limiting mag

[algol]

hi guys, does anyone know how to work out limiting mag for a scope?

i have a 130pm newt and i have seen down to around mag 10 ish as 1 or 2 of saturns moons are quite dim. anyone know how to work out the limiting mag?

cheers

[Ant]

I've just done a quick Google search and come up with...

http://www.go.ednet.ns.ca/~larry/astro/maglimit.html

That may answer your question?

Ant

[scotastro]

Nice find Ant

I've just done the calc for my 16 inch f5 and got the same answer of +15.5 mag - the same as I had visually estimated back in January.

Mike

[algol]

so it works in practise then mike? cool.

so have you turned it toward uranus yet? would you get a moon as well with that mag?

[Boris6]

I'm not sure where I got it from but I've always used the formula:

2.7+(5*logD)

D being the aperture of the 'scope.

This seems to give a reasonable guide, but it does depend on the 'scope you're using, atmospheric conditions and observers skill.

[TheThing]

Hmm! I asked the same question of the SPA back at the begining of the year and was given: -

2.7+2.5 * log (D squared)

where D is the diameter of the aperture in mm.

I don't have a calculator handy, but it would be interesting to see if the calculations come out with similar answers.

[MikeP]

I think you will find that the two are identical.

If (say) y = D⁵ then log y = 5 * log D

alternatively, since D⁵ = (D²)^2.5 then log y = 2.5 * log (D²)

Mike

[Boris6]

I think you will find that the two are identical.

If (say) y = D⁵ then log y = 5 * log D

alternatively, since D⁵ = (D²)^2.5 then log y = 2.5 * log (D²)

Mike

Yep, you're quite right, Mike. Even so, I don't understand why the SPA complicated the calculation in this way.

[beamer3.6m]

Stupid question but am I too assume that the '*' means 'times'.

[MikeP]

Not stupid at all, yes it does. Its a fairly standard form to distinguish from x which really could mean a variable x. Likewise / is used for division and ** often means exponentiation eg 2 ** 3 is 2 cubed = 8. I used superscripts instead of ** because it is more natural looking.

Mike

[beamer3.6m]

So I make the limiting magnitude for my Explorer 200 scope 14.20

Does this sound correct or has my calculator got it completely wrong.

When I used the link above it came out as about 12.87 I think.

[narrowbandpaul]

the limiting magnitude can be found from pogsons equation...m1-m2= -2.5 log10(F1/F2), where F is the flux of objects 1 and 2. F is proportional to D^2, where D is the aperture. Let m1 be the magnitude visible through the telescope and m2 be the naked eye limit for the observer at his site. Let D1 be the aperture of the telescope, and D2 be the aperture of the observers dark adapted pupil. Then ...

m1-m2= 2.5log10(D1/D2)^2, or m1-m2= 5log10(D1/D2), this is the magnitude gain over your site. Assuming a limiting mag of 5 for most UK sites then the limiting magnitude ML=5+5log10(D1/D2), where D1 is your telescope aperture and D2 is your dark adapted pupil diameter..around 6-8mm, worsening with age.

Keep D1,D2 in the same units

Paul