F5.8 72mm vs F10 150mm

[rockmattstar]

Background info: I have 2 scopes a nexstar 6se (150mm aperture and 1500mm focal length - f10) and a evostar 72ed (72mm aperture, 420mm focal length - f5.8).

I haven't had chance to use the evostar yet thanks to the clouds. 😡

Question: Does the f5.8 mean I'm going to be able to reduce my exposure time or is it cancelled out by the fact the aperture is so much smaller?

[x6gas]

Ah... it's a good question.  There is a lot written and discussed about this: lots of theory, plenty of opinion.  But from my experience focal length and image scale are more important than f-ratio.

If you are imaging a small target (small galaxies or planetary nebula) then you want longer focal length, but you also want to match the focal length of your scope to the size of the pixels on your camera sensor.  You'll ideally need an image scale per pixel that is half to a third of your best seeing conditions.

Image Scale Per Pixel (in arc seconds)= (~200 * the size of the pixel in microns) / focal length in mm (iirc)

Typical seeing in the UK is, I believe, one or (more likely) two arc seconds so you should be aiming for an image scale in the range 0.33 to 1 arc second and you ae probably pretty safe at the thick end of that...

This should ensure that you are not over or under-sampling your data.  After that, the lower f-ratio the better but it's really only fair to compare a 1000mm focal length f/5 scope with a 1000mm focal length f/10 scope when you are talking about how fast you can acquire the data...

[rockmattstar]

I was just thinking there are some things that don't fit into the view of my nexstar which is why I bought the evostar. Like m31 for example.

So I'm wondering whether the new evostar will still take the same time to capture the data as the aperture is smaller but the focal ratio is higher? But I'll get the whole object not just the centre, that's the plan anyway.

[vlaiv]

If you use same camera on both scopes and process your data the same way (no binning or other things done differently between two scopes), then despite having scope with only 72mm of aperture - it will be "faster" than 6" scope.

It won't be as fast as F/stop speed increase from photography says. That rule holds only for daytime photography when there is plenty of light and again - for same sensor and processing.

In astrophotography we are dealing with very low light levels and everything else starts to make big impact on final image - thermal noise, read noise, light pollution. All of those mean you won't get as reduced total exposure time as difference in F/ratios would lead you to believe from daytime photography rule.

You might have read that F/ratio does not describe the speed and that is correct if you have ability to control third parameter - sensor size and pixel size, whether thru hardware (choice of camera), or even in software - pixel size can be changed with binning. Things get more complicated then and F/ratio does not represent speed of imaging anymore.