Or is it a higher aperture?
-
It isn't quite like that.
First of all, larger aperture has two benefits, namely greater resolution and greater light-grasp. So, for example, if you want to be able to split two components of a double star, you need aperture to do it. And if the stars are faint, you need aperture again, to make the star and/or its components visible in the first place. All other things being equal, more aperture is always a good thing. But of course, other things (especially price) never are equal.
The focal ratio determines the image scale of the particular objective. A high ratio makes for a larger image - and therefore also a faint one - while a lower ratio makes for a smaller, more concentrated, and therefore brighter image. That's why fast systems (low f-ratios) are good for extended objects like nebulae and galaxies.
Again, this is 'all other things being equal' - and in this case, the thing which is not not equal is the ease of figuring faster and slower mirrors. It is much easier to make a sufficiently accurate f 12 surface on a mirror than an equally accurate f surface. So in that limited sense, a higher f-ratio mirror will sometimes give sharper images than a faster system.
First of all, larger aperture has two benefits, namely greater resolution and greater light-grasp. So, for example, if you want to be able to split two components of a double star, you need aperture to do it. And if the stars are faint, you need aperture again, to make the star and/or its components visible in the first place. All other things being equal, more aperture is always a good thing. But of course, other things (especially price) never are equal.
The focal ratio determines the image scale of the particular objective. A high ratio makes for a larger image - and therefore also a faint one - while a lower ratio makes for a smaller, more concentrated, and therefore brighter image. That's why fast systems (low f-ratios) are good for extended objects like nebulae and galaxies.
Again, this is 'all other things being equal' - and in this case, the thing which is not not equal is the ease of figuring faster and slower mirrors. It is much easier to make a sufficiently accurate f 12 surface on a mirror than an equally accurate f surface. So in that limited sense, a higher f-ratio mirror will sometimes give sharper images than a faster system.
-
Depends on the telescope. A higher focal ratio will reduced chromatic aberration - the focusing of different colours to different points - in non-achromatic and achromatic refractors, which will mean a sharper image. The former were the first kind of telescope and were often built VERY long to minimise the chromatic aberration inherent with a single objective lens. The achromatic refractor uses a two-element objective to bring red and blue light to the same focus, which cancels much of the chromatic aberration making the telescopes much shorter and more practical, but some CA still remains and it's worse with a lower focal ratio.
Focal ratio will have little effect on the sharpness of an apochromatic refractor (bring red, green, and blue all to the same focus), and none on a reflector or catadioptric telescope. All modern professional telescopes are reflectors excepting a few very specialised ones (there's a refracting solar telescope in use).
Focal ratio will have little effect on the sharpness of an apochromatic refractor (bring red, green, and blue all to the same focus), and none on a reflector or catadioptric telescope. All modern professional telescopes are reflectors excepting a few very specialised ones (there's a refracting solar telescope in use).
-
http://www.astronomics.com/main/definiti…