Quote:
Originally Posted by PRejto
Hi Ray,
A most interesting presentation that does back up some of what I measured when chasing my blue fringing issue a while back.
Sorry if this sounds dense! But, when you write:
"The lower data shows the same HFR data, but with each value being the average of a block of 7 shorter subs (ie this data represents 21 minute subs)"
I question whether taking an "average" of 7 subs actually represents a 21 min sub taken during the same interval. Wouldn't the 21 min sub be more heavily weighted towards whatever was the worst moment of seeing during the exposure? Taking an average would seem to make the sub better than it might be. I guess I'm thinking about an analogy towards guiding when I write this. It doesn't take much guide error, or very long, to destroy a sub that "averaging" wouldn't fix. I'm pretty sure you will adjust my thinking!!
Peter
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Quote:
Originally Posted by Bassnut
Im right out of my expertise here, but doesnt RMS guiding count more than average or PP for extended objects on long exposures?. OK, stars are destroyed by an outlier excursion, but in my experience short exposures (on extended objects) are more sensitive to bad guiding and seeing than long exposures!.
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Hi Peter and Fred.
I also had some misgivings about using an average of the HFR, so I tested it by taking results from a highly variable block of seeing and:
1. averaged the HFR of 7 individual subs
2. stacked the subs and measured the HFR of the result (this should be pretty much equivalent to a longer sub)
I got the same result for the two processes (<2% difference), so decided to use the much easier averaging in the final analysis.
I agree that the actual effect of seeing on an image is almost certainly not fully indicated by HFR, but I wanted to show that this measure at least can be improved by shorter subs. I would be grateful if you have any ideas on how to better test this idea - I am at the limit for my pay scale
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regards Ray
edit: re deconvolution, although read noise will be lower for a few long subs, the fact that you can retain so much more good data with shorter subs (in this case 40% for the chosen rejection threshold) must compensate to some degree. FWIW, with my system, the total read noise is <10% of the total broadband noise with 3 minute subs so, by rejecting less data, I can actually get better SNR than I would with longer subs (broadband of course).