It's a Dynameter and is generally considered optical test equipment and quoted by the job. I've not found any model with a published online price. Would be cool to have one of these laying around...
I don't think it would be too difficult to make one yourself. All you need is an old eyepiece with diopter adjustment (focus) from a binocular and a millimetre grid acrylic ruler.
Simply cut a circle with the gradient crosshair in the middle and glue it to the field stop. Then use what ever you have lying around to fashion a short telescoping tube element that allows the gradient crosshair to be positioned at the exit pupil. You also get bonus points for frosting one face of the gradient crosshair you cut from the ruler
I have not completely made one myself as I don't often feel compelled to precisely measure the exit pupil, but when I am... I simply use a 32mm plossel with the barrel removed, some plastic washers and a precision ruler
Here are a few pictures to give a general idea of what I mean:
Here is the setup:
And what I see through the eyepiece... of course it was a bit difficult to get everything lined up and hold the camera at the same time.
These are military 8x40's (Huet Marine) and as you can see, the 5mm EP is correct.
Edited by Mad Matt, 13 December 2018 - 05:27 AM.