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"best" per-inch magnification for Moon?

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#1 cpsTN



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Posted 10 September 2013 - 08:37 PM

Over the years, I have heard and seen statements of this or that magnification range being "best" for observing Jupiter, Mars, etc. Is there a "best" magnification per-inch of aperture for observing the Moon; one that uses the "best" balance of LGP and resolution? I would say this will be subjective because the Moon shows much more detail and contrast than other observable objects.

#2 David Knisely

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Posted 10 September 2013 - 09:17 PM

I generally like between 35x per inch of aperture to 40x per inch as the "upper end" of my powers for viewing the moon (although it can look pretty darn good at lower powers as well). In my binoviewers, I like anywhere from 20x per inch to 35x per inch in my 9.25 inch SCT. Much beyond 40x per inch, the image is larger but there is little additional detail that is revealed. Indeed, the light density at powers well beyond 40x per inch is declining rapidly (along with the size of the exit pupil). This makes things like floaters a lot more annoying, and the view seems a little softer than at lower powers. I occasionally have used more power than that on the moon (up to 60x per inch, mostly in fairly small scopes), but usually I like to stay at or below 40x per inch. Clear skies to you.

#3 GlennLeDrew


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Posted 10 September 2013 - 10:11 PM

There is no one 'best' magnification per unit magnification.

#4 azure1961p


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Posted 10 September 2013 - 10:42 PM

An open ended question like that has many answers and they are all valid. Dave pointing out 35x per inch is a good rule of the for seeing the finer features like rilles, catena- crater chains, summit craters on domes, craterlets in Platos floor. Its a good rule of thumb for a productive working magnification when the seeing allows without being too exclusive a guideline.

Where it is open ended is that the unique nature of having such an intensely detailed object THAT close is - there's a great view to be had even in a finder scope. My Nikon 8x42s provide a brilliantly riveting view of the moon. Of course there are no details like you will see at say, 350x through a ten inch but the view has the presentation of context that the narrower field higher resolution instruments can't duplicate. The 24x view through my 70 mm is one of the best views I EVER get of Luna. And if its a crescent moon with the night time earthshine present - that little low power refractor view is nothing short of an absolute beauty.

Ehhh so its open ended. There's so much there it has a lot to offer at many levels.


#5 buddyjesus



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Posted 11 September 2013 - 05:37 AM

I would think it would depend on the size of scope used for lunar.

Nearly all the detail I can see at 200x(50x/inch) I can also see at 83x(21x/inch) though it is tougher to see at times. Today for instance I saw a long and magnificent rille south of three large craters(Theophilus, Catharina, and I dont remember the third) in 200x but couldn't see a clue of it in all the hills at 83x. Therefore, I recommend using a variety of magnifications.

#6 JimK



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Posted 11 September 2013 - 12:05 PM

There is no one 'best' magnification per unit magnification.


And since it is also true that Exit Pupil = 25.4 / (mag per inch)

then the upper and lower values of exit pupil that may give "concerns" (seeing the secondary shadow or distraction by floaters in the eyeball) will also have corresponding "mag per inch" values.

[Note: The "formula" above is just a rearrangement of the standard formula for calculating Exit Pupil as = Aperture/Mag ]

#7 buddyjesus



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Posted 12 September 2013 - 03:56 AM

revision of previous post: feature observed wasn't a rille but rupes altai.

#8 BSJ



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Posted 12 September 2013 - 06:29 AM


Bigger is better and then pushed to whatever useful maximum limit the conditions will allow.

#9 macpurity


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Posted 13 September 2013 - 08:45 AM

Last night, under reasonably clear skies, I embarked on an "experiment in terror," broke all the rules and placed a Pentax XW5mm eyepiece into an Orion Shorty 2x Barlow, then mounted it on my 1000mm fl, 120mm refractor and pointed it at the Moon.

At 400x, I saw some very fine detail along the ridges of craters Ptolomaeus, Alphonsus and Arzachel, including details of craters inside of those. It was really quite a sight, even with a dimmed view having pushed beyond reasonable magnification.

Removing the Barlow was better, and the same details came through as every bit as sharp. That was at 200x, which would be pushing beyond the limit of the 35x per inch rule. Sometimes experimenting is the most fun way to determine the "best" magnification for your set-up.

#10 cpsTN



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Posted 14 September 2013 - 04:05 PM

OP here: The mag I use most often with my 12" dob is 243x (20xpi), primarily because its easy (12.5mm and 2x shorty). Last night I was looking at the Eastern edge of Walther, near Delandres, because the small series of craters along the its rim and leading away to the South, gives the effect of a valley. Anyway, I boosted the power to 506x once (42xpi) with a 6mm and 2x. Liked the mag but he FOV was too restrictive with my plossls. I might see about getting a wide field 5mm or 6mm EP.

#11 photonovore



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Posted 19 September 2013 - 09:58 AM

50x/inch, if prevailing seeing allows such (which is aperture dependant naturally) For this reason: 50X per inch is simply a round figure for 53.8x/inch, which is the magnification/inch figure required to bring the focal plane's maximum resolution frequency (found at 13.453X/inch at the Abbe limit for a 20/20 eye) to an optimal median of 7.5 cycles within the 20/20 eye's maximum acuity range of 5-10 cycles/degree. A more complete explanation can be found at the bottom of this page: http://cityastronomy...ag-contrast.htm

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