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Large telescope for moon ?

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#1 jnewton

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Posted 27 July 2013 - 11:06 PM

I love viewing the moon and often use a C11. My question is to those who have larger scopes, do you find that you see more on the moon in the larger scope, or is there diminishing returns regarding the moon? :question:
I'd look at a Webster 28 inch if I thought the views were better! Thanks.

#2 David Knisely

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Posted 28 July 2013 - 03:22 AM

I love viewing the moon and often use a C11. My question is to those who have larger scopes, do you find that you see more on the moon in the larger scope, or is there diminishing returns regarding the moon? :question:
I'd look at a Webster 28 inch if I thought the views were better! Thanks.


Under good conditions, I do like the larger aperture of my 14 inch Newtonian, although the view in my 9.25 inch SCT is nothing to sneeze at either. The 14 inch will show a lot of very very fine detail that the 9.25 just can't quite get to, but the 9.25 will do pretty well, especially with my binoviewers or when I want to do some quick imaging with my camera. Whatever scope I have, I do enjoy viewing the moon (even my 100mm f/6 is nice to get out once in a while when I don't want to drag everything else out). Clear skies to you.

#3 azure1961p

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Posted 29 July 2013 - 08:20 AM

The best most highly detailed images of the moon from amatuers with the highest amount of detail was through huge dobs normally known for deepsky observing - and under excellent seeing. But that's imaging and it could be making the most of what the eye can't see. Its a good question for folks living with jetstream interference like me. I think the safest answer is yes it'll make a big difference do long as you live in a place where the sky settles down enough for a 30" aperture. Florida comes to mind.

Pete

#4 George Tarsoudis

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Posted 10 August 2013 - 09:28 AM

Yes the large telescope give good results when the seeing and your collimation is best. Sometimes a not expensive telescope can you give you good details. Please see here my last post :

http://www.cloudynig...6017308/page...

#5 photonovore

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Posted 10 August 2013 - 12:49 PM

This is a pretty easy question to answer... to assess the level of lunar detail accessable visually with large aperture telescopes (both refractor and mirrored) all one has to do is study the LAC map series ( http://www.lpi.usra....pcatalog/LAC/). All of these maps were detailed out by a number of different observers over a number of years and thousands of collective hours of observing using optimally sited large (20-30inch) telescopes utilized specifically under excellent seeing conditions. They filled in observable details using extant photographic images as base images. Best resolution on craterlets runs abt 750m-1000m and ~400m on rille widths.

I personally consider this an accurate documentation of the limits of human telescopic resolution...and on a scientific basis, too-- as the entire mapping exercise (effectively, via hindsight) took the form of a double blind study of visual resolution, i.e. (mostly, see below) done before high rez orbiter images (LOPAM) were available to anyone (thus eliminating the possible bias of foreknowledge). And despite multiple observers, use of reflectors and refractors (both APO and achromatic) and the long term of the project (seven years), the resolution (and accuracy) is quite consistent across the visual portion of the entire set. (LAC 13,27,110,111,112,125,126 and 127 were completed *after* the Lunar Orbiter IV data was made available in May-June 1967 and their accuracy is commensurate with that advantage--IOW markedly superior to that gathered visually on the remaining 36 maps.)

If you look at LAC 12 (Plato) you will see that the *accurate* depictions of the smallest craterlets coincides with the sort of resolution one would expect to get with a 10-12" scope on a night of outstanding (=<1"arc) seeing. The takeaway is that larger apertures than this are of little practical advantage in high resolution visual observing of our Moon.

#6 azure1961p

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Posted 10 August 2013 - 11:21 PM

Foreknowledge is also a great tool in observing threshold features and objects and while there can be errors here, just as many are conjured up without foreknowledge with phantom summit crater peaks on domes that never had any coming to mind. Having a number of observers studying high and low the moon with 20"-30" telescopes without any prior knowledge of features at the diffraction limit is doomed to failure if the exercise is supposed to be indicative standard of what's humanly possible to see at the limits of perception. They simply did the best they could with what little information they had and the lighting angles particular to their experiences. Because of their lack of foreknowledge they missed features visible in far smaller scopes simply because the vastness of their undertaking was that overwhelming. Their map is clearly not a record of what's humanly possible to see on the moon with a given instrument. Rather merely a record of very careful observers who ultimately are fallible as much as they are careful and skilled.

Ignorance prior to observing is not a virtue. Pre-space age lunar cartography is ringing proof of that.

Pete

#7 photonovore

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Posted 11 August 2013 - 12:52 PM

That discreet features may or may not be "conjured up" is not a point of contention; the threshold values of such features, where the so-called "conjuring" begins, is, however, relevant to the topic of visual resolution thresholds. And this threshold is very consistent throughout the visual era observing record (Schmidt, Birt, Fauth, Krieger etc); the LAC maps are used as an example simply because of their breadth, the larger apertures used and their ready availability.

When assessing lunar resolution the simplest dimensional construct is de facto the most uniform in form and therefore indicative & reliable. This is why simple bowl craterlets are used as a base feature for resolution threshold evaluation. Diffractively they are the simplest discreet lunar features. To discern as a pit rather than a peak the light/dark high contrast orientation must be resolved. Such resolution is completely dependent upon aperture and aperture's effective resolution in turn is completely delineated by the prevailing seeing median. That the best resolution upon craterlets obtained throughout the visual era (~700m) happens to correspond to that which is expected at a sub arcsecond median seeing condition--and *not* to the theoretical resolution/aperture of large telescopes-- is no coincidence. Rather it is simply a re-statement reflecting a rather uncontroversial axiom: seeing, not aperture, controls effective resolution.

With this in mind we'll look at the case for a 12" telescope under 0.75" median sub-arc second seeing conditions (a level of exceptional seeing which most amateurs will never see BTW). Now from DIMM observations taken from various pro observatories which i have studied extensively in the past, one finds that the variance from the median seeing point ranges about 60% either way. This means that during 0.75" median conditions the moment-to-moment seeing (taken at the eye's effective cognition limit of max 60FPS, norm 30FPS) varies from between 0.45" to 1.2". Using dawes rather than rayleigh for the 12" native resolution calculation (as a best case standard which is most likely too liberal by half) we find a 0.38" resolution limit. At optimal momentary seeing windows during sub arcsecond seeing, therefore, one can expect a max resolution of .45"arc (as governed by seeing rather than aperture). This corresponds to resolution upon simple craterlets at ~800M (based upon median lunar distances). Which is still not even at the 700m resolution level..for which seeing at a median of 0.6"arc would be required (or an optimal lunar perigee). And this is just to realize the full potential of a 12" aperture. Change up the seeing to more typical backyard "best" values of 1"arc median and we find that any aperture over 8" begins to be overkill vis a vis native resolution vs. effective visual resolution. (Digital imaging being a completely different kettle of fish as you correctly surmised earlier.)

I fail to see how any of this can possibly be the least bit controversial--as it is, in sum, a really nice example of the observational record-- of the best delineated celestial object there is-- rather elegantly buttressing current resolution theories.






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