Increase contrast without activating light pollution

[Evie]

Hello,

I am just starting to take photos, and working without telescope. I have taken some wide angle sky photos, and I see they have possibilities to show more stars (they are faint in pic), however, if I try to increase the contrast, the light pollution comes into play, and while part of the sky has lots of stars, the bottom part gets white.

So, for example, I am taking this photo - sorry for small size, had to resize for attaching - quite monotone tone, several brighter stars and quite a bit of faint ones

After trying to edit it (don't have photoshop... playing with other free apps) get something like this - stars are much brighter, however, the top is almost black and the bottom is almost white...:

Any advices how should work on it? Do I need to get photoshop? I would really prefer not to, as it is quite expensive for only beginner photos...

[Alien 13]

The problem you have to deal with is overall sky colour and a gradient so you will need software that can do individual level adjustments on the RGB components and layre masks to create a copy of the sky minus the stars so it can be blended into the original layre you can then stretch out the star data using curves or RGB levels..

Photoshop is ideal for this but it can be done with free software like GIMP.

Alan

[rfdesigner]

I use IRIS..   which is free, it has a command called "subsky" which very effectively removes background gradients.

You can adjust how it works by using the command "Setsubsky" and this allows you to set the "order"..  1 = 1st order: simple straight gradient..  2 = 2nd order: simple curved gradient + straight radient..  and so on.  IRIS is looking a bit long in the tooth now, it only handles 16 bits per colour, but that still wins hands down against gimp which seems to be stuck at 8 bits despite keeping a promise of 16 bits to come for ages and ages.

Photoshop and Pixinsight are both 32bit / colour and very effective, but of course neither are free.   I'm sticking with IRIS for now as it's much more science based, will do astrometry etc. 

Derek

Edit: I had a quick go with IRIS to show what can be done...  of course things would be much better working from the RAW file.

[Evie]

I use IRIS..   which is free, it has a command called "subsky" which very effectively removes background gradients.

You can adjust how it works by using the command "Setsubsky" and this allows you to set the "order"..  1 = 1st order: simple straight gradient..  2 = 2nd order: simple curved gradient + straight radient..  and so on.  IRIS is looking a bit long in the tooth now, it only handles 16 bits per colour, but that still wins hands down against gimp which seems to be stuck at 8 bits despite keeping a promise of 16 bits to come for ages and ages.

Photoshop and Pixinsight are both 32bit / colour and very effective, but of course neither are free.   I'm sticking with IRIS for now as it's much more science based, will do astrometry etc. 

Derek

Edit: I had a quick go with IRIS to show what can be done...  of course things would be much better working from the RAW file.

Wow, really thanks for this app - it did just what I wanted As I have several photos from this trip, I can make them in something better looking...

[rfdesigner]

you can download it here.  http://www.astrosurf.com/buil/us/iris/iris.htm   there's some tutorials towards the bottom of the page, and I would strongly recommend downloading the pdf of the commands.

One point:  the "consol" or command line entry is activated from the button marked:

>=

>=

It took me way to long to twig that...

Many commands are available from the menus,   "subsky" is available on menues:   Processing>Remove Gradient (polynomial fit).

[onemoregadget]

Sorry Nebula, not to hijack your thread but I'm a newbie and having similar issues with LP gradients and DSO's....as you do when imaging from London!

First of all, this is my first post and I'd like to thank everyone for all the great advice and information on the site!  It's the first place I look when I'm having issues and many hours of headaches have been avoided.  So thanks again!

So I've been trying to image M81/M82 and the output of DSS has concentric circles of LP rather than a smooth gradient as in any single sub.  Maybe there is a setting in DSS that would give me a more consistent background gradient.  Anyway, I've tried to flatten the background with FitsWork and also manually with layers in GIMP, but the circles of LP still remain.  I installed IRIS a few months ago but haven't really had time to play with it yet.  Would the subsky command with a higher order gradient be able to handle this situation do you think?

It just feels like someone who knows what they're doing would be able to get a lot more out of my image (given the limitations of my setup and newbie-ness of course).  I've been trying to knock out issues based on priority, so my image could be in better focus, etc.

The details:

40 x 120 sec subs with unmod'ed canon 7d at iso800 (i was having polar alignment issues on the night so couldn't take longer subs)

CLS clip in filter

skywatcher 250pds on neq6 pro

orion autoguider

Any pointers would be greatly appreciated.  Again, apologies if I should have started a new thread, just seemed to be the same topic.  I don't want to get in trouble on my very first post!

Cheers,

Jeff

[onemoregadget]

Sorry Evie, just realised your name is not Nebula.  I was so anxious to post as my situation is driving me crazy I looked at the wrong thing!

[ollypenrice]

There are two main sources of gradient (uneven illumination) in astrophotos. One, which we call vignetting, is created by the optical system which will always illuminate the middle more than the corners - and dust shadows or bunnies are introduced as well by contaminants close to the chip. These are fairly easy to deal with by taking and applying flat fields, images of an even white source. There are pelnty of flat field turorials on the net.

The other source is light pollution and is best dealt with by going to a dark site a very, very long way from cities. Not always practicable! So you can use astro-specific software to attack these gradients. One, well known, is Russ Croman's Gradient Xterminator. This is a Photoshop plug in. You should be able to find a legitimate Ps CS3 or similar on Amazon for a reasonable price. The other (and best) gradient software comes in Pixinsight, a complicated programme but one which is reasonably priced and incredibly powerful. You can try a free download. Run your image first through Automatic Background Extraction and then try the more complex Dynamic Background Extraction.  Harry Page (Harry's Astronolmy Shed) has good introductory tutorials.

Olly

[onemoregadget]

Thanks for the info Olly, I'll give those a try.