ST80. Is it me?

[Louis D]

Okay, I dug out my 9mm Morpheus eye cup and its extension ring and tried them on the APM UFF 24.  Everything easily threads together.

The Baader eye cup alone is 1mm shorter than the APM's OEM eye cup.  Without eyeglasses, I preferred the view with the OEM eye cup, but I was just barely touching the eye cup, so stray light could be an issue, but jostling the scope would be minimized.  The Baader alone was just too short.

The Baader eye cup plus extension is 7mm taller than the OEM eye cup (8mm taller than the Baader eye cup alone).  I found I had to mash the eye cup into my eye socket to see the entire field without eyeglasses.  It's pliable enough that this works well.  It sealed pretty well around the perimeter, so stray light would likely not be an issue; however, scope jostling could be.

I did notice in daytime usage that just as the field stop comes into view, a shadow ring appears just inside it if you continue to look on axis.  If you pull back enough to get rid of the shadow, you lose the field stop.  If you look at the field stop and tilt your head properly, the shadow goes away.  Perhaps this is what the OP is seeing?

Here are the comparison images.  The Baader eye cup alone looks almost like the OEM one.  It's much less stiff, though.  I suppose the Baader could be unscrewed a bit to raise it up since the APM only has about 2 eye cup threads to grab onto anyway.

Left to right are the OEM, Baader, and Baader+extension eye cups:

[cwis]

32 minutes ago, Louis D said:

I did notice in daytime usage that just as the field stop comes into view, a shadow ring appears just inside it if you continue to look on axis.  If you pull back enough to get rid of the shadow, you lose the field stop.  If you look at the field stop and tilt your head properly, the shadow goes away.  Perhaps this is what the OP is seeing?

That sounds about right What  causing this effect?

I must have deep set eyes because I'm mashing my face into the eyecup to see the field stop... But if I fold it down then I get blackouts etc - too close.

[Louis D]

2 hours ago, cwis said:

That sounds about right What  causing this effect?

I must have deep set eyes because I'm mashing my face into the eyecup to see the field stop... But if I fold it down then I get blackouts etc - too close.

You might be right after all that it could be a very mild case of exit pupil aberration.  It just doesn't manifest itself as kidney beaning (SAEP), though.

I see a much more severe version of it in the Meade MWA 26mm where the outer 2 degrees is completely black until you get close enough to see the field stop.  At that distance, kidney beans start dancing around the mid-distance part of the view.  At no point could I induce similar kidney bean shadows in the APM.

I'd try the Baader eye cup in your case.  It is 1mm shorter and quite flexible compared to the OEM eye cup.  It's not that expensive to experiment with.