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Old 20-10-2012, 07:16 PM
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gregbradley
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Join Date: Feb 2006
Location: Sydney
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There are other factors that enter that make 60 minute exposures not practical.

Clouds are one!

Tracking is another.

Flexure is another.

Smoothness of PE is another.

We have to find the sweet spot for our particular setup. Too long an exposure with KAF8300 is likely to give you bloated bright stars from overexposure but with a narrowband filter that is just fine.

10 minutes is good for a lot of cameras, 15 minutes is probably better if your tracking and lack of flexure is up to it.

The reality is most systems won't allow too long an exposure due to the above factors. If its not tracking then its clouds moving in, if its not clouds it could be wind, if not then small well capacity etc.

10 -15 minutes for most CCD cameras and for narrowband longer (the images are noisier as there is less signal and you have to get the signal above the noise floor). I see John Gleason uses 40 minutes for his Ha images very often. His are about as good as you'll see so that is a good goal to be able to achieve with your system.

As far as combine methods go. Its quite educational to combine a series of Ha images with different combine methods. Ha images often have artifacts in them. Sum leaves the rubbish in place but gets a nice strong signal. Sigma reject gets rid of outliers and even satellite trails if you have enough subs. Median and mean also work well and I think mean works pretty well at clearing out rubbish from the images.

I am sure there are some who use different combines for different types of images. Perhaps sum for galaxies, sigma reject for neb shots, mean for Ha, mean or median for flats. Its worth clicking on the various types to see the differences (which tend to be subtle) when next combining some exposures.
Images Plus had a great one for DSLRs which was sigma reject on each RGB channel separately. It worked really well.

Greg.
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