Brent is absolutely correct on the theoretical limits for a given aperture. I'd like to add that many other things can affect the limit in practice. Super-smooth optics improve resolution. Contrast & baffling, even how smooth the edges of your mirrors are. Microscopic surface abrasion on eyepiece lens surfaces. Anything that sucks energy out of the central spot can affect resolution, especially low-contrast planetary details. Every little bit adds up. Mars is getting bigger now, but even just a few more arc-seconds will pay dividends when seriously trying to pick out surface & atmospheric details. When you can't detect the slight gibbous phase at medium powers, it's about right from my experience. This morning I was craving more power, but the seeing wouldn't have supported it. As it was it was barely possible to see only the largest albedo features and sharpest contrast details. This 6" mirror has supported powers over 400x on rare occassions (always in autumn & winter) and it would have been nice if the air were steady enough this morning to have gotten away with it. I'm sure that there is no more detail visible at 400x than at about 300x at this aperture, but the larger image makes some details more visible, and since there is no image degredation, sometimes the higher power pays off. For these eyes, anyway. Had I not been such a stumblebutt at 4:30 am, I might have taken out the 10"; but it's just as well since the seeing wouldn't have supported it from here. Chuck --- Brent Watson <brentjwatson@yahoo.com> wrote:
Rich,
If I might interject . . .
Dawes' Limit is 4.56 divided by the aperture in inches. This is very close to the 500 nm theoretical limit. The theoretical limit at a wavelength of 500 nanometers (greenish light)is 4.957 divided by the aperture in inches.
Dawes' Limit is an empirically derived number for just being able to discern a notch in two equal magnitude sixth magnitude stars. The theoretical limit is from your friendly neighborhood physics book.
Brent
--- Richard Tenney <retenney@yahoo.com> wrote:
Chuck,
Thanks for that great report! I noticed Mars (only naked eye object visible) in the twilight at 5:30 this morning as I was putting on my shoes to go haul a load of hay for my horses (wishing I was at the eyepiece instead of getting ready to break my backside). :o)
Is there a simple calculation or rule of thumb to determine what the diffraction limit is for a given telescope?
Rich
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