P20150726b; July 26, 2015

³Measured position corresponds to RA offset of +0.0255'' and Dec. offset of –0.179''.

⁴All "offsets" are defined in the ("corrected" – "reference") or ("observed" – "calculated") sense. The offsets should be added to reference positions to get the measured positions, which we use to calculate the prediction.

⁴Data analyzed using UCAC2 reference network. A weighted average of the data from the two telescopes was used to calculate the RA and DEC. The errors given are 1 standard deviation.

⁵The reference positions for Pluto and Charon are those given by JPL Horizon's ephemeris (Pluto source file: PLU043; Earth center source file: DE431). Our residual model is based on the assumption that the same offsets in RA and Dec apply to both Pluto and Charon.

⁶Data from the USNO 61-inch, Lowell 42-inch telescope, SARA-South 24-inch telescope, and Lowell Astrograph telescopes over the span of 5 years were reduced with respect to stars in the UCAC2 catalog. A model was developed to fit the residuals obtained from our measured positions compared to that of the object's JPL ephemeris. All residuals obtained from the different telescopes were consistent with the model. The model includes the first-order effects of errors in the orbital elements of Pluto: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, (iii) sinusoidal terms with the Earth's orbital period, and (iv) sinusoidal terms with the Pluto-Charon mutual orbit. The model was propagated to obtain the predicted position and error of the Pluto at the time of the occultation. The errors listed for the Pluto are the RMS of the models fitted.

Table 1: Prediction Details
Pluto Geocentric Mid-time (yyyy month dd hh:mm:ss)	2015 July 26 21:35:58± 00:00:41¹ UT
Pluto Minimum Geocentric Separation	0.0564± 0.032¹ arcsec
Charon Geocentric Mid-time (yyyy month dd hh:mm:ss)	2015 July 26 21:24:07± 00:00:41¹ UT
Charon Minimum Geocentric Separation	0.0181± 0.032¹ arcsec
Position Angle (Pluto relative to the star; measured north through east)	–10.90 degrees
Geocentric Velocity	22.32 km/sec
Occultation Star UCAC2 magnitude	15.13²
Measured magnitude (Lowell 42-inch frames)	15.75

Table 2: Reference Star Position
Reference star position: (UCAC2, at epoch of event)	RA (h:m:s; J2000)	Dec (d:m:s; J2000)	Notes
P20150726b Catalog	18 58 01.0592± 0.1374	–20 48 39.856± 0.129
P20150726b Measured³	18 58 01.0610± 0.0028	–20 48 40.035± 0.007	From USNO 61-inch, SARA-S, DCT, Lowell 42-inch frames
Table 3: Projected KBO Offsets from Reference Ephemeris at the Time of the Event
Body	RA (arcsec)	Dec (arcsec)
Pluto	+0.0411± 0.0410	–0.042± 0.031	See Notes 5 and 6
³Measured position corresponds to RA offset of +0.0255'' and Dec. offset of –0.179''. ⁴All "offsets" are defined in the ("corrected" – "reference") or ("observed" – "calculated") sense. The offsets should be added to reference positions to get the measured positions, which we use to calculate the prediction. ⁴Data analyzed using UCAC2 reference network. A weighted average of the data from the two telescopes was used to calculate the RA and DEC. The errors given are 1 standard deviation. ⁵The reference positions for Pluto and Charon are those given by JPL Horizon's ephemeris (Pluto source file: PLU043; Earth center source file: DE431). Our residual model is based on the assumption that the same offsets in RA and Dec apply to both Pluto and Charon. ⁶Data from the USNO 61-inch, Lowell 42-inch telescope, SARA-South 24-inch telescope, and Lowell Astrograph telescopes over the span of 5 years were reduced with respect to stars in the UCAC2 catalog. A model was developed to fit the residuals obtained from our measured positions compared to that of the object's JPL ephemeris. All residuals obtained from the different telescopes were consistent with the model. The model includes the first-order effects of errors in the orbital elements of Pluto: (i) constant offsets in RA and Dec, (ii) linear (in time) offsets in RA and Dec, (iii) sinusoidal terms with the Earth's orbital period, and (iv) sinusoidal terms with the Pluto-Charon mutual orbit. The model was propagated to obtain the predicted position and error of the Pluto at the time of the occultation. The errors listed for the Pluto are the RMS of the models fitted.

Site	Charon Immersion (UT)⁹	Charon Mid-Time (UT)	Charon Emersion (UT)⁹	Pluto Immersion (UT)⁹	Pluto Mid-Time (UT)	Pluto Emersion (UT)⁹	Pluto Altitude	Solar Altitude⁹
SAAO	-----------	21:24:11	-----------	-----------	21:35:51	-----------	77	-----------
Geocenter	21:23:36	21:24:07	21:24:37	21:35:36	21:35:58	21:36:20	-----------	-----------

PC20150726b Occultation July 26, 2015