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Old 28-05-2018, 07:55 AM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
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Originally Posted by ErwinL View Post
Hi Ray,

great work! I'm just planning my first narrowband observation and therefore find your spreadsheet particularly helpful.

Thanks for sharing,
Good luck - hope the spreadsheet helps

Originally Posted by Stonius View Post
Thanks for putting that up!

I have questions as to its use;

1) Target Brightness; Stupid question, but what does the p in p/m2/s/arcsec2 stand for? I see that it requires in-the-field measurements - does that mean the intended use is not as a preparatory tool, but more as an on-site drilling down into what the histogram is showing?
p is photons. It is useful as a preparatory tool if you specify a low value here as suggested. I don't know of any measurements of nebula surface brightness, or would have used real data. If you find any, please use.

2) Gain - I'm confused about the difference between the gain setting on the camera and its actual value in ADU's. Is unity gain (139) where all 20,000 electrons in a well are distributed among the 4096 possible values in 12 bit ADC? That would make it 4.88 e/ADU. at unity gain. Is that correct? I have no idea how to work out the gain for other levels though. Or do I just put in '139'?
"gain" for the CMOS chips is a complete dogs breakfast. The gain term in the graphs on the ZWO1600 page is specified in "gain" in ADU/e for a given gain setting (eg 139) and refers to the 12 bit ADU. It is also related to the 16 bit values measured in the output files. The spreadsheet sorts it out if you use the manufacturer's 12 bit ADU/e value from the graph (eg at gain 139, the ADU/e is 1.0) - and set the padding bits to 4

3) In band QE; The peak QE of the 1600 is 60%. Reading the relative graph which is scaled to 100% (one assumes 1 = 60%QE because it's *relative QE). So for Ha at 656nm the *relative QE is about .78, which is to say, 0.78 of 60% = 46.8% actual QE in Ha, right?

4) Total exposure time. Why whole hours only? Surely there are some who would like to do less? Maybe not for NB, but the RGB sheet is the same? Seems an odd restriction but what do I know?
you can use other than whole hours, but then the tabular results will be very slightly in error. Rather than rewrite that part of the spreadsheet or get into a post storm trying to explain/define what the problem is and why it doesn't really matter, I decided to just specify whole hours. If you are interested, the problem can be illustrated by considering that someone specifies a total time of 23 minutes of 5 minute subs - results in a total of 4.6 subs, which is nonsensical.

5) Reading the results; so I guess I'm looking for the point of diminishing returns here? The point where the graph flattens out?
yes. The spreadsheet also provides a calculation of read noise effect. Choose a sub length that provides about 5% for this value and you should be on the edge of the flatter bit of the curve


Hi Markus - answers in text above

Cheers Ray

Last edited by Shiraz; 28-05-2018 at 12:05 PM.
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