Classic refractors incl. Unitrons, Zeiss, Goto Optical, Royal Astro, Edmund; visual observer.
"Toto, I have a feeling we're not in Kansas anymore!"
Refractors Reflectors Two Cats A few eyepieces
Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schiefspiegler,60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4" f/12, 4.5" f/16 & 6" f/12 Schupmann Medial refractors, 4" Celestar, 19 Stellafane awards 9 in optics Engineering = Taking what you have and making what you need.
Quote:0.010 is more than you need for a 3" lens. The difference in expansion gives a delta of 0.0028" for a 50 C temperature change. 0.005 is more than adequate. 23-7 ppm per deg c is 16ppm x3.5" x 50 deg is 2800x10-6 or 0.0028". 0.005 is plenty. I was being conservative with all values.Rule of thumb is minimum of 0.001" per inch
George You know you're getting on when the equipment you farmed with is showing up as suburban lawn ornaments.
Being satisfied with what we already have is a magical golden key
to being alive in a full, unrestricted and inspired way " -- P.Chodrön
My small astronomy tools:
"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori