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Binoviewer Light path

Started by DonDurbin, 06/10/2006 08:22PM
Posted 06/10/2006 08:22PM Opening Post
To All,

The length of the light path through your binoviewer plays a role in how it will perform with a given EP.

My new super binoviewer scope, (My William Optics short tube with APO upgrade) can focus any known binoviewer without any needed OCA/OCS in the light path.

With a custom 5.5" extension it can also focus a single EP the way a normal scope would. Tonight I decided to measure the light path through the 5 binoviewers I have been testing with a set of digital calipers. The results were a little surprising.

Using an 18mm Radian and the 5.5 inch extension I brought the scope to focus on Jupiter. I then measured the distance between the edge of the tube and the edge of my 2" diagonal. The base number needed for focus turned out to be 204.39mm.

I removed the 5.5 " extension and focused each of the 5 binoviewers using the same 18mm Radian. I made sure any diopter adjustments were full down and that the EP was fully seated. I then made the same distance measurement I had made with the single EP and subtracted that from the base number to determine the light path of the binoviewer. The results are listed below in order of the shortest to longest light path.

1. Burgess 24 107 mm
2. Burgess BV125C 107.76 mm
3. Denkmeier Dielectric 125.47 mm
4. Siebert 22mm BN with SCD's 136.7 mm
5. Televue Bino Vue 139.08 mm

I can probably give a good guess at why these numbers are off from the advertised numbers. I think the light path was measured on units without Self Centering Diopter adjustments. I know the Siebert SCD's are very tall. The Burgess Diopter adjustments are very low profile.

In any event these are real numbers that were measured on a refractor. Try plugging these numbers into the spreadsheet that Tom Hole supplied in an earlier post and you can see just how much of a change in actual illumination the light path can make.

On a side note, if you view with the power switch attached to your Denkmeier it will add another 12.65mm to the lightpath.

These measurements were taken with the standard nose piece on the Burgess, the 1 1/4" nose piece was placed on the Denks and the power switch was removed.

Don Durbin
Posted 06/11/2006 07:30AM #1

Good data. Hard to tell what the manufacturers might have used to determine path length, but your data is what is the most interesting and normalized. Were you able to measure your 2" Elites?

Posted 06/11/2006 11:06AM #2
Useful data! Thanks!
Posted 06/11/2006 04:40PM | Edited 06/11/2006 04:45PM #3
Did you account for the fact that the focal length thru a prism is not the same as the physical light path length thru a prism.

When you measured with your mirror diagonal there was no shortening or lengthening of the light path vs the focal length. The length of the light path is the focal length.

When you measure with a prism in the path, to get the focal length you must know the physical dimensions of the prism. You must determine the physical distance the light travels thru the prism.

Light slows down thru glass. So the focal length thru the prism can be found by dividing the length of the prism light path by the index of refraction, typically about 1.52 to 1.56. If the physical length of the path thru a single prism is 50mm, the the focal length used up thru the prism is only 50/1.56 = 32mm.