Spiral Galaxy NGC7793

[gregbradley]

I also imaged this one around the same time as Mike.

Quite a nice little galaxy and relatively bright making it a good target.

CDK17, Proline 16803, Astrodon filters, PME.

http://www.pbase.com/gregbradley/image/152731706/large regular size

http://www.pbase.com/gregbradley/image/152731706 full size

http://www.pbase.com/gregbradley/image/152732117/large crop view regular size

http://www.pbase.com/gregbradley/image/152732117/original crop view large size

Greg.

[PRejto]

Nice Greg!! Lot's of fuzzies in the background too...

Peter

[gregbradley]

Thanks Peter. The CDK is a very good scope and all that aperture does tend to mean shorter exposure time still gets plenty of signal.

Greg.

[allan gould]

Greg
Your crop view image is not the same galaxy in the 200kb thumbnail posted.
But both are excellent so it's a double gimme.
Allan

[strongmanmike]

Looks good Greg with plenty of colour :thumbsup:

Your attached photo is of NGC 1232 though :thumbsup: (was going to be my current starting tonight target too...maybe I do something else now:question: :lol:)

Mike

[gregbradley]

Ah thanks Allan. Yes NGC1232 is a work in progress. I got a lot more data since processing that initial image. Still to be sorted.



Thanks Mike. Sorry to mess up your plans. Go ahead I would be curious to see how it turns out. Mine is probably now about 7 hours out of about 12 captured. Bit low in the more light polluted segment of my sky so tough to process.

Greg.

[astronobob]

Nice result Greg, dig the galaxy, havn't seen before. also not sure of the scope you use, just wondering what F/length used on this ?
Thanx.

[strongmanmike]

Hey don't apologise no one owns the sky :)...we'll see, I have other options...it'll probably be something Rick's imaging I bet :lol: ... :thumbsup:

Mike

[RickS]

Nice image, Greg! It is a pretty little galaxy. Amazing how they all look so different.



Not me, Mike :) I only get my couple of new moon nights a month at best. Other than that I'm limited to narrowband from home :sadeyes: I'm playing around with the processing of my first Hubble Palette image tonight - trying to do J.P. Metsavainio style tone mapping in PixInsight.

[gregbradley]

Thanks Bob. Its a Planewave CDK17 corrected Dall Kirkham and its just under 3 metres focal length and 17inches aperture or 432mm aperture.



hehe. Yes Rick seems to be the one to watch at the moment.

Greg.

[astronobob]

Cryyppeeeezzz Greg, , , :eyepop:

[strongmanmike]

Eeek I was planning some narrowband too while the moon is up. Hmmm?...I...wonder...what...it...is ...? :question: :P

[RickS]

I've got 30+ hours of boring old M16 and 40+ hours of Helix, Mike.

Greg: sorry for the thread hijack. May I ask how you do your colour calibration? I notice that your colours are quite different to Mike's image of the same object.

Getting decent colour is a process that vexes me greatly! I sometimes wonder if we build up a false expectation of what some objects should look like based on previous images we've seen.

Thanks,
Rick.

[alpal]

Looking very good Greg,
I wonder if you have thought twice about the processing?
your full frame:
http://www.pbase.com/gregbradley/image/152731706

It looks like a darker ring around the galaxy than the rest of
the background - which is also darker than Mike's effort here:

http://www.pbase.com/strongmanmike2002/image/152715500/original

Was it stretched in 32 bits?
I love those colours - blues & reds in the spiral arms.

cheers
Allan

[Joshua Bunn]

Cool result Greg.

[gregbradley]

Cheers Bob!



Thanks Rick. Colour is a tricky subject. I correct for any colour bias, remove any green noise which throws off colour usually, try to keep some nice colour in the stars (often a bit tricky). Otherwise I eyeball it.
Selective colour is a good tool to massage colours to the shade that you want that matches good posted images. The blues in galaxies are often hard to achieve. The blues often have too much cyan and not enough yellow which gives that nice lightish ocean blue. Rob Gendler often achieved those shades and I wondered also how he got it.

