ATM "Buena Vista" home-made Plossls- Report -

["Chuck Hards" <chuckh@companionsystems.com>]

Hi all:

I now have the first three eyepieces in the "Buena Vista" series complete,
and, with the help of fellow ATM Glen Warchol (Touring the Universe on
Crutches), gave them a test-run Saturday night:

The telescope:  4.25" f/5 DobNewt.  (Too tired to drag out anything bigger)
The eyepieces:  Homemade Plossls:  28mm, 22mm, 14mm.  (Buena Vista is
Espanol for "good view")

We compared them to:  Spectiva Plossl, Teleview Plossl, Rini Plossl, UO K-2,
and a motley assortment of refitted surplus spotter eyepieces in 1.25"
barrells.

>From a suburban, light-polluted site, (naked-eye limit about 3) we looked at
mostly bright Messier objects, the Coathanger, Epsilon Lyrae, etc.

Obviously, the TeleView Plossl was king, but not by as big a margin as you'd
think.  The Spectiva Plossl gave views almost as good, with the edge of the
field maybe a tad softer than the TV, but this may have been the beer.  The
Buena Vistas came in third, due to their slightly smaller fields, and
softness at the edges.  They were obviously better than any commercial
Kellner, and the "no-name" Plossls and Konigs.  Except for the slightly
curved field, they were about as good a view as the UO K-2's.  All three
BV's responded well to a 2.8x, 3-element Barlow.

Sharpness at the center of the field is excellent.  Several globulars showed
distinct granulation, even at low powers and small aperture.  Stars were
pinpoints, with a tight critical focus region.  There was little tendency to
wallow through a wide envelope of best focus.  They pass the "snap" test.

Very little or no color noted.  Even performing the "light bulb basement
test", in which a 40-watt bulb is examined with a bino objective from 25
feet, showed only the most subtle hint of a color fringe, and then only if
you were looking hard for it.

Eye relief is long, even on the 14mm.

Barrells were turned on my mini-lathe from 1-1/2" x 1-1/4" aluminum bar
stock.  Spacer and retainer rings were turned from PVC on the lathe;
retainers are "friction fit".  (I don't have thread-chasing gears for my
lathe)

All three used the traditional construction, two identical achromats,
"positive" elements facing each other, and within 0.5mm of contact.

The lenses are from the American Science & Surplus Optics catalog.  (Not the
junk catalog.  If you call up and just request a catalog, you get the junk
one.  Request the Optics catalog.)

For the 28mm:  Order (2)  #65171  28mm x 57mm     $8.75 ea.
For the 22mm:  Order (2)  #65073  20mm x 44mm     $8.50 ea.
For the 14mm:  Order (2)  #68019  14mm x 28mm     $8.00 ea.       All prices
less shipping & handling

The aluminum bar stock and PVC came from my scrap bin.  Each eyepiece cost
less than $20 to build, yet was optically similar to commercial eyepieces in
the $50 to $80 range.

The Caveats:

Being surplus, there is no guarantee that all lenses with the same stock
number are from the same original lot.  They may have nothing in common
other than diameter and focal length.  If you get a lens from a different
lot than I , your performance may vary.

All of the 28mm x 57mm lenses I have ordered have come with a cementing
flaw, a 1/4" hemispherical anomoly at the edge.  Align the cementing flaws
to minimize the affected portion of the field.  It is only noticeable when a
bright star is right behind the flaw.

Some of the lenses are coated, some are not.  Didn't seem to make a
difference from my light-polluted back yard.

Other focal lengths to follow, as the lenses arrive.

Image files will be available as soon as I can borrow the digital camera
from our field-service guy at the office.

Thanks to Peter Smith in Australia for re-kindling an old interest, via his
website.

Chuck Hards