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I used astronomy.tools FOV calculator to look at various magnifications and although a 40mm would be nice the exit pupil was too big.
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Using a 40mm lens with an 8mm exit pupil :
An oversized exit pupil wastes some light, some light is transmitted through the optical train that can't enter your eye. If your eye dilates to an entrance pupil of 7mm and the eyepiece has an exit pupil of 8mm, then you lose about 24% of the light. Less than the 60% loss using a 5 inch aperture mask.
At >35mm focal length on an f5, the secondary shadow may become visible as a dark shadow in the middle of the field.
The dark centre shadow becomes visible on any f ratio reflector when you use an eyepiece > 7x f ratio.
f3.5 > 24mm
f 4 > 28mm
f 5 > 35mm
f 6 > 42mm
and so on.
For use as a finder,
A GSO superview 42mm 68deg AFOV will give you a bigger true FOV than the 30mm and better image sharpness toward the edges meaning much more of the 42mm's 2.85 degree field will be usable than that of the 30mm's 2.40 degree field.
When I started out, like you, I used a 40mm Kellner eyepiece as a large aperture finder to find faint objects. It gave me 26x and a 1.7 degree true field.
GSO 42mm----23.8x----68deg AFOV----2.85deg TFOV
UWA 30mm----33.3x----80deg AFOV---- 2.40deg TFOV
AFOV-apparent field of view of eyepiece
TFOV-true field of view through telescope
The GSO might serve you better than the 30mm. With such a wide field, you will probably get coma and need a coma corrector to sharpen the edges.
Joe
