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Highest practical magnification


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Hi all!

I finally took the plunge and brought myself a half decent scope(sorry for the cloud), I've seen that it has highest practical magnification.

What happens when I exceed that magnification? (inability to focus? not enough light?) Also is there ways of increasing the max practical mag without buying a whole new scope?

I dont think I'll need to go beyond the max mag yet, I'm just curious to what happens and what options there are :)

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If you go beyond the max limit the object you are viewing will become dimmer and less sharp.

You don't need to aim for maximum magnification, I rarely go above x200 as this shows all the planetary detail there is to be seen whilst still being sharp.

In the UK the atmosphere will limit useful upper mags to around x250, however on really good nights I have been up to x400. But this is when Mars was very high in the sky - so object altitude has a bearing on max mag too.

HTH

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There are several dimensions to this, there is a theoretical limit to the resolution of a telescope, there is a limit imposed by the atmosphere - the 'seeing', and there are others. An important limit is the exit pupil.

Imagine the cylinder of light entering the front end of your telescope, the lens or mirror system reduces this to a much smaller cylinder of light emerging at the eyepiece. The diameter of the cylinder of light at the eyepiece is called the exit pupil. It is easy to calculate the size of the exit pupil - it is the diameter of the objective (the aperture) divided by the magnification you are using. So, if you have a 100mm telescope and are using a magnification of 50x, the exit pupil will be 2mm across. At a magnification of 100x it will be 1mm across.

High magnifications make the exit pupil small. The problem is that when the exit pupil is smaller that about 0.5mm it is getting similar in size to the imperfections we all have in our eyes and you will begin to see all sorts of distortions (diffraction, floaters, etc). This leads to a rule of thumb that the maximum useful magnification is equal to twice the aperture in mm (or 50X per inch). Above this you exceed the limits of your eyesight.

Here's a link to a slightly technical explanation of the limits: LINK Telescope Function

Edited by SimonR
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Another point: I would bet that the highest practical magnification quoted is beyond the scopes capabilities. In many cases it is a number on the box that is governed more by marketing then by optical capabilities.

What scope have you? That would help to be able to give an answer.

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Another point: I would bet that the highest practical magnification quoted is beyond the scopes capabilities. In many cases it is a number on the box that is governed more by marketing then by optical capabilities.

What scope have you? That would help to be able to give an answer.

Its the skywatcher 130P. Right now the max mag I could do is 130, 10mm EP with x2 Barlow. As you say the salesman would say the max practical mag is x260 because that's what it says on the box.

I'm quite happy with the magnification I have at the moment I'm just looking to the future.

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I rarely go above x200 as this shows all the planetary detail there is to be seen whilst still being sharp

Definitely agree with that. On rare occasions it may be worthwhile

to go higher, perhaps to split a very close double star, or Mars/Saturn

will sometimes be good at very high power, but does not happen often.

Max mags for scopes sometimes quoted are theoretical, and don't

take into account the state of the atmosphere.

Regards, Ed.

Edited by NGC 1502
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Two common misunderstandings are that a telescope is all about magnification and you need as much as possible to do astronomy. A telescope is primarily a light gathering tool which is why we go on about aperture so much - more light is better !. Much astro viewing is done at low to medium magnifications (say 30x - 100x) and many astronomical objects are surprisingly large. It's planets and double stars where you do need to enlarge the image and thats where it's useful to know what the practical maximums are for your equipment and your observing conditions - the latter being the real limiting factor.

I think you will find 160x or so much more useful than 260x on 99% of viewing occasions.

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I have the Explorer 150P and I regularly find that 150x is about the best for planets. I have EP/Barlow combinations that will give me 170x and 230x, but more often than not the view at 150x is sharper and the contrast is better.

Rik

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Completely agree with the above comments, magnification is mostly limited by the atmosphere, and magnification is not, generally, what it's about. I was trying to answer the original question directly - "What happens when I exceed that [theoretical maximum] magnification? ... I'm just curious to what happens..."

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Many talk about the seeing as either 'bad' - restricting max mag to less than 200X - or 'good' allowing much greater powers but I find on nights of bad seeing there are usually brief moments when the atmosphere steadies so it's still worth using high mag. The are 5 minute sessions when I never see any clarity but trying 1 hour later & conditions are sometimes better.

I use 300X for the planets regardless of the seeing conditions which gives me 1mm exit pupil (335mm aperture reduced to 300mm by a mask).

I use 443X (with no mask) giving 0.75mm exit pupil - this seems about right for observing double stars enabling the closest possible to be seen which usually occurs momentarily on most nights.

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Seems to me like the general rule is 1x mm of aperture for 'best' views on 'typical' nights,

e.g 150x in my 150mm, 200x in a 200mm, 300x in a 300mm.

and up to 2x mm for 'max' mag occasionally. I guess we all need to remember that 'maximum' practical magnification will only be achieved either fleetingly on some nights or for longer periods only on rare occasions.

Rik

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