some bad gradient, a little advice please

[Daniel-K]

taking subs on m51 last night and i have just stacked them in DSS and NEB3 and if both versions when i stretch the image im getting a bad gradient, could this be cause by a dewed secondary? any advice on what it is would be very greatfull

Dan

[rfdesigner]

I'd expect a dewed secondary to produce bloated stars.

Just stretching the background a LOT on the image you've posted, you have some very defined contours there.. are you stacking 16bit subs or something less than that?

Derek

[Daniel-K]

hi derek i think there 16bit FITS that come from the camera.

[allcart]

I get this a lot. Its really blooming annoying and I haven't found the answer yet. I suspect its to do with dynamic range or something.

[rfdesigner]

Can you post a single sub.. in RAW format.

Also did you use flats?

[Daniel-K]

no flats but there shouldnt be any vignetting as the chip is small and i was using 2inch adapters

51nocom.tif

[rfdesigner]

The file... it only appears to be 8bit... really you need more (the thermal noise in the image looks less than 1bit in the file.. so the thermal noise doesn't kill the quantisation noise)

What camera is this?

Also if you're using this on Newt, then the secondary mirror might well be vignetting you.

Derek

[Daniel-K]

its the mammut brightstar, using a 8 inch quattro. do you think it has anything to do with the capture software NEB3? also the filei have posted when download looks real bad, i have done a screen shot of what it looks like in NEB3 un processed

[rfdesigner]

Yup.. Camera is certainly 16bit. I think you might have a setting in neb3 making it save in 8bit tiff or something.. it LOOKs fine in your screenshot.. however, the gradient rings I see in your stack are also there in your 8bit tiff.

Personally I hate these tiff files people seem to use, fine for a final publish, but I wouldn't work in it. FITS is there for science/astronomy and most of the software will work in FITS, also while FITS can support 8-bit format, I don't recall ever seeing a native FITS file in 8bits, the default is 16 and can go to 64bit.

http://en.wikipedia.org/wiki/FITS

Just my opinion, not something you should blindly follow.

Derek

[Daniel-K]

i prefer fits will see if i can change the settings in NEB3 thanks for the help Derek

[AstroAndy]

Hi Daniel

Here is my solution, lately, I have been after some gradient subtraction tools, and now have IRIS, Photoshop, and no more patience, lol. Here is a handy little tool, to be used late in the process (it works on JPGs). http://micro.magnet....nddownload.html . It's free, and easy to use, here are my results, before and after.

[ollypenrice]

It's an odd effect and is clearly not from the optics. I'd be sure of that. It looks like posterization but what causes it I don't know. The only time I see anything like it is while stretching using curves in Ps. While the stretch tool is open I often see these posterizing effects but once the curve is applied and the preview turns into the modified image they disappear. So I imagine the effect arises from some kind of data compression.

Olly