M83 Southern Pinwheel

[alpal]

I took this last night from my light polluted driveway.
15 subframes of LRGB, each of 10 minutes, all 1 x binning.
2.5 hours total integration time.
Darks flats & bias frames added.
8" Newt at f6, QHY9 camera at -30°C, OAG with Lodestar.

I used an Astronomik CLS-CCD filter for all exposures
in the hope to see through the pollution but
I would have needed more subframes to cancel the noise out.
10 minute subframes were required to get enough signal
so that limited the number of subframes.
The result was a little disappointing as I still didn't have my
coma corrector properly spaced.
It is more difficult to find the proper spacing than what I thought
& explains the eggy stars.

Link to larger version here:
http://www.flickr.com/photos/24719437@N03/8703896707/in/photostream/lightbox/

[Joshua Bunn]

Nice shot there. Was your focus off a little? there appears to be some double diffraction spikes. Do you know what caused the gradients?

Josh

[alpal]

Thanks Joshua,
Yes I noticed those double spikes.
Must be a stacking problem.
The gradients were caused by shooting from a light polluted location.

I added Luminance to just the galaxy from my DSLR images from
Mt Baw Baw - a dark site at 5,000 feet in altitude.
I then did further processing & cropped it.

I think it looks better now & more natural.
It's hard to compete with a dark site.

larger version here:
http://www.flickr.com/photos/24719437@N03/8703896707/in/photostream/lightbox/

[Joshua Bunn]

Thats an improvement

[alpal]

Thanks,
when I get this system sorted I'll take it to a dark site.

[Ross G]

Hi Allan,

Shooting from a light polluted driveway.....I know your woes and desperation!

However, despite that, you have a very good photo, Disregarding the pollution gradients, it looks very sharp and well tracked and there seems some nice colour buried in there.

You've got a great grasp of QHY9 and RGB imaging. I'm still not game enough to try mine with RGB colour.

Good luck.

Ross.

[alpal]

Thanks Ross,
Why aren't you trying RGB imaging?
I will shoot from the driveway until I iron out some bugs.

I still need to do a lot of work to get better images.
I checked with Maxim DL & my FWHM figures are not good.
The best FWHM I was getting was 4.6 arc seconds
(after I correctly put in the pixel size of 5.4 microns & focal length of
1220mm which would give under ideal conditions 0.9 arc seconds)
Some stars on other frames were at 6.5 arc seconds.
Using Ha on other nights - which does give smaller stars - I have
got down to a FWHM of 2.5 arc seconds on short exposures.

There are many reasons:

The seeing in Melbourne is not good. e.g.
Jupiter is always a fuzzy ball with 2 lines through it from Melbourne
but on Mt Baw Baw at 5,000 feet it comes in sharp & clear
on the highest power eyepiece.

The EQ6 mount is not accurate enough & it's hard to drift align -
I can get it right for a north star at the equator on the meridian
but then it will be off when pointed in a southerly direction.
Maybe the mount is not orthogonal?
That means I have to use 1 second updates on the PHD guiding
which is causing the mount to be constantly moving to catch up with the star.

The focus was a bit out - I used a Bahtinov mask as
I couldn't get the focus sorted out using Ezycap software for the QHY9.

The Varilock spacer is hard to use - I need to work out
a method which won't take all night to get the correct spacing.
see it here:
http://www.flickr.com/photos/24719437@N03/8029429824/in/photostream


The secondary mirror also needs to be moved up or down as the flats
show it's not centralized.

What FWHM figures are you guys getting?

[David Fitz-Henr]

Hello Allan, some nice details in that image. You may be right about the corrector spacing; I've found in the past that a small difference can have a significant effect on the flatness of the field. One suggestion if you will be shooting from light polluted skies would be to buy a gradient tool (such as GradientXTerminator) - it can remove these gradients quite effectively.

[alpal]

Thanks David,

I reprocessed it again using some LRGB subs of only 1 minute each which I copied & pasted in.
That gave me smaller stars & more star color.
No luminance was used from my DSLR.
This is only the QHY9 data.

A full frame version is here:
http://www.flickr.com/photos/24719437@N03/8709826275/in/photostream/lightbox/

There is still some gradient but I got rid of most of it.

[David Fitz-Henr]

Yeah, the gradient looks much better Allan :thumbsup: You seem to have introduced some red rings around the brighter star cores though :question:

[alpal]

Hi David,
I will re-process M83 some day.
I worked out how to get rid of gradients perfectly using Fitsworks4 "variable flatten".
That pic has a red cast.
My latest pic of Centaurus A came out really well after I used
32 bit stretches of FITS with FITS Liberator - see it on flickr.
I will use that too on M83 when I re-do it.

cheers
Allan