(ATM) Telescope tube sizes & tube fans for 12.5-inch f/6 & f/5 Newts.

[OPTIXWIZ@aol.com]

In general, 14-inch (OD?) tubes are too small for thermal and vignetting
reasons for 12.5-inch f/6 & f/5 Newts. (Tube length should be at least equal
to the FL of primary mirror, as a general rule. A somewhat longer tube, like
longer by 1/2 diam. would benefit baffling. Then cover inside with Edmund
Scientific Optical Velvet & paint seam with Krylon Ultra Flat Black paint,
but not thick at any one coat.) 

However, IF you are willing to install one or more muffin fans, such as are
used as the main cooling fans in PCs, behind the primary mirror, to pull air
down the tube for eliminating convection currents, then go ahead and try the
14-inch diam. tubes.    (VERY IMPORTANT: PULL AIR DOWN FROM TOP OF TUBE DOWN
TO BOTTOM.  NEVER---NEVER---PUSH AIR UP FROM BOTTOM TO TOP----NEVER,
NEVER!!!)    The vignetting issue is so minor as to be a "don't care" for
most purposes.

The Parks fiberglass tubes are something like 16 or 16.5-inch OD. I used to
use these for 14.5-inch fast (f/4.3, 60-inch focus) Newts. & moderate
Cass.(f/3 primaries) telescopes. They worked out just fine without fans. 

Regards,

Gene Cross