Telescope Power

[M4lcs67]

Can anyone remind me what the calculation is for working out the usable power of a scope? I know you divide the focal length by apeture, but do you do this purely for the tube or do you add the ep in somewhere????

Also how do you know what the usable power of a particular scope is before you just see nothing but a blurr. Short of trying out various ep combinations and seeing for yourself I just wondered if there was a rule of thumb that gave the answer?

[ChrisThePuma]

if by power you mean magnification then you divide the focal length of the scope by the focal length of the eye piece.

so if you have a scope with a FL of 1000mm and use an eye piece with a FL length of 10mm you get a magnification of 100x

[Merlin66]

Divide the focal length of the main mirror/ objective by the focal length of the eyepiece to get the magnification.

Top magnification will vary with seeing conditions, usually around x200 in the UK...tops is around 50 x aperture (in inches)

HTH

[LukeSkywatcher]

The highest usable magnification of any scope is usually included in the specifications of the scope online when you buy or are researching.

Example:

For my scope the 8" SCT, the recommended highest usable magnification is 480X. Given the weather here in Ireland and the UK in general, there is no chance of ever getting that kind of magnification out of the scope. At very best it would be half of that............if even.

[ChrisThePuma]

Checking your scope online it has a FL of 1500

with your eye pieces you get the following results:

25mm ep = 60x

9mm ep = 166x

double those with your barlow.

[rwilkey]

Maximum useable magnification of the scope for a rule of thumb is twice the aperture, so a 200mm (8") scope will yield 400x. In my view the optimum magnification for any scope is = to the aperture, so for the 200mm objective you are talking 200x.

[LukeSkywatcher]

Maximum useable magnification of the scope for a rule of thumb is twice the aperture, so a 200mm (8") scope will yield 400x. In my view the optimum magnification for any scope is = to the aperture, so for the 200mm objective you are talking 200x.

I tend to agree. A 200mm scope is built to do 400X but that is in extreme good "seeing" conditions. Unless you live in the Attacama or somewhere similar you will never really get 400X.

Scopes have their "comfort zone" and if you are pushing the scope above that it will very quickly become apparent to you because the image will degrade in leaps and bounds.

How much you can push the scope all depends on local conditions. You cant ignore the maths of the scope though. If your scope specs say max usable mag = 300x................take that with a pinch of salt and stay below that number.

[M4lcs67]

Thanks guys for clearing that one up. Just needed it re-confirming.