For my RFT refractor I rather enjoy the 10-15x per inch range.
Good analysis Chuck. I have always used 3.5x per inch as my rule of thumb
for an RFT. As you point out, at any power lower than that you are wasting light.
On Mon, Oct 21, 2013 at 6:32 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
Just for fun I figured some magnifications with common focal-length eyepieces with the 70" scope. I rounded the decimals. The actual telescope specifications may be off a tad, but this is what Mike told me converstionally.
70" = 1,778 mm 427" = 35.58 ft. = 10,846 mm f/6.1
Magnification 55mm 197X 50mm 216X 40mm 271X 32mm 339X 25mm 434X 20mm 542X 16mm 678X 12mm 874X 9mm 1205X 7mm 1549X
This is not a low-power telescope! Even at it's lowest powers, the exit pupil is huge and aperture is wasted, Mike is aware of that and is OK with the compromises required for "low power" use. The secondary shadow is easily visible in the center of the field, a problem all telescopes with a central obstruction have when the magnification is pushed too low. I used to see it on my 8" f/7 Newt when using my 55mm Plossl. The scope produces an exit pupil just over 9mm @ 197X. 8.2mm 2 216X. Not until the magnification is at 271X do we get an exit pupil of 6.5mm and use the full aperture. Some of you who's pupils open a full 7mm can push the power a tad lower and still use the full aperture.
I am not putting the scope down, rather attempting to figure out which targets will be best for it. We could only see the very core of M31 on Saturday night. It takes several fields to take in all of M42. Globular clusters are incredible. Close double stars typically thought of as a challenge in smaller scopes are piddlingly easy.
It is also easy to see that seeing will limit the scope on most nights. But seeing permitting, it will yield outstanding high-powered views. It should be a planet killer, as well as showing structure in those "tiny blue dot" class of planetary nebulae.
The 32" at SPOC also has a very long focal length and has a similar situation when trying to push the magnification too low. Large apertures mean long focal lengths at reasonable f-ratios, with the attendant high magnifications, there's just no way around it in a strictly visual telescope.
I'm hoping Mike will have the time to fire it up next weekend. As of today, the weather prospects look good, with no moon in the picture for most of the night. Keep your fingers crossed.
Right now the focuser doesn't work but I'm giving Mike a new one, whether he has time to install it by then remains a question, but it still works by sliding the eyepiece and re-tightening the setscrew. I'd like to try some LPR filters, as well, and maybe some video of Jupiter if we stay at it long enough for it to rise out of the soup. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy
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-- Siegfried _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy
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