Slit spectrographs are often used to produce rasters. In fact, it is from this data product that sunraster derives its name.

A raster is produced by scanning the slit in discrete steps perpendicular to its long axis, recording an exposure at each position. Thus a spectral image over a region is built up over time despite the slit spectrograph’s necessarily narrow horizontal field of view. Another motivation can be to perform fast repeat raster scans in order to improve the chances of catching an event with the slit, e.g., a solar flare. In a raster, the slit-step axis is convolved with time.

Depending on the type of analysis being performed, users may want to think of their data as if it were in raster mode/4D (scan number, slit step, position along slit, wavelength) or sit-and-stare mode/3D (time, position along slit, spectral).

In order to access the data in the way they want, scientists may often have two copies, a 3D version and a 4D version. However, this means scientists have to keep track of two data structures which is memory intensive both for the scientist and the computer and increases the chances mistakes in analysis.

Solving this problem is the purpose of the RasterSequence class. It inherits from SpectrogramSequence but enables users to label one of the axes as the slit-step axis. This in turn facilitates a new set of APIs which allows users to interact with their data in sit-and-stare (sns) or rastering mode seamlessly and interchangeably without having to reformat their data.


A RasterSequence, is instantiated just like a SpectrogramCube. Let’s first create some SpectrogramCube instances where each represents a single raster scan. As before, we will add the timestamps and exposure times as extra coordinates.

>>> import numpy as np
>>> import astropy.wcs
>>> import astropy.units as u
>>> from astropy.nddata import StdDevUncertainty
>>> from datetime import datetime, timedelta
>>> from astropy.time import Time
>>> from sunraster import SpectrogramCube
>>> from sunraster.meta import Meta

>>> # Define primary data array and WCS object.
>>> data = np.ones((3, 4, 5))
>>> wcs_input_dict = {
...     'CTYPE1': 'WAVE    ', 'CUNIT1': 'Angstrom', 'CDELT1': 0.2, 'CRPIX1': 0, 'CRVAL1': 10, 'NAXIS1': 5,
...     'CTYPE2': 'HPLT-TAN', 'CUNIT2': 'deg', 'CDELT2': 0.5, 'CRPIX2': 2, 'CRVAL2': 0.5, 'NAXIS2': 4,
...     'CTYPE3': 'HPLN-TAN', 'CUNIT3': 'deg', 'CDELT3': 0.4, 'CRPIX3': 2, 'CRVAL3': 1, 'NAXIS3': 3}
>>> input_wcs = astropy.wcs.WCS(wcs_input_dict)
>>> # Define a mask with all pixel unmasked, i.e. mask values = False
>>> mask = np.zeros(data.shape, dtype=bool)
>>> # Define some RasterSequence metadata.
>>> exposure_times = np.ones(data.shape[0])/2 * u.s
>>> scan_meta = Meta({"exposure time": exposure_times}, axes={"exposure time": 0},
...                  data_shape=data.shape)
>>> seq_meta = {"description": "This is a RasterSequence.", "exposure time" : exposure_times}

>>> # Define uncertainties for data, 2*data and data/2.
>>> uncertainties = StdDevUncertainty(np.sqrt(data))
>>> uncertainties2 = StdDevUncertainty(np.sqrt(data * 2))
>>> uncertainties05 = StdDevUncertainty(np.sqrt(data * 0.5))

>>> # Create 1st raster
>>> axis_length = int(data.shape[0])
>>> timestamps0 = Time([datetime(2000, 1, 1) + timedelta(minutes=i)
...                     for i in range(axis_length)], format='datetime', scale='utc')
>>> extra_coords_input0 = [("time", 0, timestamps0)]
>>> raster0 = SpectrogramCube(data, input_wcs, uncertainty=uncertainties, mask=mask,
...                           meta=scan_meta, unit=u.ct)
>>> for extra in extra_coords_input0:
...     raster0.extra_coords.add(*extra)
>>> # Create 2nd raster
>>> timestamps1 = Time([timestamps0[-1].to_datetime() + timedelta(minutes=i)
...                     for i in range(1, axis_length+1)], format='datetime', scale='utc')
>>> extra_coords_input1 = [("time", 0, timestamps1)]
>>> raster1 = SpectrogramCube(data*2, input_wcs, uncertainty=uncertainties, mask=mask,
...                  meta=scan_meta, unit=u.ct)
>>> for extra in extra_coords_input1:
...     raster1.extra_coords.add(*extra)
>>> # Create 3rd raster
>>> timestamps2 = Time([timestamps1[-1].to_datetime() + timedelta(minutes=i)
...                     for i in range(1, axis_length+1)], format='datetime', scale='utc')
>>> extra_coords_input2 = [("time", 0, timestamps2)]
>>> raster2 = SpectrogramCube(data*0.5, input_wcs, uncertainty=uncertainties, mask=mask,
...                  meta=scan_meta, unit=u.ct)
>>> for extra in extra_coords_input2:
...     raster2.extra_coords.add(*extra)

The last thing we need to do before creating our RasterSequence is to identity the slit-step of the SpectrogramCube. In the above raster instances both the 0th and 1st axes correspond to spatial dimensions. Therefore let’s define the 0th axes as the slit-step. We will do this by setting the common_axis argument 0.

