calculation of sub exposure - formula

[admin]

Hi,

yes, this is fine by me.

NINA will also read sensor analysis files written by SharpCap Pro (sensor analysis written by free SharpCap are written in a binary format). I am also happy for you to do this, but beware that the format is really an internal detail, so I will change it if necessary in future updates (usually adding new fields, but not always).

I would request that you do not distribute sensor analysis data with the installed packages and instead point out the way to obtain it via SharpCap. This reflects the fact that the amount of work that went into the automated sensor analysis in SharpCap was *much* larger than the amount that went into the exposure calculation.

thanks,

Robin

[JMcGee]

Thank you Robin!

[pfleurant]

T = 10*R^2/P

The dimensions on both sides of the equation should be the same:

seconds = seconds

But,

T: optimal sub exposure (seconds)
10: corresponds to the 5% (1)
R: read noise of camera (electrons)
P: sky pollution, (electrons/sec)

Is this right?

It is not dimensionally consistent:

sec = electrons ^2/(electrons/sec)

sec = electrons * sec

What's up with this? '

[admin]

Hi,

yes, this one confused me too initially...

Actually, the number of electrons (in the read noise and in the sky background rate) is really just a number, so the read noise is dimensionless and the rate is 1/s

cheers,

Robin

[AstroSW]

Now, that doesn't mean that you have to take 14s (or 4.6s) exposures - it just means that exposures longer than that will not help reveal more faint detail in the final stacked image. You may want to take longer exposures to keep the number of images under control!

Apologies for resurrecting this, the lecture you gave on noise and exposure was a real eye-opener and I've been thinking a lot about it. But I don't quite understand this remark of taking longer exposures to manage the number of images.

Let's say my optimum exposure is 4 seconds and I want to take a stack of an hour. That's 900 subs, which I decide is rather a lot, so I decide to halve the number and take 8 second exposures giving me 450 subs. The longer exposures - an extra 4 seconds - will provide no advantage, so I would still have to take 900 of them? Or does this mean the 8 second exposure could be counted as 2*4 second exposures?

[admin]

Hi,

basically it means that the image you get from 1 hour of 8s exposures (450x8s) will be the same quality as the 1 hour of 4s exposures (900x4s). Once you hit the exposure that SharpCap calculates, you don't gain anything in terms of quality (or, rather, only gain extremely marginally), but you don't loose out in quality terms either if you go longer.

cheers,

Robin