Quote:
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.
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Hi Greg
re your question on how seeing varies, I did a couple of analyses on multi-night data from a couple of other targets (3 to 4 minute subs) - the attached data is HFR (in pixels as before) and shows how much and how quickly the seeing varies at this site. Hope it is interesting - it seems that short term fluctuations of around 0.2 pixels or more (or about 0.4 arcsec FWHM) are common, which is a fair bit really. Regards ray
edit: none of this data is normalised to remove the effect of elevation on seeing.