Blue Eye Galaxy

[daneel]

I'm finding that a lot of my images have a blueish tinge because of my light pollution filter. For example the image here is 3 hours of 5 minute ISO 800 subs stacked in DSS and processed in PixInsight taken with an EOS 1000 (un mod-ed) with an Astronomik CLS clip filter. I used 3 darks, 15 flats and 15 bias images. Is there a standard way of removing the blue-ish tinge other than playing with the blue light curve?

Also I noticed there's rotational symmetry in the background noise. I suspect this is caused by the hot pixels rotating in the image over the space of 3 hours but would have expected the darks to remove the noise. I took the darks at the end of the imaging session so the temperature should be the same as the last few images at least.

[nmoushon]

If I remember right one way is to adjust your white balance in the camera.

[Fenriz]

Doesn't adjusting the levels sort it out at all, also I would have thought that only 3 darks would not be sufficient for 3 hours worth of lights I would think a minimum of 20 should be used.

[RAC]

If you're using PixInsight then do a colour calibration using the galaxy only.

[blinky]

This is very easy to sort out if using a DSLR. Get your filter on, take an image of a white wall, go into the settings and choose custom white balance and it will ask for a sample image, give it the image of the white wall you just took with your filter on.

[daneel]

Great suggestions, I'll try all of them. Got an hour each of M3 and M13 last night so will use PixInsight on those and try the white balance trick today. And did 20 darks before hitting the sack. Thanks!

[Auntystatic]

Changing white balance settings in the camera has no effect on raw images, only if saved as jpg. However, setting a custom white balance will allow deep sky stacker to compensate for it if you set it up in the DSS options.

[Tim]

Hi Daneel,

There are many ways to skin a cat

In Pixinsight, with your autosave.tiff 32 bit output file, try using the Histogram Stretch tool.

Step 1) After selecting your image, move the central adjusting triangle thingy to the left until it comes close to, but doesn't clip into the spikes in the histogram.

Step 2) Reset the tool and do that again, less aggressively.

Step 3) reset again, and you should notice that the data spike in the histogram is stretching out. If you now adjust each channel separately R-G-B, by just moving the black point slider up to a point just before the start of the data spike, you should have a fairly colour balanced image.

At the end, if there is any colour cast left, use the SCNR noise tool to remove the cast, playing about with the aggressiveness.

Works for me

[daneel]

All of my images with the filter are coming out of DSS with a bright blue background, this is the raw image:

http://dl.dropbox.com/u/9812713/blackeye.TIF

The histogram for RGB shows that red is shifted well to the left of green and blue. Is this a problem with the way I'm using DSS or just a result of using the Astronomik CLS filter?

[Tim]

I'll have a look at the file, thanks for uploading it

[Tim]

There is a setting in DSS to colour balance the background isn't there?

It is VERY blue isn't it :s You shouldn't get that much colour cast from the filter really, I know I dont.

Have a look for the background calibration settings and play with them a bit

[daneel]

I tried both DSS settings (RGB and individual channel calibration) and it doesn't seem to make a lot of difference. The next time it's clear, assuming it's ever clear again, I'll try removing the filter to see if that's the problem. Thanks for your help Tim.