Horsehead Nebula

Dave,

In light of your description above, I think I know what has happened and this is a classic example why we should be pursuing full reduction (and not partial). If we analyze the differential sensnitivity of a camera's pixels, we will note differences which can reach 20%. In other words, a given pixel can have sensitivity "x" and for this to be the case for each of the immediately surrounding pixels except one and for the exception pixel to have 20% less sensitivity.

When we perform a non-linear stretch where our dynamic range is compressed to a smaller range, this 20% is magnified. This is the reason that as part of reduction, we should also pursue flats (and flat-darks) so as to control for this problem.

I remember about two weeks ago when I analyzed the behaviour of my camera by taking a series of 15 flats for each of the Lum, Red, Green and Blue filters and I was floored by the results. When looking at the four ensuing masters (adjusted using darks), I was seeing generally very good behaviour between neighbouring pixels but there are 15-20 pixels on my imaging chip which are "problematic" in that they have reduced sensitivity by up to 20%. For example, I would observe ADU's similar to 28900, 28912, 28890 etc etc and suddenly observe 24400.

The key comment in your description above - "the CCD behaves differently when it is exposed to light" - is where the flat frame comes into play as our saving grace. It is unfortunate that I cannot upload images (my master flats from a couple of weeks ago) which would most graphically illustrate what is going on and what every astrophotographer should be seeing using his own system so as to fully understand and appreciate the need for full reduction.

God help me if you now reply that flats were also taken! :lol:

Anthony,

I think you have hit the nail right on the head. I know Gary did not take flats for this one. What you have said sounds VERY likely to be the cause of the issue.

Your insights are an inspiration!

mr clarke
may i say that is a fine image your getting better and better at this game great image i say your chuffed with it well done mate