Norme 150mm MCT f/13, 31% CO Yellow Zone "People say I'm in denial. I disagree."
David Cotterell Toronto, Ontario "If an observer actually sees an object, there is no point in referring to a formula to find out whether he ought to see it; and if he fails to detect it, no formula will ensure his success." - W.H. Steavenson 8" f/15.5 TEC Maksutov 16" f/5 Teeter/Zambuto Dob 66mm WO SD AT 65EDQ APO Refractor Astro-Physics Mach 1 GTO Mount iOptron ZEQ25 mount Canon 60Da
Orion XT12i with Swayze-refigured primary/Protostar secondary
Televue NP101 refractor
William Optics Megrez 90 refractor
Universal Astronomics Deluxe Mounts
Quote:I'm curious because I'm trying to understand the wonderful tropical seeing patterns that seem to fit my 6" aperture so nicely.
Quote: ... seeing favors smaller apertures... Does seeing really affect larger apertures so adversely?
Quote:During moments of poor seeing, the larger scope will have reduced resolution, but not to a point worse than the small scope.
Quote:It if it is really that wonderful (and hopefully it is), it should fit a 12.5 inch that was cooled down and collimated even more nicely.
Quote:...it takes pretty poor seeing for a 6 inch to be <less than?> optimal.
Quote:Someone convince me.
Quote:Sirius B is easy to see in a good 4" apo (and possibly smaller), but hard in most 10-12" reflectors. The reasons are multiple: thermal issues, optical quality, light scatter, poor collimation, dirty optics, etc.
Quote:I don't believe this is primarily due to the reflector vs. refractor debates but most often because the small reflectors are of poor optical quality...
Quote:..p.roperly cooled and collimated, I ALWAYS see more planetary details in the big scopes, whatever the seeing. I've seen details on Jupiter in some optimized 12.5" scopes that a nearby 9" refractor could not see.
Quote:Sirius B is easy to see in a good 4" apo (and possibly smaller), but hard in most 10-12" reflectors.
Quote:The bigger aperture always does better at "cutting through seeing" than the small aperture. The main problems are poor attention to thermal issues, collimation, cleanliness and light scatter suppression, and something that is often the "luck of the draw"--optical quality.
A man is a small thing, and the night is very large and full of wonders.
“I am the only person to ever ace a 1951 USAF resolution test. My 'to observe' list says 'done'. I do not use charts or atlases when I starhop; men do not use maps. One of my sketches won an SBIG deep sky imaging contest. I am the life of star parties I have never attended. I never say anything looks like a faint fuzzy - not even a faint fuzzy. Pilots aim green laser pointers at me. Don Pensack proofreads my CN forum posts.” - The Most Interesting Astronomer in the Universe
Quote:Seeing is influenced by tube currents, too. It's all air density churning in the light path, so tube currents need to be minimized. Maybe that's the defining point, though, as tube currents can be controlled to some extent whereas seeing really cannot (except by picking an efficient location.)
Quote: Anyway, I realize my thoughts are not coherent, yet, while putting this all together. I am familiar with seeing, just would like to understand it's application at different apertures from eye ball twinkling, to 6" pleasing and rock steady Airy patterns, to total disintigration of the Airy pattern in a 12", for example. If you're 12" is in average seeing, would it still be worth keeping outside getting 8" views of plantets (on average when those fleeting moments are very far apart?)
Quote: For those of us who are fortunate enough to have no thermal problems (in a MCT, no less), and when seeing is good snough to afford nearly perfect collimation, seeing becomes the dominate aberration. That's why I want to isolate it and understand it by itself.
Quote:...it's not good enough for a 10 inch, it's not good enough for a 6 inch either. Jon
Few astronomical observations have really changed our perspective on earth's place in the universe. Those that did were made with a 20x power, 0.3° field of view telescope, sporting a singlet objective stopped down to 15 mm.
I mostly use small, fast, and (ultra)portable glass: Bushnell Legend 8x42, Pentax PCF WP II 20x60, MiniBorg 60ED, ST-80, EON-80, ST-120, 6" StarBlast.
Quote:With regard to the size of the "cells" as they relate to aperture, I doubt very much one can simply say, "the cells are 4" tonight....." These are natural phenomena which will form a normal distribution around a mean size, which may be 4" but the "cells" may have a huge range in size...
Quote:I applaud the attempt of OP to elevate the discussion on atmospheric seeing and telescope performance above the level of anecdotal evidence.I recently discovered that loads of relevant material (including Fried's paper) is available for free on the Internet. I started to look into those papers. These tend to considers the effect of atmospheric turbulence on perfect (diffraction-limited and aberration-free) optics (no luck, Jim). A key result that I reported on in a thread in the refractors forum is perhaps worth repeating here:Taking into account the effects of seeing, a large aperture diffraction-limited telescope will deliver diffraction-limited snapshots only a very small fraction of the time. In other words, the time one has to wait for what is referred to as a “lucky image” increases exponentially with aperture. Some numbers for poor seeing (at the lower end of typical seeing conditions described by optical turbulence characteristic size r0 in the range 0.1-0.2m), with the leftmost figures giving the aperture in inches, and the rightmost figures the fraction of snapshots that yield "lucky images" (diffraction-limited performance): 4 100% 8 100% 12 98% 16 46% 20 11% 24 2.1% 28 0.27% 32 0.026%When people make remarks about "pleasing views" they probably refer to getting diffraction-limited views at the eyepiece 100% of the time. The key result to note is that achieving diffraction limited performance for larger telescope apertures resents a challenge that increases exponentially with the scope's light gathering area. That's a steep uphill battle...
Quote:The key result to note is that achieving diffraction limited performance for larger telescope apertures resents a challenge that increases exponentially with the scope's light gathering area.
Quote:according to this modelling, "poor seeing" means that 98% of the time, a 12 inch scope will provide diffraction limited performance. Diffraction limited performance for a 12 inch scope is under 1/2 arc-seconds. I am quite sure many if not all would consider those views "pleasing."
Quote:So, are better corrected, unobstructed scopes less prone to seeing? I would think so, but that's just a working hypothesis. I have not yet run across anything that says so or says otherwise.
So, as Johannes said above, it does seem it is an uphill battle for larger apertures to be diffraction limited. But, is that really so important in terms of resolution and contrast? Silly question?
The Airy pattern does shrink with aperture, so while the view might not be "pleasing" in that it's not diffraction limited, it still might be good enough to render some finer details a smaller aperture is not capable of under those same conditions. Point being, there might still be a reason to have some aperture for higher resolution observing.
Quote:The question that Rutten and Venrooij asked was not how often a scopes of differing apertures could reach diffraction limited performance but rather how the different apertures compared on an absolute scale as a function of the seeing conditions.
To my mind, that is the right question to ask.
Quote:One should not describe the Fried parameter r0 as a cell size.
Quote: It (Fried parameter) can indeed be measured at a given site at a given time as an indication of seeing.