Hi All,
Tues night had to go and pick up wife from friends - left scope and camera firing off 30 sec shots. Expected to be gone 10-15mins...ended up 2 hours!!!
So I had 76 30 sec pics of M17 @ iso800.!!!!!!!!!!
Decided to get a coffee and start stacking in ImagesPlus.
Attached is the result.
Is there a law of diminishing returns with stacked images?
Expected more data as this should be the equivalent of a 38min exposures!!
Had to do my usual ham-fisted processing techniques to bring out the detail!
Cheers...
Doug
Looks alright to me Doug. Looks pretty smooth on my monitor, if a bit dark, but you can't expect wonders for only 30 sec intergrations. The advantages of stacking that many images is a strong increase in the Signal/noise ratio so your image looks cleaner.
Doug you could try summing sets of images. With 76 images you could try summing every tenth say 1,11, 21 etc and then 2,,12,22 etc these six or seven summed images could then be median averaged to minimise noise. If you sum seven images it would be eqivalent to 7x30 sec exposure.
Thanks Paul,
Ended up playing around with the compressed jpeg before posting so it's a bit rougher than the TIFF (excuses x2)
Just wondered if like reciprocity(!?) in film there is a point where stacking images makes little or no difference?
Doug
Doug you could try summing sets of images. With 76 images you could try summing every tenth say 1,11, 21 etc and then 2,,12,22 etc these six or seven summed images could then be median averaged to minimise noise. If you sum seven images it would be eqivalent to 7x30 sec exposure.
Bert
Good onya Bert, I'd forgotten all about that technique seeing as most of us go for as long as we can without light polluting our images too much.
That is right Doug, plus the summing will also increase your bit depth as well, though only modestly with only sums of 7 to 10 images. When summing try a setting that prevents sigma clipping so your stars don't go max out.
Good onya Bert, I'd forgotten all about that technique seeing as most of us go for as long as we can without light polluting our images too much.
That is right Doug, plus the summing will also increase your bit depth as well, though only modestly with only sums of 7 to 10 images. When summing try a setting that prevents sigma clipping so your stars don't go max out.
To be clear...
I stack images 1,11,21,31 etc, save as image#1. then stack 2,22,32 etc,save as image #2 - then when I've got 7 images - I stack those 7?
Please advise
Cheers
Doug
Yes but your stacking needs to be a sum/addition not an average or median. The resulting 7 images can then be average or median combined for a final shot.
Yes but your stacking needs to be a sum/addition not an average or median. The resulting 7 images can then be average or median combined for a final shot.
The other advantage Doug is that there is always a bit of drift and rotation depending how good your EQ alignment is. By summing every tenth image say, your are not summing noise in exactly the same position on the image. Median combining then gets rid of any that is left.
What I've got to do is learn to drift align so that I can break the 30 second barrier - just wish the Camera could take 60sec exposures in Manual mode, but 30 secs is the max on the Pentax.
The other advantage Doug is that there is always a bit of drift and rotation depending how good your EQ alignment is. By summing every tenth image say, your are not summing noise in exactly the same position on the image. Median combining then gets rid of any that is left.
Bert
Thanks for the tips Bert - my next project is perfecting polar alignment.
Doug
Well I reacon you've done pretty darn good job with what you've managed to capture. I'd say the marbling could be artifacts from excessive stretching (from memory I don't think I've imaged M17 ) but you can get a good idea of how yours may have become marbled by looking at this shot http://astrim.free.fr/gallery/M17CD.jpg
![[1ponders]'s Avatar](../vbiis/customavatars/avatar45_9.gif){kind=avatar}
) but you can get a good idea of how yours may have become marbled by looking at this shot 
looking pretty good there, well done.
