MAXI Reference Frame (gdt.missions.maxi.frame)

The MAXI reference frame, MaxiFrame, is the frame that is aligned with the MAXI coordinate frame, and is represented by a quaternion that defines the rotation from MAXI coordinates to the ICRS coordinate frame. This frame takes advantage of the Astropy coordinate frame design, so we can use the MaxiFrame to convert Astropy SkyCoord objects between the MAXI frame and any celestial frame.

Manually Initializing a Frame

While the MaxiFrame is typically initialized when reading from the attitude and orbit files (see MaxiAttitude and MaxiOrbit) instead of manually by a user, we can manually define the frame with a Quaternion:

>>> from gdt.core.coords import Quaternion
>>> from gdt.missions.maxi.frame import MaxiFrame
>>> quat = Quaternion([-0.218,  0.009,  0.652, -0.726])
>>> maxi_frame = MaxiFrame(quaternion=quat)
>>> maxi_frame
<MaxiFrame: 1 frames;
 obstime=[J2000.000]
 obsgeoloc=[(0., 0., 0.) m]
 obsgeovel=[(0., 0., 0.) m / s]
 quaternion=[(x, y, z, w) [-0.218,  0.009,  0.652, -0.726]]>

Notice that we can also define the frame with an obstime, which is useful for transforming between the MaxiFrame and a non-inertial time-dependent frame; an obsgeoloc, which can define the spacecraft location in orbit; and obsgeovel, which defines the spacecraft orbital velocity.

Now let us define a SkyCoord in RA and Dec:

>>> from astropy.coordinates import SkyCoord
>>> coord = SkyCoord(100.0, -30.0, unit='deg')

And we can simply rotate this into the MAXI frame with the following:

>>> maxi_coord = coord.transform_to(maxi_frame)
>>> (maxi_coord.az, maxi_coord.el)
(<Longitude [200.39733555] deg>, <Latitude [-41.88750942] deg>)

We can also transform from the MAXI frame to other frames. For example, we define a coordinate in the MAXI frame this way:

>>> maxi_coord = SkyCoord(50.0, 25.0, frame=maxi_frame, unit='deg')

Now we can tranform to ICRS coordinates:

>>> maxi_coord.icrs
<SkyCoord (ICRS): (ra, dec) in deg
    [(313.69000519, 26.89158349)]>

or Galactic coordinates:

>>> maxi_coord.galactic
<SkyCoord (Galactic): (l, b) in deg
    [(71.5141302, -11.56931006)]>

or any other coordinate frames provided by Astropy.

Combining Orbit and Attitude Frames

We typically won’t manually create a MaxiFrame object, but rather these objects are a result of reading attitude and orbit files and extracting the information from within those files. More information about how to read those ffiles can be found in Maxi Attitude Files and Maxi Orbit Files. Because the frames from the attitude files only contain attitude information and the frames from the orbit files only contain orbital information, it is often useful to combine that information into a single frame (or set of frames), and this can be done with the MaxiFrame.combine_orbit_attitude() method.

Assuming we have two frames, att_frame and orb_frame that we have extracted from attitude and orbit files, respectively, we combine them into a single frame in the following way:

>>> # the MAXI attitude frame
>>> att_frame
<MaxiFrame: 86401 frames;
 obstime=[631152082.011275, ...]
 quaternion=[(x, y, z, w) [ 0.76156184, -0.55583739,  0.14661502, -0.29928647], ...]>

>>> # the MAXI orbit frame
>>> orb_frame
<MaxiFrame: 86362 frames;
 obstime=[631152006.0929999, ...]
 obsgeoloc=[(-3988969.7208, -1428272.8356, 5300477.4864) m, ...]
 obsgeovel=[(-7318.41587813, -498.94788896, -600.77996162) m / s, ...]
>

>>> from gdt.missions.maxi.frame import MaxiFrame
>>> combined_frame = MaxiFrame.combine_orbit_attitude(orb_frame, att_frame)
>>> combined_frame
<MaxiFrame: 86325 frames;
 obstime=[631152082.011275, ...]
 obsgeoloc=[(-3834147.96033173, -1985374.36954955, 5235177.78434637) m, ...]
 obsgeovel=[(-7229.7200539, -818.54642748, -1110.50065591) m / s, ...]
 quaternion=[(x, y, z, w) [ 0.76156184, -0.55583739,  0.14661502, -0.29928647], ...]>

One thing to notice is that the attitude and orbit MaxiFrame objects have a different number of sample frames and a different range of obstime even though they represent the MAXI attitude and orbital position for the same day. This is because the attitude and orbit parameters are natively sampled at slightly different rates and phases. Therefore, when the MaxiFrame.combine_orbit_attitude() combines the two frames together, it interpolates the attitude and orbit frames onto a common array of times that must span the intersection of the two time ranges. By default, the sampling period for the combined frame is 1 second, but this can be set with the sample_period keyword argument.

Reference/API

gdt.missions.maxi.frame Module

Functions

maxi_to_icrs(maxi_frame, icrs_frame)	Convert from the MAXI frame to the ICRS frame.
icrs_to_maxi(icrs_frame, maxi_frame)	Convert from the ICRS frame to the MAXI frame.

Classes

MaxiFrame(*args[, copy, ...])

The MAXI reference frame in azimuth and elevation.

Class Inheritance Diagram