>>> from sunraster import RasterSequence
>>> my_rasters = RasterSequence([raster0, raster1, raster2], common_axis=0, meta=seq_meta)


RasterSequence provides a version of the array_axis_physical_axis_types property for both raster and sns representations.

>>> my_rasters.raster_array_axis_physical_types
[('meta.obs.sequence',), ('custom:pos.helioprojective.lat', 'custom:pos.helioprojective.lon', 'time'), ('custom:pos.helioprojective.lat', 'custom:pos.helioprojective.lon'), ('em.wl',)]

>>> my_rasters.sns_array_axis_physical_types
[('custom:pos.helioprojective.lat', 'custom:pos.helioprojective.lon', 'time'), ('custom:pos.helioprojective.lat', 'custom:pos.helioprojective.lon'), ('em.wl',)]

In the raster case, 'meta.obs.sequence' represents the raster scan number axis. For those familiar with NDCubeSequence, these are simply aliases for the array_axis_physical_axis_types and cube_like_world_axis_physical_axis_types, respectively.

The length of each axis can also be displayed in either the raster or sns representation.

>>> my_rasters.raster_dimensions
(<Quantity 3. pix>, <Quantity 3. pix>, <Quantity 4. pix>, <Quantity 5. pix>)

raster_dimensions always represents the length of the scan number axis in the 0th position. We can therefore see that we have 3 raster scans in our RasterSequence. This means that the slit-step axis is shifted by one. Since we defined common_axis=0 during instantiation, this means that the length of the slit-step can be found in the 1st element. From this we can see that we have 3 slit positions per raster scan.

To see the length of the axes as though the data is in sit-and-stare mode, simply do:

>>> my_rasters.sns_dimensions
<Quantity [9., 4., 5.] pix>

Note that scan number and slit-step axes have been combined into the 0th position. From this we can see that we have 9 (3x3) spectrograms or times in our RasterSequence.


Coordinate properties#

RasterSequence provides the same convenience properties as SpectrogramSequence to retrieve the real world coordinate values for each pixel along each axis. sunraster.RasterSequence.celestial, and sunraster.RasterSequence.spectral return their values in the raster representation while sunraster.RasterSequence.time and sunraster.RasterSequence.exposure_time return their values in the sns representation.

sns axis extra coordinates#

As well as time and exposure_time, some sunraster.SpectrogramCube.extra_coords may contain other coordinates that are aligned with the slit step axis. The sunraster.RasterSequence.sns_axis_coords property enables users to access these coordinates at the RasterSequence level in the form of an abbreviated extra_coords dictionary. Just like time and sunraster.RasterSequence.exposure_time, the coordinates are concatenated so they mimic the sit-and-stare-like dimensionality returned in the 0th element of sunraster.RasterSequence.sns_dimensions. sunraster.RasterSequence.sns_axis_coords is equivalent to ndcube.NDCubeSequence.common_axis_extra_coords. To see examples of how to use this property, see the NDCubeSequence Common Axis Extra Coordinates documentation.

Raster axis extra coordinates#

Analogous to sns_axis_coords, it is also possible to access the coordinates that are not assigned to any SpectrogramCube data axis via the raster_axis_coords property. This property is equivalent to ndcube.NDCubeSequence.sequence_axis_coords and can be used to return coordinates along the repeat raster scan axis.


RasterSequence not only enables users to inspect their data in the raster and sit-and-stare representations. It also enables them to slice the data in either representation as well. This is done via the slice_as_raster and slice_as_sns properties. As with SpectrogramCube and SpectrogramSequence, these slicing properties ensure that not only the data is sliced, but also all relevant supporting metadata including uncertainties, mask, WCS object, extra_coords, etc.

To slice a RasterSequence using the raster representation, do:

>>> my_rasters_roi = my_rasters.slice_as_raster[1:3, 0:2, 1:3, 1:4]

We can see the result of slicing using the dimensions properties.

