(OT slightly) Flatfield using photoshop

Sorry this is a little of topic, but I don't know anywhere better to ask.

Is it possible to do a flatfield calibration using Photoshop CS2? If so how?

You can do a pseudo flat that helps with vignetting but I don't know of
a way to use a real flat in photoshop to remove dust mots and pixel to
pixel variation in sensitivity as a real flat field routine does it.

To do a pseudo flat make a copy of the background. Now in the copy use
a median filter of about 60 or so. That should fuzz out most of the
detail. You may need to clone out a large bright nebula until it is
about the same background as the area around it. Then use about a 40 to
60 Gaussian blur to smooth things out. Actual values will vary
depending on image content. The idea is to leave an area that shows the
general vignetting of the image. Once finished combine using the
difference mode. Slide the % slider down to about 70%. Again the
setting depends on the image. It should remove most of the vignetting
but dust mots are too small and will remain. If they aren't severe
using a burn and/or dodge tool as needed you can often reduce these to
where you can get a usable image. Work in an adjustment layer so you
can easily back out if you screw something up.

This is the best I know how to do.

Rick


Boris wrote:
Sorry this is a little of topic, but I don't know anywhere better to ask.

Is it possible to do a flatfield calibration using Photoshop CS2? If so how?



--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Boris wrote:

Sorry this is a little of topic, but I don't know anywhere better to ask.

Is it possible to do a flatfield calibration using Photoshop CS2? If so how?


I just remembered the free version of PixInsightLE has a pretty good
pseudo flat routine built in. I prefer the more hands on PS version but
you can't beat the PixInsight one for ease of use. Again it only helps
with vignetting and some gradient removal same as PS.

Rick

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".

Rick,

Thanks for that. I knew how to produce the pseudo flat as I have to use that
as I want to remove the uneven illumination of the Sun when imaged through
my PST. I didn't think however that a difference combine would work, I'll
have to give it a try when I get back from holiday in a week.

James

"Rick Johnson" wrote in message
. com...
You can do a pseudo flat that helps with vignetting but I don't know of a
way to use a real flat in photoshop to remove dust mots and pixel to pixel
variation in sensitivity as a real flat field routine does it.

To do a pseudo flat make a copy of the background. Now in the copy use a
median filter of about 60 or so. That should fuzz out most of the detail.
You may need to clone out a large bright nebula until it is about the same
background as the area around it. Then use about a 40 to 60 Gaussian blur
to smooth things out. Actual values will vary depending on image content.
The idea is to leave an area that shows the general vignetting of the
image. Once finished combine using the difference mode. Slide the %
slider down to about 70%. Again the setting depends on the image. It
should remove most of the vignetting but dust mots are too small and will
remain. If they aren't severe using a burn and/or dodge tool as needed
you can often reduce these to where you can get a usable image. Work in
an adjustment layer so you can easily back out if you screw something up.

This is the best I know how to do.

Rick


Boris wrote:
Sorry this is a little of topic, but I don't know anywhere better to ask.

Is it possible to do a flatfield calibration using Photoshop CS2? If so
how?

--
Correct domain name is arvig and it is net not com. Prefix is correct.
Third character is a zero rather than a capital "Oh".