viewing difference between fast and slow

[Brian W (Brian)]

Hi all, Living far from the madding crowd comes with both blessings and curses. One of the curses is that I do not get to look through different scopes.

So if there are any who can describe the difference in the view between say...an f/6 and an f/12 reflector with 12" to 14" of aperture I would sure appreciate it.

I am guessing that the f/12 would have crisper views of a smaller area?

Brian

[Robh (Rob)]

For the sake of comparison, let us assume you are using a 30mm eyepiece with 80 degrees apparent field of view.

In a 12 inch (300mm) f/6 scope, primary mirror focal length = 300x6=1800mm, the magnification would be 1800/30=60.
True field of view = 80/60 = 1.33 degrees.
At f/6 the scope length is about 1.8 metres. For most, this would be too high to view through when the scope is pointed upwards and observers will need to stand on a box. A 12 inch, f/5 scope, is a better proposition.

In a 12 inch (300mm) f/12 scope, primary mirror focal length = 300x12=3600mm, the magnification would be 3600/30=120.
True field of view = 80/120 = 0.67 degrees.
Note that the scope length would be around 3.6 metres. You would need a ladder to get to the eyepiece.

Visually, the f/12 has half the true field of view but twice the power. With the smaller field you would have less issues with coma but on the other hand, the scope is far more cumbersome to handle.

If you are looking at Schmidt-Cassegrain designs, the 12 inch scopes are far more compact but have their own inherent problems with coma and field curvature.

Regards, Rob.

[Brian W (Brian)]

Thanks Rob, I have been looking into observing from the 1800's and I wondered why they made their scopes so long. Double the power for a given mirror would seem a good reason.

Standing on the ground is also possibly one of the reasons they moved to faster and shorter?

But if I understand correctly (something my wife says rarely occurs) for detail observing slower is better with the trade off of needing some way of reaching the ep.
Brian

[Robh (Rob)]

Originally Posted by Brian W

But if I understand correctly (something my wife says rarely occurs) for detail observing slower is better with the trade off of needing some way of reaching the ep.
Brian

With two scopes of same mirror diameter but different focal length, this is true to a degree. However, with the impracticality of long focal length Newtonians, there are other factors that improve detail. The ability to resolve higher detail largely depends on aperture size. The larger the diameter of the mirror, the better the resolving power (according to the Dawes' Limit, the resolution in arcseconds is R = 4.56/D where D is the aperture diameter in inches). With wider fields of view, resolution will still be good at the center of the field but somewhat affected by coma further away from centre.
Also the quality of the optics will affect detail and resolution e.g. better mirrors and eyepieces.

Rob.