[Shiraz (Ray)]

Originally Posted by strongmanmike

Once again I love the way you verify and quantify more or less why I have found what I have through trial and error, a great explanation Ray

Mike

thanks Mike it's good to find agreement.

Originally Posted by RickS

Interesting data, Ray. On a couple of occasions, when the data justified it, I have done two integrations of my luminance - one weighted for best FWHM and one weighted for best SNR. These are blended so that the high FWHM integration is used for the brightest parts of the image and the high SNR integration is used for the dim parts. It should be possible to do something similar with short subs for the high FWHM data, even with a camera that doesn't have incredibly low noise (like my KAF-16803...) Food for though

thanks for that input Rick. Much food for thought indeed - have to think about what weighting for FWHM actually means, but the idea of using different stacks for different parts of the image seems really useful.

Originally Posted by multiweb

That's interesting to see the numbers Ray. I too have had experience with shorter subs (3min) and found that the FWHM of the final was better, probably because you can be picky given the number of subs available. But read noise was an issue. I've never experienced long uninterrupted subs. Don't have an obs and there's always a wind gust, a cloud or plane that's bound to stuff one up. Want to make the most of your setup time.

Hi Marc. The ability to throw out subs that are spoiled for whatever reason seems to be very useful. But, you are right that read noise limits what you can do.

Originally Posted by codemonkey

Great topic / work, Ray! I've seen some discussion elsewhere on identifying the shortest possible exposure that you can use while keeping the total integration time and resulting SNR the same and this is another great reason to do that. Short exposures for the win!

Hi Lee. would be great if we could get hold of cameras with really low read noise - would be nice to use 1 minute subs or less and that should be possible in future.

Originally Posted by tilbrook@rbe.ne

Thanks Ray!

Thanks for doing the numbers, I'm glad to see there's some method in my madness!!

Cheers,

Justin.

Hi Justin. the other possibility is that maybe we are both bonkers?? Anyway, have also been thinking about how you might optimise sub length for your camera - will PM you.

Originally Posted by gregbradley

Nice work Ray. What does HFR stand for? ( I looked it up and it wasn't listed) Is it half full radius? Whats its definition?

Another factor here is longer subs average out guide errors as well. So if your guiding isn't super fabulous then you can see the stars getting a bit fatter with longer subs compared to shorter subs.

Of course an assumption here is a bright object or a low read noise camera where the short subs allow the read noise to be stacked out.

Doing this with higher read noise cameras or with narrowband would stop you from getting very faint detail above the noise floor of the camera.

So like anything you need to know when this would be a good approach and when it wouldn't be.

I was surprised recently comparing 5 and 10 minute subs from my CDK17 and SX694. It was hard to notice much difference in terms of the brightness of the image. Shorter subs tended to have slightly rounder stars (guide scope - yuck, but a necessary evil with that setup).

So in conclusion I think it fair to say as a general rule:

1. Shorter subs are better with variable seeing, windy or partially cloudy conditions or where you tracking is not 100%.

2. Its not good for faint maximum detail or for narrowband images.

3. If your tracking is top notch and your skies are clear and stable with no wind then longer exposures would be better.

How often do you think the seeing is variable like that? As a general trend I notice seeing often improves as the night progresses and possibly best at around 3am or so.

Greg.

Hi Greg.

The seeing behaves like that quite often at this site, but ironically it is only possible to see fluctuations like this if you use short subs (if you use long subs you will never be aware of how much the seeing varies ). I used to get major seeing improvements as the night went on, but since I put up a ROR obs, the overall seeing is much more consistent (and consistently lower) throughout a night.

You are right to point out that what I call "seeing" also incorporates tracking error - but the technique works just as well on tracking error.

HFR is the Half Flux Radius (half of the Half Flux Diameter as in http://www.cyanogen.com/help/maximdl/Half-Flux.htm). It is used in Nebulosity and Sequence Generator Pro - which I use all the time.

Totally agree that this method is not appropriate for NB, except maybe for the brightest objects. However, I think that it can be used for getting maximum detail from broadband images in good conditions - provided that the subs are long enough to cover the read noise, I think that a lot of short subs will generally do slightly better than a few long subs.

Regards Ray