Well, it’s of course a trade off.
It can be simple, but one can complicate this up – if one like ;-)
If go bigger and stay on ground it will be around 25% or more ( depends on diameter ) but very big diameter and ‘fast’ can also get down to around 20%.
But go bigger diagonal slightly get a less aperture, so in percent it’s very small, and in mag drop here is not really the issue.
But a to small diagonal will drop easy 0.4 mag or more ( lower powers ) and some say it don’t matter for visual use but that reduce ex a 18” to a 15” aperture.
But all this comes into where one observe and on what targets normally.
I has to my 18” Dobson 3 diagonals at ; 3.5”, 4”, 4.5”.
The Antares in Pyrex 3.5” is tested trimmed -5 mm ( 3.4” ) to around 1/20 w, the Antares in Quartz 4” needs to trimmed as test to a 3.6” and it is a 1/10 w and the 4.5” is tested full out to edge (?) at 1/20 w ( will be used as 4.4” )
A lip will be on them all.
Idea was L-distance/focal so that was around 3.3” so a 3.5” should be fine minus a lip at 0.1" ( mag-drop in Bartels program at around 0.3 at E21 mm )
I thought had little to much mag-drop on the 3.3” C-A, so I order a 4”, but I think it went to low in tolerance and it was to little C-A.
The 4.5” is a great diagonal and I has ‘room’ and at zero mag-drop, but will obstruct actually with the holder over 25%.
But I’m not observe planets, just deep-sky and most eye candy.
( Scope do not track and seeing is always a issue )
On Mike L’s formula.
That one is based from a TV coma corrector.
From end of tube to std focus pt it is 88.5 mm ( data from TV = 90 mm ) but add up a filter at 5.5 mm and a clearance to mirrors edge ( say 18 mm if ex E21 mm ) one get ; 18 + 5.5 + 88.5 + 2 mm extra it is 114 mm = 4.5” ( Mike said 4” or ‘whatever’ one like to use )
One idea is CAD up the light cone and check the diagonal distance and add 0.5” at least, and see what one get for diameter and then go up to next std size ( but do see your test report from manufactory ) and a lip will be at least 0.1” for best of made diagonals.
In my case I will learn if a 3.5” will gain ’my’ observing targets and sky/mag, vs to the bigger 4.5”.
In my case a 4” would be maybe best, but as if low powers and the use of wide field EP and to be coma free ( Paracorr and at f/4 ) and USE the full aperture - I like to try the 4.5” deal. ( if no gain I use it to another project or sell it )
Next thing is also the tolerance on a diagonal and stiff mounted and keep out stray light and keep heat from body and ground.
I had a thread on faint fuzzes and the use of a very small diagonal for 1.5 pupil observing, and in that case ex a FS at 7.5 mm and a full diameter to FS and no coma corrector and the Delites EP and a f/4 or 4.5 parabol and a drilled out fat blank in Zerodur.
Context was said ; today with the good fast/big ones in plano Pyrex this is a crazy idea I was into as those new will wipe what I was talking about EASY.
-Right or wrong, well...
Edited by hakann, 30 October 2019 - 10:12 AM.