Quote:
Originally Posted by MortonH
I'd heard of keeping the exit pupil at 0.5mm or greater to avoid seeing your floaters, but not the 1mm 'limit' Mark is talking about.
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Hi Morton,
I have no problems at all using exit pupils down to about .5mm, particularly in my 14" and 18" scopes and gaining additional detail to what is visible with a 1mm exit pupil. The smaller exit pupil provides a larger image scale. If the seeing supports it, the telescope aperture supports it and the observer's eye supports it, then additional detail can be obtained. Larger telescopes are better supported to use small exit pupils, as the larger telescope collects exponentially more light for a given exit pupil. Using a .5mm exit pupil in an 18" telescope is a whole lot different than using a .5mm exit pupil in a 4" telescope, simply because the larger telescope collects over 18X more light and throws up a much brighter image. The ability to use small exit pupils can also depend notably on the individuals eye physiology. To make a blanket statement that exit pupils under 1mm provide no additional details is somewhat misleading. It depends on a whole lot of variables.
Quote:
Originally Posted by MortonH
While I've looked at a few deep sky objects through large Dobs at star parties I've never seen high-power planetary views through one. I'm curious how often it's possible to use the 1000x powers being talked about here.
Not trying to be controversial - if anything I'm wondering how much time to spend pestering big scope owners at IISAC  |
Not very often at all. Maybe 4 or 5 times in the 8 years I have had the 18" telescope. I used to get excellent seeing from my backyard at Killarney Vale looking north across Tuggerah Lakes. The seeing over large bodies of water can be exceptional as the body of water does not emit thermals like land masses. This occurs because the temperature of the body of water is very stable from day to night. About 4 or 5 times a year I could push the 18" scope to 750x on the Moon, Mars, Saturn and double star splits, but the more common magnification cap under normal very good seeing nights was 525X, which I could attain about 1 in 4 observing sessions. The more common magnification cap was 300X which I could reach on 2 of the 4 nights and one night in 4 I was limited to about 200X, or just under. From my present location in Kiama, due to the close proximity of the escarpment and its prevailing thermals I haven't had any of my 3 telescopes over 400X in the 3 years I have lived here. It's worth noting that an 18" telescope at 300X shows a whole lot more detail at 300X on any given target than a smaller telescope due to its greater light gathering power and greater resolution. It's probably worth noting that in my 18"/F4.5 scope 420X represents a 1.1mm exit pupil.
Cheers,
John B