Help with M81 Colors

[midwayexpress]

As you can see from my astrobin things have been going well.

http://www.astrobin.com/users/akulapanam/

However I'm having trouble processing the image I took tonight and getting correct M81 colors. Any help would be appreciated.

The link to the stacked TIFF file is here. I have disabled any settings in DSS that can impact color as I normally do for my images.

http://1drv.ms/1ERB1VH

[admin]

Hi,

The data is a little bit tricky... Did you take flats? It also appears you didn't dither between frames nor applied darks (do correct me if I'm wrong). Did you use any light pollution filters by any chance? (nothing to indicate that you did, but it'd be good to know)
StarTools indeed can't get a proper lock on the colors, possibly due to the channels matching up poorly (a DSS problem), as can be seen in the stars.

Fortunately there are other ways to calibrate colors, for example by eye.
For galaxies, you'd be looking for a yellow core (older stars due to less star formation), are bluer (possibly purplish) outer rim due to bright young blue stars outshining everything else. You'd also expect to see some purplish HII knots/areas (a mix of Ha and Hb emissions) and brown dust lanes (white light that has been filtered by the dust to only allow the higher wavelengths).

You'd end up with something like this;

M81v2.jpg (105.48 KiB) Viewed 9250 times

For the color module;
Parameter [Dark Saturation] set to [Full] (in order to introduce color in the darker regions of the image)
Parameter [Saturation Amount] set to [291 %]
Parameter [Blue Bias Reduce] set to [1.71]
Parameter [Green Bias Reduce] set to [1.60]
Parameter [Cap Green] set to [To Yellow] to remove any spurious green hot pixels (do this last, so it doesn't interfere with trying to get the color balance visually right).

Hope this helps!

[midwayexpress]

I did take flats and bias but I currently have no way to dither and this particular camera (even after being replaced) seems to have strong noise. I have not taken darks yet but I intend to do that tonight. I always use a light pollution filter (IDAS D1). Do you have a stacking program you recommend over DSS?

[midwayexpress]

Here is the image processed in DSS with 20 darks.

http://1drv.ms/1E0Jj9L

and my best attempt so far (note I don't push the black point too far and I tend to not apply lots of noise reduction)

http://astrob.in/155057/0/

I'm working on ordering a new camera this week. Probably a Atik 460/490/4120

[admin]

I did take flats

Hmmm... the gradients are pretty severe and uneven, which led me to believe we were lookign at some sort of vignetting or worse...

I always use a light pollution filter (IDAS D1).

Ok, this particular filter doesn't seem too bad, but know that an LP filter will meddle with your colors in a way that is not recoverable by simple whitebalancing. You may be able to get close enough to.

[ChrisLX200]

I have an IDAS D1 filter also, and whilst it is effective at reducing LP it causes awful reflection problems in all bar one configuration of my optics (consisting of 4 different OTAs). It is so bad I will likely sell the thing on.

ChrisH

[midwayexpress]

Because I am a little bit concerned that I'm doing something wrong creating my flats, darks, lights, and bias frames I have zipped the entire package which can be downloaded below.

Thanks for the help!

https://www.dropbox.com/s/ajmj4obavzxo092/m81.zip?dl=0

[Rowland]

I had a look at your data set and the problem seems to be with the light subframes. I tried several calibration methods, presuming DSLR data;

1. Calibrated lights with newly created master dark and master flat - no bias subtraction of the lights, only flats. Debayer, registration and integration

this error popped up about the master flat - never seen it before - I suspect the flats are not quite right. Though a new set might make a difference dealing with the gradients in the subs which accumulate during stacking. It was impossible to get a clean image.

"** Warning: No flat frames have been selected to calibrate light frames (filter="Position 0", binning=1, exposure=210.00s, length=37)"

2. Calibrated lights with newly created master dark only - no bias subtraction

3. Traditional bias subtraction of darks and bias and dark subtraction of lights - usually suicide with DSLR data - no flats.

The result is almost identical. I didn't check the pixel values.

[midwayexpress]

Just saw this now. Are you using DSS? I think the flats are working if only because when I don't use flats there are some pronounced gradients caused by how the ICX274 is produced and I tried with nebulosity but got a similar result. I have been happy with this camera because it picks up a lot more ha than my dslr but the noise has been worse. In the process buying a new one although I hope I don't have some underlying calibration issue.

[midwayexpress]

Here are my DSS settings:

FITS Files:
AHD Interpolation

Stacking Settings:
Intersection Mode
(Align RGB Channels is NOT checked)
Light Frames - Kappa-Sigma Clipping with Kappa of 3 (if not startools has tough time on automatic mask for stars) and 5 iterations
No Background Calibration
Dark Frames - Median Kappa Sigma Clipping Kappa of 2 5 iterations
Hot Pixel Detection and Dark Optimization are checked
Flat Frames - Median Kappa Sigma Clipping Kappa of 2 5 iterations
Bias Frames - Median Kappa Sigma Clipping Kappa of 2 5 iterations
Cosmetic - Detect and Remove remaining hot pixels filter size of 1 px
Alignment Method Automatic