>>> print(my_rasters.raster_dimensions)  # Check dimensionality before slicing.
(<Quantity 3. pix>, <Quantity 3. pix>, <Quantity 4. pix>, <Quantity 5. pix>)
>>> print(my_rasters_roi.raster_dimensions) # See how slicing has changed dimensionality.
(<Quantity 2. pix>, <Quantity 2. pix>, <Quantity 2. pix>, <Quantity 3. pix>)
>>> my_rasters_roi.sns_dimensions  # Dimensionality can still be represented in sns form.
<Quantity [4., 2., 3.] pix>

To slice in the sit-and-stare representation, do the following:

>>> my_rasters_roi = my_rasters.slice_as_sns[1:7, 1:3, 1:4]

Let’s check the effect of the slicing once again.

>>> print(my_rasters.sns_dimensions)  # Check dimensionality before slicing.
[9. 4. 5.] pix
>>> print(my_rasters_roi.sns_dimensions)  # See how slicing has changed dimensionality.
[6. 2. 3.] pix
>>> print(my_rasters_roi.raster_dimensions)  # Dimensionality can still be represented in raster form.
(<Quantity 3. pix>, <Quantity [2., 3., 1.] pix>, <Quantity 2. pix>, <Quantity 3. pix>)

Notice that after slicing the data can still be inspected and interpreted in the raster or sit-and-stare format, irrespective of which slicing representation was used. Also notice that the my_sequence.slice_as_sns[1:7, 1:3, 1:4] command led to different SpectrogramCube objects to have different lengths along the slit step axis. This can be seen from the fact that the slit step axis entry in the output of my_sequence_roi.raster_dimensions has a length greater than 1. Each element represents the length of each SpectrogramCube in the SpectrogramSequence along that axis.

As with SpectrogramSequence, slicing can reduce a RasterSequence dimensionality. As in the Exposure Time Correction section, let’s slice out the 2nd pixel along the slit. This reduces the number of dimensions in the raster representation to 3 (raster scan, slit step, spectral) and to 2 in the sit-and-stare representation (time, spectral). However, the raster and sit-and-stare representations are still valid.

>>> slit_pixel_rasters = my_rasters.slice_as_raster[:, :, 2]
>>> print(slit_pixel_rasters.raster_dimensions)
(<Quantity 3. pix>, <Quantity 3. pix>, <Quantity 5. pix>)
>>> print(slit_pixel_rasters.sns_dimensions)
[9. 5.] pix

This demonstrates that the difference between the raster and sit-and-stare representations is more subtle than simply a 4-D or 3-D dimensionality. The difference is whether the raster scan and slit step axes are convolved into a time axis or whether they are represented separately. And because of this definition, the raster and sit-and-stare representations are valid and accessible for any dimensionality in which the raster scan and slit step axes are maintained.


To quickly and easily visualize slit spectrograph data, RasterSequence supplies simple-to-use, yet powerful plotting APIs. They are intended to be a useful quicklook tool and not a replacement for high quality plots or animations, e.g. for publications. As with slicing, there are two plot methods for plotting in each of the raster and sit-and-stare representations.

To visualize in the raster representation, simply call the following:

>>> my_rasters.plot_as_raster() 

To visualize in the sit-and-stare representation, do:

>>> my_rasters.plot_as_sns() 

These methods produce different types of visualizations including line plots, 2-D images and 1- and 2-D animations. Which is displayed depends on the dimensionality of the RasterSequence and the inputs of the user. plot_as_raster and plot_as_sns are in fact simply aliases for the ndcube.NDCubeSequence.plot and ndcube.NDCubeSequence.plot_as_cube methods, respectively. For learn more about how these routines work and the optional inputs that enable users to customize their output, see the NDCubeSequence plotting documentation.

Extracting Data Arrays#

It is possible that you may have some procedures that are designed to operate on arrays instead of SpectrogramSequence or RasterSequence objects. Therefore it may be useful to extract the data (or other array-like information such as uncertainty or mask) into a single ndarray. A succinct way of doing this operation is using python’s list comprehension.

To make a 4-D array from the data arrays in my_rasters, use numpy.stack.

>>> print(my_rasters._dimensions)  # Print sequence dimensions as a reminder.
(<Quantity 3. pix>, <Quantity 3. pix>, <Quantity 4. pix>, <Quantity 5. pix>)
>>> data = np.stack([cube.data for cube in my_rasters.data])
>>> print(data.shape)
(3, 3, 4, 5)

To define a 3D array where the data arrays of each SpectrogramCube in the sequence is concatenated along an axis, use numpy.vstack.

>>> data = np.vstack([cube.data for cube in my_rasters.data])
>>> print(data.shape)
(9, 4, 5)

To create 3D arrays by slicing sequences, do:

>>> data = np.stack([cube[2].data for cube in my_rasters.data])
>>> print(data.shape)
(3, 4, 5)