Black spots on image

[Davey-T]

Just doing a bit of preliminary processing on some luminance subs of M81 / M82  to see if I had enough decent data to produce an image.

7.5 hours X 10 minute subs.

QSI683 so do bad pixel map as part of calibration then stack and got a couple of places with these dark bits.

I seem to recall reading something about this and may even have made a note in one of my many notebooks but can't find it.

Dave

 

[steppenwolf]

There is quite a good explanation of this effect here.

[Davey-T]

7 minutes ago, steppenwolf said:

There is quite a good explanation of this effect here.

Thanks for that Steve, I was thinking it maybe something to do with the bad pixel map setting in Maxim, I was only trying to remove the burnt out pixels but presumably it removes dead ones as well ?

Should a flat be used for removing dead pixels ?

Dave

[steppenwolf]

Quote

I was thinking it maybe something to do with the bad pixel map setting in Maxim

The bad pixel map in MaxIm DL can indeed cause this sort of issue, again through over-correction but I haven't found a solution to the problem as it doesn't always give grief. I prefer to dither my images to resolve these sorts of issues.

As an aside, MaxIm's documentation also mentions how hot pixels can also appear black after dark frame subtraction "A dark - but not dead - pixel may also occur in images after dark frame subtraction, usually because of a ”hot pixel” which has a much higher rate of dark current accumulation than the average." which does make sense.

I have never used a flat frame specifically for he removal of dead pixels and am not sure how this would work in practice but it would be worth experimenting with. My suspicion is that as flat frames don't remove hot (white) pixels then they won't correct for dead (black) pixels.

[Davey-T]

Thanks Steve, there are only a couple of little groups of black pixels so I can just clone stamp them out, just wondered what was causing it.

Presumably the bad pixel map can get confused by two or more adjacent bad pixels if it's based on detecting single pixels.

Using 10 Micron mount so not tried dithering  [yet] 

Dave

[steppenwolf]

9 hours ago, Davey-T said:

Using 10 Micron mount so not tried dithering  [yet] 

Nice mount! Dithering is my best friend!