Max Magnification

[fauxpas (Tony)]

I saw a tutorial stating (generally) the maximum magnification of a scope was twice the aperture in mm... so a 130mm aperture would be a max of 260x.

Then I also heard that with amateur scopes, because of the atmosphere, there's a maximum practical magnification of around 250x.

Is this all hogwash?

[barx1963 (Malcolm)]

Originally Posted by fauxpas

I saw a tutorial stating (generally) the maximum magnification of a scope was twice the aperture in mm... so a 130mm aperture would be a max of 260x.

Then I also heard that with amateur scopes, because of the atmosphere, there's a maximum practical magnification of around 250x.

Is this all hogwash?

Tony
A very rough rule of thumb for maximum "theoretical" magnification is 50x per inch of aperture, which is not far of the 2x per mm of aperture. So a 130mm scope which is 5" gives 250x. The critical word is theoretical!
It assume very good to perfect optics, perfect alignment and collimation and above all, perfect seeing (a perfectly still atmosphere). A better bet with smaller scopes is more like 10x to 15x per inch (50x to 75x in a 5"/130mm scope) On some occasions it can be pushed beyond this, but I never managed satisfactory results, I never managed more than 90x in my 130mm scope, or at least the results were very ordinary!

With your second point, yes in most every use, 250x is pushing it even with a big scope. I do most of my observing in my 20" at 150x or 195x and find that does the job 9 times out of 10. Occasionally I slip in a 10mm EP for 254x but usually I am disappointed.
Having said that, when conditions are good, nothing beat cranking up the power on Eta Carina, Jupiter or the Trapezium!!

Malcolm

[Wavytone]

Tony,

The maximum differs for most reflectors vs refractors:

- for reflectors, 1.5X per mm of aperture;
- for refractors 2X per mm of aperture.

And yes you are reasonably correct in suggesting 200-250X is about the upper limit for most scopes if the seeing is average ("average" seeing being about 1" of arc).

With the right night, the altitude of the object above the horizon, and your location you can do better, of course; though for most amateurs these nights are about 1 or 2 a year.

[Blue Skies (Jacquie)]

Originally Posted by Wavytone

And yes you are reasonably correct in suggesting 200-250X is about the upper limit for most scopes if the seeing is average ("average" seeing being about 1" of arc).

With the right night, the altitude of the object above the horizon, and your location you can do better, of course; though for most amateurs these nights are about 1 or 2 a year.

Ditto to that. Yes, you can push past 200x but its rarely worth it - except on those rare nights of rock-steady seeing! They don't come around that often, unfortunately. When they do, you'll remember them for a long time after, too, because they're just so amazing compared to what we normally get.

[doppler (Rick)]

Originally Posted by Blue Skies

Ditto to that. Yes, you can push past 200x but its rarely worth it - except on those rare nights of rock-steady seeing! They don't come around that often, unfortunately. When they do, you'll remember them for a long time after, too, because they're just so amazing compared to what we normally get.

Those nights are rare and memorable. Last year I was observing saturn through the 10" newt and thought gee thats sharp tonight went down to a 6mm eyepiece still sharp, dropped in a 2x barlow and wow what a view at 400x.
Rick

[wasyoungonce (Brendan)]

Max magnification...rough guide, 75X~ 125X per inch aperture.

Problem is that as you magnify the image your scope captured (with it's objective & given Focal length), it becomes spread across a larger image size and thus becomes dimmer, more difficult to see details. You reach a point of diminishing returns.

Couple this with atmospheric disturbances.

You can get away (with the right atmospheric conditions) higher magnification of planets but Deep Space objects are dimmer by nature thus you need less magnification. Also wide field low magnification is more pleasing to view.

FWIW.

