pyephem FixedObject() for given RA/Dec

Question

I'm looking to determine the alt/az of (un-famous) stars at given RA/Dec at specific times from Mauna Kea. I'm trying to compute these parameters using pyephem, but the resulting alt/az don't agree with other sources. Here's the calculation for HAT-P-32 from Keck:

import ephem
telescope = ephem.Observer()
telescope.lat =  '19.8210'
telescope.long = '-155.4683'
telescope.elevation = 4154
telescope.date = '2013/1/18 10:04:14'
star = ephem.FixedBody()
star._ra = ephem.degrees('02:04:10.278')
star._dec = ephem.degrees('+46:41:16.21')
star.compute(telescope)
print star.alt, star.az

which returns -28:43:54.0 73:22:55.3, though according to Stellarium, the proper alt/az should be: 62:26:03 349:15:13. What am I doing wrong?

EDIT: Corrected latitude and longitude, which were formerly reversed.

Answer 1

PyEphem always uses UTC for time, so that programs operate the same and give the same output wherever they are run. You simply need to convert the date you are using to UTC, instead of using your local time zone, and the results agree fairly closely with Stellarium; use:

telescope.date = '2013/1/18 05:04:14'

The result is this alt/az:

62:27:19.0 349:26:19.4

To know where the small remaining difference comes from, I would have to look into how the two programs handle each step of their computation; but does this get you close enough?

Answer 2

First, you've got long and latitude backwards; second, you need to provide the strings in hexadecimal form; and third, you need to provide the RA as hours, not degrees:

import ephem
telescope = ephem.Observer()
# Reversed longitude and latitude for Mauna Kea
telescope.lat =    '19:49:28' # from Wikipedia
telescope.long = '-155:28:24'
telescope.elevation = 4154.
telescope.date = '2013/1/18 00:04:14'
star = ephem.FixedBody()
star._ra  = ephem.hours('02:04:10.278') # in hours for RA
star._dec = ephem.degrees('+46:41:16.21')
star.compute(telescope)

This way, you get:

>>> print star.alt, star.az
29:11:57.2 46:43:19.6