201.6. ForcedSourceOnDiaObject table

201.6. Forced Source on DiaObject table¶

For the Rubin Science Platform at data.lsst.cloud.
Data Release: Data Preview 1
Container Size: large
LSST Science Pipelines version: r29.2.0
Last verified to run: 2025-09-02
Repository: github.com/lsst/tutorial-notebooks

Learning objective: To understand the contents of the ForcedSourceOnDiaObject table and how to access it.

LSST data products: ForcedSourceOnDiaObject

Packages: lsst.rsp, lsst.daf.butler

Credit: Originally developed by the Rubin Community Science team. This notebook also utilizes a transient detection from LSSTComCam first identified by Dan Taranu. Please consider acknowledging them if this notebook is used for the preparation of journal articles, software releases, or other notebooks.

Get Support: Everyone is encouraged to ask questions or raise issues in the Support Category of the Rubin Community Forum. Rubin staff will respond to all questions posted there.

1. Introduction¶

The ForcedSourceOnDiaObject table contains forced PSF measurements in individual visit_images and difference_images at the sky coordinates of all DiaObjects.

DiaObjects are the union set of all diaSources detected in difference_images.

It is recommended to use the ForcedSourceOnDiaObject table for light curves.

• TAP table name: dp1.ForcedSourceOnDiaObject
• butler table name: dia_object_forced_source
• columns: 28
• rows: 196,911,566

Related tutorials: The TAP and butler data access services are demonstrated in the 100-level "How to" tutorials. There are 200-level tutorials on difference_images and the DiaObject table.

1.1. Import packages¶

Import standard python packages re, numpy, matplotlib and astropy.

From the lsst package, import modules for the TAP service, the butler, and plotting.

In [1]:

import re
import numpy as np
import matplotlib.pyplot as plt
from lsst.rsp import get_tap_service
from lsst.daf.butler import Butler
from lsst.utils.plotting import (get_multiband_plot_colors,
                                 get_multiband_plot_symbols,
                                 get_multiband_plot_linestyles)

1.2. Define parameters and functions¶

Create an instance of the TAP service, and assert that it exists.

In [2]:

service = get_tap_service("tap")
assert service is not None

Create an instance of the Rubin data butler, and assert that it exists.

In [3]:

butler = Butler('dp1', collections="LSSTComCam/DP1")
assert butler is not None

Define the colors and symbols to represent the LSST filters in plots.

In [4]:

filter_names = ['u', 'g', 'r', 'i', 'z', 'y']
filter_colors = get_multiband_plot_colors()
filter_symbols = get_multiband_plot_symbols()
filter_linestyles = get_multiband_plot_linestyles()

2. Schema (columns)¶

To browse the table schema visit the Rubin schema browser, or use the TAP service via the Portal Aspect or as demonstrated in Section 2.1.

2.1. Retrieve table schema¶

To retrieve the table schema, define a query for the schema columns of the ForcedSourceOnDiaObject table and run the query job.

In [5]:

query = "SELECT column_name, datatype, description, unit " \
        "FROM tap_schema.columns " \
        "WHERE table_name = 'dp1.ForcedSourceOnDiaObject'"
job = service.submit_job(query)
job.run()
job.wait(phases=['COMPLETED', 'ERROR'])
print('Job phase is', job.phase)
if job.phase == 'ERROR':
    job.raise_if_error()
assert job.phase == 'COMPLETED'

Job phase is COMPLETED

Retrieve the query results and display them as an astropy table with the to_table() attribute.

In [6]:

results = job.fetch_result().to_table()
results

Out[6]:

