Eta Carina Nebula - help with LP gradient

[naskies (Dave)]

Here's a shot of the Eta Carina Nebula from early this morning using my Canon 5DmkII + 70-200 f/2.8 IS II at 200 mm, f/5.6, ISO 400, 52x 185 sec subs (2 hr 40 min total), processed with darks/flats/dark flats/bias frames.

The warmer weather resulted in a crazy amount of noise on my SLR. The dark frames helped, but without dithering it's almost like salt-and-pepper field of black dots.

I tried to shoot a wide field including Eta Carina, the Southern Pleiades, and the NGC 3532 open cluster, but I haven't been able to remove the light pollution gradient across the image. See the second image - it's most intense in the bottom-right corner.

Any tips/suggestions?

[LightningNZ (Cam)]

Do you have photoshop? Regardless you can use GIMP if you don't. I'd split your image into RGB and in each channel, go the brightest corner and select the average of the darkest pixels you can find there, then do the same for the darkest corner (with the secondary colour). Now create another layer and create a gradient that goes from one corner to the other with these colours - but in the opposite direction to the nasty colour in your image. Use lighten layer blending and reset your black balance. That's my process and it works ok. Hope that helps.
Cam

[alan meehan (Alan)]

HI Dave
i have run your shot through pixinsight dbe tool and it has removed some of your light pollution ,light pollution can always be a problem i shoot from newcastle and have the same problems which i overcome using a cls filter with the telescope and camera ,not sure if you can get these for a lens but a bit of processing will remove most of your problem
ALAN

[irwjager (Ivo)]

Originally Posted by naskies

Any tips/suggestions?

Cam is giving some great advice. I've attached the sort of results you should be looking for, as well as a sample of a gradient Cam says you should generate using his method.

Alternatively you could use a dedicated tool such as GradientXterminator if you have Photoshop, ABE/DBE if you have PixInsight or Wipe if you have StarTools. If you're not afraid of command-line utils, there's StarTools Wipe's great-great grandpa StarWipe as well which is a free download (he's getting on though ). If you use StarWipe or Cam's method then make sure you crop that stacking artifact (dark line) at the bottom as they don't constitute 'real' dark pixels - they're artifical.

Cheers,

[naskies (Dave)]

Hi guys,

Thanks for your advice. Yes, I have Photoshop CS 5 and LR 3... and I've tried to do the gradient removal process as you describe, but I can't get my gradient to match up with the light pollution gradient - so it results in weird artefacts.

Alan & Ivo - I like your efforts so far but I think they've inadvertently removed some of the real nebulosity in the Milky Way. Here's a screen shot from http://www.skysurvey.org - you can see that the Southern Pleiades area has the darker brown/gold nebulosity while the central area with Eta Carina has a lighter shade of nebulosity.

Showing this in my image is proving rather challenging!


Dave