This is a copy of a masterflat taken with my Atik 383 l+ osc, firstly i need to seriously clean my optics ,
The time is 1.72 secs which gives mw about 30,00 adu
When i use this to clean my stacked image it removes the dust bunnies but introduces a gradient in the lower right hand corner that corresponds to the shadow that appears there. A poorly processed running chicken nebula ( looks more like Woody Woodpecker for those old enough to remember ) shows the result
Am i correct to assume it isn't vignetting but something else, amp glow? shutter?
Any ideas as to the cause?
IMHO, flat is a file with informations about the differents responses from pixels of sensor. Same intensity of light cause differents output level from each pixel.
If your flat was done in the right way, e. g. with equal distribution of white light over sensor, it is normal that you observe diferents levels in the flat image.
The optic, vignettting, and dusts cause artificial pixels (sensor area) level output and will be showed in the flat image also.
IMHO, flat is a file with informations about the differents responses from pixels of sensor. Same intensity of light cause differents output level from each pixel.
If your flat was done in the right way, e. g. with equal distribution of white light over sensor, it is normal that you observe diferents levels in the flat image.
The optic, vignettting, and dusts cause artificial pixels (sensor area) level output and will be showed in the flat image also.
Thanks Jorge,
I understand what flats are for and how to take them, in the flats image there is also some shadowing in the top left hand corner, why did his not also have the same effect on the image as the lower right hand corner.
George
Im just wondering whether you actually had anything in the bottom right of the Lights? If these are ok to begin with, the flat is subtracting from nothing, which adds the gradient.
What im getting at is that maybe the camera shifted a little between the lights and flats?
It depends from some conditions:
1. the flat was done at the same session of photos ?
2. the flat was done with the same camera position ?
Perhaps the photo had some external influence of light that the flat hasn't. For example: flat doesn't solve problems with light pollution, sky glow and others. Flat only solve problems of sensor response.
Im just wondering whether you actually had anything in the bottom right of the Lights? If these are ok to begin with, the flat is subtracting from nothing, which adds the gradient.
What im getting at is that maybe the camera shifted a little between the lights and flats?
Goran.
Hi Goran,
I also think something is being added, not sure what, i did take the scope and camera off the mount the next day and took it inside and took the flats then, up here it does get quite hot and i did not want to leave it outside but took great care not to change anything.
George
It depends from some conditions:
1. the flat was done at the same session of photos ?
2. the flat was done with the same camera position ?
Perhaps the photo had some external influence of light that the flat hasn't. For example: flat doesn't solve problems with light pollution, sky glow and others. Flat only solve problems of sensor response.
Jorge , i did take the flats the next day and i also took the scope off the mount but took great care not to alter focus or camera position.
Thinking on, as you stated, would sky glow cause this?
yes, it is possible since the different brightness of regions of sky aren't inside flat information. Before I use skyglow filter my photos always have a side more dark than the other. Different from vignetting, that has the center of image more bright.
At that time, many people told me: "make more flats, you need dozens of flats, make new flats ..." ... for what ? I tried to do a "flat" for my sky glow, and it was a disastre ! Solution, only with filter .
Turns out that with the Atik 383 which has a shutter, you can get this shadowing with low exposure time, when I bumped it up to about 4 secs it disappeared and a normal flat appeared.
With the longer exposure will I have to take flat darks as well, or is it short enough that noise will be minimal ?
, 
