Smart Histogram and Focal Ratio

Discussion of using SharpCap for Deep Sky Imaging
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Topographic
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Smart Histogram and Focal Ratio

#1

Post by Topographic »

I may have missed something but does Smart Histogram have a way of factoring in the speed of a Scopes optics. An exposure length for an 8" Newt and a RASA 8" may be very different.
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Re: Smart Histogram and Focal Ratio

#2

Post by admin »

Hi,

the measurement of the sky background brightness automatically factors this in - the faster scope will collect more skyglow brightness per pixel in the same amount of time in exact ratio to the inverse square of the f-number.

Cheers, Robin
Topographic
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Re: Smart Histogram and Focal Ratio

#3

Post by Topographic »

Well obviously :lol:

I thought that there was some relationship but never saw anything written down.

The potential for shorter subs is a joy to those of us whose age brings with it less efficient bladders and, other such inconveniences. :oops:
Prudentis
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Re: Smart Histogram and Focal Ratio

#4

Post by Prudentis »

admin wrote: Fri Nov 15, 2019 6:42 pm the faster scope will collect more skyglow brightness per pixel in the same amount of time in exact ratio to the inverse square of the f-number.
Sorry for zomie here, but I found your video on exposure times and you have become my nr. 1 source Dr. Glover ;)

So do I understand it correctly, that if you are interested in the actual object itself and not the surrounding star-field, a smaller f-ratio (leading to a smaller object in the center and bigger FOV) will actually decrease the S/N for the object although increasing it for the whole image?
I have seen so many posts stating that f-ratio leads to shorter exposures ... this seems impossible to me, from a pure mathematical perspective. Reducing the f-ratio would lead to a wider FOV while simultaneously increasing sky glow and spread the incoming photons from the object onto a smaller area of the chip, thus reducing the details.
In that case, would a Barlow lense actually HELP if, like on my setup, Triangulum is perfectly placed on the whole sensor with a 2xBarlow and fills only the center without?
Or am I off here somewhere?

The first and most important thing in choosing the setup would then be to shoot the image with the object filling as much of the sensor as possible?
Of course, apperture would be even a bigger factor, but well ... that's a topic for a discussion with my wife :lol:
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Re: Smart Histogram and Focal Ratio

#5

Post by admin »

Hi,

I think you have things backwards – you add a Barlow lens then the telescope is still collecting the same number of photons, but the image is now four times bigger – which means that the photons are spread out over an area that is four times larger, which means that in turn only 1/4 of the number of photons are landing on each pixel of the camera. If you want to bring the number of photons collected by each pixel backup to the original level then you need to make the exposure four times longer. That pretty much shows why longer focal ratios (or Barlow lenses) lead to longer exposures and shorter focal ratios lead to shorter ones.

Cheers, Robin
Prudentis
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Re: Smart Histogram and Focal Ratio

#6

Post by Prudentis »

Oh yes ... I had a brain fart at how the barlow lense works. However, if I get 1/4 of the image but all I am interested in is the 1/4 that the Barlow is now cropping to... I am still collecting the same amount of photons in that region as I was before, right? Only spread over the whole sensor now. So I am still getting the same amount of photons of the object only less signal per pixel and a worse S/N ratio ... right?
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Re: Smart Histogram and Focal Ratio

#7

Post by admin »

Hi,

yes, that's right – whereas before each pixel might've been receiving 100 photons in a particular exposure time, now it will be receiving 25 in the same amount of time.

Cheers, Robin
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