Gradients :/ what am I doing wrong?

[badgerchap]

Hi people.

I took 2 images last night, both stacks of 90s exposures, around 30 mins total for M33 and about 45 mins for the Pleiades, all unguided. Apart from my focus which could do with some work, I'm fairly pleased. I'm still fairly inexperienced compared to most, so they're about at the level I was expecting. However, there's a gradient on both shots that I'd like to know how to get rid of or minimise.

Both have 15 darks and 15 flats. Camera was Canon EOS 1100D unmodded, stacked and processed in DSS. Taken through an f4 6" Newt. I took the flats by pointing the scope at a clean whiteboard illuminated evenly by the LED light on my phone, with the camera set to auto. Focus setting unchanged from light frames. The pic of M45 has been turned through 90 degrees so in truth, the gradient goes the same way in both shots.

I feel like I'm slowly getting somewhere with AP now, but this gradient problem, which I've had on most of my images, is detracting from an otherwise steady improvement. Is there something I can do in PS, or DSS itself, that can help me get rid of it?

Many thanks,

Badgerchap

[Steve Ward]

Try here ...   http://www.youtube.com/watch?v=yTEVMH_WE80

[badgerchap]

Thanks Steve, great tutorial there.

Well that's certainly calmed it down a bit, although I think I may have undone it a bit with a new curve-fiddle. I was rather hoping that there was something I could do either during initial capture or the stacking phase that would eliminate this. Is processing the only thing I can do?

[Steve Ward]

Dunno , it's all gobbledygook to me ... 

[glappkaeft]

Well that's certainly calmed it down a bit, although I think I may have undone it a bit with a new curve-fiddle. I was rather hoping that there was something I could do either during initial capture or the stacking phase that would eliminate this. Is processing the only thing I can do?

First thing to check is if the gradient is there on single uncalibrated subs or if it is an effect from bad calibration images. Looking at all the calibration images using high contrast is also useful. 

The number one method to avoid gradients are truly dark skies and no direct lighting. Even if you skies are perfect indirect light from LEDs, torches, head lights can enter the tube (indirect light hitting the inside, coming in around the mirror cell, etc.)  or camera (entering backwards via the finder).

[badgerchap]

First thing to check is if the gradient is there on single uncalibrated subs or if it is an effect from bad calibration images. Looking at all the calibration images using high contrast is also useful. 

The number one method to avoid gradients are truly dark skies and no direct lighting. Even if you skies are perfect indirect light from LEDs, torches, head lights can enter the tube (indirect light hitting the inside, coming in around the mirror cell, etc.)  or camera (entering backwards via the finder).

This could be the culprit - I'm not as strict with ambient lighting as I should be. I will have a look at some single subs in badger diagnostic mode... Thanks!

[badgerchap]

Yes, having messed around in PS I'm sure it's an ambient lighting thing. Time for a flocked tube and some thicker curtains...

[2stroke]

You could get that out easy in photoshop, lol those kill nearly any image i get from my backyard. Have you got gradientXterminator? its a 2 second job with that.

[ollypenrice]

I have a world class dark site and still get colour gradients.  Not bad ones, but we don't leave lights on around here! When you stretch an image you stretch everything, including whatver little colour imbalances lurk in the faint data. Good flats and a good site will minimize them but they will be there.

By far the best software solution is Dynamic Background Extraction in Pixinsight. It also calibrates your colour balance into the bargain. You can then apply SCNR Green to zap pesky green noise, which is oddly prevalent in AP.

Photoshop plugins that do a similar job are Gradient Xterminator (Russ Croman) and Rogelio Bernal Andreo's free Hasta La Vista Green (Deep Sky Colors website.)

What you MUST do is resist the considerable temptation to black clip your image in order to remove them. This just robs you of precious data.

Olly