The target star, and the pre-point line of declination, is shown in a series of
finder charts of different scales that have now been posted at
https://occultationpages.com/rasc/charts.html ;(it is the âFinder Chartsâ
section of Steve Messnerâs general new page for the occultation at
https://occultationpages.com/rasc/20191024_16Psyche.html ;). Except for the
color-inverted Google Sky (GS) chart at the end, which has north up, all of the
other charts have the zenith up, and north towards the upper right, for
Phoenix, Arizona at the time of the occultation, when the altitude will be 13.8
deg. in the southwest (azimuth 238 deg.). However, the general appearance will
be virtually the same for all of the mainland US areas crossed by the path from
s. Calif. to Colorado. My late Sunday night message is in a Word file,
mp016o20.docx, that you can obtain by clicking on the âDocumentâ tab of the
Web site.
The finder charts on the Web site are as follows (documentation is given in the
lower left of each one):
3endNakedEye: Naked-eye view to 6 th mag.
4-1: First finder-scope pre-point chart from dusk to 7:30pm MST (= PDT). The
field of view (FOV) rectangle shown is for a mighty mini, about 2 x 3 deg. on a
side. Typical finder scopes have a 5-deg. circular FOV, a little larger than
the rectangle.
4-2: 2nd finder-scope pre-point chart from 7:30 to 9:00 pm MST
4-3: 3rd finder-scope pre-point chart from 9:00 to 10:30 pm MST (the rho Sgr
pre-point discussed below is near the center)
4-4end: Finder scope chart centered on the target star (too faint to be shown)
and showing the prepoint line from 10:30 pm MST. The rho Cap pre-point is at
5:46 UT.
6end: 5-deg. telescopic view centered on the target. The rectangle is the FOV
for a 10-in. f/4 telescope using a 0.5 focal reducer and camera with ½-in.
chip, like the IOTA Runcam cameras. The bright star above and left of the
target is 5 th -mag. 19 Capricorni, while the brightest star on this chart,
near the bottom to the right, is 4.2-mag. upsilon Cap.
7-1 is like 7end (see below), but for the good rho Sagittarii prepoint
opportunity close to 9:39 pm MST (4:39 UT).
7-2 is like 7end (see below), but for the rho Capricorni prepoint opportunity
close to 10:46 pm MST (5:46 UT).
7end: 2-deg. telescopic view centered on the target. The rectangle is the FOV
for an 8in. f/10 telescope using a 0.5 focal reducer and camera with ½-in.
chip, like the IOTA Runcam cameras. The brightest star on the chart is 5th-mag.
19 Capricorni. Note that the target star on both 6end and 7end is shown as mag.
12.4, while in fact, it is almost as bright as the two 10.2-mag. stars to its
right; see below for more about the starâs visibility from last nightâs
tests.
GSinv: This is from Google Sky (from Google Earth) but Iâve inverted the
colors for a more printer-friendly chart. The total view is a little smaller
than the rectangle shown in the 7end chart, and since north is up, it is
rotated about 45 deg. counter-clockwise from all of the other charts above it.
_ _ _
With the low altitude, the target star will be quite difficult to find with
telescopes that donât have a robust go-to capability. If thatâs the case
for your scope, there are some pre-point opportunities that will allow you to
point your telescope to the precise altitude and azimuth that the occultation
star will be during the occultation, including one with 8.5-mag. SAO 163929
(J2000 RA 20h 51.0m, Dec -17 deg. 46â) only 1m 1s (61s) before the
occultation. The other brighter earlier prepoint opportunities should be used
by all, at least to check for visibility of the target altitude and azimuth
from the site you chose, early enough so you can change the location, if
necessary. Iâll describe these below. The idea is to point your telescope at
the specified star at the pre-point time for it, then turn off your
telescopeâs tracking and clamp it so that it does not move. Then at the time
of the occultation, the target star will appear close to where the pre-point
star was when the occultation occurs. Since the pre-point star will rarely have
the same declination as the target star, there will be an offset distance,
north (positive) or south (-), that the target star will be offset from where
the pre-point star was; this offset value will also be specified for the
individual cases below.
