Focal length a Magnification? (1 Viewer)

I'm here again seeking advice. If an EP gives you 37x on a scope with a focal length of 570mm, what would it be on a scope with 390mm?
Also the zoom gives you 27x72 on 570mm what would that be on a scope with 390mm?
The reason I'm asking is because I've just got my Pentax PF65ed11 and at the moment I'm using the Opticron Ep's
Thanks in advance

Ron

Ron,

It should be 25,3x. Focal length of scope divided by focal length of E.P = magnification, and focal length of scope divided by magnification is focal length of eyepiece.

Kimmo

kabsetz said:

Ron,

It should be 25,3x. Focal length of scope divided by focal length of E.P = magnification, and focal length of scope divided by magnification is focal length of eyepiece.

Kimmo

Click to expand...

Thanks for that Kimmo, it's sounds about right looking through the scope.
But how do I find the focal length of the EP? In this case the ep I'm using is an Opticron 40809, which gives 38x on the HR80 and 28x on the HR66.
On the Pentax the XW14 gives you 27.85. Your calculations make the 40809 EP a 15.4. Is that correct?

on

Hi,

I would call the 40809 a 15mm EP with roughly 72 degree AFOV. The Pentax XW14 is 14mm and 70 degree AFOV so if the Opticron delivers a good image for you the XW14 will probably give a rather similar result at a higher price. The XW14 might be a wee bit brighter on dark skies... but who cares in bright daylight.

I would get other focal length EPs or a zoom. I think the usual astro zoom recommendation Baader Hyperion is out on the Pentax though since it doesn't focus to infinity... the pentax zooms are nothing to phone home about either...

Joachim

I still don't know how to work it out, but it looks like the Opticron EPs will work out about the same as on most 65/66mm scopes. With the the HDF 40810 I get 27x on the HR80 and with the HDF 40809 I get 25x on the Pentax.

Hi,

as Kimmo stated, magnification is focal length of objective divided by focal length of EP - or solve for the variable you need to know. Unfortunately non-astro optics manufacturers usually don't state focal lengths but just the magnification.
For Opticron I found some body focal lengths on the web somewhere and thus was able to calculate that my SDLv2 is in fact an 8-24mm zoom. From this you can obviously derive the focal length of any opticron body which has magnifications stated for the SDLv2 and from that the focal length of any opticron EP which has magnifications stated for the body with a now known focal length.

As an example, the HR80 body with SDLv2 is listed as giving 24-72x with the SDLv2 (remember this has 24-8mm) so it the focal length of the HR80 body is 24x24 or 72x8 = 576mm.
So the HDF EP #40810 listed at 27x for the HR80 must be 576/27 = 21.3333mm or probably just 21mm and the rest was rounded.

Joachim