RA, Dec and object name assignment to 1D extracted spectra

Version History

Version

Author

Date

PypeIt

1.0

Debora Pelliccia

25 Jan 2021

1.3.1dev

1.1

Debora Pelliccia

02 Apr 2021

1.3.4dev

1.2

Debora Pelliccia

28 Jul 2021

1.4.3dev

1.3

Debora Pelliccia

21 Oct 2021

1.6.1dev

1.4

Debora Pelliccia

6 Sep 2023

1.13.1dev

Basics

The procedure used to assign RA, Dec and object name to each 1D extracted spectrum is currently available for these Slit-mask design Spectrographs only and is performed right after the object finding procedure described in Object Finding.

Procedure

RA, Dec and object name assignment is primarily performed by pypeit.slittrace.assign_addobjs_alldets().

First, Pypeit, using pypeit.slittrace.get_maskdef_objpos_offset_alldets(), computes for every detectors that the user wants to reduce the offset of the observed slitmask from the position expected by the design file (see Slitmask ID assignment and missing slits for a description on how the slitmask design matching is performed), and stores it in the SlitTraceSet datamodel (see Slits for a description of the provided information and for a way to visualize them). There are four different options that the user can choose to compute the offset: using objects with high significance detections, using only one bright object in a selected slit, using alignment stars, or using the dither offset recorded in the science frame header (available only for MOSFIRE). Or, alternatively, the user can input a known offset in pixels (see Application).

Then, pypeit.slittrace.average_maskdef_offset() determines an average slitmask offset over all the detectors (this does not happen for MOSFIRE observations, which have only one detector), which is used by pypeit.slittrace.assign_addobjs_alldets() to assign RA, Dec and object name to detected objects and to force extract objects that were not detected (for the latter see Object position on the slit from slitmask design).

The function assign_addobjs_alldets(), for each detector, goes through all the slits and checks if the measured distance of the detected objects from the left edge of the slit (corrected for the offset computed in the previous step) is within a certain tolerance (see Application for details on how to control the value of this parameter) of the distance expected from the slitmask design (differences between the expected and the measured slit length are taken into account). Correcting for the slitmask offset allows PypeIt to deal also with dithered observations. If the measured distance is within the set tolerance, the RA, DEC and MASKDEF_OBJNAME of the detected object are updated with the coordinates and name of the targeted object. If the measured distance is not within the tolerance, the detected object is considered a serendipitous object. Using the coordinates of the slit center available from the slitmask design and the distance in pixels (converted then in arcsec) between the traced object and the center of the slit, the coordinates of the serendipitous object are estimates and recorded in RA and DEC while MASKDEF_OBJNAME is set to “SERENDIP”. For both cases, the MASKDEF_EXTRACT attribute is set to False (see Object position on the slit from slitmask design).

Application

To perform the RA, Dec and object name assignment to extracted spectra, the parameters described in the Application section of Slitmask ID assignment and missing slits must be set. Moreover, the assign_obj flag in SlitMaskPar Keywords must be True. This is the default for DEIMOS (except when the LongMirr or the LVM mask is used) and MOSFIRE (except when the LONGSLIT mask is used). Seven other parameters control this procedure. Six are for the slitmask offset determination and one is for the RA, Dec and object name assignment. They are the following.

  • snr_thrshd: objects detected above this S/N threshold are used to compute the slitmask offset. This is the default behaviour for DEIMOS unless slitmask_offset, bright_maskdef_id or use_alignbox is set. Default value is snr_thrshd=50.

  • bright_maskdef_id: maskdef_id (corresponding to dSlitId and Slit_Number in the DEIMOS/LRIS and MOSFIRE slitmask design, respectively) of a slit containing a bright object that will be used to compute the slitmask offset. This parameter is optional (default value is bright_maskdef_id=None) and is ignored if slitmask_offset is provided or use_dither_offset = True. However, this parameter is highly recommended for MOSFIRE observations if a bright object is present in the slitmask, since it allows to trace the small drifts of the objects position that have been typically seen in MOSFIRE data.

  • use_alignbox: flag to use stars in alignment boxes to compute the slitmask offset. This is available only for DEIMOS observations and it is set as the default for this instrument. If use_alignbox = True PypeIt will NOT compute the offset using snr_thrshd or bright_maskdef_id.

  • use_dither_offset: flag to use the dither offset recorded in the header of science frames as the value of the slitmask offset. This is currently only available for MOSFIRE reduction and it is set as the default for this instrument. If use_dither_offset = True PypeIt will NOT compute the offset using snr_thrshd or bright_maskdef_id. However, it is ignored if slitmask_offset is provided.

  • slitmask_offset: user-provided slitmask offset (pixels) from the position expected by the slitmask design. This is optional (default value is slitmask_offset=None), and if set PypeIt will NOT compute the offset, i.e., the above parameters will be ignored.

  • obj_toler: sets the tolerance in arcsec for the matching process between the measured coordinates of the extracted spectrum and the expected coordinates of the targeted object. The default value is obj_toler = 1.

Access

PypeIt users can access the objects information in several ways.

  • Ra, Dec, object name and maskdef_id are visible in the .txt file with a list of all extracted spectra, generated at the end the PypeIt reduction.

  • Ra, Dec, object name are also visible by running pypeit_show_1d –list (see pypeit_show_1dspec)

  • Object names are visible in ginga when running pypeit_show_2d (see pypeit_show_2dspec)

Testing

  • PYPEIT-DEIMOS (PD)

    Requirement PD-8 states: “As a user, I expect products that are associated with my mask definition (object names, object positions).”

    Requirement PD-35 states: “As a user, I expect to be able to reduce data taken with a nodding pattern.”

  • PYPEIT-MOSFIRE (PM)

    Requirement PM-9 states: “As a user, I expect products that are associated with my mask definition (object names, object positions).”

    Requirement PM-12 states: “Use slitmask information to determine object position in the slit.”

PypeIt meets this requirement as demonstrated by the tests at pypeit/tests/test_slitmask.py. To run the test:

cd pypeit/tests
pytest test_slitmask.py::test_assign_maskinfo_add_missing -W ignore
pytest test_slitmask.py::test_dith_obs -W ignore

The tests require that you have downloaded the PypeIt PypeIt Development Suite (including the folder compressed in “Cooked_pypeit_dev_vX.XX.X.tar.gz”, where vX.XX.X indicates the version of the file) and defined the PYPEIT_DEV environmental variable that points to the relevant directory. These tests are run using only one instrument setup and for DEIMOS only one detector.

First test pytest test_slitmask::test_assign_maskinfo -W ignore. The algorithm of the test is repeated twice (once for a DEIMOS dataset and once for a MOSFIRE dataset) and is as follows:

  1. Load the information relative to the specific instrument (DEIMOS, MOSFIRE).

  2. Load the Instrument-Specific Default Configuration parameters and select the detector.

  3. Build a trace image using three flat-field images from a specific dataset in the PypeIt Development Suite.

  4. Update the instrument configuration parameters to include configurations specific for the used instrument setup. Among others, this step sets the assign_obj flag in SlitMaskPar Keywords to True.

  5. Run the slit tracing procedure using EdgeTraceSet, during which the slitmask ID assignment is performed (see Slitmask ID assignment and missing slits), and the maskdef_id and object information associated to each slit are recorded in the SlitTraceSet datamodel.

  6. A file containing previously extracted 1D spectra is loaded from the Cooked folder in the PypeIt Development Suite and the objects information re-initialized, i.e., RA, DEC and MASKDEF_OBJNAME are set to None for each spectrum.

  7. pypeit.slittrace.average_maskdef_offset() is run to determine an average slitmask offset over all the detectors, which is used by pypeit.slittrace.assign_addobjs_alldets() to assign the object RA, Dec and object name to each extracted spectrum.

  8. Read RA, DEC and MASKDEF_OBJNAME for a selected slit and check if those correspond to the expected values. The expected values are taken from the SlitTraceSet datamodel. See Slits for a description of the provided information and for a way to visualize them.

Second test pytest test_slitmask::test_dith_obs -W ignore is performed only on DEIMOS data. The algorithm of this test is identical to the first test, but using a different DEIMOS dataset obtained with dithered observations.

Because these tests are now included in the PypeIt Unit Tests (GitHub CI), this check is performed by the developers for every new version of the code.