Home Forums Hunting & Shooting Ask The Gunwriters Ken! I must have been out of the room when they explained muzzle flash...help!

" ...according to all the optical engineers I've consulted, photopgraphic light meters don't work very well for telescope brightness tests....."
<br>The SEI Photometer isn't built and doesn't operate on the same principles as ordinary photographic spot meters, which generally rely on photosensitive cells. The SEI uses an inner light, first set to a standard brightness by rheostat, to which it then compares the brightness of the image covered by its small spot. But the difference may not be significant in this application. Also, the SEI is more nearly analagous to the human eye peering into a scope -- I don't stick my head through a snug collar into a brightly lit sphere to look through a scope, and I'm sure you don't either. As for me and my druthers, I druther test a scope (or binoculars) with a method that relates as closely as possible to normal, practical field use. I once had a monster of an old military telescope with a humongous ocular (smaller than a foxhole but not by much) and an exit pupil the diameter of a soup can -- not all usable by the average human eye's "entrance" pupil, so not as relevant to field use as lab tests would've implied.
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<br>"The big problem is their small field. If held close to the ocular lens, you're only testing the brightness through the center of the optical system, not overall brightness. If held farther back, at the apex of the exit pupil, ambient light from the side interferes with the reading."
<br>Not a problem with a black rubber shield around the scope's ocular and extending back over the objective of the SEI's scope I have a couple of commercial shields and could easily make another from a couple of inches of bicycle inner tube. And here again, the frequent difference between the diameters of the scope's exit pupil and the "entrance" pupil of the human eye renders lab measurements of total full-field brightness moot at best -- whether the scope's exit pupil is larger or smaller than the "front door" of the peering eye.
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<br>Straying is the name of the game on these boards -- including, often, wandering far afield from common sense. So merely straying from the subject of the original post carries no stigma.

i want to say thanks, but not for the muzzle flash chemistry and light measurement physics.
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<br>certainly those topics were interesting.
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<br>no, i want to say thanks for the sterling demonstration of courteous discourse.
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<br>well done, gentlemen.
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Interesting instrument. The only problem then would be to create a repeatable light source for consistent results. This isn't as easy as it sounds.
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<br>My present brightness testing system uses my own chart of alternating black and white lines, in ever-decreasing widths, set up 25 yards from a 100-watt bulb on DARK nights. Scopes can be repeatedly rated according to the smallest line that can be discerned.
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<br>The problems here are four:
<br>1) Variations in the tester's eye.
<br>2) Variations in the actual brightness of 100-watt bulbs--which I have found varies noticeably! Have found one type of GE that seems brightest and try to stick to that.
<br>3) Keeping the scope absolutely steady, which I do with V-blocks on a window-sill.
<br>4) Having all the scopes tested on the same magnification, which significantly affects "twilight factor." Tested 11 scopes on my last outing, and trying to set all of them on 6x was trial, as the 6x marked on one scope is not exactly the same as 6x on another. It took 30 minutes to get the image the same size on all 10.
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<br>The eye isn't a problem as long as it's relatively youthful, so can open to up to 5mm or so. Older observers tend to rate scopes differently, as their pupils can't open up as much.
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<br>You would be surprised at how closely my results follow lab results from integrating spheres. It would be interesting to see how your system does.

Your .30-378 would still show considerable muzzle flash even with a 32" barrel, or even a 40" barrel. As long as hot gas hits the atmosphere, there'll be a flash. It's reduced with longer barrels, but doesn't disappear.

That was certainly the answer I was expecting from this thread. It was just differant from what I had mistakenly always thought previously and wanted to make sure.
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<br>Cheers
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<br>Tex

Maybe I'm just terminally dense, but I don't see the "problems" that you see.
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<br>(a) The SEI photometer adjusts its own internal light to a standard brightness, to which it then compares the brightness of the image transmitted -- as both are seen through its built-in scope.
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<br>(b) My set-up uses the SEI to measure (1) the brightness of the object as seen by the unaided eye IN THE FIELD and (2) the brightness of the same object's image as transmitted, then and there, through the lenses of the subject scope or binocular.
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<br>Since both sets of readings are direct comparisons, the second using ordinary field daylight (with whatever the Kelvin color temperature may be at the time), there's no need for a separate, day-in, day-out, year-to-year artificial brightness standard.
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<br>I'm much more interested in what the eye sees, afield, with and without the help of the lenses, less in the theoretical technical characteristics of the lens system itself. I think my tests with the SEI are field-realistic in this regard, irrespective of whether any technical lab tests say the same or otherwise.
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<br>And this is a test system that I CAN AFFORD -- not a small consideration, though of course purely secondary to its practicality and reliability.

Ken--No, you're not being dense, I didn't quite grasp your instrument's capabilities. In what terms does it measure?
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My take is that it produces a light at a given value then reads what comes through and then through what ever formula it uses comes up with a comparison of how successfull it was in transmitting said light source.
<br>Now I am no gun writer so maybe I understand better because of that.
<br>The above is my take and also meant to be a little humorous at the end.

In general have found that if somebody really wants to track you down, they will

I've been looking for you for years now. I just want to sniff your hair... <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" /> J/K

It's definitely good to see that you're here. I think it really allows for a bit of input from the readers.


Turok

To the top!

Mike