[AstroPy] Convert from AltAz to ICRS

[Eric Jensen]

> On Nov 26, 2019, at 12:06 AM, keithmclaurin at gmail.com wrote:
> 
> The RA and dec changes mostly due to precession is my understanding. 

Thanks for responding.  A quick back-of-the-envelope calculation shows that precession (360 degrees in 26,000 years) should be less than 0.1 arcsecond over 12 hours, whereas the difference here is quite a bit bigger, about 7.5 arcminutes.  So it must be something else - maybe some internal rounding or (more likely) some more subtle misunderstanding by me of some conversion / reference frame issue.  In any case, it’s not really germane to the original query, but still has my curiosity piqued. 

Eric 

> 
>> On Nov 23, 2019, at 15:20, Eric Jensen <ejensen1 at swarthmore.edu> wrote:
>> 
>> Hi Franco, 
>> 
>> You can use the same transform_to method that is given in the example docs, just in the other direction, e.g. 
>> 
>> object_radec = object_altaz.transform_to(‘icrs')
>> 
>> Referring to the example here:  
>> https://docs.astropy.org/en/stable/generated/examples/coordinates/plot_obs-planning.html#sphx-glr-generated-examples-coordinates-plot-obs-planning-py
>> 
>> Here is some code based on that example, that extends it to show a transformation back to RA and Dec: 
>> 
>> import astropy.units as u
>> from astropy.time import Time
>> from astropy.coordinates import SkyCoord, EarthLocation, AltAz
>> 
>> # Get M33's coordinates
>> m33 = SkyCoord.from_name('M33')
>> 
>> # Define a location for observation: 
>> bear_mountain = EarthLocation(lat=41.3*u.deg, lon=-74*u.deg, height=390*u.m)
>> utcoffset = -4*u.hour  # Eastern Daylight Time
>> time = Time('2012-07-12 23:00:00') - utcoffset
>> 
>> # Find the alt/az at that time:
>> m33altaz = m33.transform_to(AltAz(obstime=time,location=bear_mountain))
>> print("M33's Altitude = {0.alt:.2f}, Azimuth = {0.az:.1f}".format(m33altaz))
>> 
>> # prints:  M33's Altitude = 0.13 deg, Azimuth = 47.3 deg
>> 
>> # Look at the same alt/az, 
>> # but half a sidereal day later: 
>> new_altaz = SkyCoord(AltAz(obstime=time + 0.5*u.sday,
>>                    az=m33altaz.az,
>>                    alt=m33altaz.alt,
>>                   location=bear_mountain))
>> 
>> # Transform back to RA, Dec;
>> # should get the same Dec as before but RA 12 hours different:
>> newradec = new_altaz.transform_to('icrs')
>> print(m33.to_string('hmsdms'))
>> print(newradec.to_string('hmsdms’))
>> 
>> # Prints 
>> # 01h33m50.904s +30d39m35.79s
>> # 13h33m59.1873s +30d47m03.8071s
>> 
>> Interestingly, the declination isn’t *exactly* the same and the RA doesn’t differ by exactly 12 hours - I wonder why?   But overall you can see the idea for doing what you want. 
>> 
>> Best,
>> 
>> Eric
>> 
>> 
>>> On Nov 23, 2019, at 2:00 PM, franco basti <franco.basti at gmail.com> wrote:
>>> 
>>> 
>>> Can anyone help me converting from a alt az to ra,dec ?
>>> all I can find are examples that convert from ra,dec to alt,az
>>> 
>>> Thx
>>> Franco
>>> 
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