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Old Tuesday 22nd November 2016, 07:38   #26
Binastro
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Join Date: Aug 2012
Location: S.England
Posts: 3,343
Actually for long distance work a classic 5 inch or 6 inch f/15 or f/16 refractor would be best.
A heavy duty mount is needed.
An observatory quality instrument if possible.
The main problem is temperature gradient and local Seeing conditions. Early morning and late evening are usually best, with the Sun behind you.

Sticking with Pentax, a good example of the 100mm f/12 would be a smaller option. This can take high magnification say 250x at least at 3 a.m. but in daylight maybe 200x in the best conditions. My one was very good indeed.
A Russian Tal 100mm f/10 has good reports.

For a present day option I would choose a Skywatcher 120mm or 150mm refractor with as long a focal length as possible. The simple doublets are the cheap option.
There are older 6 inch f/10 refractors also. D and G I think.
A 120mm or 150mm ED doublet or ED triplet Skywatcher would be good. These are 1,000 to 4,000 for the Optical tube assembly and more with a mount. A really heavy duty mount is needed.
I used an ex. gov tripod able to take an adult's weight easily.

For me 5 miles was the usual limit to give good views.

For 20/15 vision I think that 7 miles is the furthest one would have a chance at reading a car number plate in optimum conditions with a fine 120mm aperture refractor.
I used the refractors without any diagonal or prism. Views are upside down, which is not a problem for an astronomer.
A 1/10th wave or better star diagonal would be the next option giving a laterally reversed image. I would avoid prisms, although one could try a Televue or Williams.

Long focus refractors are not only better because of optical considerations, they also are less affected by tube currents and changing temperatures than short focus refractors. Maybe the shallower curves on the objectives also help.

Another choice is a 6inch or 7inch Maksutov left outside for an hour before observing to stabilise the temperature. Either Skywatcher, Russian or a Quantum 6inch or Questar 7 inch. Again a very heavy mount is needed.
My 5 inch SCT was not good as there are temperature effects. Same with the 8 inch, but on occasion they were good.
The 12.5 inch Dall Kirkham would make easy work of a number plate at 5 miles in optimum conditions.

Last edited by Binastro : Tuesday 22nd November 2016 at 14:00.
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