ep equation ??

[hemihaggis]

hi there does anyone know the proper (rough) equation for working out the highest and lowest usable power eyepieces

the one i know is this ...mutiply your f number by 8 ie.. f5 x 8 = 40 (add decimal point 4.0) ...so the highest mag ep you could use in an f5 scope is around 4mm

any idea how to work out the low power ep ??

cheers

[ollypenrice]

Hmmm, what's going to impose a lower limit, I wonder? The exit pupil can get too big. Over 7mm it won't go in your eye and it might be nearer 5mm for some. You might start seeing effects from the secondary in a reflector. I don't think there is a lower limit in the same way that there is an approximate upper one but I might be wrong.

Olly

[John]

I think the lowest useful power is often quoted as 3x or 4x per inch of aperture but I could be wrong with that. This web page has some good information on all this:

Starizona's Telescope Basics

[hemihaggis]

cool thanks john and ollie , i think that was the equation i was looking for its somthin like the size of your pupil x your f number , so a 6mm pupil x f4 = 24mm .... i think , bern from modern astronomy told me this once , but i kinda forgot lol cheers

[marcopolo]

Upper limits:

reflector approx x40-50/inch

entry level refractor x30-60/inch

high quality refractor up to x100/inch

This is all seeing permitting of course.

Most nights i'm stuck at x20/inch in bonnie Scotland.

[hemihaggis]

thanks marco , but the low power eypiece is also dictated by your age and the size of your pupil , as you know this will change with age !!

[John]

As a 50 year old, I try and aim for a maximum exit pupil of around 5mm.

[ollypenrice]

As a 50 year old, I try and aim for a maximum exit pupil of around 5mm.

As a nearly 60 year-old I first try to find the eyepiece...

Olly

[Mr Spock]

I'd work on the exit pupil.

For a high magnification, 1mm. You wont nornally see any more detail above this - more magnification just makes things bigger.

For low magnification, 7mm to 5mm depending on your age and dark adaptation. For me it's 5mm...

[cs1cjc]

Exit pupil = focal length of eyepiece / f-number of scope

Exit pupils of less than 0.5mm generally show no more detail except for the floaters in your eye (and some would put it higher at perhaps 0.7mm), and have little use apart from perhaps splitting doubles.

Exit pupils above perhaps 5mm to 7mm means that you are "wasting light" so not getting the full benefit of your telescope aperture, but can usefully give you a wider view. With a reflector the shadow of the secondary does put an absolute upper limit, but there is no real upper limit for refractors. It is common for example to have an exit pupil of perhaps 4mm on binoculars, but noboy thinks they are wasting light when they use such binoculars when their pupil in much smaller than this, in daylight.

There is a good article on this here:

Tele Vue Optics: Choosing Eyepieces

Under typical British shies, there is an upper useful limit imposed by the quality of seeing typically at perhaps 200x, but of course one is always waiting for those nights when you can push things up to 300x!