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Hi all, I'm an astronewb looking for advice :)

On the websites of many telescope vendors I see this term "Highest Useful Magnification" used and I've been wondering about how strictly to interpret that.

For example, I see a scope with a focal length of 2032mm. Does this mean that I could fit it with a 2mm eyepiece and get a magnification of 1016x?

I'm mainly curious about the image quality at that level. Like for the scope with the 2032mm focal length they claim a Highest Useful Magnification of 480x. Which I'm guessing means one can't get decent images with an eyepiece smaller than 4.50mm or 4.25mm?

I'd like some clarification on "decent" :icon_scratch:

Like I realize extremely high magnification levels come with certain temporal issues... planets would just be racing across the field as if they were in the indy500 lol... but I'm more curious about image quality issues... what happens when you go beyond the manufacturer's Highest Useful Magnification? Is it just a blurry mess or something?

As images speak louder than words I'd be very appreciative if any of you could post the actual results you obtained by fitting a long focal length scope with an eyepiece that had a very small focal length like 2mm or so, in the form of a .JPG or .PNG attachment or what have you. Even if the images are just blurry mess I'd still like to see them because I'm curious just what form the distortion takes...

Thank You

Edited by dogon512
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SOme mor9e

This is the highest visual power a telescope can achieve before the image becomes too dim for useful observing (generally at about 50x to 60x per inch of telescope aperture). However, this power is very often unreachable due to turbulence in our atmosphere that makes the image too blurry and unstable to see any detail.

On nights of less-than-perfect seeing, medium to low power planetary, binary star, and globular cluster observing (at 25x to 30x per inch of aperture or less) is usually more enjoyable than fruitlessly attempting to push a telescope's magnification to its theoretical limits. Very high powers are generally best reserved for planetary observations and binary star splitting.

Small aperture telescopes can usually use more power per inch of aperture on any given night than larger telescopes, as they look through a smaller column of air and see less of the turbulence in our atmosphere. While some observers use up to 100x per inch of refractor aperture on Mars and Jupiter, the actual number of minutes they spend observing at such powers is small in relation to the number of hours they spend waiting for the atmosphere to stabilize enough for them to use such very high powers.

This image gives you an idea I found somewhere earlier


e.g. the other day I was viewing jupiter at 120x, and with a 2x barlow it was just too blurry

2mm would be a miserable experience

Edited by coffee_prince
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Hi , max magnification as a rule of thumb is 50x apeture in inches, ie 10" mirror 500x.

Any higher and no more detail is seen only a bigger fuzzier image.

However in the real world with bad skies and rising heat from roof tops. A better bet would be 25x per inch.

I use 80 through to 250x most on a 10" newt and only go higher on rare occasions.

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In the UK, seeing conditions only allow up to 200x, but the safe option although rule of thumb is twice the aperture (200mm objective = 400x), I say stick to the objective as guide for seeing conditions in the UK, ie. 200mm = 200x for comfortable viewing. I see no benefit in ramping up the magnification, I prefer quality.

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Same conclusion here. The typical best high mag views I get are with a 5mm EP. This gives me 150x in my 150P or 240x in my 250PX. The Saturn image above is a fair representation of what you see when when you use too high magnification.

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Agreed. The highest mag. is around 200x under typical UK conditions. I have pushed my Mak-Cass to around 300x with the very best of seeing conditions. It works very well but only with exceptionally stable air. I was able to get a very clean and clear image of the Double Double recently with a 6mm Plossl in this 1800mm focal length scope.


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Sometimes seeing conditions can be misleading. Last week I was looking at Albireo at x107 (22mm Nagler) and it was very fuzzy, moving up to x138 and x181 just made it worse. The same was true on other targets. However, on Eps Lyrae x392 had a slow shimmer but a clean airy disk and a single broken diffraction ring - just what I'd expect to see with well collimated high quality optics on a good night.

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The main thing here is to try progressively higher magnifications till the image degrades enough you don't want to use it. In typical UK seeing you'll find this max's out around 200x-250x on average for most scopes that are capable of it. Times when 300x and above are possible, are few and far between. :)

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the other day I was viewing jupiter at 120x, and with a 2x barlow it was just too blurry

That is a damn strange looking Jupiter.

What scares me is that no-one seems to have pointed it out.

What scope is it that you have, that makes a difference. It is also useful to know what you intend to look at or tried to look at.

Somewhere I read that the best image comes when the magnification was equal to the diameter in mm. In easy terms you use an eyepiece of the same focal length as the f number of the scope. Whoever posted the detail did supply the maths behind it and it made sense.

After that I suspect that yes the magnification goes up but by an increasing amount the image quality goes down.

Somewhere you end up with (as above) a big but blurry image. Then it is dependant on what you consider most important:- a large image of poorish quality or a small image of good quality.

Edited by ronin
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