I have been dreaming of building a binocular scope for a while now. While designning different systems I came to the realization that many of the finest wide-angle eyepieces were simply too large for binoculars. This is very unfortunate, since binocular scopes would perform most impressively in wide field views of the milky way.
Most large binoculars are designed with an inter-pupilary distance of 56-72mm. This range should match the inter-pupilary distance for more than 90% of adults. The eyepiece size is restricted by the lower bound of 56mm. Only eyepieces with a maximum outer diameter of 56mm or greater can be used in a binocular telescope. This places restrictions on the widest FOV vs. f/#.
This restriction rules out some of the best and biggest eyepieces. The following partial list identifies eyepieces which are too big for binocular use. I am not sure about several eyepieces, so I would appreciate feedback about them. Also, there may be mistakes in the table, so please post to the list if you find any.
AFOV Too Big Not Sure Small Enough Tele vue Nagler 82 deg 20,13 16,12,9,7,4.8 Panoptic 68 35 27 22,15 Wide Field 65 40 32,24,19,15 Mead Ultra Wide 84 14 8.8,6.7,4.7 Super Wide 67 40,32 24.5,18,13.8 Orion MegaVista 67 40,34 16.8,10.5 Erfle 65 32 Parks Erfle 65 32
It would be great if other ATM's would measure the maximum outer diameter of their eyepieces and post their results to the list. My eyepiece collection is very limited, but I would like to purchase eyepieces which could later be used in a binocular scope (someday).
Lets look at an example assuming the largest available eyepiece is the TeleVue 32mm Wide Field with a 65 degrees Apparent VOF. To get a 7mm exit pupil, you would need an f/4.57 instrument. To eliminate coma in a Newtonian this fast I would recommend a Coma Corrector like a TeleVue Paracorr. Which adds a x1.15 magnification. So the Newtonian will need to be even faster, an f/3.975 to get a 7mm exit pupil. This binocular scope would theoretically have the largest FOV available, given current eyepiece selection. For example, an 8 inch Newtonian F/3.975 with a Paracorr will give a focal lenght of F=36.57 inch. Using the TV 32mm WF eyepices will provide a x29.0 magnification with an amazing 2.2 degrees field of view for both eyes.
As this example illustrates larger eyepieces would be required to get a faster f/# (with less coma) for a 7mm exit pupil. If you are willing to accept a smaller exit pupil (and smaller AFOV) then a faster f/# can also be used. For example: an 8 inch f/6 instrument will give a 5.33mm exit pupil (=32/6) with magnification of x38.1 and a FOV of 1.71 deg.
I would appreciate your comments and eyepiece recommendations.
Thanks, Tal TAL@Johnson.Cornell.edu