pupil_gauge.htm

Pupil Size Gauge
Maximizing the aperture of your telescope requires that you consider your personal optical equipment!

When you are choosing the eyepieces to use with your telescope, do you take your eyes into account? Many folks don't realize that the maximum size of your eye pupil is what determines what your optimum low power eyepiece should be. By using a lower power eyepiece than the optimum, one effectively shrinks the aperture of the scope. Since pupils tend not to open as widely as we get older, it is clear that one needs to adjust to get the maximum performance out of our equipment (personal and purchased).
You can search the net and find many references on the importance of pupil size in determining your optimum low power eyepiece, here are two good ones:

Sky & Telescope, "A Pupil Primer"

When does your 8" telescope deliver less light than a 3"? This title gives an indication of how important paying attention to this can be! This article by Jim Sapp has apparently become unavailable on the internet and the link to it has failed. It is such a good article that I had printed it out, and convinced my wife to retype it in. I scanned in the diagrams and converted it to a PDF of about 9 pages and 250K in size. If you send a request to duchekcs@duchekconsult.com, we would be pleased to email you a copy.

It also has a nice chart of pupil size vs scope speed (F/?) if you don't like the math. I have reproduced the table in the article below.

Find your largest acceptable exit pupil in the left column, and your telescope’s focal ratio across the top. At the intersection of the row and column is the longest usable eyepiece (in millimeters) for conserving the light collected by your telescope.

			F	O	C	A	L		R	A	T	I	O
		3.5	3.8	4	4.5	5	5.6	6	6.3	7	8	10	11	12	15
	3.0	10	11	12	14	15	17	18	19	21	24	30	33	36	45
E	3.5	12	13	14	16	18	20	21	22	25	28	35	38	42	53
X	4.0	14	15	16	18	20	22	24	25	28	32	40	44	48	60
I	4.5	16	17	18	20	23	25	27	28	32	36	45	49	54	68
T	5.0	18	19	20	23	25	28	30	32	35	40	50	55	60	75
	5.5	19	21	22	25	28	31	33	35	38	44	55	61	66	83
P	6.0	21	23	24	27	30	34	36	38	42	48	60	66	72	90
U	6.5	23	25	26	29	33	36	39	41	46	52	65	72	78	98
P	7.0	25	27	28	32	35	39	42	44	49	56	70	77	84	105
I	7.5	26	28	30	34	38	42	45	47	53	60	75	83	90	113
L	8.0	28	30	32	36	40	45	48	50	56	64	80	88	96	120
(mm)	8.5	30	32	34	38	43	48	51	54	59	68	85	94	102	127
	9.0	32	34	36	41	45	50	54	57	63	72	90	99	108	135

$5 includes US shipping; Email for international shipping

I have a 14" F/4.7 and reasonably good eyes for my age (62). My exit pupils measure about 4.5-5.0. I find that my best, ie brightest images come with a 23-24mm eyepiece with that telescope(both by calculation and experience). Even if I had 7.0 mm pupils (about the best the normal young person could expect), ~ 32 mm would be the maximum for this scope. To use the 30-50mm eyepieces effectively one needs an F/10 or higher scope.

Several people put out pupil gauges to test your eyes. The second article tells you how to make one and use it. Mine are adjusted to let in as much light as possible since they are meant to be used in the dark. They aren't hard to construct and so I have. It is basically a series of holes on clear plastic at measured distances apart. Hold your eye close to the scale (too close to actually focus). You move your eye up and down the scale until the holes seem to just touch, and read the distance between the holes. If you do that in a bright room, you will find that your pupils contract considerably (mine are at ~2.5mm). If you do it with just enough light to see the holes, with dark adapted eyes you will get a measure of what your eyes really do under a night sky. Take it with you when you observe and see how dark adapted you get under different nighttime conditions.
If you would prefer not to make your own, I have made these available at $5 including shipping. The picture shows it printed on white paper background so I could scan it in. The tester is on clear plastic so you can see through the holes.

Also, since I relearning to program computers, I have developed a program that will calculate the match between your maximum pupil size and over 200 eyepieces. It will also estimate how much light will be lost if any in the combination of its exit pupil in your telescope and your eye. It also calculates their FOV while it is at it. It is listed below.

Whether you make your own or buy one here it is important to match your scope to your personal pupil parameters. It may be the easiest and cheapest way to start using a larger aperture instrument!

Using the pupil gauge:

Hold it up close to one of your eyes and look through it, It will not be in focus, it will be too close to your eye for that. Start at the bottom. The black line in the middle should be overlapping itself since your pupil is seeing around both sides of the 1mm black center slit. Move the instrument slowly downward (or your eye upward). As the slit gets wider, there will come a point where it will appear that the clear areas just touch (they are out of focus, but one can still see them). At that point the black is completely blocking the pupil. Move the instrument away from the eye and read which one you were looking at. That tells you the width of your pupil under current conditions. If you move the instrument further downward, the black line gets thicker. Repeat with your other eye.

If you try it in a completely dark room you will see nothing as there is no light to see with. If you try it under the stars AND let your eyes dark adjust first, there is enough light to see where the edges touch (if you are under very dark skies it can be tough). It also works in a fully lit room where your pupils are only open 2 or 3 mm but they will read 2-3 mm, not 4-7 mm. You can practice in a lit room, but expect lower values. The number will change depending on how much light is available since your pupils open and close based on the brightness of the area you are in. If your pupils only open 3 mm (and there are people like that, especially older ones), then they may be fully open in a somewhat lit room as 3mm is usual for normal interior lighting. Those people usually do not have good night vision in general. Young people in the dark get numbers up to 7 and even occasionally 8. People vary widely and some young folks don't open as much as others. Some old folks can still manage a 7 also. In general your pupils will open less the older you get.

Give this program your maximum pupil size, telescope aperture and speed (F/ratio). It will tell which of 284 eyepieces will have exit pupils your size or less. Delivered by email attachment, $5.00 Runs on Windows 2000 and later. If your eyepiece of interest is not in our database, just email its name, size, and Apparent Field of View and I will send you back a copy of the program with your eyepiece(s) included. Program is available in Windows and in Linux. For $5 registration, you get either or both by email. Upgrades with current eyepiece database are free for registered users by email request.

(c) John R. Duchek, 2006-2010

Note: On first run, you may get a text file error. This is because when you quit it writes a data file to save your latest pupil size, aperture and F/ratio.
Simply clear the error and the program will run. Press quit and the file will be written. Run again and it will find the file it wrote. Calculations are done whenever you click on an eyepiece.