Here are some additional data points (the Rayleigh Criterion or resolution limit for a 72mm aperture at various wavelengths of light). The calculation is done by:

theta (radians) = 1.22 wavelength/ D

-- and --

arc seconds = (radians x (360 / 2pi)) x 60 arc minutes per degree x 60 arc seconds per arc minute

-- and where --

"D" will be 72mm = 72,000,000nm

Or, simplifying all of the above (constants and unit conversions):

arc seconds = 0.252 wavelength(nm) / D(mm)

Rayleigh Criterion for 72mm aperture (0.252 wavelength(nm) / 72mm))

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635nm (center of the 610-675nm bandpass): 2.22 arc seconds

562nm (typically used for Dawes' Limit): 1.96 arc seconds

540nm (Baader Solar Continuum filter): 1.89 arc seconds

495nm (Baader H-beta): 1.73 arc seconds

455nm (center line Baader Blue CCD filter): 1.59 arc seconds (approximate, fairly wide bandwidth of 390nm to 510nm)

Note that Dawes' Limit is a slightly more stringent test than the Rayleigh Criterion and is usually represented as:

Dawes' Limit = wavelength / D

Thus, the conversion factor is just (Rayleigh Criterion / 1.22) and going back to the earlier simplification of the Rayleigh Criterion we have:

Dawes' Limit = 0.206 wavelength(nm) / D(mm)

Dawes' Limit (as "R") is also commonly written as (when converting diameter, D, from millimeters to centimeters):

R = 11.6 / D

where D is in centimeters and thus for the 7.2cm AT72ED that comes to:

R = 11.6 / 7.2 = 1.61 arc seconds

Which is equal to the earlier Rayleigh criterion at 562nm when divided by 1.22, thus:

1.96 arc seconds / 1.22 ≈ 1.61

So, given the earlier Rayleigh Criterion calculations a table for Dawes' Limit at various wavelengths is as follows.

Dawes' Limit for 72mm aperture (0.206 wavelength(nm) / 72mm)

===============================================

635nm (center of the 610-675nm bandpass): 1.82 arc seconds

562nm (typically used for Dawes' Limit): 1.61 arc seconds

540nm (Baader Solar Continuum filter): 1.55 arc seconds

495nm (Baader H-beta): 1.42 arc seconds

455nm (center line Baader Blue CCD filter): 1.30 arc seconds (approximate, fairly wide bandwidth of 390nm to 510nm)

Of course, both the Rayleigh Criterion and Dawes' Limit are based upon resolving two equal magnitude point sources (i.e. stars) and a crater on the moon isn't that. Furthermore, it's easier to see a line-type feature (fault line or rille) than it is a crater or point source so all of these limits need to be taken with some latitude.

In terms of going from 635nm to 455nm, that's equivalent in terms of resolution to switching between a 72mm aperture and a 100mm (approximately 3 inch to 4 inch refractors).

Now, how about my wished for 4" f/8 Newtonian?

Dawes' Limit for 101mm aperture (0.206 wavelength(nm) / 101mm)

===============================================

635nm (center of the 610-675nm bandpass): 1.30 arc seconds

562nm (typically used for Dawes' Limit): 1.15 arc seconds

540nm (Baader Solar Continuum filter): 1.10 arc seconds

495nm (Baader H-beta): 1.01 arc seconds

455nm (center line Baader Blue CCD filter): 0.93 arc seconds (approximate, fairly wide bandwidth of 390nm to 510nm)

This MIGHT get me down to about 1 arc second in blue light on a night with nearly perfect seeing conditions and that would represent a crater of about 1.9km (significantly better than the 4.26km that I did with the 72mm refractor in red light). More realistically and given less than perfect seeing I might expect something around 1.3 arc seconds or 2.5km (resolved). In any case, I think that could exploit the full resolution and scale of my IMX183 sensor (given my desire to capture the entire disk of the moon in a single framing).

Note, I've clearly split an approximately 0.92 arc second double star with my Tele Vue NP127is (5") refractor using a Baader Blue CCD filter. However, with the red CCD filter the star was just an elongated barbell shape (below, my image of the double star SAO 46152, more on the latter).

**Edited by james7ca, 16 January 2019 - 06:29 AM.**