Highest useful magnification?

[Jim Smith]

I have heard of a general "rule of thumb" that there is a practical maximum limit of 50x magnification per inch of aperture.

For visual astronomy, even if you have an aperture greater than 5", does anybody find that they can see more detail by going over 250x ?  Or do atmospheric conditions make pushing magnification higher a waste of time?

[John]

The seeing conditions are the big factor in how much magnification is useful. Then comes the state of cooling and collimation of the scope and the target being observed makes a difference as well.

My largest aperture scope is a 12" which is of good optical quality but I've never used 600x with the scope on any target. Under decent conditions 265x or 318x is my more usual max with that scope on targets such as Jupiter, Saturn or Mars and sometimes 400x is good on the Moon. For smaller apertures proportionately lower magnifications prove to be the upper limit. With my 4.7" refractor 225x or 257x is as far as it's useful to go although 300x is sometimes useful on really close binary stars under good viewing conditions.

These are just rules of thumb though and practical experience. There is no harm in trying something - the worst that would happen is a fuzzy image !

[Charic]

Jim Smith......Hi, On a bright target like the Moon, I often view at  over 300x but DSO's will be harder to view due  to the constraints of  seeing and transparency. If you take your rule of thumb, or just double your aperture, that is your theoretical limit. What you can achieve up to that  limit is governed by the weather and the telescopes set-up.

[ronin]

50x magnification per inch of aperture will deliver an exit pupil of 0.5mm after which you get problems of the eye coming into everything as a factor.

I would think that what has been said is below an exit pupil of 0.5mm the eye is the limiting factor so greater magnification is pointless. And to get an exit pupil of 0.5mm means 50x dia in inches.

However that does not mean that a scope will deliver an exit pupil/magnification of 0.5mm with a good image, so the scope may simply be incapable of delivering the required magnification.

[Mr Spock]

I find the best balance of detail, sharpness and contrast for planets comes around x1 per mm; generally that balance is limited by seeing conditions for larger scopes. That's why there's this 'mystery' sweet-spot for planetary scopes somewhere between 200-250mm aperture. Larger scopes are capable of higher resolution but are limited by seeing conditions most of the time. x250 is a maximum most nights - combine that with an ideal x1 per mm and you get a scope of 250mm.

Above x1 per mm can be used, but only in the very best seeing conditions. The image will get larger, but also dimmer - not always a good trade on planets. Stars are different and doubles can take quite high magnifications.

[Stu]

I would say the potential is certainly there with larger scopes to go to higher magnifications, but it is often limited by the seeing conditions.

With my smaller refractors, I am often pushing to 0.5mm exit pupil on planets or doubles to get the magnifications needed to pull out detail, but with these that still means being below or around the x250 mark. Floaters are an issue but I have to look around them! I have a very annoying one right in the middle of my observing eye which really interferes on planets.

Under good seeing, larger scopes can be pushed well beyond x250, it becomes a game of waiting longer for the moments of good seeing.

I have had the odd occasion where nice used x300 or even x400 on Saturn and the image has stayed sharp but these are rare.

I find that different planetary targets take different magnifications, Jupiter has low contrast features so tends to get washed out if you push the mag too high. Saturn has high contrast features and so takes mag well, and Mars is small so needs magnification to show up the detail. Doubles you can push very high.

I think it should be said that even at x250, the larger scope will show more detail because of the additional resolution available so there is still benefit to be gained from the larger aperture.

Stu

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[Dreamweaver]

For what it's worth I have been enjoying Mars whenever I get the chance using my 80mm SW APO with 2.5mm Nagler the magnification being 200x.  I get better resolution and detail using this set up rather than using the lager much heavier LX10, 8" SCT with a 7mm ep yielding 285x magnification!

I would rather have a slightly smaller better quality image to look at than a bigger less detailed one. 

[Jim Smith]

Thanks everyone.

My current maximum magnification is x180 in my 120mm refractor.  I might try getting a new eyepiece to push that up to somewhere in the 220x to 250x range.

[Tiki]

I use 216x, 154x and 110x with my 4" refractor. I often try 216x on the planets but it is only really of benefit a tiny proportion of the time.

[acey]

I have heard of a general "rule of thumb" that there is a practical maximum limit of 50x magnification per inch of aperture.

For visual astronomy, even if you have an aperture greater than 5", does anybody find that they can see more detail by going over 250x ?  Or do atmospheric conditions make pushing magnification higher a waste of time?

The rule of thumb is very old (19th century or older) and really means a minimum exit pupil of 1/50th of an inch (about 0.5mm). This is imposed by the optics of the eye itself, but everyone's eye is different so it's not hard and fast.

The atmosphere imposes a limit by means of the "Fried parameter", r0 (a critical length scale). For aperture smaller than this the resolution is limited by diffraction in the telescope, for aperture larger than the Fried parameter it's the atmosphere that imposes the limit. In UK the Fried parameter is typically about 5 or 6 inches. The seeing disc of a star seen through a telescope is an Airy disc if the aperture is less than r0. For larger scopes the image in good conditions is a small blurry disc made of lots of tiny Airy discs ("speckles"), because of atmospheric turbulence. So in UK with a telescope bigger than 5" it's generally the atmosphere that dictates things, though that doesn't mean you can't go higher than x250.

Visual observation at observatories such as Mount Wilson was done with magnifications of over 1000. At Mount Wilson the seeing is typically about an arcsecond, better than you can expect in UK. But Herschel also used powers over 1000 (with an 18.7 inch telescope in UK) and very high powers were also used on the 72" Leviathan in Ireland. Those observers made use of brief moments when the air became steadiest (which is what planetary observers still do).

In practice you just start low and work up through available magnifications until nothing more is gained. A similar procedure is advisable when buying eyepieces - don't rush out and get something that might provide uselessly high power. Work up gradually and find your own preferred limit, with very high power kept in reserve for the few occasions when you might actually need it.

Currently with my 12" f4.9 I have a 4mm Nirvana as my highest power eyepiece - exit pupil 0.8mm, magnification x375. I use it on DSOs (small faint galaxies) when lower magnification doesn't suffice - and on planets when I occasionally look at those. But I usually find my Baader Hyperion 8-24mm zoom (top magnification x187.5) goes as high as I need.

[RichM63]

In practice you just start low and work up through available magnifications until nothing more is gained. A similar procedure is advisable when buying eyepieces - don't rush out and get something that might provide uselessly high power. Work up gradually and find your own preferred limit, with very high power kept in reserve for the few occasions when you might actually need it.

Even as a relatively 'new to half decent' telescope user, I couldn't agree more.

All the retailer hype is about xxxMagnification with no mention of Seeing conditions that we all suffer from.

When I decided that I could afford to purchase an EP my question was the exact opposite, wanted a wide FOV with a decent exit pupil and sensible TFOV to get as much of the area as possible.

A member either here or elsewhere supplied this Excel EP calculator (sorry can't remember your name)  but it is great for punching in EP scenarios.

EP_Calc_v2.xlsx

Cheers,

Rich

[Charic]

Al Nagler once said "Frame the subject properly...

This was good advice for me, and the reason I invested in a set of EP's rather than the advisory 1x High 1x Medium & 1x Low.

Whatever target I have, I will use the full range of EP's rather than try to second  guess what I should need, then whichever looks the best is good enough for me. 

[RikM]

I also go with the 'generally best at about 1x per mm' thinking. A nice 5mm eyepiece gives me 240x in my 250PX. This is quite often usable. I also have 4mm (300x) and 3.5mm (343x) available  and these do get a look in but more rarely. I use these for planets, the moon, doublestars, globular clusters, planetary nebulae and even some small galaxies. I don't think of them as 'planetary' eyepieces at all.

