Magnification formula

[mangrovedutch (Dutch)]

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

Regards, Dutch

[mental4astro (Alexander)]

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,

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

[mangrovedutch (Dutch)]

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

Regards, Dutch

[ZeroID (Brent)]

Originally Posted by mental4astro

Hi Dutch,


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

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

Cheers
Brent

[Robh (Rob)]

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 (Brent)]

Thanks Rob, more useful information.

[multiweb (Marc)]

Originally Posted by 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

Regards, Dutch

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

[AstroJunk (Jonathan)]

Originally Posted by Robh

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.

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 (Jonathan)]

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

Have fun!

[mangrovedutch (Dutch)]

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 (Allan)]

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]

Originally Posted by multiweb

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