Bad gradient

Bruce Donzanti

I just ran WBPP for a galaxy (M94) using LRGB filters on a C11" EdgeHD @f/10 with an ASI6200mm pro (0.27 arcsec/pixel), under average/below average seeing under Bortle 7 sky.  So, there is some oversampling going on.  Camera gain is 100 and offset is 50 with subs are 120s each and dithering occurs after every other frame.  When I use this setup with narrowband filters, I can produce excellent images, even from faint planetary nebulae.  However, the integrated frames from these broadband filters have a horrible gradient.  Attached are two examples (Lum on the left and green on the right) that are fully stretched - no DBE has been performed.  My question: Does this harsh gradient seem normal for BB images under a light polluted sky or does it seem I am doing something wrong?

BlockHead

Something else is wrong.

This looks like an obstruction... kind of. 

Please verify filters are centering properly... an no other issues.

Also, check your dark frames to make certain no light leaks. (Do the RAW Red and Green images look like this??).

-the Blockhead

Bruce Donzanti

yes- the red and green look the same.

I can redo my dark frames.

I'd be surprised if the filters are not centering properly as when I do NB, I don't notice this and they are all on the same 7 position FW.  However, I will check.

Thanks

Bruce Donzanti

Not sure if this helps, but here is a red flat which does not show any obstruction and appears to be centered.

BlockHead

But do the RAW Red and Green frames show the problem?

Bruce Donzanti

A single raw red frame does not...... and the green, blue and lum individual frames look similar to the red.  So, I am guessing there has to be something wrong with my darks or dark flats?

BlockHead

Yes..definitely.

I think you got light leaked... 

-the Blockhead

BlockHead

One of the things I teach in FastTrack training is to blink ALL data (even calibration data)... just look at it- it only takes a second- and make certain it makes sense before processing.  This bit of time spent is likely better than the extra time spent trying to figure out the consequential issue.

-the Blockhead

Bruce Donzanti

thanks- will do-  for the heck of it, I just ran manually star alignment and image integration (no flats/darks/dark flats) on the 90 red frames

I will redo my darks, and dark flats and flats at night.  I usually do them during the day in my observatory but I do cover the end of the scope.  There must be a light leak elsewhere.

BlockHead

Yes. Cameras are not light tight in general... nor is the sensor end of most telescopes where adapters are and so on. I always recommend taking all calibration data at night away from any sources of lights. 

Score Card for nighttime darks in my experience.

Dark Frames                          Light Leaks

30,000                                        0 

:)

Actually early in my career they was a devilish case of IR light leaks. The filters were selected by turning on and off an IR light to measure the index position. But if the IR light stayed on... even briefly... it would cause light leaks.

-the Blockhead

Bruce Donzanti

Wow- this hobby is starting to give me a headache! Thanks again for your help!!

BlockHead

The headaches get better once you have pulled out all of your hair.

Bruce Donzanti

I finally figured out the problem- so my headache is gone and hair pulling has subsided.  A light leak was coming in through the camera tilt plate in which one of the 3 push/pull screw sets was not tight.  I doubt it was more than 0.5mm of an opening.  I took new darks, flats, and lights, with no signs of the ugly dark corner.

BlockHead

Good work figuring it out. 

-the Blockhead