Maximum Useful Magnifiaction?

[Connor]

I've just bought a Celestron Astromaster 70AZ from amazon that should arrive Friday, and the seller says it's maximum useful magnification is 165x. The focal length is 900 mm and the aperture is 70 mm. Is the maximum useful magnification about right?

[DirkSteele]

Rule of thumb for good optics is 50-60x per inch of aperture. Seeing conditions will normally limited max magnification to around 300x regardless of aperture size.

[daz]

It's usually quoted at 50x per inch of aperture, so, somewhere between 130x and 150x

The UK doesn't usually get good enough seeing to go much beyond that!

[daz]

garn! type faster Daz!!

[Cornelius Varley]

Practical (useful) maximum magnification is usually calculated as about 50x per inch of aperture, in reality in the UK this usually comes down to aboUt 30x depending of the atmospherics. For a 70mm this figure would be about 138x mag at 50x per inch or about 83x at 30x per inch. Manufacturers usually tend to exaggerate the maximum magnification - there are a lot of cheap telescopes which claim 675x magnification.

Peter

[Connor]

Thanks guys

[michael.h.f.wilkinson]

twice the aperture in mm is simpler (sometimes metric helps ) 70mm -> 140x max.

[Capricorn]

I wouldn't bet on anything over 100x being of much use.

Always seems odd that we say 2xaperture in mm, but do not take into account achro, ED or apo triplet.

If an apo triplet operated the same as an achro we wouldn't pay out for the triplet would we.

[michael.h.f.wilkinson]

The above makes the assumption that the scope is diffraction limited for its aperture, and it makes assumptions on the visual acuity of the observer. Achro or triplet is actually irrelevant if you do not take focal ratio into account. An F/15 achro can be pin-sharp on planets. I think it was Abbé who came up with the 2x aperture in mm rule, and optimum magnification of 1x the aperture in mm. People have commented that his visual acuity was probably very good, as he claimed no more detail appeared beyond the optimal magnification. For most of us, this does not hold, and about 1.5x the aperture is useful. Above that, it depends on the objects, and it always depends on sky conditions.

[LukeSkywatcher]

twice the aperture in mm is simpler (sometimes metric helps ) 70mm -> 140x max.

Thats the rule i use, but it really is all down to local seeing conditions also.

In reality, when you buy a scope and it says:

Maximum Useful Magnifiaction = xxx

You really can take that with a large pinch of salt. You may get the advertised Maximum Useful Magnifiaction stated if you live in a dark sky location with no light polution.

In saying this, i have a 200mm SCT which should (in theory) give me a maximum useful magnification of 400x. Maybe it can and will under ideal seeing condition. So far i have only been able to push it to between 250-300x where i live.

[michael.h.f.wilkinson]

Actually, the twice-the-aperture rule is useful in one respect: whenever a seller claims he can go beyond that (60mm scopes with 340x magnification supplied have been around), you know you should be wary (or simply steer clear!)

[Mr Spock]

I always feel the point where things just start to get bigger instead of showing more detail is about x1 per mm. So the optimum magnification for a 200mm would be x200 and a 250mm x250.

Beyond x1.5 per mm the image gets dark and loses contrast.

The only time x2 per mm and beyond is useful is for splitting double stars.

[Carbon Brush]

Then of course if you are looking at groups of stars, or galaxies (bright enough to see) you may well choose magnification in the tens for a good view, rather than 100+. Only going for high magnification on planets or the moon.

Maximum magnification, whether theoretical or claimed on the box, is not really important. Generally if the scope is decent quality, on many nights, the sky quality limits magnification before the glass.

[acey]

The rule-of-thumb for maximum magnification is based on the optics of the eye, not the optics of the telescope. The first assumption is that the telescope is diffraction limited, i.e. top quality. The assertion is then that if the exit pupil is less than about 0.5mm then diffraction effects in the eye will dominate, so higher magnification is not useful.

Exit pupil E is aperture A divided by magnification M (E = A/M).

Hence maximum magnification is aperture divided by minimum exit pupil (M = A/E).

If E = 0.5mm and A is in mm then this is equivalent to 2 times aperture in mm.

If E = 0.02 inches and A is in inches then it is saying 50 times aperture in inches.

The figure of 0.5mm or 0.02 inches is a convention: some users may find benefit in smaller exit pupils. Try looking at stars with the naked eye through a pinhole smaller than 0.5mm, or just try bumping up the power on your own telescope, and see how small an exit pupil you can use. 19th century double-star observers frequently went below exit pupil 0.02 inches, hence the frequently stated 50-60 times aperture, which would be going down to an exit pupil of 1/60 of an inch = 0.42mm.

The assumption is that the telescope is diffraction limited: an entry-level scope probably isn't, and aberration in the telescope may dominate before diffraction in the eye does. The other assumption is that the air is steady enough to make such high magnification useful. In practice you just find out what the limit is for your own scope, site and eye.

[Moonshane]

I'd agree with around 1.5x aperture in mm generally. seeing governs the reality of the situation and as said above is often poor in the UK generally and made even worse by central heating plumes and warm rooftops. astronomy is more about power (i.e. light gathering) than magnification.

that said I believe the usable magnification depends on the object. even with larger apertures, different objects will 'allow' different magnifications. e.g. Jupiter often allows lower magnications (maybe 100-120x) than say Mars (200-300x) or Saturn (usually 150-200x). the moon often allows very high magnification when the seeing is good although you rarely see more detail at higher than maybe 250x with a 6" scope. double stars can allow very high magnification with even small scopes as they are very high contrast and you are not looking for detail, just a split generally. on this latter point, I have used my 6" f11 newt at 533x on the moon and double stars which is theoretically 'impossible' yet the views were superb on a few nights of seriously good seeing.

I also routinely use my 16" newt (masked to 170mm) at 400x plus on double stars and even up to 500-600x on occasion which again is not sensible yet works.

don't be afraid to push your scope if you have the eyepieces and you'll find that every now and again you'll be surprised.

[Naemeth]

Usually, pushing above 50x per inch (2x per mm) is only possible on the Moon or double stars, but you will find on all but the best nights, the Moon doesn't appear any clearer (you may even lose detail), but sometimes that extra magnification is needed to split double stars (of course, the view won't be pretty). In my experience anyway, it's a pain to keep even the Moon in view at 325x on a Heritage 130P (2.5x per mm or 63.7x per inch)... fun to try though!