasonably pleased with the result, nevertheless.  I need to shoot some new flats and re-process.

Flats - my usual approach is to point the scope at the TV screen in the lounge (displaying something uniformly white - I connect my PC to achieve that).  But it does mean moving the scope indoors.

Also the zenith of the sky shortly after sunset is pretty uniform - but it's not always available!

I don't find a lot of difference between the two methods but the sky at zenith tends to work better - possibly because the light "source" is at "infinity" instead of right up close - this probably affects the way the light enters the tube.

In my experience with a large CCD, the flats never work 100% - I'm always left with brightness either at the edges or in the middle or some kind of ring.  This results from numerous things including:
1) Stray light getting into the tube and reflecting around inside is not always repeatable
2) On my SCT, an illuminated corrector plate will cast it's own very out of focus image on the CCD - again not repeatable.

I always have to apply some gradient removal technique to remove the residual gradients left behind after applying flats.

Mark

[Carole]

Lovely image mark.

You're all making me very jealous, it's going to be months before i can do any imaging again, even if i get my laptop sorted.  Will have to "shadow" some-one else for a bit, even assuming i can get mobile again.

carole

[JohnP]

Have to agree it looks very good Mark - Nicely resolved stars to core & lovely colours. There are some slightly elongated stars but I am sure you can sort that - All in all a great result.

John

[The Thing]

You seem to be getting some serious resolution despite what you see as the short comings of your optical train. I think I can see stars to the core. Can't wait to see the perfected version, it'll be amazing.

[MarkS]

Reprocessed after taking a new set of (white TV screen) flats: