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View Full Version here: : Modern refractors are too short!


The Mekon
29-04-2012, 12:33 PM
Tried to do some photography last night using an eyepiece projector. Found I had to have the camera adaptor, projector extension + 35mm extension just to reach focus. The whole affair was 230mm long! These new refractors are just made too short in the tube. The astro-tech 106 that I am using has 80mm of focus, and will not reach focus with a 1 1/4' prism. It is about 15mm too short. Last night it was about the same short of working without the 35mm extension. Why cannot the builders of these scopes give us a decent length tube? An extra 20mm on the tube would not make the case any bigger, and make using the scope so much easier.
I am sure Astro-tech are not the only makers guilty of this, but they must be one of the worst offenders looking at some of their other scopes which hardly have much main tube at all.

Wavytone
30-04-2012, 12:02 AM
Old-school refractors normally had a large-diameter stiff sliding sleeve that was used to make large changes to the back focal distance to accommodate eyepiece projection and other odd bits of gear such as filar micrometers. The rack & pinion was for fine focussing.

Modern cookie-cutter refactors are really only set up for CCD's.

Wavytone
30-04-2012, 12:03 AM
Then you'd be wanting one of these new-fangled old-style f/13-15 refractors instead of the usual CNC cookie-cutter short-focus things that are made for "imaging":

http://www.skylightelescopes.co.uk/

Now that is what I remember as a decent refractor...

Profiler
30-04-2012, 06:32 PM
If you look at their catalogue Vixen still do sell various bits and pieces for rear projection so presumably there is some thinking/design for these options in their scopes.

bert
30-04-2012, 09:18 PM
The shorter the better for me....

Focusers, ao, rotators all chew focus. Tube extenders are far easier than cutting tubes.

Brett

bmitchell82
30-04-2012, 09:51 PM
Plus 1 for me with Brett!!!!

you add in all the imaging fruit next minute that 80mm turns into 20mm. And you don't want 2kgs of imaging kit hanging at maximum draw tube extension!

With the advent of the Dob and massive light buckets why the bugger would you want to spend the dollars on a high end refractors with 1/4 the light capturing ability for visual work? Smart play i thinks by the manufacturers.

BM