[Stu]

I also go with the 'generally best at about 1x per mm' thinking. A nice 5mm eyepiece gives me 240x in my 250PX. This is quite often usable. I also have 4mm (300x) and 3.5mm (343x) available and these do get a look in but more rarely. I use these for planets, the moon, doublestars, globular clusters, planetary nebulae and even some small galaxies. I don't think of them as 'planetary' eyepieces at all.

I completely agree with that for larger scopes Rik, as you say 1mm is a good minimum.

For decent quality, but smallish apo refractors I think you can, and need to, push it further to get the best out of them on planets and doubles.

The optics are generally capable of taking the magnification, the limiting factors are the eye and the seeing conditions but as I have said, going down to 0.5mm on these scopes does have benefits if you can live with the floaters!

Stu

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[jetstream]

After sorting out my refractor it is taking 40x-50x in. aperture very often,with a sweet spot of about 44x (aperture).My 10" reflector has very rarely used 40x aperture(400x),with 30x(300x mag) being a useful maximum.As has been said the 1x per mm magnification is a great mag to use in bigger scopes on lunar/planetary.

[spaceboy]

I have an app on my phone https://play.google.com/store/apps/details?id=com.blackdot.telescopecalculator which recommends lowest and highest magnifications for any given scope. This also seems to show a trend for highest magnifications as x1 magnification for every mm of aperture. I don't think this is so much a recommendation based on the full potential of a scope but more so on a minimum exit pupil of 1mm. This makes sense to me as is it not the light reaching your eye that determines how much detail you see? Although in my experience as in so many others, high magnification is no use to anyone if seeing and transparency is poor.

[stevend]

From my own experience; it is only on good nights that I get clarity with the 8mm BST (188x) - Moon, Jupiter & Saturn, so on that basis I see no need to push the scope, or my eyes, beyond that. I often get my best views at 125x. I think the 50x rule seems reasonable but I would suggest that buy EPs based on what you can see with your scope. eg If you can regularly get good views at 250x then consider pushing further, but I would not just drop the ££ on an EP without seeing lesser magnifications first.

[stevend]

My current maximum magnification is x180 in my 120mm refractor.  I might try getting a new eyepiece to push that up to somewhere in the 220x to 250x range.

I find the quality falls away quickly as you increase magnification (experience also with a friend's 8" scope) so I recommend you buy a 220x EP first as it will get used much more that a 250x.

[acey]

I don't think this is so much a recommendation based on the full potential of a scope but more so on a minimum exit pupil of 1mm. This makes sense to me as is it not the light reaching your eye that determines how much detail you see?

For an extended object (e.g. planet or galaxy) seen in a telescope, as you reduce exit pupil (increase magnification) the image surface brightness of both object and background are reduced (made fainter). That in itself may or may not affect visibility of detail, which is dependent on image size as well as brightness.

The 50x per inch rule (0.5mm exit pupil) is based on the fact that such a small pencil of light produces diffraction in the eye, hence a loss of visible detail. It also makes floaters more apparent. To see the effect, just look through a 0.5mm pinhole in daylight.

[Jim Smith]

acey, I think I see why floaters could be more of a problem with a small exit pupil as the light is only passing through a tiny part of the eye's lens, so any imperfections in that part of the lens would be more obvious.

But, why does "diffraction in the eye" become a problem?

[Jim Smith]

stevend, I think I might hold off buying anything for now. When I get my new 368mm f/5 scope that will give me a focal length of 1840mm. Using the 8mm setting of my zoom eyepiece will give me...let me see...230x. I'll try that before I do anything else.

[swamp thing]

Al Nagler once said "Frame the subject properly...

This was good advice for me, and the reason I invested in a set of EP's rather than the advisory 1x High 1x Medium & 1x Low.

Al Nagler would say that wouldn't he. He not got much to lose from you buying a huge collection of eyepieces has he?