Quote:
Originally Posted by Wavytone
So far no-one with a scope faster than f/5 has fessed up concerning min/max magnifications ... Anyone ?
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Well my 18"/F4.5 isnt really typical of the new breed of compact faster (sub F3.5) newtonians but I have no trouble running to .5mm exit pupils, in this scope, when conditions are ideal.
I have on several occasions used my 5mm Pentax XW combined with a 2.5X TV powermate for 1025X. It is very rare that conditions are good enough to support this but I can often get to 735X with my 7mm Pentax XW in the 2.5X Powermate. Of course you can't get there all the time but the thermal stability of the optics and the atmospheric conditions are the limiting factor, not the optics themselves.
I have no doubt that a 20"/F3.3 telescope could run .5mm exit pupils under ideal thermal and atmospheric conditions. For instance a 3.5mm Pentax XW in a 2X TV barlow gives a .53mm exit pupil and 950X. A 5mm Pentax XW in a 2X TV barlow will give .75mm exit pupil and 670X.
Of course not all observers can run with .5mm exit pupils and as you get older this ability diminishes slightly. I am 53 years old and can use .5mm so I see the limiting factor to be factors other than the telescope and the optics, namely, the atmospheric conditions and thermal equilibrium.
I haven't factored the paracorr into the equation here as all my scopes have tracking and I do not use the paracorr for high power work as the tracking enables me to keep the view "on axis". If you wanted to use the paracorr for high power views you would just need to adjust the eyepiece/barlow accordingly.
At the low end of the scale, I am happy to push to a 7mm exit pupil on a regular basis. In a 20"/F3.3, with paracorr a 26mm Nagler will give a 7mm exit pupil for 75X and a TFOV of 1.1 degrees. I have on many occasions used larger exit pupils, but that just wastes some aperture IMO.
Cheers,
John B