One of the better pre-point opportunities will occur 1h 30.1m before the
occultation. At that time (9:38.9 pm MST Oct. 23 or 4:38.9 UT Oct. 24 for the
Phoenix region), point your telescope at 3.9-mag. rho1 Sagittarii (= SAO
162512) and stop tracking. At the occultation time (11:09.0 pm MST in the
Phoenix area), the target star will then be 8 arc minutes north of where rho1
was. The pre-point star, rho1 Sgr, is in northeastern Sagittarius, northeast of
the âTeapotâ and six degrees directly above (northeast of) of Saturn, at
J2000 RA 19h 21.7m, Dec. -17 deg. 51 arc min. Around 4:05 UT (9:05 pm MST),
Saturn itself can be used for a rough check of the visibility (testing for
obstructions) of the occultation; at that time, the pre-point area will be
about 6 deg. to the right of Saturn.
But 8 arc minutes is too large for some narrow fields of view (FOVâs).
Especially for them, a better (smaller pre-point offset) opportunity occurs
23.1 min. before the occultation (= 10:45.9 pm MST for the Phoenix region) with
4.8-mag. rho Capricorni = SAO 163614 at J2000 RA 20h 28.9m, Dec. -17 deg. 49
arc min.; when the occultation occurs, the target star will be 4.7 arc min.
north of where rho Cap was. A list of a few dozen prepoint opportunities is at
https://occultationpages.com/rasc/20191024_16Psyche.html ;(click on the
âPre-point buttons; the top one is the generic list , just with times before
the occultation, while the second one gives the times for the Phoenix area) or
in the attached file st016PHX.txt. Note that in that latter list, the U.T. date
is Oct. 24 while the local (MST = PDT) date is Wed. Oct. 23. The occultation
times, and then also the pre-point times, will be nearly the same throughout
the US mainland part of the path. In Orange county, they will be 0.4 min.
earlier, and in San Diego, 0.3 min. earlier, than in Phoenix and central AZ,
while near Colorado Springs, 0.2 min. later and Albuquerque, 0.3 min. later.
Also, I wrote late Saturday night: Thereâs uncertainty in the starâs
magnitude; 10.2 is from Gaia. But the GSC says 12.4 (thatâs whatâs used in
the finder charts produced by Guide that weâve posted in the âFinder
Chartsâ section of https://occultationpages.com/rasc/20191024_16Psyche.html - ;
itâs way too pessimistic ) and APASS gives the V-magnitude as 11.4 with B
13.0. The large B-V indicates a red star, like the star occulted by Phaethon on
Sept. 29 where there were similar differences; the star may be variable.
With Google Sky, the star seems to be about as bright as two nearby 10.2-mag.
stars, so maybe it will be not so difficult to observe, as the Gaia magnitude
implies. That was confirmed in the tests that Joan and I performed Sunday
evening, with 80mm and 120mm refractors. As Google Sky shows, the target was
about the same brightness as the two stars shown to the right of the target
star on the charts â6end.jpgâ and â7end.jpgâ in the âFinder Chartsâ
section of the above Web site. We used Runcam cameras which, like most black
and white video cameras, are a little red-sensitive. But extinction at the
13-deg. altitude in the west made the star appear fainter (by about 1.0 mag.).
The target star was quite marginal (but âthereâ barely) with our 80mm
scopes, but good with our 120mm systems. So our plan is to try to run 8
stations with our 120mm and larger easily portable telescopes, and if we have
time after setting those up, we will also set up one, or maybe two, 80mm
systems with higher-integrating Watec cameras in the northern-limit region.
With the above plan, we have one or two extra recording systems, if anyone
wants to borrow them, to use with their telescope, if you donât already have
occultation video equipment, or an astronomical CCD system that you could use
for a drift-scan observation, as described at
http://www.asteroidoccultation.com/observations/DriftScan/Index.htm . ;
David Dunham, dunham@xxxxxxxxxxxxx , Mon. Oct. 21, 5pm MST
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