M31 = strange dots in stacked image?

[Dave 2112]

Hi all,

My first ever attempt at imaging, using a new DSLR (first camera that wasn't point and click) then stacked in DSS (never used anything but Paint) came out sort of ok. But there is a pattern of dots that appear several times in the image, is this a usual sort of thing that is tidied up in processing or is it likely to be an issue with the camera (or camera operator)?

Full image

Close up of dots, seen top left and top right.

Its a stack of 10 x 30s, and there are 10 dots in the pattern. Is this a hot pixel that is fixed with darks? All settings in DSS were left to default. The images were jpegs as DSS seemed to think my RAW were greyscale, and its saved with DSS adjustments applied. They don't appear in the unadjusted image, but it looks totally different anyway, small version below

Any help or feedback appreciated.

[steppenwolf]

These are hot pixels. They 'appear' to have moved because there has been movement between each of your subframes. Your subframes have now been aligned with one another using DSS - as the subframes are now aligned with one another, the hot pixels (which are in a fixed position on your sensor) are no longer aligned with one another!

Hope that makes sense.....

[Ant]

Wot he said.

Take some darks at roughly the same temperature and add that into your processing and they should go away

Ant

[Dave 2112]

It does, thanks Steve. So this could be fixed using darks, and is that many normal in a camera as there seems to be half a dozen or so in the image?

[Kropster]

Half a dozen seems like not many at all. I am sure I get many more than that.

Considering how many million pixels there are, it is a tiny percentage.

[steppenwolf]

and is that many normal in a camera as there seems to be half a dozen or so in the image?

Yes, quite normal and as Ant says, Darks will help although they will not totally alleviate the issue on their own. When you stack, select Kappa-Sigma Clipping as your method as this removes the more outlier pixels.

I am guessing that currently you are not autoguiding? If/when you do start autoguiding you should consider 'dithering' your images (automatically shifting the telescope slightly between subframes) to distribute the hot pixels randomly so that the Kappa-Sigma Clipping sees them as outlier pixels.

[red dwalf]

i used to have a meade dsi 2 pro and that was littered with dead pixels, i looked as if someone had taken a shot gun to it, but when darks added it was greatly improved.

even my new atik 314l+ has a few dead pixels, very hard to see as they are so small but you get them on the best of chips, infact they get more over time and it`s totally normal. 

[wxsatuser]

Hi all,

 All settings in DSS were left to default. The images were jpegs as DSS seemed to think my RAW were greyscale, and its saved with DSS adjustments applied.

I would'nt use jpegs.

When the RAWs are loaded they will be greyscale, DSS will debayer them and you will need to increase saturation.

[Psychobilly]

You can also remove hot and cold pixels by using the options in the cosmetic tab in DSS... Not ideal but it's there as an option.... Sometimes the hot pixel removal can remove the center pixel from stars.

With my setup I used to get sub pixel drift between frames without deliberately dithering and with enough frames this was enough for the hot and cold pixels to be taken care of by Kappa Sigma Stacking...

I have even found KS stacking does a good job of removing passing sub frame clouds...

Peter...

[red dwalf]

i like to use kappa sigma clipping also, gets rid of a lot of oddments

[wxsatuser]

Aircraft for one.......I get lots of planes in my subs.

[red dwalf]

i`m on the flight path also, depending on which way the winds blowing, it does a great job of getting rid of those streaks right across the middle of your 5 minute sub.

[Dave 2112]

Thanks to everyone for the pointers, much appreciated.

Redoing the stack using Kappa Sigma removed all the hot pixels and the aircraft trail. Its also a relief knowing its normal rather than a duff camera.

Next time I'll do some darks, and work with RAW, at some point get an autoguiding set up, and the camera modded. First though I'll go with the advice that is included in most answers to new imagers and get a copy of Every Photo Counts.

Thanks all.