a little math help

Started by Greg Shaffer, 11/14/2009 05:35PM
Posted 11/14/2009 05:35PM Opening Post
In the planning stages of expanding my observatory with the idea of eventually installing an 8" F/18 OTA. Just how does one figure out how long the tube is going to be on an 8" F/18 refractor?

Thanks
Greg

20" MidnighTelescopes f/5
8" f/16 Muffoletto Achro
Meade 6" AR6 f/8.0 Achro
Celestron C11
Parallax PI250 10" F/5 Newt
Vixen FL102S,NA140SS,R200SS
80mm Stellarvue Nighthawk with WO APOupgrade lens, 90mm f/16.67 Parallax Achro
LXD75/LX200, Mini-tower, GP-DX, CG5 ASGT, LXD650, GM-8, G11, GM-100, GM-150EX, GM-200 and a Gemini G40
4" portable AP convertable 44" or 70" pier
Stellarvue M7 Alt-Az, TSL7 Pier/Tripod
12.5" f/5 MidnighTelescopes DOB Swayze optics
Monroe GA
Posted 11/14/2009 06:20PM | Edited 11/14/2009 06:22PM #1
In the planning stages of expanding my observatory with the idea of eventually installing an 8" F/18 OTA. Just how does one figure out how long the tube is going to be on an 8" F/18 refractor?

Thanks
Greg

A reasonable estimate is to assume that the OTA plus the Dew shield will equal the focal length. This would be 8" x 18 = 144inches = 12 feet. This should be sufficient for observatory planning.

I am curious if you were aware of the length of the tube when you decided to go with an 8 inch F/18? By the time you add the mount height required for such a tube, it makes for a large observatory.

Jon