Celestron f6.3 reducer - do not use ep more than 35mm?

[Starslayer]

As per title.  I have my new focal reducer and have played with it daytime. Good result with solar and also with terrestrial  'photogs' using phone mount.   So having, as usual, referred to the instructions supplied after playing with it ( well it is not difficult to work out is it? ) I see they state in BOLD type that with this reducer attached you should not use with EP over 35mm!  What is all that about.? It does not say why.  I have used today with my low power 40mm plossl and now I read this but google does not help.  Any ideas guys?  Edit - it further says this is because of the short focal length of the telescope. Still does not explain why. 

2nd edit - found this which sort of suggests why but does not say if it causes damage to kit or user.

Visually, some reducer lenses also allow rich field viewing with medium focal length 1.25” eyepieces. For example, the standard 26mm Plössl that comes with most f/10 Schmidt-Cassegrain scopes effectively becomes a 41mm eyepiece with an f/6.3 reducer, with a 149% brighter image and 56% wider field than its normal view. Some vignetting and a deterioration of the image quality at the edge of the field may be visible, however, particularly with wide field eyepieces. Eyepieces longer than 35mm in focal length are not recommended for use with a reducer lens.

Edited August 4, 2021 by Starslayer

[Louis D]

It has to do with vignetting.  It's not so much the eyepiece focal length as the field stop diameter.  It's just that 32mm to 40mm 1.25" eyepieces generally have the max field stop size possible, about 27mm.

Some people have put 2" diagonals on focal reducers and used with much larger field stops in 2" eyepieces with some noticeable vignetting.  Some claim it goes to complete blackness near the edge of a 40mm SWA class eyepiece with a 46mm field stop.

All this is the result of the focal reducer shrinking the effective size of the image circle at the output end of it.  It squeezes the existing image circle to 63% of its original size.  This allows eyepieces with 63% of the field stop diameter of a max field 2" eyepiece to show the same amount of true field.  Thus, a 27mm field stop shows as much sky as a 27/0.63=43mm field stop diameter 2" eyepiece.  It's simple to show that using a 46mm FS eyepiece with an FR would try to show as much as a 46/0.63=73mm FS eyepiece.  This just isn't going to happen with the size of the SCT's rear baffle and the size of the FR lenses.

Edited August 4, 2021 by Louis D