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A visibility calculator for the 2.7 m Harlan J. Smith Telescope.

Purpose: Show the hour angle and declination of your object's Apparent Observed Place, compute the visibility limits and other observing ephemera, and return a saftey verdict.
 
The current version is April 8 2026 (see LEGEND). You may need to clear your history or cache if you have used an older version. At the moment, you cannot copy/paste in the worklist because of browser and operating system security requirements. In Firefox, if you release the mouse button outside the application's boundary the program will freeze and you'll need to reload. See Compatibility below.
 
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Enter your targets into the worklist, one target per line. Input is case insensitive. Click anywhere on the line: if the calculation is successful the parsed target will be highlighted in red and the chart updated. The formats are:
 
1. RA and DEC
The basic format is sexagesimal right ascension (RA) and declination (DEC), optional J2000 or B1950, and optional proper motion px py in milliarcseconds per year; single-space separated (colons are allowed in the sexagesimal values); and optionally embedded in freeform commentary text:
[prefix text] HH MM SS[.S] [+|-]DD MM SS[.S] [J2000|B1950[ px py]] [suffix text]
with J2000 and zero proper motion the default. This format is always tried first.
 
Examples: 
15 17 20 +55 1 20
15 17 20.3 55 1 20 J2000
Prefix comment. 15:17:20.3 +55:1:20 B1950 10.3 137 Suffix comment.
2. Sun, Moon, Planets
Enter the name of solar system object - Sun, Mercury, Venus, Moon, Mars ... Pluto, with optional suffix comment:
 
Examples: 
Sun
MOON -- displays approximate phase!
Jupiter
Pluto Poor Pluto!
3. Internal historical catalogs
Catalogs are FK5, Messier, New General Catalog and Index Catalog (Steinicke), using the single letter F, M, N, or I followed by a space and a catalog number and optional suffix comment:.
 
Examples: 
F 732  Albireo is a good test of seeing for the Visitor Center programs.
M 33
N 6365 Pair of spiral galaxies in Draco
4. External SIMBAD search
Query a SIMBAD identifer using the single letter S followed by a space and the identifier. The retrieved 
 RA DEC J2000 px py
will be inserted after the S command, and subsequent clicks will use this cached position, treating the original S command as a prefix comment in the basic RA DEC format.
 
Examples: 
S alf Ori
S IRAS 23077+6707
S Andromeda Galaxy
S Sirius
Changing the date and time
The default date and time is NOW, but you can also enter any reasonable date and time by clicking on the Set button. For convenience entering UTC, you can omit the : separator and use just a space, in which case you can also enter just the hour, hour and minute, or hour minute second.
 
Note that after changing the date or time, updates are not automatic: after you click on the worklist line, the results are correct for the date and time shown.
 
Verdict
Your object will be either ABOVE HORIZON or BELOW HORIZON, and if above, it can additionally be VISIBLE or BLOCKED. If it's blocked, the reason will be either the pier limits, which takes into account all the stuff around the north pier, or a curtain limit, which is dictated by the lower curtain's fully dropped position. Whichever, it will be apparent in the graphical depiction.
 
The Limits in Hour Angle reports the visible hour angle interval, i.e. the setting-circle hour angle where your object first becomes visible and where it sets, taking into account the horizon (90º) zenith distance, curtain limit, and pier. Similarly, the Limits in UTC reports the visibility interval in UTC for the dates indicated.
 
Zenith Distance is reported even when the object is below the horizon, useful for civil (90º, 96º), nautical (96º, 102º), and astronomical twilight (102º, 108º). Note that while the object is above the horizon the ZD and AZ are for the Apparent Observed Place, which takes into account refraction. When the object is below the horizon, ZD and AZ are calculated using the just the Apparent Place to get the actual geometric position. Hence, crossing below the horizon you will see a jump in ZD as the refraction calculation is omitted.
 
Mouse-over information
Mouse-over the blue target to see its Apparent Observed RA, HA, and DEC.
 
Mouse-over the compass rose to see the LST (RA + HA).
 
Cow-over the moon to see the mean lunar day.
 
Switching Tube East/West
A target that is above the horizon but blocked may be visible in the opposite tube (east/west). When you click East or West, the chart is redrawn and the controls flip to the opposite side. The collision limits in tube west may not be correct at the f/33 cage, elevator, and Hi-Ranger, and I will probe those limits as I develop the system.
 
Compatibility
Modern versions of Edge (Microsoft) and Safari (Apple) work best. Avoid clicking in the application window until it's fully loaded - depending on the browser you may to reload if you jump the gun. In Firefox, don't click in the application and then release the mouse elsewhere: this may cause the program to freeze, and you'll need to reload it. In iOS, the proxy connection to Simbad is blocked, so the S command won't work. Public computers, kiosk, and family protection modes may disable this application.
 
Credits
The north pier limit functions are computed from data mapped by Judson Richards in 2025.
 
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Why Cartesian coordinates? Don't all astronomers prefer radial/polar coordinates which match the sky?
 
The answer is that HJST Calculator is also my tool for designing the mathematical model of HJST's safety system.
 
An efficient way to move from (HA_1, DEC_1) to (HA_2, DEC_2) is to move each axis motor at top speed until reaching the desired point. A refinement is to reduce the speed of one axis or the other so that they both arrive at the desired point at the same time. The great circle route on the celestial sphere is never faster, and is often much slower if it overshoots the target declination and has to come back. The reason is that the pole of rotation of the telescope is fixed, so the only great-circle routes are at DEC=0 or HA=constant.
 
In cartesian coordinates, this produces essentially straight-line moves, and these lines are useful for designing a safety system that can unconditionally go rapidly from one point to another without hitting the north pier or exceeding the curtain limit or horizon limit.
 
This means dealing specially with the convex boundary of the north pier and the convex curtain limit on the other side. Here's a simple prescription: on the north pier side, draw a line from near DEC=20 up to HA=1 that just misses the convex curve, and on the other side a line tangent to the curtain (or horizon) curve at HA=6 DEC=0 up to the north pole. Within these restricted boundaries, all moves are straight-line safe. To enter a convex zone, first move to the boundary line stopping at the desired declination, and then finish the move in HA. Similarly, to leave a convex zone, first move in HA to the line boundary.