Quote:
Originally Posted by Steffen
No, not quite, only as far as the final magnification is concerned.
The f/10 scope will generally be more pleasing to look through, most aberrations will be diminished compared to the f/5 one, and it will have better apparent contrast due to the darker sky background.
IMO fast scopes are "only" good for astrophotography, or when you can't handle the longer OTA size for some reason (large OTAs can be an issue in windy conditions).
Cheers
Steffen.
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Yes that was the point I was trying to make. For visual use (and it is entirely different for imaging) the "speed" of a scope makes no difference to the overall brightness of an image. How can it? It's the aperture which determines how much light is collected hasn't changed. BUT the magnification has changed. So a scope with a fl of 650 with a 10mm ep will give 65x while a scope of fl 1250 and the same ep will give 125x. The available light from an extended object will be spread over a larger area so its
surface brightness will appear lower. But overall it is just as bright, the same number of photons are being collected. And logically a point source such as a star will exhibit no change in brightness.
In imaging you are accumulating signal over time. A "faster" scope will concentrate the available photons on your sensor from an object on a smaller area of the sensor obviously so it will produce enough signal to create an image in a shorter period of time, hence "faster" means something to an imager.
The above is how I think of it and explained in simple terms. I am not an optician or trained expert, so I have this wrong I am happy to be corrected.
There is an interesting discussion involving Les and Alex on this thread a few years back
http://www.iceinspace.com.au/forum/s...ghlight=faster
Cheers
Malcolm