Sky & Telescope - May 2023 - 6

HARVARD LIBRARY ARCHIVES / PUBLIC DOMAIN
FROM OUR READERS
Preserving Astronomical History
I enjoyed reading Diana Hannikainen's encouraging article
" Star Notes " (S&T: Mar. 2023, p. 57).
I have been an active volunteer for over nine years with
the Harvard College Observatory's Digital Access to a Sky
Century at Harvard (DASCH) and Preserving Harvard's
Early Data and Research in Astronomy (Phaedra) projects.
Tens of thousands of pages have already been transcribed/
reviewed, and, currently, around 174,000 Star Notes pages
have been reviewed.
Some unique historical items struck me, e.g., a handwritten
note from 1862 indicating that Sirius' companion was
discovered the night before; sketches of Halley's Comet in
1910; written notes by E. C. Pickering and E. E. Barnard; a
note referring to the first image ever taken of a star (Vega)
in 1860; and H. P. Tuttle's description of his comet discovery
of 1858. There is even a side note by Henrietta Swan Leavitt
about the armistice ending WWI on November 11, 1918!
I hope others will enjoy participating in these projects to
help preserve astronomical history.
Al Lamperti * Royersford, Pennsylvania
pVolunteers are working hard to transcribe and preserve historic notes
recorded in the notebooks of Harvard astronomers and " computers. "
What Speed Is Your Car?
The first line of Ted Forte's " Winter
Outreach " (S&T: Feb. 2023, p. 18) is,
" What's the power of your telescope? "
I've heard that so many times. My usual
response is, " That's like asking: What
speed is your car? With a car, it depends
on a bunch of factors, including what
gear you are using. With a telescope, it
also depends on several factors, including
what eyepiece you are using. " I then
explain what magnification they are
using and that I like to use a low-power
eyepiece because it shows the most area
of sky with the sharpest view and is
easiest for most people to focus. That's
simplistic, but it's enough for most
folks. On rare occasions I find someone
interested enough that I can begin to
discuss apertures, focal ratios, eyepiece
focal lengths, and the concept of " useful
magnification. " I try to stop myself
before their eyes glaze over.
Ray Harris
Macungie, Pennsylvania
Respecting SchmidtCassegrains
Just
after I purchased a new SchmidtCassegrain
Telescope (Celestron C8
6 MAY 2023 * SKY & TELESCOPE
Powerstar III), I was barraged with
articles attesting to the poor planetary
performance of these telescopes. A fellow
SCT owner, who supervised a small college
planetarium, asked me one evening,
" Aren't you sick of people bad-mouthing
these scopes? " I certainly was. That
is just one of the reasons why I found
Tom Dobbins' " What Makes a Good
Planetary Telescope? " (S&T: Jan. 2023,
p. 58) so enjoyable. Recently, I have read
many more positive evaluations of these
scopes. It was valuable to see some of the
factors contributing to this explained
in Dobbins' article. Some of the best
planetary images are now produced by
amateurs who own these instruments.
I have a question concerning the
effect of the central obstruction of
SCTs as it relates to the atmospheric
distortion of our scintillating skies. As
explained and illustrated in the article,
the amount of atmospheric distortion is
dependent on aperture. With an 8-inch
aperture and a 2-inch central obstruction,
would that result in the scope's
image receiving a 6-inch dose of distortion
or the full amount of distortion?
Gene Santagada
Newburgh, New York
"
Thomas Dobbins replies: SCTs
have been unjustly maligned as
planetary telescopes for many years.
Undoubtedly, much of the criticism arose
from variability in optical quality, which has
been much reduced in recent years due
to improved optical fabrication and testing
techniques combined with more stringent
quality control. The central obstruction has
often been cited as a cause of inferior performance,
but central obstructions of SCTs
are similar to those of Maksutov-Cassegrains,
like the Questar, instruments that have
always enjoyed a sterling reputation. The
old proverb that the proof of the pudding is
in the eating has never been more apropos
- the planetary images obtained by SCT
owners speak for themselves.
Like resolution, the sensitivity of a telescope
to atmospheric turbulence is a function
of its aperture. An 8-inch telescope with
a 2-inch central obstruction would suffer
the same amount of distortion as an 8-inch
scope without a central obstruction.
Comet Conundrum
I'm interested in a theme of Javier
Barbuzano's " Earth's Wellspring " (S&T:
Mar. 2023, p. 34) - that most of the
water on Earth probably wasn't deliv
Sky & Telescope - May 2023

Table of Contents for the Digital Edition of Sky & Telescope - May 2023
