DSS stacking artifacts

[Carole]

Had some artifacts when stacking my Bubble nebula done over two days:

When I stacked the subs for 20th, there were no artifacts, just lots of noise as it needed more subs:
http://gallery.orpington-astronomy.org.uk/albums/userpics/10047/Bubble_20-8-11_5mins_7_x_5mins_13_degrees_new_flattener_13_degrees_OAS_Forum.jpg

When I stacked the subs for both 20th and 21st I was getting both vertical streaking and Red green and blue artifacts (RGB is this a clue?)

Vertical streaking:


RGB artifacts:


I tried several ways of stacking, i.e. Stacked in Groups, stacking the stacks for each night, then I tried stacking both nights as if they were all done on the same night, I used the darks from the hottest night.  It just seemed that every time I added the subs from 21st I got these strange artifacts.  

Even a single stack of 21st (without 20th subs) produced the same results.  Nothing was different except the temperature, it was 13 degrees on 20th and 19 degrees on 21st, and everything was still set up from the night before.  Same settings on DSS.

Well in the end I used Noel's actions "Vertical banding reduction, and processed out the artifacts.

I have had this vertical banding before, just wondered whether any-one had any thoughts on this.

Thanks
Carole

[JohnP]

looks like hot pixels & you have vertical shift between the frames (due to flexure or something) - this is evident as when you stack all the frames you have a dark black band along bottom. For some reason your darks haven't successfully removed hot pixels from the two different nights of imaging.

[Mac]

Im assuming the vertical banding you are referring to are the green stripes,

looking at the bottom image, i can count 13 green spots, and im assuming that there were 13 images that you stacked.

these are hot pixels, that, as John said have not been removed by the darks,

Just noticed that you have used the same darks for the two different nights.
a big NoNo, the two temperatures are different as you said yourself, so the pixels
are going to react differently, you need to use the darks for each night on their own subs.

Mac

[Carole]

looking at the bottom image, i can count 13 green spots, and im assuming that there were 13 images that you stacked.

OK, understood, yes that makes sense.

Just noticed that you have used the same darks for the two different nights.
a big NoNo, the two temperatures are different as you said yourself, so the pixels
are going to react differently, you need to use the darks for each night on their own subs.

Yes I know, but if you see above I only did this after stacking "correctly with the right darks for each group" to try to eliminate the problem. I then tried stacking in two groups, each with it's correct own darks, and got vertical streaks and hot pixel artifacts even when I used the correct darks for both.  I also tried stacking each night separately and then stacking the resulting stacks. 

Im assuming the vertical banding you are referring to are the green stripes,

No Mac, I'm referring to the streaky sky background bottom of top image.

looks like hot pixels & you have vertical shift between the frames (due to flexure or something) - this is evident as when you stack all the frames you have a dark black band along bottom

John I do have a slight dark band along the stacked image for 21st but put this down to stopping the guiding for a short while (during clouds).  It would be difficult to tell which is was due to.  As I say the set up was the same for both nights, I never touched anything (being in the observatory), so if there was flexure one night surely it would be on both nights images, and this problem did not happen on the first evening's subs.

Carole

[MarkS]

The darks aren't well matched to the data.  Hence the "warmer" pixels leave streaks along lines parallel to the obvious hot pixels.  These streaks will will in all colours.

Mark

[Carole]

The darks aren't well matched to the data.  Hence the "warmer" pixels leave streaks along lines parallel to the obvious hot pixels.  These streaks will will in all colours

Hmmmm, interesting.  I took the darks for the 2nd night from my Darks library.  I have no way of measuring the temperature on the sensor itself, unless some-one knows how, so using air temperature may not be a good guide.  The darks on the first night I did some before and some while there were clouds and that night's resulting stack does not have this problem.

So maybe this is a good lesson on NOT having a darks library unless you can measure temperature on the sensor.

Thanks

Carole

[Mac]

I took the darks for the 2nd night from my Darks library

ah there in lies your problem,
during winter this would not be as greater problem as the temperature is low, i.e 0 less noise
during the summer its considerably warmer say 19, so there will be more hot pixels, because of the warmer sensor,
you need to match the temperature with the time length, so idealy you will have

50 folders from -30 to +20 in 1c steps, and in each of those folders other folders rainging from 30s to 900 secs in say 30sec intervals each with 10 images

I have no way of measuring the temperature on the sensor itself, unless some-one knows how

you dont need to, you just need to measure the ambient temperature,
If you leave the camera outside it will reach equilibrium with the air temperature, sensor temp roughly the same as air temp

You could always do the following if you want a perfect library.

Darks can be taken anywhere. so your camera can be indoors ect, it dosent have to be attached to the telescope

Set the camera up in a room with a thermometer and leave it for say 15 mins so it changes temperature to match the room,
not the temperature and set up your darks, taking say 10 of each.
Save these in a folder at that temperature.

Now change the temperature of the room and repeat.

(how can i do this easily you ask)

Simple, put the camera, thermometer and heater into a sealed box, sort of large cardboard size.
that way you can get darks from the temperature that night up to whatever temperature you want to set.
obviously the colder the starting temperature the more sets you can get.
all you need to do is to wait for the temperature to lower to add those temperature.

MAc.

[Carole]

Mac what you have described is exactly what I do. 

When I said I took darks from my darks library, I meant darks taken at the same temperature on another occasion, same length etc etc.

Basically when I do darks, I copy them into a folder in the darks library recording temperature and length of subs.

Carole