OBSERVINGBASICS
Apparent field of view (AFOV)
Large
Small
and time on my calendar
because Jupiter will be a readily
seen telescopic sight for months
to come, but other Jupiter-
related events require precise
times, too. For instance, if the
skies are clear on the evening of
the 8th, I may want to take out
my telescope and try for the
Great Red Spot (GRS). Will it be
visible then?
To find out, I’ll need to know
the time when the GRS will be
near the center of the side of
Jupiter’s disk that faces us. I
turn to Project Pluto (specifi-
cally, www.projectpluto.com/
jeve_grs.htm), which supplies
transit times for the GRS. A
transit is predicted at 1h17m
Last January, I described
three simple ways to determine
the true field of view (TFOV)
of any eyepiece/telescope com-
bination. Mike Bertin, of
Orange County Astronomers
in California, offers another
method for those of you who
own a go-to scope that displays
altitude and azimuth. The
beauty of this method is that it
works during daylight by using
a distant terrestrial target, like
the top of a light tower.
In a world ajumble with some two dozen
time zones, we need a standard way to express
the time of astronomical events.
UT on May 9, which corre-
sponds to 9:17 P.M. EDT on May
8. This is an hour after sunset,
so Jupiter should be comfort-
ably positioned above the east-
ern horizon.
What about your time zone
and the correction you need to
make to convert to UT?
Astronomy simplifies the pro-
cess by posting times in Eastern
Standard (or Daylight) Time.
You can also refer to a chart and
article on EarthSky (earthsky.
org/astronomy-essentials/
universal-time). The article
refers to Universal Coordinated
Time (UTC), which, for all prac-
tical purposes, is identical to
Universal Time.
He writes, “I level the scope
— eyeballing it is OK, but I usu-
ally use a small level that I have
mounted on top of the optical
tube. Next, I move the telescope
in azimuth so the object is at
one edge of the field of view, and
I write down the azimuth value.
Then I move the telescope so the
object is at the other edge of the
field of view. I write down the
new azimuth value, and by tak-
ing the difference, I have the
TFOV in azimuth.”
Bertin then double-checks
his result by repeating the pro-
cess in altitude. “I once again
center on my reference object
with the scope still level. I move
the telescope so the reference
BROWSE THE “OBSERVING BASICS” ARCHIVE AT www.Astronomy.com/Chaple.
70
A ST R O N O M Y • MAY 2018
N
oting the time
that a predicted
astronomical event
will occur can be
problematic. If I
told you that a comet is pre-
dicted to impact Jupiter at 9:00
P.M. on May 1, you’d rightly ask,
“9:00 P.M. where?” In a world
ajumble with some two dozen
time zones, we need a standard
way to express the time of astro-
nomical events.
To this end, astronomers use
Universal Time, abbreviated UT.
To avoid the confusion between
A.M. and P.M. hours, UT utilizes
a 24-hour “military time” clock,
and it is based on the time at the
0° longitude meridian in
Greenwich, England.
For any place on Earth out-
side this zone, you have to make
a correction. Here’s how it
works for me. I live in the
Eastern time zone. During
Eastern Daylight Time (EDT),
which runs from the second
Sunday in March through the
first Sunday in November, my
clock is four hours behind
Universal Time. The rest of the
year when clocks “fall back” to
Eastern Standard Time (EST), I
subtract five hours from UT.
Unfortunately, there will be
no comet/Jupiter collision, but
the big planet arrives at opposi-
tion this month. According to
the 2018 Observer’s Handbook
— an annual publication put
out by the Royal Astronomical
Society of Canada — the pre-
cise time of opposition is May 9
at 1h00m UT. That translates to
May 8 at 9:00 P.M. EST.
I don’t plan to mark that date
Timing can make or break an observing session, but
don’t let converting to Universal Time bog you down.
I know where,
but when?
BY GLENN CHAPLE
object is at the bottom of the
FOV, and I note the altitude
value. Then I move the tele-
scope so the object is at the top
of the FOV, and I write down
the new altitude value. The
difference gives me the true
field of view in altitude. All of
this takes less than 5 minutes.”
Al Nagler, a pioneer in the
production of ultra-wide-field
eyepieces, notes that the appar-
ent field of view divided by
magnification does not give an
accurate TFOV because of eye-
piece focal length and distor-
tion variations and tolerances.
He notes, “The actual
calculated true field of ANY
eyepiece is its field stop diam-
eter divided by the telescope
focal length, times 57.3°.”
Field stop data for all Tele Vue
eyepieces are published on the
company’s website. If you’re
not sure of the field stop
diameter for a different brand
eyepiece, the methods I men-
tioned in the January column
will suffice.
Do you have questions,
comments, or suggestions of
your own? Email me at
[email protected]. Next
month: The missing Messier
object. Clear skies!
Glenn Chaple has been an
avid observer since a friend
showed him Saturn through a
small backyard scope in 1963.