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| He... OK, that's a good visual argument (if not mathematically rigorous) for downsampling pre-stretch, but why pre-integration? And in particular subsequent linear steps like deconvolution? |
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On top of what rightly Björn is saying in the previous post, consider this: In my near-sea level, Bortle 6/7 site I get seeing for long integrations between 2.2" and 3.5" (but never that good below 45 degrees). My pixel scale is around 0.78"/px hence my sampling of the integrated PSF is between 2.75 to 4.4 pixel so I get a fair stab at deconvolving the image, both in terms of proper sampling of the PSF and of, more importantly, of the SNR of the same. This said I really struggle at times to collect enough signal to stretch the image to the level I'd require and not showing the background noise, In your case the same seeing distribution would yield between 8.4 and 13.5 pixels while the recommended (in terms of SNR) would be between 3 and 4. Putting it an another way: the ratio of apertures between my 8" and your 11" (let's ignore the different COs) is 1.89, i.e., your 11" has 89% more collecting area than my 8". But the area you're collecting the light into is 900% larger in your case. That is a factor of 9! Even with binning it would be still 225% larger. Going bin2x2 in camera or in software? To me it doesn't make sense other than bin 2x2 for the reasons posted before but this is something that could be easily decided by a simple test . Shooting the same subject in bin 2x2 and bin 1x1, process both to have an integrated light, convert the bin1x1 image to the same (lower) image scale and measure the noise and/or the SNR of both. Assuming at least 30min worth of integration that should decided whether in your case is worth going one way or the other |
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How do you measure seeing? Here, when the jet stream moves north, I have periods of pretty good seeing. I can guide then in the low 0.2 or even high 0.1s (I realize that is not exactly seeing). During this capture seeing was awful, and guiding was in the high 0.3s at best, often in the 0.5-0.7 range. So everything is soft and bloated in comparison to good nights. And the seeing argument is very valid for this image. But... how do you attach a number to seeing? |
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Linwood Ferguson: I'm not sure how you measure seeing yourself, but you can estimate it. Even just by looking up at the sky, if the stars are really twinkling above 45 degrees you are in for a bad time. In Montreal, that always seems to be the case but back home in the west where I have the privilege of being around 1KM above sea level with a dryish climate, there are nights when the stars above are almost still. There is this website: https://www.meteoblue.com/en/weather/outdoorsports/seeing/montreal_canada_6077243 And this one: https://www.cleardarksky.com/c/Montrealkey.html For an explanation on what the squares actually mean in terms of seeing on the second website, see here: https://weather.gc.ca/astro/seeing_e.html While I can't help you put an EXACT number on the seeing, you can estimate it so its "good enough" for what we do as amateur imagers. |
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Thanks, @SemiPro, the problem comes in trying to compare, e.g. Andrea assuming our seeing is similar. Maybe it is, maybe not, it would be useful to know. It's hard to compare qualitative opinions. I use the Meteoblue already, and especially its clouds forecast is usually quite at odds with our local forecast and relaity. But let's take that as an example: tonight the Seeing Index 1 is 5 almost all night (two 4's early), with index 2 4 most of the night. But that hardly seems to be possible since the associated pointers list V as "Perfect motionless diffraction pattern". It's been my experience that the seeing it gives is optimistic at best. Especially since the jet stream is still present (though not for London apparently). Really "for what we do" all I really need is to know how well I can guide (and lots of things play into that), since if I wait for both excellent seeing AND clear skies, I may be too old to carry the OTA outside. But if one is trying to compare between two people's experience, it would be nice if there was an objective measure we could make, e.g. some sharpcap routine to run on a known target or some such. Another consideration is since when I start imaging for a night and go to bed, I never know if most of the images will be with seing mediocre or terrific or somewhere in between. So if (emphasis on If) I am binning in anticipation of mediocre seeing, it would be a shame to wake up to the best of the year. Whereas if binning=downsample, then I'm good by not binning. That presumes (which I am not trying to argue) that I should not bin even with the best of seeing. I'm still trying to get my head around the math. I should note when I started all this (about 18 months ago) I binned much of the time. Discussions and experiments made me think there was no point - sure, it processed faster, but it seemed worthwhile to decide late in the process how much to downsample. So I just stopped. Never regretted it, until I posted in this thread.  |
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| While there are ways and means to measure the seeing the only way that matters to you is to measure the FWHM of a mid-range (not too bright, not too faint) star with a short exposure, let's say between 1 and 5 seconds. That is your seeing and it would compare to what I get here or anywhere else. FWHM in terms of arcsec, not pixel. |
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Ah... that makes sense. Obviously will need to try with short exposure. They are huge on a 4 minute sub (which I just looked at, in the 3" range). Thanks. |
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| PS. You gotta love all the information sources we have. Meteoblue says 5 = best, cleardarksky says 3/5, Astrospheric says 2/5 to 3/5 depending on hour. It's a good thing we now are digital, I can't imagine all the film I would have wasted on bad forecasts. |
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Linwood Ferguson: Ah, maybe I should of been more specific but the link I posted from the Canadian weather folks only applies to the charts on Cleardarksky. I am not sure what sort of meteorological voodoo MetoBlue uses in their seeing calculations. Another point: I am pretty sure MeteoBlue uses their own model that only has a 30km resolution in North America, while Cleardarksky pulls all their stuff from the CMC/GEM model which has a resolution of 2.5km in Canada and the Northern US, and 15km for the rest of North America. I tend to prefer Cleardarksky just because it has been more accurate for me in Canada, which given the information I just typed out makes a lot of sense. I'm not sure how Astrospheric manages to forecast seeing by the hour since they also pull from the CMC and they only forecast seeing in three hour blocks... |
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andrea tasselli: I finally looked these up, sorry for delay. $10,000! Wow. Better? Probably (but only for my C11). Worth it? Can't envision how it is that much better. Maybe I am just blind. |
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Linwood Ferguson: Whether they're worth the outlay of dosh, I can't judge. Me, I'd bin the ASI6200 and be done with it. And make that FR work, too. |
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andrea tasselli: The specs certainly support that, but I also spent a couple nights doing some careful comparison and have converted to always using gain 100. In less time I got better results with LRGB. Thanks for that advice. |
