On Saturday, April 18, 1998 10:25 PM, Chris Heapy wrote:

> I've just imaged M13 tonight, this is 4 stacked exposures of 25 secs
> each. I seem to have a lot of noise in the background which I've seen
> others remark on with a 416. You can find it temporarily at the link
> below:
>
> http://easyweb.easynet.co.uk/~chrish/m13_x4.jpg
>
> This was using Pview 6.4r1, I would liked to have used the shift and
> combine function, but try as I might it *will not* align the images
> properly. I reckon that function is still broke.
>
>
> Chris
> --
> *Model Engineer Support Page*
> Engineering: http://easyweb.easynet.co.uk/~chrish/homepage.htm
> Astro Stuff: http://easyweb.easynet.co.uk/~chrish/lx200.htm
>
>

Chris, I took the liberty to look at your image. To me, the most
impressive part is the tight point spread and good focus.

For non-binned (high resolution) images on the 0400 chip, best
results are obtained with good S/N (signal/noise) ratios. Simply
stated, measure the background and compare that intensity to
the fainter portions of your image. Only very bright objects will
produce a good S/N ratio with so short an exposure, especially
when stacking images. In this case any amplifier noise is added
with each image, so you will need to increase the overall exposure
times to compensate (outrun the noise).

For M-13 at f/6- f/7 on non ABG 0400 chips using shift and combine
or track and accumulate, I recommend 8 X 1 minute to 10 X 3
minutes. The higher later exposure times will result in displaying
fainter stars without exceeding individual pixel well capacity and
spoiling the resolution potential.

For the longer exposures, you will want PEC peak to peak values
of 5 arc seconds or better. Good seeing, no wind, 800-1200 X and fast
reflexes enabling 1-3 manual corrections per second will keep the star
well centered providing accurate PEC recording every 2.4 seconds.

Overexposed (flat) stars increase the point spread and map to white.
Only the best drive action can produce similar star sizes in your image
with one exposure and require guiding corrections every 3-5 seconds
for a 250-300 seconds exposure and guiding errors not exceeding
0.3 pixels.

I would not be too concerned at this time with the slight patterns
at such low signal levels. Many 0400 chips produce these to some
degree.



--
Michael Hart
Husen Observatory

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