Excess green is the usual enemy and comes from light pollution and also air glow. You can use the G2V star calibration method (I never have) but the top guys often do. That's where you have a list of G2V stars (a free list on Astrodon's site) and find one in your image. When you process the colour that star should be white if its high in the sky. Others correct for colour extinction as the angle of the scope lowers, blues extinct first and reds last. In your acquisition routine you should try to capture the blue subs when the scope is at a steep angle. If you are using CCDSoft this annoyingly means you have to wrongly label the filter name so even though you can select blue as the colour in the red filter dialogue box it will be labelled as red in the saved file - annoying little point. I read somewhere you can change it with some dive into the code - I never did. Just be sure to remember to rename the files the next morning or you are in for some really challenging processing!!





I do some minor stretching in CCDstack (click auto) and process the callibration, the data rejection and the registering and combine into masters and the initial colour combine. I don't see much evidence of a dark ring around it on my monitor but I can a slight one in the thumbnail.

I don't use DDP for stretching of my images but rather curves and levels.
In Photoshop that is 16 bits. Do you think 32 bit files in Photoshop would help?

Unfortunately this was shot low in the sky where I have the most light pollution. I then use a few gradient handling techniques to get rid of that. That's probably some minor residual of that routine. If you saw the process along the way its a mile from where it started. Yes the background being a bit darker is part of that process as well. At a dark site I would allow the background to be a bit lighter like Mikes - a dark grey My image is also only 1/3rd of exposure time of his and he is now in a dark site area. If the object were higher or past the Meridian and I was imaging to the west where I have little light pollution it would be a nicer result.

So in summary I don't think it came from the stretching process but the gradient handling process - Gradient Xterminator requires you to lasso the object and invert the image before running.



Cheers Josh. I was happy with it considering everything.
What I am realising is the gain from the 17 inch aperture is also the shorter total exposure time to get a good signal to noise ratio and the fact it also picks up Ha areas even without imaging some Ha.

[alpal]

Greg,



Hi Greg,
I am only a beginner compared to you so take all this with a grain of salt if you want.

I find that stretching in 32 bits is well worth the effort.
You have DDP in Maxim but I haven't tried that yet.

I now save my stacks from Deep Sky Stacker as 32 Bit FITS files
& stretch them using the free program FITS Liberator.
I find that the compression stretch function x^1/5 works well.
It expands out the lower levels which allowed me to show
the halo of Centaurus A which was almost missing on previous
Photoshop 16 bit stretches. ( in my flickr photos )

I was surprised that I could get any halo at all as I took the image
from a semi light polluted location - outer suburb of Melbourne
& only 2 hours of integration time.

Yes - gradient handling can cause the darker area around a galaxy.
I use Fitswork 4 & - background flatten - variable flatten.
Yes it also requires a masking of the brighter areas which can
cause a dark band to form around a galaxy.

Whatever we do we are always limited by light pollution.
Mike had the luxury of more time & super dark skies.
Your image is remarkable given the conditions.


cheers
Allan

[Ross G]

A beautiful galaxy photo Greg.

Great colours and detail.

I love the wide composition.

Ross.

[Shiraz]

very fine galaxy image Greg - seeing must have been good to get that much detail. regards ray

[gregbradley]

I am always open to new ways of doing things. Thanks for those tips. Where do you get a copy of Fitswork 4?



Thanks Ross. Widefield views of galaxies are nice as it gets them oriented and I also like a nice clear crop view to show the detail when I get it.



Thanks Ray. Yes there have been several nights where the seeing is quite good. If not initially then later on in the night.

Greg.

[Paul Haese]

Nice Greg, heaps of detail. Back ground looks a little dark to me, but not really a problem. I like the fact you can gobble down so much data so quickly with that diameter. One day.....

[alpal]

Greg,


Hi Greg,
I hope those ideas can help you & others.
Fitswork 4 is here:

http://www.fitswork.de/software/softw_en.php

cheers
Allan

[gregbradley]

Thanks Allan.

Greg.

[RobF]

Beautiful work Greg. Stars and galaxy colours feel so natural - doesn't look like you've overcooked it one bit. Very soothing on a tired astronomer's eyes :)

[gregbradley]

Thanks for the nice compliment Rob. Processing of this one came along fairly easily unlike some others I have done recently. I think it was because it was higher in the sky.

Greg.