I was wondering if you start seeing a shadow of the secondary at low magnification (large exit pupil) when using low power, for example the 30mm Nagler. I notice this sometimes when observing the moon with my f4.5 telescope, but of course, my eye's pupil is greatly reduced under those circumstances. I would imagine at some point, with increasing exit pupil, it could become an issue even looking at DSOs, but I'm not sure from a practical point fo view at what point this would happen, if ever.
While I'm not the owner of this telescope ( I do have a NMT 20" F/3.5 on order - a little longer FL for significant $$ and time-to-produce savings ) -- I have observed thru a few other F/4 and faster Dobs (F/4 32", 36", and F/3.8 22" ) - including NMT owner Ryan Goodson's personal 20" F/3.3 on about 6 nights -- with Paracoor = F/3.8. It was only under very dark sky ( SQM = 21.7+ ). Ryan himself had a 31mm Nagler and, being a 'star hopper' with no electronics on his scope - he regularly used the N 31 at least for hunting down targets.
The Televue 'eyepiece calculator' says N31 + Paracoor + 20" F/3.3 = 61.2 power, 1.3 degree true field of view, and 8.2mm exit pupil == that would be - in theory, way too big of an exit pupil that would certainly show the secondary shadow. Televue recommends not going larger than 7.5mm EP and my own eyes only open up to 6.5mm. Yet -- I remember quite pleasing views thru Ryan's 20" F/3.3 using his Paracoor and N31. I would guess that the shadow could be seen under brighter light polluted skies. Using a N31 with a scope this fast is probably a compromise - but usable at times - especially if looking for a target, or maybe with an O-III filter at something like M-42 or the Veil. For everyone but the 18 year old Jolly Green Giant - you would be thro'ing away a significant part of the light gathered by that 20" primary - but for certain targets - it might not be an issue.
Looking at Televue's 'eyepiece calculator' the widest field of view that can meet their recommendations of 7.5mm exit pupil is -- a 21mm Ethos = 90.4 power, 1.1 degree field, 5.5mm exit pupil. This is probably the widest true field that will use all of this telescope's collected light, with no 'secondary shadow' for most people. "Close enough" other TV eyepieces include the discontinued 26mm Nagler ( I own one and like it ), the 22mm Nagler ( I don't own one, but friends do - excellent ep ) and the 27mm Pan ( never looked thru one ).
BTW - I have observed thru a friend's 36" F/4 and an Explore Scientific 30mm 100 degree 3-inch eyepiece -- which probably violates all 'the rules' -- but scanning over M-31 looked like a CCD image -- and could keep me happy all night -- or until I fell off the ladder!
Bottom line - in my opinion: Owning a telescope like this - 'premium' 20+ incher faster than F/4 -- will mean living with some compromises - but still fantastic views from a telescope that does not require a ladder and most adults would find very easy to move and assemble and use - with only minor to no difference compared to a 'classic' 20" at F/5 or F/6 ( something that I've owned for 18+ years ). A scope like this NMT *will* require very high end eyepieces (but you could live with only 3), some extra precision with collimation, and a desire to buy a neat little APO refractor for wide-field views - or in my case, 20x80 binoculars. After being convinced by the views I've seen, and realizing at age 73 my 'ladder days' are coming to an end - I ordered a NMT 20" F/3.5 -- and would have gotten a 22" if not for the $$ increase. If -- you can 'live easy' with ladders, more than twice the weight, and longer more complicated to assemble structure - at a considerable $$ savings (probably used only today) -- then a 20" F/5 'Classic' Dob could be a better choice.
Edited by George N, 01 May 2021 - 11:12 AM.