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Using a CCD camera, each pixel records the photons and displays an ADU - the relationship between them being the gain of the camera.

Every star image produced by a telescope is basically an Airy Disk.

Due to atmospherics etc (ignore guiding errors and aberrations for a moment) a PSF (Point Spread function) curve is seen, basically the same Gaussian shape as the Airy disk but much larger in diameter.

The usual measure of this curve is the FWHM ( the width/diameter of the curve at half the peak intensity)

Still with me?

I do a 3 min exposure.....

The fainter stars appear smaller than the brighter stars ( get's even worse if the star intensity causes satuaration...)

Why is this so??

The full extent of the PSF curve doesn't change.

Why doesn't the intensity just build up in each pixel within the PSF curve? Why does it appear to enlarge the PSF curve??

The reasons given say that as the exposure increases the CCD can record "fainter outer regions of the curve" and these show up as an increased diameter.....Sorry, I can't see that!

If ALL the light from the star is within a defined PSF curve and that curve, for a faint star is say 5 pixel diameter, then surely for a brighter star the total size should still be 5 pixel allbeit with a higher peak intensity.

Add 10 x 30 sec star images.....compare the size with a 5min star image????

I'm finding that if the star image is NOT saturated, then the PSF curve remains close to constant...when the star becomes a saturated image it starts to grow outwards ie 5 pixel max profile for a non-saturated star image v's 15-20 pixel for a saturated star!!!

( Ron Wodaski in his "The New CCD Astronomy", p 50-53 demonstrates this issue but gives no reasoning or answer to the possible cause.)

Can anyone add to my confusion?????

Edited by Merlin66
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Whoosh....as that goes above my head..!

Apologies, but I really love learning more and it's only when people ask difficult questions that I realise I'm just scraping the surface. I'm interested in this answer too! But it's a little beyond me to make any suggestions...

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Diffraction causes the stars to grow and then bloom in my camera... As you point out already, all things being equal the airy disk determines the size of the star on the ccd, which due to the Nyquest theorem needs to cover at least two pixels to be correctly sampled.

As the pixels saturate, they leak into the adjacent pixels causing them to fill faster and faster. With a NABG camera the effect is limited, with my ST10 it is not.

Brighter stars will become more bloated and the fainter star will be revealed up to a limiting magnitude of course. Therefore, you always need to find a balance between exposure and detail, this is especially true when stretching an image.

I hope this helps?

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Neil,

The PSF should take care of all the atmospheric effects....

"As the pixels saturate, they leak into the adjacent pixels causing them to fill faster and faster. With a NABG camera the effect is limited"

Can you confirm this statement? I'd be interested in seeing some hard data for various CCD's.

What it could mean is that multiple subs may infact be "better" than longer subs - the stars wouldn't saturate and therefore should/ would be smaller and tighter????

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Weill my imaging has become a lot better, so thats one fact, right.

If you really want real data best to just do it, experiment with a ccd on a clear night for the first hour at least.

One of the common mistakes is not to reduce your exposure time when binning the ccd.

Lums is taken at 4mins with 1x1 (No binning)

RGB are taken at 4mins with 2x2 binning - mistake!!!

They should have been taken at 1min!!! 1/4 of the Lums time! Binning increases sensativity but people still think more time is better!!!

I never take subs longer than 600s and normally my colour subs are 300 or 400sec depending upon the subject. 600s is normally for the Ha, OIII and SII when I finally get one for Christmas.

The only issue with taking more subs is "READ NOISE" which should be a very small contribution to the total noise in the image / sub.

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Neil,

I agree.....

All my CCD camera's are "tied up" into spectroscopes. I don't do any actual imaging any more - just spectroscopy.

The aspect of star size growth made me question the "optimum" slit size for the star - which is based "nominally" on the FWHM 3" image size, not the larger images found after a 5 min exposure.......

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Come over and have a look at our Yahoo group:

astronomical_spectroscopy : Astronomical Spectroscopy for Amateurs

We have members using various designs of spectroscopes - gratings etc and observations of everything from planetary spectra, variable stars, Be and WR stars as well as Quasars and supernova.....

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