Table length=28

column_name	datatype	description	unit
str64	str64	str512	str64
band	char	Abstract filter that is not associated with a particular instrument
coord_dec	double	Fiducial ICRS Declination of centroid used for database indexing	deg
coord_ra	double	Fiducial ICRS Right Ascension of centroid used for database indexing	deg
detector	long	Id of the detector where this forced source was measured. Datatype short instead of byte because of DB concerns about unsigned bytes.
diaObjectId	long	Id of the DiaObject that this DiaForcedSource was associated with.
diff_PixelFlags_nodataCenter	boolean	Source center is outside usable region on image difference (masked NO_DATA)
invalidPsfFlag	boolean	Forced source has an invalid PSF.
parentObjectId	long	Unique ObjectId of the parent of the ObjectId in context of the deblender.
patch	long	Skymap patch ID
pixelFlags_bad	boolean	Bad pixel in the Source footprint
pixelFlags_cr	boolean	Cosmic ray in the Source footprint
pixelFlags_crCenter	boolean	Cosmic ray in the Source center
pixelFlags_edge	boolean	Source is on the edge of an exposure region (masked EDGE)
pixelFlags_interpolated	boolean	Interpolated pixel in the Source footprint
pixelFlags_interpolatedCenter	boolean	Interpolated pixel in the Source center
pixelFlags_nodata	boolean	Source is outside usable exposure region (masked NO_DATA)
pixelFlags_saturated	boolean	Saturated pixel in the Source footprint
pixelFlags_saturatedCenter	boolean	Saturated pixel in the Source center
pixelFlags_suspect	boolean	Sources footprint includes suspect pixels
pixelFlags_suspectCenter	boolean	Sources center is close to suspect pixels
psfDiffFlux	float	Flux derived from linear least-squares fit of psf model forced on the image difference	nJy
psfDiffFlux_flag	boolean	Failure to derive linear least-squares fit of psf model forced on the image difference
psfDiffFluxErr	float	Uncertainty on the flux derived from linear least-squares fit of psf model forced on the image difference	nJy
psfFlux	float	Flux derived from linear least-squares fit of psf model forced on the calexp	nJy
psfFlux_flag	boolean	Failure to derive linear least-squares fit of psf model forced on the calexp
psfFluxErr	float	Uncertainty on the flux derived from linear least-squares fit of psf model forced on the calexp	nJy
tract	long	Skymap tract ID
visit	long	Id of the visit where this forced source was measured.

The table displayed above has been truncated.

Option to print every column name as a list.

In [7]:

# for col in results['column_name']:
#     print(col)

Option to use the regular expressions package re to search for column names that contain the string temp.

In [8]:

# temp = 'Err'
# temp = 'Flux'
# temp = 'psf'
# for col in results['column_name']:
#     if re.search(temp, col):
#         print(col)

Delete the job, but not the results.

In [9]:

del query
job.delete()

2.2. Key columns¶

The few most commonly used columns of the ForcedSourceOnDiaObject table.

2.2.1. DiaObject Id¶

The diaObjectId that identifies the row of the DiaObject table associated with the ForcedSourceOnDiaObject entry.

• diaObjectId

2.2.2. Coordinates¶

The sky coordinates in decimal degrees for the associated DiaObject is held fixed during the forced measurements, and has no quoted uncertainty in the ForcedSourceOnDiaObject table.

• coord_ra, coord_dec

2.2.3. Observation¶

The band, visit, and detector in which the ForcedSourceOnDiaObject was measured.

• band, visit, detector

2.2.4. Photometry¶

A forced fit of the Point Spread Function (PSF) at the object's coordinates in each individual visit_image and difference_image.

• psfFlux, psfFluxErr
• psfDiffFlux, psfDiffFluxErr

Forced fluxes on difference images can be negative.

It is not recommended to convert forced fluxes to magnitudes, but if they are positive, the fluxes ($f$) are in nanojanskies and the corresponding AB magnitude ($m$) is:

$m = -2.5\log(f) + 31.4$

2.2.5. Flags¶

A variety of flags indicating whether pixels that are saturated, or affected by cosmic rays, contributed to the source's measurements.

• pixelFlags_*

2.3. Descriptions and units¶

For a subset of the key columns show the table of their descriptions and units.

In [10]:

col_list = set(['diaObjectId', 'coord_ra', 'coord_dec', 'band', 'visit', 'detector',
                'psfFlux', 'psfFluxErr', 'psfDiffFlux', 'psfDiffFluxErr'])
tx = [i for i, item in enumerate(results['column_name']) if item in col_list]
results[tx]

Out[10]:

