AltAz -> Galactic, with multiple observation times #5364

I created a stackoverflow question for this here.

I have event tuples of (azimuth, altitude, time) in horizontal coordinates, which I store as 2 numpy arrays azimuth and altitude, and a list(datetime). I also have an EarthLocation where these events are observed. The number of events is on the order of 100k.

Now I'd like to transform these to galactic coordinates. I tried

my_loc = EarthLocation.from_geodetic(...)
my_frame = AltAz(my_loc, obstime=times)
evt = SkyCoord(
        alt=my_altitude*rad, 
        az=my_azimuth*rad, 
        frame=my_frame, 
    )
evt_gal = evt.galactic

But each of these events apparently use the same observation time. (Judging from plots where all events are suspiciously neatly sitting on a curve).

Looking at a single event, it seems that each event has multiple observation times associated to it, which is not quite what I want.

e = evt[1]
print(e.frame)
<AltAz Coordinate (obstime=[datetime.datetime(2015, 5, 17, 16, 58, 17)
 datetime.datetime(2015, 12, 14, 0, 15, 26)
 datetime.datetime(2015, 9, 25, 4, 16) ...,
 datetime.datetime(2015, 9, 27, 6, 5, 9)
 datetime.datetime(2015, 2, 5, 1, 31, 8)
 datetime.datetime(2015, 12, 21, 13, 15, 49)], location=...> 

Supplying the obstimes directly to SkyCoord yields identical results.

evt = SkyCoord(
        alt=df_cscd['altitude_cscd'] * rad, 
        az=df_cscd['azimuth_random'] * rad, 
        location=my_earth_loc,
        obstime=my_times,)

The next idea was to not pass the observation times to the frame, but to the SkyCoord:

frame_without_time = AltAz(location=my_location)
evt = SkyCoord(
        alt=df_cscd['altitude_cscd'] * rad, 
        az=df_cscd['azimuth_random'] * rad, 
        frame=frame_without_time,
        obstime=my_times,
)

But this raises a

ValueError: cannot specify frame attribute 'obstime' directly in SkyCoord since a frame instance was passed in

It works perfectly fine if I have just a single time, e.g. astropy.time.Time.now(). But looping over 100k events and creating a SkyCoord instance for each of them is incredibly slow, obviously.

I don't have a pull request for this, since I wonder if there is a built-in better way to do this?

What do you think?