Occultation by (171) Ophelia Tues. night, 2023 July 25/26 - New 2023 July 24

This large asteroid will occult a 12.4-mag. star in Ophiuchus in a wide path crossing N. America from Nfld. to s. California

For practice, and hopefully to train others to make the observations, we will 
be trying an occultation by the large asteroid (171) Ophelia late Tuesday evening; 
if skies are clear enough, we will distribute a go-no go decision by email to those 
who express an interest, Tuesday afternoon. The event will be visible across the 
continent, from Newfoundland to southern California, so many will be trying it. 
The 12.3-mag. star is at J2000 RA 17h 40 13.1s, Dec -23 deg 06' 19", about 2.5 deg.
east and a little north of 4.8-mag. 51 Ophiuchi and about 4.5 deg. east and a little
north of 3.3-mag. theta Ophiuchi - the finder charts below will help you find the 
star, which is only about 4' southeast of 8.3-mag. SAO 184602 = HIP 86455, star "A" 
on our finder charts below with the target star centered. Our charts are prepared
with zenith up as seen from Greenbelt, MD but their appearance will be very similar 
for other locations within the path across the continent, with some rotation of a 
few to several degrees; keep in mind that the time shown on the chart is for the 
occultation in Greenbelt, MD; you should determine the occultation UT for your 
location either from this Occult map, from Occult Watcher, or from the OW cloud 
interactive map described in the next paragraph. The pre-point line of declination 
is shown on our charts, ending at the target star at the time of the occultation.

Naked eye chart to mag. 6.6.
Finder scope chart to mag. 10.0.
Telescopic chart to mag. 13.5.

This is a good occultation to record for those with eVscopes. The exposure should 
be the shortest possible that shows the target star clearly; probably an exposure 
of 300 ms should work, but it should be no more than 500 ms (half a second).

In Washington, DC, central Maryland, and northern Virginia, the occultation will 
last up to 18s centered at 11:59:14 pm EDT (July 25; 3:59:14 UT of July 26 UT)
with a strong 1.8-mag. drop since Ophelia will be 14th mag.  The time
uncertainty is +/-5s, but if you can, record the target for 3 min. before
to 3 min. after the predicted time for your location, to catch any possible
occultations by satellites of Ophelia (none are known). Those outside of the 
greater Washington, DC region can find the local occultation UT and circumstances 
by clicking on your location shown the Google map (click on the world map to get it) 
on the Occult Watcher cloud page for the occultation. It also has zoomable Aladin star 
charts, but our charts below will probably be easier to find the target. 

There is a great pre-point opportunity 100 minutes before the occultation with 
2nd-mag. Dschubba (delta Scorpii), the brightest star in the head of Scorpius, for 
either finding a place that will have an unobstructed view of the event (25 deg. high in 
the south as seen from the path across the northeastern US) or for precisely pre-pointing 
a non-tracking telescope to obtain a drift-scan CCD image to time and record the occultation.
The Dschubba pre-point time is around 10:19pm EDT (2:19 UT of July 26) for the 
DMV (Washington, DC region) to find a location with an unobstructed view, or 
more precisely at the times given on the most detailed Dschubba pre-point chart 
below (center your scope on one of the faint stars B, C, D, or E on the chart 
at the times shown to accurately pre-point your telescope to the occultation 
altitude and azimuth for drift-scan observations, or for just getting on target 
without finding the target star directly, if your scope does not have a good 
go-to capability. The pre-point charts are valid for the DMV; for those outside 
of the DMV (or in it if you want a more precise time for your location), adjust
the pre-point times by the difference, your occultation time minus the occultation 
time for Greenbelt, MD (3:59:14 UT). On our charts, the pre-point line has marks 
at 2-minute intervals with the UT time given at 10-min. intervals.

Consult the naked-eye chart for the target star above; it is wide enough to show 
Dschubba and the pre-point line close to it.
Dschubba finder scope pre-point chart to mag. 10.0;
  Antares in the mag. legend box is also close to where it actually is relative to 
  Al Niyat and other nearby stars..
Dschubba telescopic pre-point chart to mag. 13.0
  showing the pre-point stars B - E with a table showing their pre-point times.

This NWS cloud cover forecast map
This NWS cloud cover forecast map shows that it will be mostly clear in the 
path across the US except for monsoon clouds over much of Arizona. However, 
Occult Watcher shows conflicting forecasts for Maryland observers, with 
7Timer! giving 90% cloud and Meteoblue giving 0% cloud. Hopefully, the 
forecasts will converge before the event, but we will find out Tuesday.

David Dunham, 2023 July 24
e-mail:  dunham@starpower.net; cell phone:  301-526-5590