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Bruce,

Oil works for bright stars.  Blackening the pan (spraying with ultra
flat black enamel) helps a lot, especially when street or houselights
get in the line of sight.  I think it also obviates the need for
darkened oil.  This on the lines of Bruce Bauer's black-painted
horizon mirror.  I'm wondering whether a thin layer of oil on top of
water, especially maybe black-dyed water, would have a higher
reflectivity.

The mirror I was using was a standard wall mirror, about 20 x 30
inches.  The large size was very helpful in identifying targets, as I
could see whole constellations.   However, the frame prevented
leveling with a long level, and the surface additionally was
detectably unflat.  Leveling with thin wedges worked well enough.  My
fancy new (1948-vintage) Husun has a dark lens that fits on the
telescope, for determining index error, which considerably reduces
sun glare from mirrors.  There were double reflections from the top
and bottom surfaces of the mirror; shades may have helped here.

When purchasing spirit levels, make sure they read the same when
pointed in both directions and turned upside down, if capable of
that.  Only the more expensive do so, in my experience. I can barely
detect a movement of 14 minutes of arc in my masons level.  Perhaps
other levels are more sensitive.  I tested the sensitivity by wedging
with sheets of paper from a thick ream of measured thickness and
known number.

Thanks for the help with the Gaussians.  Hope to get back on that shortly.

Fred

   

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