Exit Pupil Size and my own eyeballs

[Poita]

How does one measure the pupil size of their own eyeball?

My understanding is that if the exit pupil size of your eyepiece is larger than your own eyeball's open pupil size then you will have a lot of light not entering your eye, so contrast and detail etc. will suffer.

(Please correct any of this if it is wrong, I'm still getting my head around it).

Exit pupil size is calculated by the diameter of your scope (e.g. 10"/254mm) divided by the magnification of the eyepiece.

The magnification of the eyepiece is calculated by the Focal length of your scope (e.g. 2500mm) divided by the eyepiece size (e.g. 20mm)

So for a Meade 10" SCT with a focal length of 2500mm and a 50mm eypiece you get:
Magnification = 50x (i.e. 2500 / 50)
Exit Pupil = 5.08mm (i.e. 254 / 50)

With a 10mm eyepiece you get a magnification of 250x and an exit pupil of only 1.016mm! is that correct?

I have no idea how to measure the night-time pupil of my own eye, would the 50mm eyepice with an exit of 5.08mm be likely larger than my pupil and make the eyepiece somewhat useless, or are you pretty right with any size eyepiece when you only have a 10" scope?

[Merlin66]

A 10mm eyepiece as you say will only give a exit pupil of around 1mm.
I find a 32mm is as far as I can realistically go....(I do have a 40mm which works occasionally)

You could try measuring your pupil in subdued light, a clear plastic ruler and a mirror....the other method is to hold various width objects ( drill bits are handy) infront of the eye until the object being observed disappears....

[Poita]

I see a drill-bit related injury coming on...

[mishku]

Maybe it's a job for the pros? Have you had your eyes tested in the last twelve months? Perhaps if you politely explain to your friendly local optometrist, they'll be able to dim the lights and measure for you as part of a standard, bulk billed exam... :thumbsup:

[Robh]

Hi Peter,

Here is a fairly safe way to measure your pupil size.
Make a template similar to the one below using cardboard.
The width between each of the slit pairs is in millimetres.
Cut out the template using a stanley type knife carefully cutting out the slits. Just don't cut yourself.
The fold is to give it a bit more rigidity.

Procedure:
1. In a pitch dark room put a subdued light about 3 to 5 metres away. A red light torch turned down is ideal.
2. Put the cut template on the bridge of your nose with the fold forward.
3. Close one eye and slide the template slits past your open eye. I usually work down from 9mm to 3mm.
4. At the higher end, you will only see light through one slit. Move down until you just see light through both adjacent slits. That is approximately your pupil size.

Regards, Rob

[Poita]

That is fantastic, I've been wracking my brains trying to come up with a method. I'll give it a go.

Thanks!