(ATM) Re: tube diamater

[Chuck Dethloff <dethloff@uofport.edu>]

Several posts regarding tube size and currents have recently appeared. 
Tube currents can be a problem if the ID of the tube is not sufficient. 
Installing a fan on the bottom of the OTA, or mirror box, will help reduce 
tube currents if you use a smaller than normal spacing. I suggest open 
ended mirror boxes to allow for air to flow down and out of the telescope 
without pooling in the bottom of the mirror box. 

I do find that on truss tube telescopes allowing .75" spacing around the 
edge of the mirror for sizing the ID of the uppper end to be 
satisfactory, even when running a shroud. Tube currents do occasionaly occur 
but they are easy enough to remove by pulling the shroud up for a few 
moments. 

Chuck Dethloff
Forest Grove, OR