Quote:Hello cloudy-nightersI think my inexperience may be showing...The 6" dobs from Skywatcher and Saxon both have 2" (51mm) focussers, but only 34.5mm secondary mirrors. As a 1.25" (31.7mm) eyepiece is just about big enough to capture the available light from a 34.5mm mirror, is there any any point buying a 2" eyepiece for either of these telescopes?thanks...Mark
Quote:Thanks JohnI don't really understand about the effect of non-illumination of the outer portion of an eyepiece 'input aperture' (should that be field stop?).Would there be any added field of view at all if the small secondary only directs light through the middle portion of a large field stop eyepiece?My concern was that telescope manufacturers might put in big focussers as a mere fashion statement. I feel a bit silly saying so. Getting cynical in old age perhaps.cheers...Mark
Quote:One way to think about this is to consider that you are looking at the primary mirror through a pinhole in the focuser.
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Quote:Thanks Jon and Ted.I am now equipped to demonstrate a greater depth of ignorance...How come a bigger eyepiece gives a bigger real field of view if even a pinhole will see the whole primary mirror. Surely the eyepiece diameter doesn't increase the FOV seen by the primary mirror?Newbishly yours...Mark
Quote:Glenn and JonThank you for your valiant attempts at clearing the fog. I think I'm getting it about off-axis partial illumination in the eyepiece, though questions remain. I assume that if an eyepiece field stop can 'swallow' the whole focal plane, the maximum real field of view is seen through the exit pupil. Here's the thing: if the curvature of the primary is designed so that the whole focal plane is 'swallowable' by a 1.25" eyepiece, I don't get why a 2" eyepiece should increase the total field of view.
Quote:This raises another question (sorry). If the entire light cone from the primary, when it reaches the eyepiece, is smaller than the eyepiece field stop, why isn't the real field of view of the eyepiece the whole 7 degrees of the scope?
Quote:why isn't the real field of view of the eyepiece the whole 7 degrees of the scope?
Quote:This brings me back to the title of this thread. I suppose that if the minor axis of the secondary mirror (34.5mm) of a 6" F8 dob is only 0.75 x the max field stop diameter of a 2" eyepiece(46mm), and the secondary is close to the field stop, the max real FoV may be constricted a bit by the secondary, but who maxes out the FoV anyway?
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Quote:is this still the beginners forum?
Quote:there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece".
Quote:Wow! Quite a few interesting posts here. I don't know that anyone answered your practical question though. If you're like me and you don't use a finder scope, then there is a point to buying a 2" EP for any scope that will take it: the wider field of view will help you in a "finder eyepiece". Poorly-illuminated, ridiculous giant exit pupil, no matter. Having a 2 or 3 degree true field of view to look through makes star hopping *much* easier. To say nothing of the aesthetic delight of such a rich field.
Quote:OK then! Guess I'd better start saving for a 2" wide field eyepiece and a 2" barlow/telextender.A pity the Orion 6" dob only has a 1.25" focuser, but the Saxon/Skywatcher 6" dobs don't have intelliscope. Oh well.Thanks All...Mark
Bryan Stone Past President, NSAAC (North Shore Amateur Astronomy Club) Visual Astronomer with: "GoldenEye" XT8 Classic, flocked, Moonlite CR2(Gold), Telrad, 1st Base mount Serial #001 "Night Hawk" SV 80mm f/7 Aplanat on M1/GNG TV 11NT6, 32P, 1.8xB, Pentax XF 8.5 ES 82 N2 6.7, GSO SV 30, UO KII 16 Lumicon OIII,Orion Ultrablock, Moon Filter, 4 color filters, 25mm Sirius P, 10x50 Explorer Binocs, Celestron broadband filter,Tectron collimation tools, Pelican 1500 Divider Case, StarDust chair
Quote:the real FOV is identical to the angular width of the field stop if viewed from the centre of the objective lens.
Quote:1. Why is it assumed that the rays from the distant object (the top, middle and bottom of a distant tree, say) all pass through the centre of the objective lens?
Quote:Surely they spray all over the objective.
Quote:Is it that the spray from a particular bit of the tree is either focused to the same point as the ray through the middle
Quote:2. Just confirming: Is the focal plane located at what I used to think of as the focal point? (ie one focal length from the objective/primary?)