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Limiting Magnitude and Excel

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#1 CollinofAlabama

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Posted 21 October 2021 - 12:46 PM

Does anyone know how to calculate the Limiting Magnitude Formulas of Roger N. Clark and Patrick Moore as demonstrated on this chart in Excel?  When I tried copying them right off the chart, I got bogus results, so there's a bit more to it.  I realize that the entire Limiting Visual Magnitude Formula concept is very individual, and many would argue, not worth much, but I am interested in the results with my scopes in Excel, and am unconcerned about those issues, so please don't bother with comments in this thread if you cannot speak to the point -- the formulas in Excel.  Thank you.



#2 Mark Lovik

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Posted 21 October 2021 - 01:03 PM

i have have a page in Excel doing this (along with the Airy disk and Daws limit as a function of aperture)

 

Reference calcs - http://twcac.org/Tut...itude_table.htm

 

Limiting Magnitude (clarke)

=LOG10(B16^2)*2.5 +3.7

          B16 --- just a cell reference with the diameter in mm - pick your own cell or hardwire the diameter

 

Limiting Magnitude (Moore)

=LOG10(A16) * 5 + 9.5

          A16 -- just a cell reference with the diameter in inches. 


Edited by Mark Lovik, 21 October 2021 - 01:05 PM.

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#3 CollinofAlabama

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Posted 25 October 2021 - 11:52 AM

Mark Lovik, thanks SO much for your post.  This is EXACTLY what the Doctor ordered, and the numbers come out perfectly.  My figures are slightly below those listed on the website I reference in my original post, but I am taking into account mirror absorption and the area of the obstruction in my figuring, so a 10" mirror (254mm) is catching the same number of photons as a perfect 228.84 diameter unobstructed scope would.  Of course, there's no such thing as a "perfect" scope, and even when figuring the LM in my refractors I deduct the tiny amount of absorption (about 1.15%) lens' remove from starlight photons.  But the numbers finally all look "right", something I wasn't getting before.

 

I had to add a formula for good Sir Patrick Moore's use of the antiquarian Imperial system (net size in millimeters/25.4) since I am only using the world standard that actually makes mathematical and physical sense in my spreadsheet.  About the only genuine criticism I've ever heard of the metric system is for range of temperatures.  For example, in the Imperial system, one can say it'll be in the 60's today and you know it'll be cool, but pleasant, and not cold (unless it's raining).  Likewise, the 30's, 40's, 50's, 70's, 80's, 90's and even 100's tell a person a whole lot more about what to wear than the metric system's 10's, 20's and 30's do, there being such a huge range between, say 20ºC and 29ºC (68ºF to 85ºF) that such expressions are less valuable.  I mean, a person could say "lower 20's" and you'd have an idea, but just saying "in the 20's" in Celsius isn't a lot of help in terms of dressing for the day.  But other than that small caveat (a benefit in having each Fahrenheit degree almost twice as precise [1/1.8]), the metric system is far superior, having much greater precision, and tying volume, weight and distance all back to a convertible standard, instead of the haphazard Imperial system.  But far be it from me to try and get Americans on board with the metric system.  We have bigger fish to fry right now, like deciding if we will remain a democracy or not.


Edited by CollinofAlabama, 25 October 2021 - 11:56 AM.



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