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!
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?
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
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!
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
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!?
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
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
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!
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
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.
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.