Aperture - Shutter Speed

jgs001

jgs001

Brian Cox
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John
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I get the basics of this, but I'm getting a new scope and can't get my head around the numbers...

I know what I can achieve with a 5 minute exposure at f/7.5 with my refractor... I'm getting an f/9 scope, what exposure time do I need to hit the same equivalent exposure, keeping the ISO the same (ISO800).

Sorry, I've been pondering this all morning, and I just can't work it out
:thinking::thinking:
 
To find out the difference in exposure between two aperture values follow this procedure:
Divide one by the other:
9/7.5=1.2
Then square this number:
1.2*1.2=1.44

So, your f/7.5 scope lets in about 44% more light than the f/9. Rounding that to 50%, a 7min 30s exposure should be ok.
 
I think about 7mins 20sec.
The F number is proportional to the diameter of the aperture so if you calculate the proportional decrease in area between F7.5 and F9 that should give you the proportional increase in shutter speed.
 
Last edited:
Excellent thanks very much guys. I can do that long easily... My tracking test exposures have always been 20 minute exposures at 1200mm...

Iain, the aperture and focal length of both scopes is totally different.. the f/7.5 is 80mm aperture, 600mm focal length, and the f/9 is 150mm aperture, 1370mm focal length, but I don't believe that the actual aperture makes any difference to the exposure time in astrophotography, just the focal ratio. The aperture makes a big difference to resolution however.
 
Thanks Derek. My new scope should be here today, hopefully allow me to better my lunar and planetary... and it's not slow enough to be a major league headache for deep sky, although I might stick to the brighter targets till I can afford to add a focal reducer.
 
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