Limiting magnitudes

[glenc]

How much aperture do you need to see a given limiting magnitude?
Here are some figures.

mag cm inch
11.0 5.0 2.0
11.5 6.3 2.5
12.0 7.9 3.1
12.5 10.0 3.9
13.0 12.6 5.0
13.5 15.8 6.2
14.0 20.0 7.9
14.5 25.1 9.9
15.0 31.6 12.4
15.5 39.8 15.7
16.0 50.1 19.7
16.5 63.1 24.8
17.0 79.4 31.3

Formula limit = 7.5 + 5 x log(cm)

[mickoking]

Looks good to me Glen :thumbsup: But with a transparent dark sky and a good eye you could probably improve on those magnitudes.

[glenc]

I was looking to see what size scope I need to buy to get another one magnitude. Looks like I need a 20", it sort of increases exponentially.

[Don Pensack]

Your formula is extremely pessimistic--and probably works from right in the city among the bright lights.
I favor the Schaefer-derived calculator which works much better for me, observing at a dark site (sky brightness magnitude 21.4 or better):
http://www.go.ednet.ns.ca/~larry/astro/maglimit.html
This calculator has worked for me with several different scopes.
I personally have reached magnitude 15.1 to 15.3 on an average night with my 8" SCT, and have reached (though I haven't really pushed it) 16.4 with my 12.5".
The Schaefer-derived calculator suggests I should reach about magnitude 17.5 with a 20", and I have heard of a few observers going quite a bit deeper with the same aperture, so 17.5 would be quite conservative.
My 5" has reached better than magnitude 14 several times.
I've see conservative formulae that work better for city viewing. Here is one based on Clark's work that is conservative for a dark site (column 1) and conservative for city (column 2)
http://www.twcac.org/Tutorials/limiting_magnitude_table.htm

[stephenmcnelley]

Thanks for those links Don, last year i found i could resolve Pluto at an apparent magnitude of 14.5 easily with a 25cm Newt with the planet at its prime for starhopping and viewing, there was only some small-moderate local light polution.

I think an area's limiting magnitude for a scope size is subject to too many local area independant variables to be succesfully modeled with any great accuracy => barring electronic aids... and might be best considered when you are way out in woop woop under decent dark skies.
For anyone that is not out in the sticks then they are only a guide to what might be through a certain apeture or optical design/focal ratio.

Still, a scope can never be big enough regardless, if the skies are reasonably good.:lol:

[glenc]

Thanks Don. Is that at sea level? I found Pluto difficult with an 8" when it was mag 13.9.