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I'm wondering how do you work out the Maximum Magnification (or Optimum) for your telescope.

I have just bought the Sky Watcher Evostar 150 Ed Pro and am planning on which Ep's will suit the scope best.

Is there a formula for working this out?

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20 minutes ago, Soligor Rob said:

I'm wondering how do you work out the Maximum Magnification (or Optimum) for your telescope.

I have just bought the Sky Watcher Evostar 150 Ed Pro and am planning on which Ep's will suit the scope best.

Is there a formula for working this out?

The formula is  50 per inch of aperture or 2x per mm of aperture. A 150mm telescope would therefore have a practical maximum magnification of about 300x. In practice, this would only be possible under absolutely ideal conditions (ie never). 200x is probably a more realistic maximum magnification depending upon the conditions. 

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Hi Rob,

Depending on the focal length of a scope, generally, the best views can be achieved up to a 1mm exit pupil. As your scope is f8 and has a focal length of 1200mm, a 1mm exit pupil is achieved when using an 8mm eyepiece. This would produce 150x magnification (1200 / 8 = 150). However, a refractor can work at higher magnifications than this, but the exit pupil continues to reduce in size. If your eyes don’t mind a few ‘floaters’ you can go up to magnifications that result in an exit pupil of 0.5mm which, in your scope’s case, would produce 300x magnification. Most people struggle to go much beyond this as the floaters can be very distracting. However, you will not be improving detail beyond 150x... just making it ‘bigger’, if that makes sense.

Edited by Rob_UK_SE
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1 minute ago, Cornelius Varley said:

The formula is 250 per inch of aperture

Oops! I would love x1000 from my Tal! Best ever was x375 in the best seeing I've ever experienced. 

Realistically, it is seeing conditions that will determine maximum practical magnification. On one night it might only be x50, the next could be x500 (unlikely though), so buy your eyepieces accordingly.

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5 minutes ago, Roy Challen said:

Oops! I would love x1000 from my Tal! Best ever was x375 in the best seeing I've ever experienced. 

Realistically, it is seeing conditions that will determine maximum practical magnification. On one night it might only be x50, the next could be x500 (unlikely though), so buy your eyepieces accordingly.

That should have read x50 per inch.

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7 minutes ago, discardedastro said:

depend a lot on what you're observing

Specifically, using higher magnifications on bright objects (like planets) with a faster achromat will give you some chromatic aberration (colour fringing). Although yours isn't as fast as some others, and the ED glass apparently controls the CA quite well.

I have a 150mm reflector so my theoretical maximum is around 300x as stated above. Most of the time I find the maximum reasonable magnification is between 150x and 200x.  A 1mm exit pupil is 150x for me. On very steady nights I have pushed it a little over 200x when trying to split tight doubles, but this is unusual.

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Much depends on seeing conditions as always.

The top and bottom limits are usefully defined by using exit pupil, the max generally making sense as your maximum dilated pupil size and the minimum of around 0.5mm which is where floaters become intrusive if you suffer from them. The 0.5mm actually drives the ‘Max mag = x2 the aperture in mm’ formula.

What I tend to find is that a 1mm exit pupil lower limit makes far more sense in larger reflector or SCT scopes under U.K. skies eg a 250mm scope with a x250 max. Apply a 1mm exit pupil limit to a small apo refractor and you end up leaving a lot of unused capability. If I only ever used my FC100DC at x100 I would miss a major part of the capability of the scope.

Under excellent conditions, such as occasionally happened with Mars last Opposition, I used up to x300 with my Vixen FL102S (102mm apo) and x360 with my 8” f8 dob. 0.33mm and 0.55mm exit pupils respectively.

I guess if planning then would look for being able to use maximum mags of between x200 and x300 depending on the seeing conditions, the target and its altitude above the horizon. Of course, a range of mags below that is also useful/needed. You may also be able to go higher on nights of excellent seeing and when trying to split close doubles if you enjoy that too, but x300 is probably a useful indication of maximum.

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