Comparing scopes - light gathering

[Drac0]

Hi all,


Trying to get my head around the light capturing abilities of different scopes and if it can in some way be used for comparison, especially taking into account different focal ratios & how it all affects exposure times.

So I have 102mm F7 & 150mm F8 refractors. Now from calculations, if I'm correct, the 150mm will gather a little over twice the light of the 102mm. So if they both had the same focal ratio I could expect to need exposures of half the time on the 150mm? But how is this affected by the different focal ratios? And if I stick a x0.8 reducer on the 102mm, reducing the focal ratio to ~F5.6, how does that affect things?

I'm used to the photography version of things like F-stops where doubling/halving the light changes things by one stop of light (in this case the 102mm @ F5.6 would ~= 150mm @ F8) but can't figure out what it means when it comes to telescopes. Would the 150mm @ F8 collect about the same amount of light as the 102mm @ F5.6?

Anyone know where I can find some information that won't be too confusing?

Cheers,
Mark

[Startrek]

Hi Mark,
I strongly recommend reading all the relevant articles from Craig Stark ( original inventor of PHD guiding )

https://www.cloudynights.com/articles/cat/column/fishing-for-photons/

It should answer all your questions about SNR , light gathering power , focal ratio , focal length, Noise in general , Sampling and so on …

I found them most useful

Cheers
Martin

PS : Some of the information is high tech but hang in there , Craig does eventually bring things into perspective

[Drac0]

Thanks Martin, will give it a good read.

Cheers,
Mark

[Wilso]

Hi Mark,
I think easiest way to visualise it is the camera is the aspect ratio and the telescope as the focal ratio.
102mm @ f8 = 816mm which is the point the telescope comes to focus.
102mm @ f5.6 = 571mm which is the point the telescope comes to focus thus allowing more field of view and extra light gathering power(picture a cone)
Hope this helps

[Didge]

Hi Mark,
Yes - it all gets very confusing very quickly once you start getting into the technical details of focal ratio, SNR, Airy disks, resolution, point-spread functions and Gaussian profiles.

I think the simple answer to your question is this - the only thing that matters with respect to light-gathering power is the diameter (and hence the area) of the aperture.

The f-ratio (or the focal length) simply determines the area over which the light gathered through the aperture is spread when it arrives at the image sensor.

So, light gathered by a 50mm telescope is spread over a very small area on the sensor, and hence is very bright. The same amount of light gathered by a telescope with the same aperture but a longer focal length is delivered to a much larger area on the sensor, and hence is less bright - so requires a longer exposure - all else being equal.

It is a function of the areas - which is why standard f-stops increment by a factor of √2.

Telescopes are just big cameras - so the same principles apply.

I hope that helps :)

Gary