Resolution, Seeing, Magnification and Aperture

Lunar Observing: Resolution, Seeing, Magnification and Aperture

Tacoma, WA


How far can an observer expect to be able to 'see through' the prevailing seeing levels (nominal seeing)? I could find nothing on this specifically so I thought I'd try to see if I could arrive at some sort of a quantification through examination of Lucky Imaging techniques and data combined with examination of the differences in image aquisition frequencies between the human eye and the CCDs the Lucky Imaging Group at Cambridge University uses...and were so kind as to detail on their extensive website.

What I found was that the CCD enables them to see through prevailing seeeing by a factottr of five. This however is due to the high recording frequency of the CCD imager which operates at a frame rate of 100hrz or 1/100th second per frame captured. The human eye is disadvantaged here with an aquisition/recognition/cognitive minimum frequency of about 30hrz--IOW to absorb the information that a clearing in seeing provides, it must be at least 1/30 of a second in length--a reduction of the ability of a CCD by a factor of three, which leaves the human eye's ability to see through the prevailing seeing to be a positive factor of 1.6 vs. the factor of five of a CCD.

I examined a few dozen observing reports and a similiar number of CCD images of various resolution subjects (lunar and multiple stars primarily) and found that the data seems to follow along with the 1.6 factor quite consistently.

I quantified the results of all this in the following graph:

lunar seeing

This chart is based upon the following assumptions:

*Best visual 'seeing windows' exceed nominal seeing by a factor of 1.6.
*Minimum separation necessary for visual diffetrentiation is 180" or 3 minutes of arc.
*Resolution per aperture based upon 140/aperture (mm) ; Raleigh's Limit.
The factor for best visual window per nominal seeeing was calculated as follows:

Lucky Imaging data support an increase in resolution obtainable per nominal seeing by a factor of five. Since the CCD imagers used operate at a frequency of 100hrz (images obtained at 1/100th second intervals) and the human eye operates at approximately 50hrz threshold for flicker response and about 30 hrz (1/30th second minimum time to register a change in image), the human eye is therefore less sensitive to seeing windows than a CCD by a factor of three, leaving a remainder factor of 1.6 to nominal seeing levels.


The next issue was to find a way I could objectively quantify my local seeing with some other method than measuring CCD star images as is done by professional observatories and some advanced amateurs. The Moon provides a good setting for objective quantification as it offers a plethora of various sized measured objects with which to calibrate one's resolution and thus one's absolute seeing in arc-seconds. By establishing absolute (maximum 'see through' resolution) one can make a consistent estimate of the average seeing by using the next table:

lunar seeing

So how is your seeing...really? Just for interest's sake & for some comparisons I surveyed some observatories that actually make an effort to measure and track their seeing. Here's a sampling of what I found:

*Cloudbait Observatory, 40km W Pike's Peak, CO, winter seeing averages 4-5"arc, summer slightly better. (amateur)
*Van Vleck Observatory, Middletown, Connecticut, median 2.5"arc (professional)
*High Energy Astro, Rockville, MD--2.8"arc seeing summer nominal. (amateur)
*Hume Observatory, Santa Rosa, CA Summer nominal 3" arc. (professional)
*Vedeler Obseervatory, Catalina, AZ nominal annual range 1.8-2.4"arc (amateur)
*Apache Point Observatory, Sacremento Peak, NM nominal 1.5"arc (professional; see graph)
*Stony Ridge Observatory, Angeles National Forest, CA 2-3"arc nominal annual average. (advanced amateur)
*BTA telescope, Caucasus Mountains, Russia annula 90%>1.5"arc seeing. (professional)
*MRO Observatory, Magdalena Mountains, SW Socorro, NM reports 1"arc annual average (<1.0" 49%; professional)
*NCO Lu-Lin Observatory, Yu-Shan National Park, China, 1.39"arc nominal annual (professional)
*Keck Observatory, Mauna Kea, HI 0.55"arc median seeing. (Professional)
*Dome C, Antarctica, nominal 0.27"arc seeing (professional)

I think it's possible that amateurs frequently overestinmate their local seeing conditions. The frequency I see 1"arc seeing reported just doesn't jive with the objective data, especially for suburban observing sites. I feel that the *very best* I get at my suburban site is on the order of 1.5"arc in midsummer within 20degrees or so of the meridian. The average, is definately >2.0"arc; probably around 3"arc. I feel this is probably typical suburban seeing in the US generally.

What is strikingly obvious is that in any case it requires exceptional suburban seeing to realize the resolution potential of apertures larger than 8". As resolution is the primary paradigm upon which lunar telescopic performance is based, this information gives a useful guide to what size aperture is cost effective for a given area.

Comments, more data etc, welcome! This is all a premiminary effort, so I don't consider anything here 'cut in stone'.

e-me at: photonovore (at) gmail (dot) com


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