[ralph1]

I also use a 130mm scope, a reflector, and can use 216X on 9/10 nights with the loss of image quality outweighed by the increased image scale. At 108X the image is almost always sharp, bright and steady. On 50% of nights, higher magnification could be used if I had the necessary eyepieces. As mentioned before, altitude is also critical as is collimation. An object that looks great 70+ degrees up will be terrible just above the horizon.
Ralph

[SkyWatch (Dean)]

Interesting that Televue say on their website: "Tele Vue recommends maximum magnification of 60x / inch (2.5x / mm) of telescope aperture and 350x maximum regardless of aperture."
- and that is for premium refractors! A lot of commercial scopes simply don't have good enough optics to go that high.
On a practical note, once you start to get an exit pupil smaller than 0.5mm (ie. a magnification of 2x per mm or about 50x per inch) you are really starting to battle with "floaters" and what your eye is capable of (not to mention needing a better than "diffraction-limited" scope!).
I have taken my TSA102 to 270x-300x, but the seeing has to be just about perfect, and generally I get a sharper view with better contrast at around 200x.
Having said that, I remember using an ETX125 at Arkaroola, looking at Saturn at 500x in sub-arcsecond conditions: and the view was crisp and contrasty. I suspect it was a freak scope in freak conditions!

- Dean

Footnote: I just conducted an experiment. The seeing is quite good tonight, and I just looked at the Trapezium in M42 with my Tak TSA102, and a 3-6mm Nagler zoom. This gives magnifications of 136x to 272x. The images were sharp at each step, with text-book-perfect star images all the way. I was able to easily see the "E" and "F" stars with direct vision at 136x and 163x. Not so easy at 204x, and difficult at 272x; better with averted vision at both 204x and 272x. This shows the effect of contrast reduction as the power is increased, with the best, highest contrast images at 136x and 163x, or 1.33x and 1.6x per mm (or 34x and 41x per inch) respectively...

[barx1963 (Malcolm)]

Originally Posted by wasyoungonce

Max magnification...rough guide, 75X~ 125X per inch aperture.

75x per inch? That means for my 20" I can go to 1500x?? and at 125x per inch...hmmm 2500x??

Really?

Malcolm

[wasyoungonce (Brendan)]

Originally Posted by barx1963

75x per inch? That means for my 20" I can go to 1500x?? and at 125x per inch...hmmm 2500x??

Really?Malcolm

Well...I cannot help it if the optics are not up to this.....

It was a brain fade moment...should have referred to the TV site on this.

[Asterix2020 (Paul)]

All good advice so far. But don't be afraid to ti higher mag if it seems a good night - if you don't try you won't know. One night at Astroron's it was incredible stable. On Saturn with Ron's 16.5" scope we got to 500x I think before it became a soft. On my 12" Dob, I had a Pentax XW 5mm with a 2.5x powermate giving 750x. My notes say, "Cassini division all the way around, Encke division visible. Some banding visible. 6 moons, Enceladus, Mimas (just) Dione, Tethys, Rhea, Titan visible."

It was an exceptional night though. The image was only a little soft at 750x, but it was quite useful still. I had collimated at the start of the session (SW Flextube so I always check it after travelling), which probably helped quite a bit.

[astroron (Ron)]

I agree with Paul, I really don't think there is a hard and fast rule you must stick too.
On the night of Anthony Wesley's discovery of the Jupiter impact nearly 7 years ago.
I and a number of people were observing Jupiter in my 16" through mist at 700xmag and the detail was extra ordinary.
The Oooos and Arrrs coming from the other observers was testament to the magnificence of the apparition through the eye piece.
I am sure we could have upped the magnification but at the time I didn't have any eyepieces to do that,I do now.
BTW if we had observed Jupiter for about another half an hour we would have seen the "Bird Strike" visually.
Cheers

[PSALM19.1 (Shaun)]

Hi Tony,
I think the 50x guide is reasonable and not quite in the hogwash category! I have had a few (and ONLY a few!) memorable nights with Jupiter at 400x in my 8" Dob...they were fantastic - good enough even for a half decent I-phone 5 photo! But mostly, 200x is much more pleasing...that's with Jupiter...not every planet, DSO is the same of course, but I find my 6mm EP is lovely for Jupiter, Saturn, Uranus and Venus...and the Moon of course! Sometimes I'm very naughty with the moon and up the mag to 800x...and yes, on those rare occasions, it looks great! But then I peg it back to 200x and you know what? More pleasing than a great big 800x moon 99% of the time! I guess the answer then is, just experiment and take it night by night! I usually use my 6mm EP and my 32 because they cover most bases and are decent EP's. My others come out on rare occasions!

Keep pointing up!