Greetings,
I am 17 and new to astrophotography. I have recently seen the video that gave the equation to calculate the Optimum Sub Exposure Time. That equation was "C times Read Noise Squared Divided by the Light Pollution Electron Rate. I am a little confused about where the Magnitude would come into play in this equation. If the magnitude of a deep sky object varies, shouldn't that change the optimum exposure time for such objects?
Video: https://www.youtube.com/watch?v=3RH93UvP358 (49:28)
Cheers,
-Nathan
Optimal Exposure Calculating
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Re: Optimal Exposure Calculating
Hi Nathan,
the amount of light collected from a deep sky target depends on how long your total imaging time is and (clearly) how bright the target is. If you capture for 4 hours, you collect 4x as many photons as for 1 hour.
However, the formula is not aimed at calculating the total imaging time, it's aimed at calculating the optimum *sub exposure* time - that is, should we divide our 1 hour of imaging into taking 4 images of 15 minute exposure each, or 60 of 1 minute exposure, or 240 of 15s exposure?
Making the wrong choice of sub-exposure time will give poor results
* Too long - individual sub-exposures may be spoiled by tracking problems, satellites or planes passing through or becoming too saturated
* Too short - the noise level in the final stacked image will be higher than optimal, reducing the visibility of the deep sky target.
In fact, the formula helps calculate what is 'too short', and for that the key brightness level is not the target itself, but the background brightness of the image around the edges of the target - you are optimizing for the faint parts of the target where the brightness is really not much above the background level. Optimizing for the faintest parts gives them the best chance of showing nicely above the noise, and of course the brighter parts are brigther anyway...
cheers,
Robin
the amount of light collected from a deep sky target depends on how long your total imaging time is and (clearly) how bright the target is. If you capture for 4 hours, you collect 4x as many photons as for 1 hour.
However, the formula is not aimed at calculating the total imaging time, it's aimed at calculating the optimum *sub exposure* time - that is, should we divide our 1 hour of imaging into taking 4 images of 15 minute exposure each, or 60 of 1 minute exposure, or 240 of 15s exposure?
Making the wrong choice of sub-exposure time will give poor results
* Too long - individual sub-exposures may be spoiled by tracking problems, satellites or planes passing through or becoming too saturated
* Too short - the noise level in the final stacked image will be higher than optimal, reducing the visibility of the deep sky target.
In fact, the formula helps calculate what is 'too short', and for that the key brightness level is not the target itself, but the background brightness of the image around the edges of the target - you are optimizing for the faint parts of the target where the brightness is really not much above the background level. Optimizing for the faintest parts gives them the best chance of showing nicely above the noise, and of course the brighter parts are brigther anyway...
cheers,
Robin