Quote:
Originally Posted by multiweb
Well I've owned a Celestron 130SLT for 5yrs which is exactly what the OP is talking about. It's a 5" F/5 system. I imaged with it and observed with 32/25/9/6 & 5mm eyepieces. I barlowed it x2, x3, used an AO on it as well. The view at native F/5 is brighter than at F/10. The contrast is also much better at F/5.
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That's because you halve the exit pupil diameter when you insert a 2x barlow.
The f-ratio has no effect whatsoever in terms of image brightness to a visual observer. It has an effect for imaging purposes only.
100mm f10 telescope used with a 20mm eyepiece will give 50x with a 2mm exit pupil.
100mm f5 telescope used with a 10mm eyepiece will give 50x with a 2mm exit pupil.
in each scenario the visual image will be of equal brightness.
The advantages of an f10 telescope:-
Greater depth of focus
Collimation accuracy is less critical for a newtonian
A newtonian will have a smaller secondary mirror which improves contrast
A newtonian will have less coma and field curvature and a refractor will have less chromatic aberration and field curvature.
Because of the shallower light cone the f10 scope will work a lot better with cheaper eyepieces and simple eyepiece designs like plossls and orthoscopics.
Higher optical quality in cheaper telescopes is sometimes likely because it is a lot easier to make f10 optics well than it is to make f5 optics well.
Advantages of an f5 telescope:-
Greater field of view
A lot easier to mount, particularly on an equatorial mount.
Easier to store and transport.
For visual astronomy image brightness and contrast are not a consideration in deciding between an f5 and an f10 telescope. The f ratio can be a major determining factor in selecting a scope for imaging as it affects the exposure time significantly.
Cheers
John B