Re: [ATM] Ideal Magnification for Public Terrestrial Telescope?

[aart@insightbb.com]

One thing to consider is the exit pupil.  For example at 16X your 80mm Nexstar objective has a 5mm exit pupil, which is much larger than the daylight adapted iris diameter.  Assuming people go hiking to your mountaintop on pleasant, bright days, their irises would be contracted to about 2mm.  To utilize the full diameter of your objective lens the telescope's exit pupil therefore should not be larger than 2mm, which corresponds to a minimum 40X magnification.  But more practically, if you have to use the telescope standing up with your legs wobbly from the hike and your head weaving about it's nice to have a large exit pupil to help keep your iris on target.  So I would use an objective and eyepiece combination to achieve a magnification that results in a 5-7mm exit pupil.  Your smaller iris will effectively stop down the objective lens and so it will theoretically lose a bit of resolving power but daytime seeing over horizontal distances isn't very good anyway, and if the objective isn't too good to begin with it will perform better when stopped down.

By the way, Bill Vorce sells what appears to be a nice cheap 32mm eyepiece kit consisting of lenses and spacers that might be good for this purpose.  Hunt around

http://stores.ebay.com/Telescope-Warehouse

 

Aart Olsen
Urbana, Illinois
_______________________________________________
ATM mailing list http://www.atmlist.net/