More Colour Issues - Losing the will to live - help!

[midnight_lightning]

Hi,

I have spent literally days of effort trying to get a good image of M31 from some subs I took in July - the data looks good but I can't get rid of the blue/green cast. I have spent hours reading the posts on this forum but still struggling.

Would anyone do me a big favour and take a look and see where the problem might be?

DSS Processing.
Basically everything off, no calibration, no RGB alignment, just left Hot Pixel detection on. Used the FTS autosave file.

Star Tools
I just did some basic processing with pretty much default settings - not bothered about noise etc, just trying to sort out the colour:
AutoDev
Crop
Wipe
Autodev
Colour

The files are in dropbox on the link below and I have included:
Lights
Darks
Flats (I have some dark flats but am not confident in them so have not included them.) The Flats are very blue but I read somewhere that doesn't matter and the Master flat that DSS produces is a dark grey so I'm assuming thats ok.
DSS Autosave.fts (this is around 250MB!)

https://www.dropbox.com/sh/0fa9rdc396r9 ... 4jH5a?dl=0

It may be that ths is the best I can expect from my set-up (which would be disappointing!) but I have seen other folks images from similar and they have much more colour, particularly in the dust lanes and core.
I am still a relative noob (have done perhaps 5 or 6 imaging sessions to date) so apologies if I've done something obvious.

If anyone has time to try and get a decent image out of my files that would be brilliant - at least I would know it's just me that's at fault!

Thanks

Mid


Equipment : HEQ5 Pro, Esprit Pro 80/400, Sky Watcher 9x50 Guide Scope with Lodestar X2 (PHD2), Nikon D750 Un-modded (BYN), IDAS D1 LPS. (Could the IDAS LPS be the issue?)

[Starry Eyes]

Try using a mask using fat stars and eliminate the galaxy from the mask. then Color>keep mask>sample. Then still in color>mask>clear>invert(fills Mask)>keep. now you'll be closer!
this uses the nearby galactic stars for white color balance. Don't forget to use the max color icon to help fine tune what you got. Good luck!

[admin]

Great tips by Starry Eyes.

Are you absolutely 100% sure you got the right debayering pattern specified for your camera?
It's almost as if some channels are swapped.
A light pollution filter can certainly skew colors quite a bit, however this is far too extreme...

[midnight_lightning]

Thanks Starry Eyes, I will give that a try and see what happens - will post back next week.

Ivo, I'm too new at this to be 100% sure of anything but I'm using the Generic RGGB in the FITS settings of DSS. I spent quite a bit of time googling the correct setting for my Nikon D750 and the consensus was that this is correct but no one seemed to actually know for sure - more a case of that's what most DSLR manufacturers use.

I just finished working on my latest data (Heart and Soul) below and whilst this was also heavily green biased (I cleared a lot of it with the Cap Yellow), it's probably my best processing result so far and used the RGGB FITS setting - it's a different type of DSO but I'm hoping this confirms the RGGB is ok(?). I don't know how much Impact the LP filter has but, again, this result has given me some confidence that I am getting usable data - would you agree?

I'm finding my lack of knowledge so hard, too many permutations of things that could be causing issues so I am trying process of elimination - if I could even just eliminate the LP filter/RGGB as causes it would be a major step forwards.

Thanks for responding, very much appreciated.

Mid

[admin]

The colors in that heart & soul image are absolutely spot-on. It looks amazing! (the detail in Andromeda like wise).

It must be the LP filter then causing the blue Andromeda. In my experience, CLS filters create the biggest color skews, but the one you're using typically shows the least color skew. The thing that made me think it was not the LP filter is the color of the stars, which do exhibit some variation.

For the Andromeda image, was this really the default color balance StarTools came up with?

[midnight_lightning]

Hi,

Thanks Ivo, praise indeed - I was in need of a bit of encouragement

I agree that it looks like the LPS filter is the issue. I have done some more analysis today trying various de-bayer filter combinations in DSS (RGGB, BGGR, GBRG, GRBG) and they all give surprisingly similar (and disappointing results) but I think rule out this as the issue.

Some screen shots at the link below - these show results of the Colour auto processing using Starry's Mask/Sample suggestion for each combination above. I used a very basic workflow - Autodev/Crop/Wipe/Autodev/Colour.

The 1st autodev is very interesting - bright blue/green and doesn't look right at all - I included an example - 3rd screen shot from the bottom of the attached. (I tried this with and without Flats/Dark Flats - no significant difference)

https://www.dropbox.com/s/6b0f0cidltrog ... .docx?dl=0 .

I have also spent a couple of hours researching how people get on with the IDAS D1 and oddly whilst there are some reports of processing difficulties most don't seem to have many issues. I think my next step will be to get out and take some rough short exposures of M31 without the filter and see how they compare.

At least if I can nail it on the filter I can then consider whether there are processing solutions, or perhaps whether I need to look at a different camera.

[admin]

Hi,
Some screen shots at the link below - these show results of the Colour auto processing using Starry's Mask/Sample suggestion for each combination above. I used a very basic workflow - Autodev/Crop/Wipe/Autodev/Colour.

The 1st autodev is very interesting - bright blue/green and doesn't look right at all - I included an example - 3rd screen shot from the bottom of the attached. (I tried this with and without Flats/Dark Flats - no significant difference)

If using a DSLR and DSS (which color balances the data beforehand), then the bright blue green is consistent with skyglow filtered by a light pollution filter and nothing to worry about. Data from a DSLR that has not been white-balanced can also often show a distinct teal/green which is simply non-whitebalanced light pollution, for example, you'll find PixInsight produces this type of data). In either case, Wipe should be able to deal with this bias quite effectively.

https://www.dropbox.com/s/6b0f0cidltrog ... .docx?dl=0 .

The RGGB auto colour adjusted image looks reasonably close to what I would expect the Color module to generate on first launch. Some galaxies (M31 included) are fairly good white balance sources, as they "contain a bit of everything" (similarly there is a reason why the Milky Way is called the Milky Way - its light is a nice milky white on average). However, since you're missing part of the spectrum (notably some lines in the yellow part), the image is skewed towards blue more, making the core in particular seem a stark white. Colour balancing will of course not be able to recover those fine bands that you're missing, but pulling back on the blue a little (and keeping an eye on any dominant green using the MaxRGB mode) may help.
For the same reason, using the foreground star field to colour balance will not yield correct results. LP filters will cause a distinct lack of yellow stars.

I have also spent a couple of hours researching how people get on with the IDAS D1 and oddly whilst there are some reports of processing difficulties most don't seem to have many issues. I think my next step will be to get out and take some rough short exposures of M31 without the filter and see how they compare.

At least if I can nail it on the filter I can then consider whether there are processing solutions, or perhaps whether I need to look at a different camera.

You could always shoot with and without light pollution filter and use the colours of the non-lp-filtered stack, combining them with the luminance of the lp-filtered stack; best of both worlds!

[midnight_lightning]

Thanks Ivo, much appreciated. I'm going to spend some more time today working on my processing and will also take some new lights without LP for comparison. Then see what's next, I think if I need to take images with and without LP, which is a good suggestion, I may start to look at CCD, possibly narrow band.

Cheers

Mid