Quote:
Originally Posted by alistairsam
Thanks for the info Ray.
didn't pay much attention to these aspects before. it was more winging it.
it makes sense now cause the other day, I tried a 5 min sub of M83 on my 8inch F4 with the qhy8 and then a 10min sub. There was hardly any difference, but it was less grainy on the 10min sub, I'm guessing because there was slightly more signal. I've noticed that with 2 and 5 min subs. the mild graininess disappears with the 5 min sub.
would you have a link to the SS?
Else I'll the online calc at starizona.
I have a qhy8 and an 8inch F4 and a 10inch F4.
whilst determining exposure time, shouldn't it be based on the sky background measurement as well as explained in the site?
so the optimum exposure length would vary based on seeing conditions as well?
the specs on the qhy8 are
Spectral Response:
QE max at 540um ~60%
Read Noise:
less than 11 -e RMS (typically less than 9 -e)
Full-well capacity:
Greater than 25K -e un-binned
so does this mean the dynamic range is between 11e- and 25Ke-?
Cheers
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Hi Alistair
I haven't made my spreadsheet available anywhere - might do so soon.
suggest that you use the Smith equation on Starizona rather than the calculator - you need to take off bias/pedestal signal to get the true background sky and the calculator assumes a pedestal of 100. as you know, you can set the pedestal anywhere you want with the QHY8 and you can also change the gain any way you want - the calculator assumes some sort of fixed gain for a given camera.
to get around these problems, I used a typical dark sky background brightness in my spreadsheet, rather than measured data on a specific night, and went back to basic CCD parameters - that way you can work out how long the optimum subs are under dark skies. If sky is brighter you can use shorter subs, but you will not stuff up by using the longer ones.
the dynamic range is indeed between 11e and 25ke. it is usually expressed as the ratio of the two measures - ie about 2200. this is a bit less than some other cameras, but still pretty good.
be wary of the quoted QE for OSCs. this is the maximum inband QE for any of the Bayer filters. At a given wavelength, the ccd can have a QE of 0.6, but if illuminated with broadband light it will only detect about <.2 of the total incoming photon flux in any pixel. This is not a fault, but makes it more difficult to compare OSCs and mono cameras on anything like an apples-to-apples basis.
as I see it, the optimum sub calculation is a guide only - you can use what you want. But it does give you a suggestion for the best combination of sensitivity and exposure times for your setup.
And on the original topic of the thread, my expensive new SX 694 camera just died - Peltier cooling stopped working. whoopee... So now I find out how good the after sales service is while all the interesting galaxies head steadily westwards.
regards Ray