Long focal length for deep sky astrophotography

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4 hours ago, vlaiv said:

Why not? I put my trust in fellow SGL members who recommend said video when discussion is had on topic of read noise and sub duration. I'm also aware that SharpCap (software which author is Dr Robin Glover) can do needed calculations to determine read noise of camera and proper sub duration.

No, we are actually not having civil conversation about this - you keep bashing at what's been said without any arguments.

You showed absolutely nothing. You keep giving vague statements on topic.

This is wrong on several accounts. I will list them:

1. First part is partly correct - it indeed requires signal to be above read noise, but signal needs to be above total noise - not just read noise. SNR needs to be higher than 1 (or in fact about 5 for reliable detection).

2. Increase in total integration time won't necessarily raise SNR above certain threshold. If we use exposures that are very short - for example few milliseconds, then you will introduce too much read noise for SNR to be above limit. In fact - for any total integration time - I can select sub duration that will keep SNR below 1 due to read noise.

3. Using faster optics is not the only way to increase photon flux per pixel. That is also achieved by using larger pixels or by binning smaller pixels to larger size.

I agree on most of what you said here - although I don't know what you mean by "SNR of object signal vs read noise". That statement makes no sense. You should not compare ratio of different quantities (SNR is ratio of signal to noise) to one of such quantities (read noise). I'll just assume you meant SNR (signal to noise ratio).

I never claimed differently - although you tried to misinterpret what I've said in such way several times.

However, I will repeat what LP is good for - it shows you at which sub duration it virtually makes no difference for given read noise and you don't need to use longer subs. In fact - this is not exclusive to LP - it applies to any noise source that depends on time - when any noise source other than read noise becomes significantly larger than read noise - read noise stops having significant impact on total SNR (unlike above example where we use sufficiently short exposures and read noise overpowers signal for any integration time).

Having this capability brings equality to cameras with different read noise. If one camera has 1.5e of read noise and other has 3e of read noise - there are sub lengths for each of them that will produce the same final SNR for the same integration time.

I agree. No point in further "debating" this topic.

Nice, you accuse me of not being civil, and I haven't insulted you in any way, just pointed out where you are wrong. Then you dismiss all my arguments telling me I showed you nothing when I addressed yours point by point and showed your errors.  This is indeed not a way to debate.

I'm not even going to go into the issues with your latest statements except to say that when you say you don't understand some of the statements I make I suspect English may not be your first language?  The word vs can imply a ratio in English, like the amount of nickel vs silver in an alloy.  Just saying.