Table length=10

column_name	datatype	description	unit
str64	str64	str512	str64
band	char	Abstract filter that is not associated with a particular instrument
coord_dec	double	Fiducial ICRS Declination of centroid used for database indexing	deg
coord_ra	double	Fiducial ICRS Right Ascension of centroid used for database indexing	deg
detector	long	Id of the detector where this forced source was measured. Datatype short instead of byte because of DB concerns about unsigned bytes.
diaObjectId	long	Id of the DiaObject that this DiaForcedSource was associated with.
psfDiffFlux	float	Flux derived from linear least-squares fit of psf model forced on the image difference	nJy
psfDiffFluxErr	float	Uncertainty on the flux derived from linear least-squares fit of psf model forced on the image difference	nJy
psfFlux	float	Flux derived from linear least-squares fit of psf model forced on the calexp	nJy
psfFluxErr	float	Uncertainty on the flux derived from linear least-squares fit of psf model forced on the calexp	nJy
visit	long	Id of the visit where this forced source was measured.

Clean up.

In [11]:

del col_list, tx, results

3. Data access¶

The ForcedSourceOnDiaObject table is available via the TAP service and the butler.

Recommended access method: TAP.

3.1. Advisory: avoid full-table queries¶

Avoid full-table queries. Query by diaObjectId or visit on the ForcedSourceOnDiaObject table.

The ForcedSourceOnDiaObject table is a large, inclusive union set of forced measurements made in all the visit and difference images at the locations of all DiaObjects.

There are two main use-cases for queries on the ForcedSourceOnDiaObject table.

Case A: Return all forced photometry for a given DiaObject.

• Step 1. Query the DiaObject table to get the diaObjectId.
• Step 2. Retrieve all forced photometry by diaObjectId.

Case B: Return all forced photometry measurements from a given visit_ or difference_image.

• Step 1. Query the Visit table to get the visit identifier.
• Step 2. Retrieve all forced photometry by visit.

These two steps can be performed simultaneously with table joins as shown in Section 3.2.2.

The future data release ForcedSourceOnDiaObject tables will contain trillions of rows. It is important to develop good habits in making efficient queries.

3.2. TAP (Table Access Protocol)¶

The ForcedSourceOnDiaObject table is stored in Qserv and accessible via the TAP services using ADQL queries.

3.2.1. Demo query¶

For this example, start with the coordinates of a known extragalactic transient:

• RA = 53.125
• Dec = -27.740

First get the diaObjectId for this target. Search the Object table with a radius of 1 arcseconds.

In [12]:

query = "SELECT diaObjectId, ra, dec " \
        "FROM dp1.DiaObject " \
        "WHERE CONTAINS(POINT('ICRS', ra, dec), " \
        "CIRCLE('ICRS', 53.125, -27.740, 0.00028)) = 1 " \
        "ORDER BY diaObjectId ASC "
job = service.submit_job(query)
job.run()
job.wait(phases=['COMPLETED', 'ERROR'])
print('Job phase is', job.phase)
if job.phase == 'ERROR':
    job.raise_if_error()

Job phase is COMPLETED

In [13]:

assert job.phase == 'COMPLETED'
results = job.fetch_result().to_table()
print(len(results))

1

In [14]:

results

Out[14]:

Table length=1

diaObjectId	ra	dec
	deg	deg
int64	float64	float64
611255759837069401	53.124767650110215	-27.739814591611168

Then retrieve the forced photometry for that diaObjectId.

In [15]:

query = "SELECT diaObjectId, band, visit, detector, " \
        "psfFlux, psfFluxErr, psfDiffFlux, psfDiffFluxErr " \
        "FROM dp1.ForcedSourceOnDiaObject " \
        "WHERE diaObjectId = " + str(results['diaObjectId'][0]) + " " + \
        "ORDER BY diaObjectId ASC "
job = service.submit_job(query)
job.run()
job.wait(phases=['COMPLETED', 'ERROR'])
print('Job phase is', job.phase)
if job.phase == 'ERROR':
    job.raise_if_error()

Job phase is COMPLETED

In [16]:

assert job.phase == 'COMPLETED'
results = job.fetch_result().to_table()
print(len(results))

446

Option to display the table.

In [17]:

# results

As an example, calculate and print the minimum, mean, maximum, and standard deviation in PSF flux in the difference_images per filter. Negative fluxes demonstrate that the transient was likely present in the reference template used for the image subtraction.

