help with the colour of my images

[little col]

details are
30 secs 800iso , prime focus on a 8" newt with a baader nedymium filter/ 350d baader modded dslr
seems that every image has a purple cast to it and its driving me knuts , do you think its l.p ?:eyepop::shrug:

really appreciated for any help

[iceman]

Hi Col.
Probably a combination of sky pollution and the modded 350D.
What do longer exposures, or more exposures + stacking bring out?

Do you capture in RAW?
What's the WB set to?

[Zuts]

Hi,

I wouldnt worry about the cast. I image from inner city sydney and have tried an orion lps filter (orange cast), atronomic UHC filter (blue cast). I think all filters give a cast to the LP in the sky background.

I just stack as many photos as possible and process the cast out with PS2 (or equivalent). Also since you have severe LP i would consider capturing at ISO 400, so you will be able to expose longer, stack more subs and rely on PS2 to bring out the details.

Paul

[Tamtarn]

Col you posted this image on 12/12/07

30sec ISO800 same camera same scope. Looks great. What have you changed since then??

39248

[JohnH]

Hi Col,

I have been experimenting with an NBP fiter recently for imaging - it also produces a color cast. To get back to a "natural" colour balance I have found using the RGB curves in PS to be the most effective - adjust the curves so the peaks for each color are at the same point on the x-axis and the colours seems to be "natural". There may be better ways - eg use of a standard "white" star in the image (gv2 class) or using a white/grey card and setting your camera to use the reference shot for a custom color balance.

Hope you do not mind but I have had a quick got at re-processing your image to get it close using this technique (and got rid of the vignetting with a synthetic flat)...

[Terry B]

Open the pic in Iris, define a small area of blank sky, type "black" and magically it is gone.:)
I get the same problem with the moon.
A nice demonstration of the same problem is at http://astro.ai-software.com/articles/dslr_iris/dslr_iris.html

[little col]

after the stack it was a white colour and i was imaging in jpeg auto white balance

cheers paul i was going to get a narrowband filter but as you say it might not be that beneficial

ah yes i remember this , if i remember this was taken from a darker site and i processed the balls out of it to get it as good as this

i am very interested in the synthetic flats as i am having problems with vignetting

wow cheers , had a lookk at this and it looks the same colour as my images

[JohnH]

Col, A synthetic flat is made by making a duplicate layer in PS, then applying a Gaussian blur untill all the details you wish to preserve are fuzzed out. What you are left with is the background gradients and vignetting effects. Set opacity to 50% and suntract the new layer from the original image, adjust the blur and opacity to give the most pleasing result.

It is, of course, much better to shoot real flats and process your lights with those.

[little col]

cheers john i will have a go and see if i can get a better result with this method :)