To provide effective observations WSO-UV needs a high precision coordinate support. In order to put a target in a narrow (about 1 arcsec width) slit of one of WSO-UV spectrographs, and keep it in during a long (typically several hours) exposure one needs a number of secondary targets in the object vicinity, which provide the object identification, and fine control of the telescope pointing. The role of such secondary targets can be played by stars with known positions and magnitudes.
Why one needs a dedicated Master Catalogue for WSO-UV?
In Fig. 1 an explanation how the pointing of the telescope will be organized is presented. One can see there the central fragment of the telescope focal plane with the stellar images superimposed by the Fine Guidance System (FGS) detectors field of views (FOVs). The FGS needs for the position identification typically a few stars per a FOV. The latter condition being coupled with currently adopted parameters of the FGS (and especially size of the FOV) imply a dense grid of the guiding stars over the sky (condition A). This requires special efforts to construct the corresponding list of guiding stars (or mission Master Catalogue, WSO MC). Apart from the density requirement the catalogue should conform several other constraints. In order to provide accurate placement of the target into the spectrograph slit the stellar positions it lists, influenced e.g. by stellar proper motions, should be sufficiently accurate, and their epoch should be close to the operational period of the mission (condition B). In order to facilitate the identification and selection of the guide stars the catalogue should be complete to some magnitude in photometric band compatible with that of the FGS (condition C).
Figure 1. The image of the central area of the T-170 focal plane, reproducing the Sky region near the North Pole. The stars are shown as yellow filled circles. The circle size is proportional to magnitude according to the scale shown at the right side of the plot. The coordinates are measured in linear units (mm) from some arbitrary origin at the focal plane. The large cross marks the position of optical axis of the telescope. The red arrow shows some dedicated direction (e.g. to the Sun). The innermost ring indicates the radial distance where spectrograph slits (not shown) will reside. The two outer rings indicate positions of the FGS detector FOVs shown with green-border windows.
Figure 2. A relation between the size of the FGS detector and the limiting magnitude R of the catalogue (given at least one star falls in the detector window) for different galactic latitudes b.
What can one cook the Master Catalogue of?
There are a few all-sky catalogues nowadays, which can suit the mission purposes. They all contain from 500 to 1000 mln. stars, and provide sufficiently dense grid of guiding stars (i.e. conform the Condition A). The best of them in the context of condition B is 2MASS ("The 2MASS all-sky catalog of point sources" Skrutskie et al., AJ, 2006) with the position epochs 1997-2001. The 2MASS contains highly accurate (with position accuracy better than 0.1 arcsec with respect to the Hipparcos reference frame, and photometric accuracy of order of 3% for brighter stars), and homogeneous (within 2% over the sky) data on stellar positions and infra-red magnitudes (J, H and K) for about 470 mln. point sources (stars). This catalogue, however, is a result of near infrared survey and produces stellar patterns, which can in many cases be hardly recognisable in optics (i.e. violates the condition C). In order to adjust this catalogue to the mission aims one needs to transform infrared magnitudes to the optics. This work is currently in progress. Some preliminary results can be seen in this page for the subset of about 150 mln. 2MASS objects, reduced to optical R-magnitude brighter than R=17 (R-band is centered at 0.61 mkm, i.e. is close to the FGS specifications) and referred herewith as SUV UV MC.
Figure 3. A comparison of different all-sky catalogues over the average surface density of stars and the accuracy of their coordinates. Heavy dots show average relations between the values; the bars show the the density range (the left one indicates the density in the South Galactic Pole direction, the right one the density in the direction to the Galactic Center).
How the Master Catalogue was built?
We have transformed 2MASS near infrared magnitudes J into optical system RJ which is close to the FGS photometric system. A distribution of RJ magnitudes if compared to that of J-magnitudes is shifted to fainter values. The fraction of stars with RJ > 20m is about 7% (see Fig.4a). The catalogue is complete at RJ<17m. About 64 percent of all MC objects (and 96% of stars brighter than RJ=17m) have error in RJ less than 0.5 mag.
Figure 4a. The differential and integral distributions of stellar magnitudes J (black curves) and RJ (red curves) in 2MASS and MC. Vertical magenta line indicates the completeness limit of the MC.
Figure 4b. Distribution of photometric errors in 2MASS (black, long-dashed for H-magnitude, short-dashed for J-magnitude) и МC (red and magenta). Magenta curve shows the distribution of all stars, red curve is the distribution of brighter stars (RJ ≤17m).
The sharp contrast in the appearance of optical and infra-red skies can be seen from comparison of top and bottom maps shown in Fig. 5. One can see in the maps a relation of the observed distribution of stars with the Galactic structure and can easily see signatures of typical galactic sub-systems. The difference between the maps is caused by larger transparency of the Galaxy in the infrared. This stresses once again the necessity of reduction of the 2MASS photometry to the FGS optical system.
Figure 5a. The distribution of MC objects in optical light with Rlim≤17 over the sky in galactic coordinates. The Celestial Sphere is ripped up along the latitude circle at galactic longitude l=180 deg and unwrapped. The North Galactic Pole is at the top, the South Galactic Pole at the bottom. The density is indicated with gray scale colors. The light color corresponds to the high density, the dark one to the low density. The highest density concentration is observed at the Galactic center, slashed with the dash of dark clouds, the surrounding Galactic bulge, and along the Galactic plane. The dark filaments indicate the galactic dust layer. The two light spots below the Galactic plane are Small and Large Magellanic Clouds.
Figure 5b. The same as in Fig.5a, but in infrared band J.