Creating Astrometric Masks

We use astrometric masks to check/remeasure the rotator zeropoint after the instrument has been mounted. A set of astrometric masks based on the Stone fields (Stone, Pier and Monet 1999, AJ 118, 2488) was created in 2015/2016. With the launch of GAIA in 2013, new astrometric all-sky catalogs have become available, and a new set of MODS astrometric masks is being designed using Gaia DR2. In August 2021, a first batch of Gaia astrometric masks was cut, and more are being created now (2022).

Table of MODS Astrometric Masks

The table below lists all of the MODS astrometric masks.

Field	RA(J2000)	DEC(J2000)	mask ID	N	mms	Epoch	catalog_file	notes
Stone A	00 56 20.127	-01 14 12.57	571722	2	mods.37.StoneA.mms	2015.68
StoneD1f	04 29 53.371	00 25 10.72	533218	2	mods.49.StoneD1f.mms	2016.25
StoneE1f	07 50 41.492	-01 12 48.08	590945	2	mods.44.StoneE1f.mms	2016.25
StoneI1	12 35 46.043	-01 01 00.99	550828	2	mods.50.StoneI1.mms	2016.25
StoneO	21 37 22.232	00 31 54.19	535231	2	mods.36.StoneO.mms	2015.68
g20d25ep2	20 00 00.91	25 00 28.1	531956	2	mods.3.g20d25ep2.mms	2022
g00d25ep	00 00 00.139	25 00 17.09	531713	2	mods.2.g00d25ep.mms	2022
g04d25ep	03 59 58.669	25 00 40.05	532732	2	mods.4.g04d25ep.mms	2022
g08d25bt	07 59 36.311	25 07 29.24	520017	2	mods.38.g08d25bt.mms	2023
g12p25n	12 01 58.157	25 00 15.49	521950	2	mods.42.g12p25n.mms	2023	r12d25_pmlt8_plus1_a_bt.from_c2000_to_ep2023.out	a
g16d25r3	15 58 26.656	24 34 01.25	520140	2	mods.39.g16d25r3.mms	2023	r16d25_pmlt8_gaia.from_c2000_to_ep2023.magsort.out
g06d80ep	05 26 16.3	80 13 11.05	515404	2	mods.37.r06d80ep.mms	2023

a. Star in top box is double, select SE component; bright star in NE has proper motion ~ 30 mas/yr. The rest have proper motions < 8 mas/yr.

Procedure for creating astrometric masks from Gaia DR2

Steps to produce the Gaia catalogs (used by autoslit in mms) follow:

1. Search the GAIA catalog for low proper motion stars around a certain field center, with magnitudes between 15.5-19.5 (these are loose ranges with the rationale that in 30-sec they do not saturate nor are they hard to centroid on). I use vizquery, but you can search in the web or use cdsclient. To find all of the stars in Gaia DR2 in a 7 x 7 arcmin box (c.bm=7) around a certain (RA, DEC) that have Gmag between 15.5-19.5 and proper motions less than 8 mas/year, and print out a selected set of columns (-out=""), the syntax is:

    vizquery -mime=text -source=I/345/gaia2 -c=J160000.000+250000.00 -c.bm=7 "pmRA"="-8 .. 8" pmDE="-8 .. 8" "Gmag"="15.5 .. 19.5" -out="_RAJ2000 _DEJ2000 RA_ICRS DE_ICRS Source Epoch pmRA e_pmRA pmDE e_pmDE Gmag e_Gmag" > 16+25/r16d25_pmlt8_gaia.dat 

   I chose to output both sets of coordinates, the FK5, Equinox=J2000, Epoch=J2000 coordinates calculated by vizier and the original barycentric ICRS coordinates, epoch 2015.5 from the Gaia catalog.
2. The output will have a certain number of lines of explanatory text followed by a table, see, e.g. the output of the previous command, r16d25_pmlt8_gaia.dat.
3. The python script, pmcorr_2ways.py, reads in this output file and creates a catalog formatted for MMS/LMS with coordinates proper-motion corrected to the indicated epoch and in the FK5 equinox 2000 frame. The syntax is: ./pmcorr_2ways.py infile.dat pmcorr_to_epoch nlines_skip, or for the example above:

    ./pmcorr_2ways.py r16d25_pmlt8_gaia.dat 2023.0 48

   . I get the following warning, but the output looks fine:

   /opt/anaconda/lib/python3.8/site-packages/erfa/core.py:154: ErfaWarning: ERFA function "pmsafe" yielded 1 of "distance overridden (Note 6)"
     warnings.warn('ERFA function "{}" yielded {}'.format(func_name, wmsg), 

4. This creates 3 output files, choose the one c2000_to_epoch. The 3 files differ in the coordinate transformations: from_icrs_to_epYYYY.Y takes as input the ICRS, ep2015.5 coordinates, applies proper motions and transforms these to FK5 J2000 coordinates at epoch YYYY.Y; from_c2000 starts with the FK5, J2000 coordinates calculated by vizier and uses astropy.coordinates to apply proper motions, and brute_force is a by-hand calculation. The c2000_ and brute_force_ outputs are identical and these differ by about 0.03" from the coordinates output when starting from the ICRS coordinates. Masks created in this way and based on the from_c2000 output have been used successfully.
5. I've attached the catalog files used by MMS, although in the cases where these were not written to the *mms file, it is a little doubtful whether the uploaded file is the exact version used. (This information may be written to the mms file at the final step, when upon saving the popup box appears asking you to select the catalog file. I don't always do this, because I thought it made no different to the saved mms file. To be confirmed.) The ambiguities are for the 00+25, 04+25 and 20+25 fields.

-- OlgaKuhn - 19 Nov 2022

• r16d25_pm2000_gaia.dat: Result of the vizquery command in step 1.

• pmcorr_2ways.py.txt: pmcorr_2ways.py.txt