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Max usable mag on moon


Kaminovs
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Hello,

My question is usable magnification on moon also caps at about 200, or we can go higher on moon to observe its details in more close up?

I use sw200p and with stock 2xbarlow and celestron 10mm x-cell i get very good views. I am thinking is it worh investing in more X barlow or less mm eyepiece for moon closeups, or 200 mag is about it...

Appreciate your replies!

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As with most things astronomical the max magnification is usually determined by aperture, optics quality, seeing conditions and type of object viewed. A decent 8" on a good night, should be capable of 400x on the Moon.

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I agree with Peter

on nights of good seeing and particularly when the moon is higher in the sky than currently, I have managed 400x and possibly a shade more with my 6" f11 (focal length 1600mm) newt and TV Nagler 6-3mm zoom. When half illuminated or less, the terminator is just stunning in such conditions. in truth though, many of the features observed can be seen with much less magnification.

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I'm surprized by the 400x, I must say. Living at altitude (...ish, 3000 feet) I just once had 500X hold up in the very early hours as dawn was breaking. But when I say 'hold up' was I really seeing more than at 250X? I had an incredible feeling of intimacy with the moon but was I really seeing more detail? This was in a 10 inch SCT. A lot may depend on your eyes and the way that you 'see.'

Olly

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hi Olly, I am a complete novice compared to yourself and many/most of the long term observers on SGL, but I certainly use 400x regularly on double stars, especially those towards the zenith (where you are not looking for detail of course, just a clean split) with ease. on the moon it depends mainly on declination of the moon and of course seeing, but in the winter when the moon is high, it's certainly regular to use 300-400x and even very occasionally 500x).

although I am a big believer in try what works rather than worry what should work in theory, I am interested in trying to work out the resolution I should see with my scope.

I think this is correct:

Moon = 0.5 degrees across = 30 arc minutes or 1800 arc seconds

Dawes limit of my scope = 11.6 / diameter in centimetres = 0.76 arc seconds resolution.

so in theory in perfect seeing with perfect optics (neither of which I get - the figure on my 6" is 1/6PV)

moon diameter = 3476 km so each arc second = 1.93km so I should be able to see features which are 1.46km across. Assuming this figure is less due to never getting perfect seeing and optics, it seems that perhaps around 2km would seem reasonable and ties in nicely with the craterlets in the Crater Plato which I have seen which I think are all over or close to 2km.

Quite how this ties in with magnification I have not worked out though :)

I do agree though that you don't really see much more with more magnification than around 200-250x and I usually prefer to observe at around 250x mark on the moon assuming even this is possible.

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I seem to get the best views with 1x per mm of aperture. 150x with my 150P, 200x with an 200mm f/6 dob I borrowed for a while and now 250x with my 250PX. My collimation is pretty good. I think it's down to my local conditions. I observe from the edge of a housing estate and all the heat rising from rooftops makes a mess of things.

Edit: maybe the exit pupil I get with a 5mm eyepiece just suits my eyeballs?

I did get to use 375x with the 250PX (sketching the craterlets in Plato) but I don't think I saw any more detail than I did at 240x which is my next mag down. The view didn't mush at 375x, like it does when you overdo it in smaller scopes so it was quite usable.

Edited by RikM
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  • 2 weeks later...

Hi,

I just thought I would chime in on this one, I have read up on the maximum 'usable' magnification etc etc, I agree that for optimum viewing in general the rule of thumb of roughly 50x per inch is probably true, however, this Saturday evening just gone I noticed that 'seeing' seemed quite crisp and I could not resist the temptation to stay up to 4am and make good use of unusually clear skies just outside of Birmingham.

I have a Celestron Omni XLT 120 frac and recently purchased a Celestron Ultima 2x barlow, I have a Celestron Xcel 8mm and a Hyperion 5mm. I just could not resist pushing my scope on the moon and was surprised to say the least, with the 8mm Xcel plus barlow giving me roughly 250x the views were very crisp and detailed, experimentation got the better of me so I popped in the Ultima and the 5mm Hyperion giving me 400x and to be honest although the 'seeing' popped in and out as I viewed, the detail blew me away, considering this is only a 120mm achro frac I was really shocked, yes the brightness was slightly lower but not by much, and on the moon this is not such a problem so I would say experiment and see what works for you.

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hi Olly, I am a complete novice compared to yourself and many/most of the long term observers on SGL, but I certainly use 400x regularly on double stars, especially those towards the zenith (where you are not looking for detail of course, just a clean split) with ease. on the moon it depends mainly on declination of the moon and of course seeing, but in the winter when the moon is high, it's certainly regular to use 300-400x and even very occasionally 500x).

although I am a big believer in try what works rather than worry what should work in theory, I am interested in trying to work out the resolution I should see with my scope.

I think this is correct:

Moon = 0.5 degrees across = 30 arc minutes or 1800 arc seconds

Dawes limit of my scope = 11.6 / diameter in centimetres = 0.76 arc seconds resolution.

so in theory in perfect seeing with perfect optics (neither of which I get - the figure on my 6" is 1/6PV)

moon diameter = 3476 km so each arc second = 1.93km so I should be able to see features which are 1.46km across. Assuming this figure is less due to never getting perfect seeing and optics, it seems that perhaps around 2km would seem reasonable and ties in nicely with the craterlets in the Crater Plato which I have seen which I think are all over or close to 2km.

Quite how this ties in with magnification I have not worked out though :)

I do agree though that you don't really see much more with more magnification than around 200-250x and I usually prefer to observe at around 250x mark on the moon assuming even this is possible.

You are quite right. I'm sorry, I should have excluded double star observing because there, as you say, more mag means a cleaner split. I was really thinking about casual aesthetic observing since I read that as the OPs interest.

It is ocassionally worth pumping the mag for aesthetic observing, too, but if this means buying a posh EP for rare occasions it might be a bit of an indulgence!

Olly

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Very true Olly, planets are really fussy when it comes to seeing/magnification etc.

the best views aesthetically are often at much less than the magnification you can get away with an in truth as we know, you can often see the detail there too compared with higher mags if you take the time to observe for a while.

it's nice to have a bigger more juicy image in the eyepiece though if you can do. :)

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