Lunar Observation: Magnification, Aperture and More

How far can an observer expect to ‘see through’ the prevailing seeing levels (nominal seeing) during lunar observation?

A way to approach this question is to quantify data formulated by examining Lucky Imaging techniques and data combined with analysis of the differences in image acquisition frequencies between the human eye and the CCDs the Lucky Imaging Group at Cambridge University uses. They provide such detail on their extensive website.

The CCD enables them to see through prevailing seeing by a factor of five. It is due to the high recording frequency of the CCD imager, which operates at a frame rate of 100 Hrz or 1/100th second per frame captured.

The human eye is disadvantaged here with an acquisition, recognition, and cognitive minimum frequency of about 30hrz–IOW to absorb the information that a clearing in seeing provides, and 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.

A woman adjusting her magnification for an clear night lunar observation.

An examination of a few dozen observing reports and similar CCD images of various resolution subjects (lunar and multiple stars primarily) found that the data follows along with the 1.6 factor quite consistently. The results are in the following graph:

A chart created displaying Best Visual Resolution via different levels of magnification at Normal Seeing Levels.
A graph representing the Best Visual Resolution at Normal Seeing Levels.

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 differentiation 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 Seeing 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:

A chart displaying Normal seeing in arc seconds, max resolution in arc seconds and min. craterlet in km.

So How is Your Seeing, Really?

For some comparisons, data were used from observatories (as seen below) that make an effort to measure and track their seeing.

  • 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)

Less experienced observers may frequently overestimate their local seeing conditions.

The frequency of 1″arc seeing reported may not be aligned with the objective data, especially for suburban observing sites. The average seen here for urban locations is closer to > 2.0″ arc, probably around 3″arc, and may be more typical suburban seeing in the US generally.

Final Thoughts

What appears to be obvious is that it requires exceptional suburban seeing to realize the resolution potential of apertures larger than 8″. 

Resolution, magnification, and contrast are the variables that determine a significant portion of the result, with “resolution” as the primary paradigm upon which lunar telescopic performance is based.

This information can be helpful as a guide to what size aperture is cost-effective for a given area.

Written by CityAstronomy.com

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