Posts Made By: Roger Persson

I've followed a variable star for some months now. The problem is that I have never been able to see it! So I want to try a film shot through my Dobsonian on Equatorial Platform. What is the fastest film available - say for a 30 seconds exposure time?

Roger

An optically perfect and collimated 6" Newton telescope is going to show more details on the planets than a 4" APO refractor according the laws of physics. But there is a big caveat with the Newton and that is the time it takes to cool down to near ambient temperature. If you use a fan and are willing to wait an half an hour until the heat turbulence layer above the mirror disappears I recommend the Newton. The optical path in a Newton is going twice through the tube and thus is twice as much affected by air density irregularities than the refractor.

Personally I would choose a high quality 10" f/5 Newton for planetary studies because I can not afford 5" to 8" APO refractors and I'm willing to wait for the cool down time. Also you get more resolution, light and contrast than the 6" Newton.

Google for Thierry Legault. He has some good informative pages about high resolution. Especially read his page about obstruction.

Roger Persson
(8" f/6 and 20" f/5 dobson)