Photometry

class daschlab.photometry.Photometry(data=None, masked=False, names=None, dtype=None, meta=None, copy=True, rows=None, copy_indices=True, units=None, descriptions=None, **kwargs)

Bases: Table

A DASCH photometry data table.

Photometry is a subclass of astropy.table.Table. You will in turn almost always deal with DASCH photometry using one of two more specialized subclasses: Lightcurve for lightcurves (the same source over time) or Extract for “extracts” (a bunch of sources from the same exposure).

This class defines methods that apply to either of these use cases. You can also use all of the usual methods and properties made available by the astropy.table.Table class. Items provided by those classes are not documented here.

The actual data contained in these tables — the columns — are documented elsewhere, on the main DASCH website.

You should not construct Photometry instances directly. Instead, obtain lightcurves using the daschlab.Session.lightcurve() method.

See the lightcurve module-level documentation for a summary of the filtering and subsetting functionality provided by these classes.

Cheat sheet:

• time is HJD midpoint

• magcal_magdep is preferred calibrated phot measurement

• legacy plotter error bar is magcal_local_rms` * `error_bar_factor, the latter being set to match the error bars to the empirical RMS, if this would shrink the error bars

Attributes Summary

count	An action returning the number of selected rows
drop	An action returning a Photometry copy dropping the selected rows; all non-selected rows are retained.
keep_only	An action returning a Photometry copy containing only the selected rows.
match	An action returning a boolean array identifying selected rows.
reject	An action modifying the photometry in-place, rejecting the selected rows.
reject_unless	An action modifying the photometry in-place, rejecting rows not matching the selection.
split_by	An action returning two Photometry copies, partitioning by the selection.

Methods Summary

apply_standard_rejections()	Apply a standard set of data-quality rejections to the photometry.
export(path[, format, drop_rejects])	Save a copy of this photometry to a file.
scatter(x_axis, y_axis[, rejects])	Make a Bokeh scatter plot of photometry data.
summary()	Print a brief textual summary of the photometry contents.

Attributes Documentation

count

An action returning the number of selected rows

Unlike many actions, this returns an int, not a new Photometry

drop

An action returning a Photometry copy dropping the selected rows; all non-selected rows are retained.

keep_only

An action returning a Photometry copy containing only the selected rows.

match

An action returning a boolean array identifying selected rows.

Unlike many actions, this does not return a new Photometry. It can be used to implement arbitrary boolean logic within the action/selection framework:

lc = sess.lightcurve(some_local_id)
subset = lc.keep_only.where(
    lc.match.series("a") & lc.match.brighter(13)
)

reject

An action modifying the photometry in-place, rejecting the selected rows.

Usage is as follows:

lc = sess.lightcurve(some_local_id)

# Mark all points from meteor telescopes as rejected:
lc.reject.meteor(tag="meteor")

The tag keyword argument to the selector is mandatory. It specifies a short, arbitrary “tag” documenting the reason for rejection. Each unique tag is associated with a binary bit of the "reject" column, and these bits are logically OR’ed together as rejections are established. The maximum number of distinct rejection tags is 64, since the "reject" column is stored as a 64-bit integer.

If a verbose keyword argument is true (the default), a message will be printed summarizing the number of rejected rows. If false, nothing will be printed.

If a dry_run keyword argument is true (not the default), the "reject" column will not actually be modified. Instead, a message about what would have changed is printed.

reject_unless

An action modifying the photometry in-place, rejecting rows not matching the selection.

Usage is as follows:

lc = sess.lightcurve(some_local_id)

# Mark all points *not* from narrow-field telescopes as rejected:
lc.reject_unless.narrow(tag="lowrez")

# This is equivalent to:
lc.reject.not_.narrow(tag="lowrez")

The tag keyword argument to the selector is mandatory. It specifies a short, arbitrary “tag” documenting the reason for rejection. Each unique tag is associated with a binary bit of the "reject" column, and these bits are logically OR’ed together as rejections are established. The maximum number of distinct rejection tags is 64, since the "reject" column is stored as a 64-bit integer.

The logical negation used by this function can cause problems if your photometry contains nondetections. For instance, if you intend to reject points unless they are brighter than a mag of 15.0, you will also reject all of the nondetections, even shallow upper limits consistent with that cutoff.

split_by

An action returning two Photometry copies, partitioning by the selection.

Unlike many actions, this returns a tuple of two new Photometry instances, not just one. The first returned instance contains only the selected points; the second instance returns all others. This can be useful for statistical comparisons:

from astropy import units as u

lc = sess.lightcurve(some_local_id)
neardet, fardet = lc.keep_only.nonrej_detected().split_by.sep_below(20 * u.arcsec)
near_std = neardet["magcal_magdep"].std()
far_std = neardet["magcal_magdep"].std()

Methods Documentation

apply_standard_rejections()

Apply a standard set of data-quality rejections to the photometry.

Warning: this function is in development. Do not rely on it to fully clean your data.

Notes

Until this functionality is more settled, the rejections will not be documented here. See the source code to see what it does.

export(path: str, format: str = 'csv', drop_rejects: bool = True)

Save a copy of this photometry to a file.

Parameters:

pathstr

The path at which the exported photometric data will be saved.

formatstr, default "csv"

The file format in which to save the data. Currently, the only allowed value is "csv".

drop_rejectsbool, default True

If True, rejected points will not appear in the output file at all. Otherwise, they will be included, and there will be a "reject" column reproducing their rejection status.

Notes

The default "csv" output format only exports a subset of the table columns known as the “medium” subset. It is defined in the lightcurve table documentation.

scatter(x_axis: str, y_axis: str, rejects: bool = False) → figure

Make a Bokeh scatter plot of photometry data.

Parameters:

x_axisstr

The name of the column to use for the X axis.

y_axisstr

The name of the column to use for the Y axis.

rejectsoptional bool, default False

Whether to include rejected points in the resulting plot.

Returns:

bokeh.plotting.figure

A plot.

Notes

The function bokeh.io.show (imported as bokeh.plotting.show) is called on the figure before it is returned, so you don’t need to do that yourself.

summary()

Print a brief textual summary of the photometry contents.