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waldi
member
Reged: 07/14/06
Posts: 41
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RA-88 or Refractor 120mm , F-5?
what will be better for DS?
Edited by waldi (09/14/09 07:05 AM)
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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Considering that the RA88 works at a real aperture closer to 77-78mm due to light cone clipping by the prisms, I'd probably lean more toward the 120mm short tube. And this is coming from a dyed-in-the-wool, foaming-at-the-mouth zealot when it comes to two-eyed viewing!
Another limitation I just can't abide re. the RA88; it's largest field can never get above about 2.5 degrees, no matter what eyepiece you could slap on the back end. That's because of the small-ish prism rear aperture, which moreover the focal surface lies *very* close to, thus limiting eyepiece choices.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
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charen
Pooh-Bah
Reged: 05/28/05
Posts: 1083
Loc: New Zealand
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It is difficult to compare one eyed viewing to two eyed viewing. I use my Binoviewer with my ED120 almost 90 % of the time and mainly for planetary use. I also use my Oberwerk 100-45 B.T. for most of my dark sky use. I can go from 20x -80 x and I can get really jaw dropping images esp. on low power, wide field, dark sky, Milky Way viewing. With 2x eyes I 'see more' as it is so much more relaxing and 2x eyes I can view for hours without eyestrain. Hovewever if you have to choose from the 2x you mentioned above the 120 mm S/T may be superior for the reasons Glenn said - but it is still one eyed viewing. If however if was between a 100-45 B.T and a 120 S/T I would certainly go for the 100-45 B.T.
Chris
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35 binos.
80mm Cat.
WO66ED
SV NH 80mm / EQ3
Meade 8in.LX90
Skywatcher Equinox ED120 / Goto HEQ5.
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waldi
member
Reged: 07/14/06
Posts: 41
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you wrote "real aperture closer to 77-78mm" Ok I agree but if we will count like this:
78 x 1.4 = 109 aperature!! and more comfortable observation
am I think right?
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14732
Loc: Cumberland, R I , USA42N71.4W
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Quote:
you wrote "real aperture closer to 77-78mm" Ok I agree but if we will count like this:
78 x 1.4 = 109 aperature!! and more comfortable observation
am I think right?
Sorry, not correct
Pi r squared x 1.4 then take square root = effective aperture = 92
compared to the 120ST with a binoviewer
area sqrd / 2 x 1.4 = 100
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