Magnification formula

[mangrovedutch]

G'day all, I was talking to the other half yesterday about limitations of magnification of a scope, and do you think I could remember the formula for working it out? Could someone please tell me what it is and I'll write it down this time :shrug:

Regards, Dutch

[mental4astro]

Hi Dutch,

There are a few formulae to keep in mind, each for a different purpose:

* Magnification = focal length of scope ÷ focal length of eyepiece

* Max. magnification for a scope = 50X per inch of apeture

* Longest focal length eyepiece for a reflector = 6 X focal ratio of scope ( with a 7 X margin to allow for variations in eyepieces. )

* Typical max. magnification allowed by the atmosphere = 100X to 120X

* Rarely max. magnification allowed by atmosphere = 250X

* Very rarely allowed max magnification (altitude dependent too) = 600X

There are probably a few others that escape me right now, :lol:

And then there are the technical explanations too! Do you want me to go into them?... :lol: :lol:

[mangrovedutch]

Thanks Alex, No mate that is great. Thank you for coming to my rescue again (LOL)

Regards, Dutch

[ZeroID]

?? Could someone please explain that one. Not one I've ever heard of or the reason for it.

Cheers
Brent

[Robh]

Brent,

The average person is assumed to have a night-time pupil size of about 6mm.

The exit pupil of an eyepiece is D/m where D is the diameter of the scope's aperture and m the magnification achieved by the eyepiece in the scope.
Ideally, D/m=6 to match the eye's pupil size. If the exit pupil of the eyepiece is larger, the extra light is wasted.
Rearranging the formula, m=D/6.

Now m=focal length of aperture/focal length of eyepiece =D/6.
Rearranging the formula,
focal length of eyepiece=focal length of aperture/D x6=focal ratio x6.

This is not too say you can't have a longer focal length eyepiece but the extra light it sends to your eye will be wasted. Of course, if you have a night-time pupil size of 7mm, you can use focal ratio x7.

Regards, Rob

[ZeroID]

Thanks Rob, more useful information.

[multiweb]

One measure of vodka, two measures of orange. Works on almost everything even bad focus. :thumbsup:

[AstroJunk]

And further on this point, just because you will loose light due to a too large exit pupil, it doesn't mean that you can't get a splendid view from an eyepiece outside the normal range.

Peter Brooks did a nice spreadsheet of eyepieces, and I modified it to include the use of a paracor. I can't add it here, so I will upload it to the SDM Telescopes website shortly and post a link.

[AstroJunk]

http://www.sdmtelescopes.com.au/extras.html

Have fun!

[mangrovedutch]

Thank you for that link Jonathan, as I get to understand this more in time that will be a realy useful reference. Thanks mate.
Shame I don't drink, Marc, it does sound tempting.

Thanks all, Dutch

[Allan_L]

Theoretical maximum magnification obtained from eyepiece with half the f ratio of the scope.

Therefore my CPC1100 has f ratio of f10.
The biggest magnification I can expect is from an eyepiece of 5mm.
Don't even bother trying a smaller mm ep.

For that matter, the 5mm will only be viable in extremely good seeing conditions.

[Liz]

:lol::lol: