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Old 20-11-2011, 12:42 PM
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ozstronomer (Geoff)
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misalignment in Nebulosity

I have been successfully using Nebulosity for a long while but I can't work out what has gone wrong with these frames.

I imaged Ngc1333 in RGB+Ha last night and the night before. Today i've run the usual processing routine with Nebulosity; Darks. Flats and Lights and PS3. After processing I noticed a series of dots through the image that I can't explain.

I've gone back and checked the images straight from the camera (no darks or flats applied). The attached is 3 frames of Ha 10min each and 4 frames of blue 10min each, stacked via Nebulosity.

It appears to be background noise that has not aligned. If I stack 3 images there are 3 dots, 4 frames = 4 dots etc

The frames seem to be miss aligned somehow. The main stars are aligned but the background ones seem to move.

I wondered if the darks I have are not subtracting out the noise which is why is shows up on the final image?

I have included the final image from PS3

Any suggestions appreciated
Attached Thumbnails
Click for full-size image (ngc1333 blue miss align.jpg)
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Click for full-size image (Ngc1333 Ha miss align.jpg)
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Click for full-size image (ngc1333 RGB miss align.jpg)
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  #2  
Old 20-11-2011, 12:56 PM
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multiweb (Marc)
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A proper dark subtraction will reject those dots are they are hot pixels on your sensor. They trail because your frames are offset when registering and stacking them back. You can also make a BPM (Bad pixel map) in neb 2 I believe. Just subtract it from all your subs prior to registering.
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Old 20-11-2011, 01:29 PM
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RickS (Rick)
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Geoff,

Best to get rid of the hot pixels as early as possible, as Marc suggested, but aren't you doing some form of data rejection when you stack? That should remove the hot pixels as well if it is working properly.

Cheers,
Rick.
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Old 20-11-2011, 01:43 PM
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mldee (Mike)
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QHY9 bad pixels

I get the same thing, only I have a lot more hot pixels, to the point where I'm almost ready to ask Theo if it's normal to have so many. I made a bad pixel map in Neb2 last night that seems pretty good, I'll attach it.

As I want to make some reliable Darks and Flats now, I also did the Test as described in http://www.cool.id.au/astronomy/Neb_tut/ by Benko and Felix, which I think is also here on IIS if you do a search. My results were good apart from the unexplained drop in mean from ~64000 to 32742 at saturation (Gain 10). Any comments on this are welcome.

Anyway, my final settings appear to be G9, Offset 115, and they seemed to work OK last night, although still lots of hot pixels in my normal 5 minute exposures.

I was hoping to do a new set of bias, darks then flats at this setting this afternoon, but it's 42C in the obs and the poor old QHY9 will only hit -14C at the moment.

Hope this may assist the OP.
Attached Thumbnails
Click for full-size image (Bad Pixel map Q9 5min  -20c G9 O115.jpg)
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File Type: pdf QHY9 Gain and offset test 111120.pdf (4.2 KB, 6 views)
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Old 20-11-2011, 02:44 PM
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ozstronomer (Geoff)
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Quote:
Originally Posted by multiweb View Post
A proper dark subtraction will reject those dots are they are hot pixels on your sensor. They trail because your frames are offset when registering and stacking them back. You can also make a BPM (Bad pixel map) in neb 2 I believe. Just subtract it from all your subs prior to registering.
Marc, I'll give Neb Bad Pix Map a go and see if that helps

Quote:
Originally Posted by RickS View Post
Geoff,

Best to get rid of the hot pixels as early as possible, as Marc suggested, but aren't you doing some form of data rejection when you stack? That should remove the hot pixels as well if it is working properly.

Cheers,
Rick.
Rick, I'm using the standard Neb dark subtraction which has worked ok in the past, always something to keep us on our toes

Quote:
Originally Posted by mldee View Post
I get the same thing, only I have a lot more hot pixels, to the point where I'm almost ready to ask Theo if it's normal to have so many. I made a bad pixel map in Neb2 last night that seems pretty good, I'll attach it.

As I want to make some reliable Darks and Flats now, I also did the Test as described in http://www.cool.id.au/astronomy/Neb_tut/ by Benko and Felix, which I think is also here on IIS if you do a search. My results were good apart from the unexplained drop in mean from ~64000 to 32742 at saturation (Gain 10). Any comments on this are welcome.

Anyway, my final settings appear to be G9, Offset 115, and they seemed to work OK last night, although still lots of hot pixels in my normal 5 minute exposures.

I was hoping to do a new set of bias, darks then flats at this setting this afternoon, but it's 42C in the obs and the poor old QHY9 will only hit -14C at the moment.

Hope this may assist the OP.
Thanks for all the suggestions.

I am taking another set of darks and will redo the processing again to see if that will clean it up.


Cheers Geoff
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