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Old 10-02-2011, 09:07 PM
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dugnsuz (Doug)
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Angry Strange stacking artefact - any ideas!?

Hello all,
Advice needed...
Recently noticed some weird things after using my new 400mm f5.6L lens.
Stacked/Combined images seem to have some strange 'wiggly worm' artefact throughout the image - it's not present on ANY of the individual subs either...checked them all.

Below images...
Left: Stacked/Combined using Adaptive Add (problem obvious when stacked using Average too)
Artefacts circled!

Right: same data combined using Sigma Average

Stacked in Images Plus.

Any ideas as to what's going on?

Doug
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  #2  
Old 10-02-2011, 09:20 PM
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dugnsuz (Doug)
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Similar BS in my M42 stack (Sigma Avg Combined)
Once again - not present in any sub!!!
Grrrrrr
WTF?
Doug
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  #3  
Old 10-02-2011, 09:40 PM
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h0ughy (David)
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not being funny but is your autoguiding keeping the image centered, its like the autoguiding is moving it around, making each individual image ok but when stacked any pixels that are singular and dont line up will show the tracking error. the drift may be on the mount and the autoguiding compensates - maybe?
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Old 10-02-2011, 09:46 PM
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They look like hot pixels that are not properly removed by dark frames and they don't move with the alignment process.
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Old 10-02-2011, 10:00 PM
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dugnsuz (Doug)
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Quote:
Originally Posted by h0ughy View Post
not being funny but is your autoguiding keeping the image centered, its like the autoguiding is moving it around, making each individual image ok but when stacked any pixels that are singular and dont line up will show the tracking error. the drift may be on the mount and the autoguiding compensates - maybe?
Autoguiding seems fine - the artefact has only appeared with the 400mm lens not the 70-200. The error you suggest shouldn't be stack/combine dependant though should it?
Doug
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Old 10-02-2011, 10:03 PM
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Quote:
Originally Posted by Terry B View Post
They look like hot pixels that are not properly removed by dark frames and they don't move with the alignment process.
Interesting - as I'm using a cooled DSLR I've not used dark frames, flats only.
Still perplexed about the artefact being present or absent in different combine functions!

My Brain Hurts
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Old 10-02-2011, 10:18 PM
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Quote:
Originally Posted by dugnsuz View Post
Interesting - as I'm using a cooled DSLR I've not used dark frames, flats only.
Still perplexed about the artefact being present or absent in different combine functions!

My Brain Hurts
you still need to take darks - and flat darks and bias shots
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Old 10-02-2011, 10:26 PM
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This artefact seems peculiar to this lens - I have had no similar problems with my half-hearted approach to calibration with images prior to the purchase and use of this lens...please explain!

Surely a slight increase in FL doesn't necessitate taking 2-3x as many different calibration frames!?

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Old 10-02-2011, 10:41 PM
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Octane (Humayun)
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They're probably dodgy internal reflections of some kind.

H
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Old 10-02-2011, 10:41 PM
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always take images with the lens you put on - i have two cooled 40D's - i use different lenses but i always take a set of flats darks and bias's with the lenses on before changing
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Old 10-02-2011, 11:33 PM
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Cosmic ray hits can look like that. Although they don't look the same one to another.

Greg.
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  #12  
Old 11-02-2011, 12:19 AM
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Doug,

Have you checked your flats ?
Stack 5 copies of the same flat together on their own and see what you get.

I am just wondering if its an additive problem of say out of focus dust motes being diffracted/affected by the 400mm aperture blades ?

Try some different apertures and see if it changes.

Otherwise Octane is on to it.

Just guessing really

Of course the Worminids were passing through that night you know, with their radiant being in Orion

Rally
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Old 11-02-2011, 10:14 AM
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Doug

I think these are just hot pixels showing the total drift over the course of the exposure.

Sigma clip stacking removes them, but averaging or adding leaves them in.

You could try stacking all your images withoug aligning them - I think you will find the hot pixels strong and all your stars take that shape instead.

Dark frames will help and the sigma clip stacking as well.

cheers
Phil


Quote:
Originally Posted by dugnsuz View Post
Hello all,
Advice needed...
Recently noticed some weird things after using my new 400mm f5.6L lens.
Stacked/Combined images seem to have some strange 'wiggly worm' artefact throughout the image - it's not present on ANY of the individual subs either...checked them all.

Below images...
Left: Stacked/Combined using Adaptive Add (problem obvious when stacked using Average too)
Artefacts circled!

Right: same data combined using Sigma Average

Stacked in Images Plus.

Any ideas as to what's going on?

Doug
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  #14  
Old 11-02-2011, 11:43 AM
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dugnsuz (Doug)
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Thanks for the comments and ideas - the "hot pixel drift" theory looks the most plausible. Just can't understand why the effect is repeated over the whole image with varying degrees of brightness/intensity.
Thought the effect would be confined to one spot.
Doug
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Old 11-02-2011, 01:23 PM
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Hrm, yeah, now that I think about it, if you haven't created dark frames for that session, nor, created a hot pixel/cosmetic map, then, that would likely be the cause of the issue.

H
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  #16  
Old 11-02-2011, 01:31 PM
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mill (Martin)
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I had the same when using my D40.
These are hot pixels.
I always took them out in the final pic with the magic wand in cs.
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Old 11-02-2011, 01:37 PM
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Quote:
Originally Posted by dugnsuz View Post
Thanks for the comments and ideas - the "hot pixel drift" theory looks the most plausible. Just can't understand why the effect is repeated over the whole image with varying degrees of brightness/intensity.
Thought the effect would be confined to one spot.
Doug
It is multiple hot pixels. The intensity of each one will vary depending on how hot it is.
If you use a pixel mask to remove the hot pixels as well as a dark frame it stops this happening.
Not all reducing software allow pixel masks but lots do.
Examples that I know of that do are Iris, Mira and AIP4WIN.
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Old 14-02-2011, 09:57 AM
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dugnsuz (Doug)
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Thanks for the comments - the hot pixel theory was correct...easily removed by dark calibration (should've done that in the first place!)

Cheers all
Doug
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  #19  
Old 14-02-2011, 10:17 AM
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Looks like its going to be a corker mage Doug. What are the details?
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  #20  
Old 14-02-2011, 10:26 AM
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dugnsuz (Doug)
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Quote:
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Looks like its going to be a corker mage Doug. What are the details?
Astro 40D (Avg Temp -13ºC)
400mm f5.6L @ f5.6, iso1600
Darks(!) and Flats calibrated
150min (10x15min subs)
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