In [18]:

for filt in filter_names:
    fx = np.where(results['band'] == filt)[0]
    print('%s %3i %8.1f %8.1f %8.1f %8.1f' %
          (filt, len(fx),
           np.min(results['psfDiffFlux'][fx]), np.mean(results['psfDiffFlux'][fx]),
           np.max(results['psfDiffFlux'][fx]), np.std(results['psfDiffFlux'][fx])))

u  22 -17794.6  -1184.0   1651.0   5280.5
g 121 -38815.1   -275.9   3379.5   4096.1
r 116 -38037.2  -1497.7   5476.7   5217.8
i  88  -9918.1   1432.2   9298.7   5791.1
z  87 -64061.1  -4087.0   5891.5   9607.9
y  12  -2178.1   -306.9   1851.8   1119.6

Clean up.

In [19]:

job.delete()
del query, results

3.2.2. Joinable tables¶

The ForcedSourceOnDiaObject table can be joined to:

• the Visit table on column visit
• the DiaObject table on column diaObjectId

The Visit table contains information about the observation, such as the date and time.

The DiaObject table contains measurement statistics made on the difference_images.

To plot a light curve, join the ForcedSourceOnDiaObject and Visit tables.

To the query used in Section 3.2.1, add a table join to the Visit table and retrieve the column expMidptMJD. Change the WHERE statement to select all rows of the ForcedSourceOnDiaObjectId catalog for the diaObjectId identified in the previous section. This query would also work the same with the CONTAINS(POINT(), CIRCLE()) statement.

In [20]:

query = "SELECT fsodo.diaObjectId, fsodo.coord_ra, fsodo.coord_dec, fsodo.band, fsodo.visit, " \
        "fsodo.detector, fsodo.psfFlux, fsodo.psfFluxErr, fsodo.psfDiffFlux, " \
        "fsodo.psfDiffFluxErr, v.expMidptMJD, " \
        "diao.u_psfFluxMean, diao.g_psfFluxMean, diao.r_psfFluxMean, " \
        "diao.i_psfFluxMean, diao.z_psfFluxMean, diao.y_psfFluxMean " \
        "FROM dp1.ForcedSourceOnDiaObject AS fsodo " \
        "JOIN dp1.Visit AS v ON fsodo.visit = v.visit " \
        "JOIN dp1.DiaObject AS diao ON diao.diaObjectId = fsodo.diaObjectId " \
        "WHERE diao.diaObjectId = 611255759837069401 " \
        "ORDER BY v.expMidptMJD ASC "
job = service.submit_job(query)
job.run()
job.wait(phases=['COMPLETED', 'ERROR'])
print('Job phase is', job.phase)
if job.phase == 'ERROR':
    job.raise_if_error()
assert job.phase == 'COMPLETED'

Job phase is COMPLETED

Fetch the results as an astropy table.

In [21]:

results = job.fetch_result().to_table()
print(len(results))

446

Option to display the results table.

In [22]:

# results

Plot the light curve for this object, as measured from the difference_images (psfDiffFlux) and visit_images (psfFlux).

Warning message: The following cell will produce a pink warning about masked elements being converted to nans, which is just saying that some of the flux measurements in the array were masked (i.e., were not successful) and that the corresponding values are converted to "not a number", "nan". They are not plotted.

In [23]:

fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(10, 4))
for filt in filter_names:
    fx = np.where(results['band'] == filt)[0]
    if len(fx) > 0:
        ax1.axhline(results[filt+'_psfFluxMean'][fx[0]], color=filter_colors[filt],
                    ls=filter_linestyles[filt], alpha=0.2, label=filt)
        ax1.plot(results['expMidptMJD'][fx],
                 results['psfDiffFlux'][fx],
                 filter_symbols[filt], ms=5, mew=0, alpha=0.7,
                 color=filter_colors[filt])
        ax2.plot(results['expMidptMJD'][fx],
                 results['psfFlux'][fx],
                 filter_symbols[filt], ms=5, mew=0, alpha=0.7,
                 color=filter_colors[filt], label=filt)
ax1.set_xlabel('MJD')
ax1.set_ylabel('PSF Diff Flux')
ax1.legend(loc='lower left', ncol=2)
ax2.set_xlabel('MJD')
ax2.set_ylabel('PSF Flux')
ax2.legend(loc='lower left', ncol=2)
plt.tight_layout()
plt.show()

