D-Files and Amateur Telescope Making



Further Binocular Observations
Jack Eastman, DAS Chief Observer

While I was on vacation over Christmas I made some more observations and tests with binoculars and a low power telescope. These were on Dec. 26-29; the objects were the moon, Jupiter, Saturn and Albireo, starting with a bright sky as soon as I could find Jupiter. The two instruments were the 8X30 Zeiss and a 10X40 Unitron finder. I had prepared aperture stops of 12.5mm for the bino and 11mm and 20mm for the finder. Careful measurement showed the actual magnification of the 8X30 is 7.5X (4mm exit pupil) while the 10X40 is actually 10X(also 4mm EP.)

Against a fairly bright twilight sky the disk seemed apparent at full aperture with the bino and was easy stopped to 12.5mm. It was obvious with the 40mm finder at 11mm, 20mm and full (40mm) As the sky darkened the aberrations and glare swamped the disk in the 8X30s at full aperture. Stopped to 12.5mm it was as described in my earlier reports. It was much easier in the finder, although at full aperture there was some degradation from glare and eye aberrations.

Much as reported in my earlier discussions. By the time it was dark enough to find Saturn my pupil was probably opened up at least to the exit pupil size of these instruments. It did look like a tiny Saturn in the 40mm at 20mm aperture. Not as obvious at 11mm (diffraction?) or full aperture (glare, eye aberrations.)

The central peak of Copernicus could be distinguished with the 8X30s at full aperture, but was sharp as a tack at 12.5mm. This was Dec 27, 5:30 local time with the shadow of the crater wall crossing very close to the peak. The gibbous moon was beautiful sharp and full of detail with the bino at 12.5mm, somewhat ragged at full aperture.

In some of the earlier lists there was a question of seeing Albireo with binos. With the 8X30's after the sky darkened I thought I could see the companion directly above the primary. I couldn't really be certain but maybe 50% sure. Going to the 10X40, at full aperture I could see the companion directly below the primary. The finder produces an inverted image so this is consistent with the bino observation. With the finder at 20mm it was loud and clear. Back to the bino, yes I could see it. At full 30mm too much aberration, I couldn't see it at all. I could see it easily in the finder at 11mm, and although ragged it was visible at 40mm I repeated these observations the next two nights, with similar results, the star being fairly easy in the 8X30s at 12.5mm after seeing it the first time. At no time could I honestly say I saw it split at full aperture in the 8X30s.

On the analytical side you'll remember the wavefront calculation a few lists back where the bino objective sort of stunk at a minimum OPD error of about 0.8 waves. I took an optimized 12-cm f/15 airspaced objective, opened the aperture to f/7.5, added thickness to the crown element so it didn't pinch out and then scaled it to a 300mm focal length. It would seem close to the specs for the 10X40 finder. The wavefront calculation showed this lens was essentially perfect with what looked like .005 waves of high order spherical near the edge of the pupil. I tried to optimize this thing further for grins, but the computer knows that perfect ought to be good enough and kept making this thing worse! The punchline is that our eyes go to pot much above 2 or 2.2mm aperture. The finder doesn't add aberrations but the binocular does.

Jack Eastman

The Denver Astronomical Society
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