Request for Video/CCD Observations of First and Last
Contacts of this Christmas' Partial Solar Eclipse
Updated: 2000 Dec. 22
by Constantino Sigismondi, Terrence Girard,
and David Dunham
______________________________________________________
[New information is given first,
then the general information that was here before]
Below I mention my successful test run with the standard
equipment that we would like used as much as possible.
I did not mention before that we would like to match, if possible,
the scale, that is, the field of view or number of pixels per arc
second. We want to use a relatively long focal length, the approx.
200 cm of popular 8-inch Schmidt-Cass telescopes, and the PC-23C
Supercircuits camera (now common since it is the least expensive),
or any other camera with a CCD chip about
5 mm across (horizontal length). For filters, it is best to
use a red filter and broad-band LPR filter to best match the
Solar Disk Sextant observations. I used these with good results
with my C-8 yesterday morning, using my C-5 mylar solar filter
with a kluged cardboard 5-to-8-inch adaptor that worked fine.
I was able to get a good focus on a complex sunspot group, and
saw some faculae.
If you can't get or borrow an LPR filter, then use just the
red filter; the result seemed similar when I did that. Also, the
filters are made so that only one can be screwed onto the back of
the Adironacks Video adaptor. I used the Celestron/Meade back
LPR filter from Orion, and that worked fine, with both filters
then shortly in front of the CCD camera chip.
Also, H-alpha filters can be used, as Costantino Sigismondi
describes in his message:
Return-Path:
Subject: Re: H-alpha filter all right for partial eclipse observation?
To: Joan and David Dunham
In my opinion H-alpha having a narrower waveband
should be better, but the waveband of the SDS is 80
nm, larger.
So the combination of the two filters can be better
in matching the SDS device and from an economical
point of view it is also better (less expensive).
[but if you already have one, may as well use it.]
Costantino
______________________________________________________
[This is modified slightly from the e-mail version to
reflect the more detailed prediction information
that is now available elsewhere on this Web site]
We would like to alert, as soon as possible,
North American observers to an opportunity for
precisely measuring the angular diameter of the Sun
during the partial solar eclipse of 2000 December 25.
Past efforts to derive such information from partial
eclipse observations have resulted in dismal failure
due to atmospheric seeing, but we believe that
processing of video CCD images using adoptive-optics
techniques modified for the smooth solar edge and the
irregular (but constant and determinable during the
observation) lunar profile (rather than a stellar
point source) can succeed. The proposed method
requires fast CCD imaging, at video (standard NTSC
rate of 60 half-frames per second) or faster rates
with telescopes of aperture 20-cm or greater (we'll
adopt 20-cm as the standard), and a filter of
approximate passband of 6300 +/-800 Angstroms, for
comparison with observations with the Solar Disk
Sextant (SDS) that observes in that passband (the
solar radius is expected to have some dependence on
wavelength).
Many members of IOTA who have videorecorded
annular solar eclipses have most of the necessary
equipment. The only thing that they might not have
are the necessary filters, which can be obtained from
most major telescope dealers. One such combination
that should work are a broadband light pollution
reduction (LPR) and a red filter. An example is the
SkyGlow LPR filter (item #5660 for 1.25" eyepiece
holder, $59.95, or item #5658 for Schmidt-Cassegrain
rear cell, $79.95) and the #25 red filter (item #5550
for 0.965", $9.95; unfortunately, the 1.25" version
of this filter doesn't seem to be in stock for
individual sale, but it can be obtained by ordering
their "Starter set" of 4 filters, including the #25
1.25", for $44.95) sold by Orion, phone 800-447-1001
or can be ordered via the Web at http://www.telescope.com
The red filter blocks out the blue part of the LPR
filter to give a passband from about 6300 to 7000
Angstroms; the red filter by itself might be good
enough for the planned observation. I notice that my
1.25" to C adaptor (allowing cameras such as Astrovid
and Supercircuits PC-23C to be used with a 1.25"
eyepiece holder) obtained from Adirondack Video,
phone 888-799-0107, has about 3/4 inch of interior
threads at the telescope end, like most eyepieces,
for mounting the filter or filters. Also, of course,
a full-aperture solar filter, orange or neutral
density, should also be used, and may need to be
stopped down (with two holes on opposite sides of the
aperture to allow good focusing) to prevent blooming
or saturation.
The recording should be made for two to three
minutes around both the first and the last contact
with WWV or CHU time signals recorded on the audio to
allow accurate determination of Universal Time.
Observations from three different latitudes are
required. Since the sky is often cloudy this time of
year, and most observers won't be able to travel away
from their families on Christmas morning, hopefully
more than a dozen widely-distributed observers will
attempt these observations. Especially valuable will
be observations from the northeastern U.S.A. and
eastern Canada, where the partial eclipse will be
deepest, but also where the weather tends to be
worse. The position of each telescope should be
known to 10m or better in each coordinate, obtainable
by averaging GPS data for 15 minutes if the horizon
is relatively unobstructed by buildings, mountains,
heavy foliage, and thick clouds above 15 deg. altitude
in most directions. Before and/or after the eclipse,
whenever conditions are best, slowly sweep the edge of
the Sun both vertically and horizontally across the
field of view, so that any distortions can be measured
and used to properly deconvolve the images. It might
be necessary to sweep slowly across the Plieades
cluster (of course without the solar filter, and
perhaps with and without the other filters) at night
also for this purpose.
Predicted times and altitudes of the eclipse
contacts for many North American cities are given at
http://www.skypub.com in Sky Publishing's coverage of
the eclipse, but better information is given here,
including the angles of first and last contacts.
We prefer that you image first contact, but useful
information should still be obtainable if you miss
the exact instance, but can image the small lunar
"bite" for several seconds while it is small enough
that you can image at the same time segments of the
uncovered Solar edge on both sides of the "bite".
A precision of about 40 milliarcseconds in the
determination of the solar diameter is expected with
this method. The absolute value of the Solar
diameter, measured with this method, will be useful
for calibrating the balloon-borne instrument, SDS,
and the upcoming space mission, Piccard.
Sometime late Monday, December 18, more
information about this project can be found at
elsewhere on this Web site (predictions have already
been added; more will be added soon).
Good luck with your observations!
Costantino Sigismondi and Terrence Girard,
Yale University and
David Dunham, IOTA