/opt/lsst/software/stack/conda/envs/lsst-scipipe-10.1.0/lib/python3.12/site-packages/matplotlib/cbook.py:1719: UserWarning: Warning: converting a masked element to nan.
  return math.isfinite(val)
/opt/lsst/software/stack/conda/envs/lsst-scipipe-10.1.0/lib/python3.12/site-packages/matplotlib/cbook.py:1355: UserWarning: Warning: converting a masked element to nan.
  return np.asarray(x, float)

Figure 1: At left, the PSF forced photometry light curve on the difference_images for the diaobject. Horizontal lines mark the diaobject's mean flux from associated diaSource measurements in the difference_images. At right, the PSF forced photometry light curve on the visit_images for the diaobject.

Note that the photometry statistics from the DiaObject table (e.g. r_psfFluxMean) are calculated from associated DiaSources, where the signal-to-noise ratio of the detections are $\geq5$, not from the ForcedSourceOnDiaObject measurements.

Clean up.

In [24]:

job.delete()
del query, results

3.4. Butler¶

TAP is the recommended way to access the source table, but the butler can also be used.

Show that the dimensions for the object_forced_source table are the skymap's tract and patch, and that both are required.

In [25]:

butler.get_dataset_type('dia_object_forced_source')

Out[25]:

DatasetType('dia_object_forced_source', {skymap, tract, patch}, ArrowAstropy)

In [26]:

butler.get_dataset_type('dia_object_forced_source').dimensions.required

Out[26]:

{skymap, tract, patch}

3.4.1. Demo query¶

Retrieve all dataset_refs for dia_object_forced_source tables for patches that overlap the coordinates that were also used in Section 3.2.

In [27]:

ra = 53.125
dec = -27.740

refs = butler.query_datasets("dia_object_forced_source",
                             where="patch.region OVERLAPS POINT(:ra, :dec)",
                             bind={"ra": ra, "dec": dec})

Print the number of dataset references returned.

In [28]:

print(len(refs))

1

In [29]:

refs

Out[29]:

[DatasetRef(DatasetType('dia_object_forced_source', {skymap, tract, patch}, ArrowAstropy), {skymap: 'lsst_cells_v1', tract: 5063, patch: 34}, run='LSSTComCam/runs/DRP/DP1/v29_0_0/DM-50260/20250419T073356Z', id=53be93ed-96c6-4e4c-a45d-a7551a9f3a7f)]

Print the dataId of the second element of the refs.

In [30]:

for ref in refs:
    print(ref.dataId)

{skymap: 'lsst_cells_v1', tract: 5063, patch: 34}

Define the columns to retrieve.

In [31]:

col_list = ['diaObjectId', 'coord_ra', 'coord_dec',
            'band', 'psfDiffFlux', 'psfDiffFluxErr']

Get the data from the butler for the patch in the list of returned dataset refs.

In [32]:

results = butler.get(refs[0],
                     parameters={'columns': col_list})
print(len(results))

3428663

Option to display the results.

In [33]:

# results

Without a small constraint on the region as in Section 3.2.1, data for many more unique objects has been returned.

In [34]:

values = np.unique(results['diaObjectId'])
print(len(values))
del values

6344

Check that the transient is in the results table.

In [35]:

tx = np.where(results['diaObjectId'] == 611255759837069401)[0]
print(len(tx))

446

Print the same table of psfDiffFlux statistics in Section 3.2.1.

In [36]:

for filt in filter_names:
    fx = np.where(results['band'][tx] == filt)[0]
    print('%s %3i %8.1f %8.1f %8.1f %8.1f' %
          (filt, len(fx),
           np.min(results['psfDiffFlux'][tx[fx]]),
           np.mean(results['psfDiffFlux'][tx[fx]]),
           np.max(results['psfDiffFlux'][tx[fx]]),
           np.std(results['psfDiffFlux'][tx[fx]])))

u  22 -17794.6  -1184.0   1651.0   5280.5
g 121 -38815.1   -275.9   3379.5   4096.1
r 116 -38037.2  -1497.7   5476.7   5217.9
i  88  -9918.1   1432.2   9298.7   5791.1
z  87 -64061.1  -4087.0   5891.5   9607.9
y  12  -2178.1   -306.9   1851.9   1119.6

In [37]:

del ra, dec, refs, col_list
del results, tx, fx

In [ ]: