The specs for my 130mm, 650 focal length (f/5.0) reflector indicate that the lowest useful magnification I can have is 18x. I calculated a 36mm eyepiece would give me this magnification.
So, for example, if I went to say a 40mm eyepiece (16x) would I not obtain focus? What is the manufacturer getting at? Thanks.
As others have stated, this specification is based on a 7 mm exit pupil. Pretty much a standard in how the manufactures provide this spec.
FL eyepiece / FR scope = exit pupil
35 mm eyepiece / F5 scope = 7
On some reflectors you can go larger than 7 mm. On my Orion XT8 I used a 38 mm that worked fine.
38 / 5.9 = 6.44 mm exit pupil. No shadow of the secondary
I now have an Apertura AD12 - FR F5
38 / 5 = 7.6 mm exit pupil. Works fine
However on my little 100 mm table top Orion SkyScanner 100 I do run into a problem. F4 Newtonian reflector If I go any shorter than 25 mm on the eyepiece the secondary mirror shadow becomes an issue. Even the 25 mm shows a hint of the secondary mirror.
25 mm / F4 = 6.2 exit pupil. Should be fine but if I go longer I get the shadow. I normally don't go any shorter than 20 mm on this scope.
On your scope, with that short focal length, I would not go longer than a 32 mm Plossl eyepiece. You gain nothing and you invite problems. See if you can borrow a 32 mm Plossl from someone to try to be sure you don't get the shadow, but I would expect you would be OK.
Edited by aeajr, 08 July 2020 - 04:39 AM.