Last updated: 15 September 2003

Star Test in Daylight

Subject:	Star testing your ETX scope: in daylight!
Sent:	Wednesday, September 3, 2003 11:44:31
From: (Bob Morris)
My neighbour's ashphalt shingled shed is about 50 ft from a picnic table
in my back yard. Its roof is maybe 8-10 feet off the ground.

I'd thought the fine grained shingle granules would be a neat object to
test focus on.

However, at a specific time of a sunny day, with the scope slightly out
of focus, I noticed that certain isolated random granules were picking
up the sun and giving intense diffraction ring patterns, standing out
from the background.

In focus, you can see a single ceramic granule reflecting the sun with
adjacent granules not.

Typically with a 26 mm eyepiece, about 1/2 dozen "stars" of varying
intensity shine out. There is usually one brighter than the others. This
is on a roof about 7 years old. On a new one, there would be more
reflecting granules.

I had observed before that new black-shingled roofs are very "sparkly."

Real stars have atmospheric effects. These very bright artificial stars
are a near-point source of a very bright light with only 50 feet of
atmosphere between the scope and them.

Bottom line: I have never seen such intense, steady, contrasty
diffraction ring patterns on my ETX-90 as from ashphalt shingles at the
correct sun angle.

In recent weeks, observing at about 4 pm in the afternoon seems to work

Try it, you'll like it!

Bob Morris
Mike here: Interesting tip. Sort of like the "Christmas Tree Ornament" test, which also uses a pinpoint reflection of the Sun as a light source.

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