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## Creating Spectral Cubes

You can use a SpecFrame to describe the spectral axis in a data cube containing two spatial axes and a spectral axis. To do this you would create an appropriate SpecFrame, together with a 2-dimensional Frame (often a SkyFrame) to describe the spatial axes. You would then combine these two Frames together into a single CmpFrame.

```AstSkyFrame *skyframe;
AstSpecFrame *specframe;
AstCmpFrame *cmpframe;
...
skyframe = astSkyFrame( "Epoch=J2002" );
specframe = astSpecFrame( "System=Freq,StdOfRest=LSRK" );
cmpframe = astCmpFrame( skyframe, specframe, "" );
```

In the resulting CmpFrame, axis 1 will be RA, axis 2 will be Dec and axis 3 will be Frequency. If this is not the order you want, you can permute the axes using astPermAxes.

There is one potential problem with this approach if you are interested in unusually high accuracy. Conversion between different standards of rest involves taking account of the Doppler shift caused by the relative motion of the two standards of rest. At some point this involves finding the component of the relative velocity in the direction of interest. For a SpecFrame, this direction is always given by the RefRA and RefDec attributes, even if the SpecFrame is embedded within a CmpFrame as above. It would be more appropriate if this ``direction of interest'' was specified by the values passed into the CmpFrame on the RA and DEC axes, allowing each pixel within a data cube to have a slightly different correction for Doppler shift.

Unfortunately, the SpecFrame class cannot do this (since it is purely a 1-dimensional Frame), and so some small degree of error will be introduced when converting between standards of rest, the size of the error varying from pixel to pixel. It is hoped that at some point in the future a sub-class of CmpFrame (a SpecCubeFrame) will be added to AST which allows for this spatial variation in Doppler shift.

The maximum velocity error introduced by this problem is of the order of , where is the angular field of view, and is the relative velocity of the two standards of rest. As an example, when correcting from the observers rest frame (i.e. the topocentric rest frame) to the kinematic local standard of rest the maximum value of is about 20 , so for 5 arc-minute field of view the maximum velocity error introduced by the correction will be about 0.03 . As another example, the maximum error when correcting from the observers rest frame to the local group is about 5 over a 1 degree field of view.

Next: Handling Dual-Sideband Spectra
Up: Spectral Coordinate Systems (SpecFrames)
Previous: Using Default SpecFrame Attributes

AST A Library for Handling World Coordinate Systems in Astronomy