2 The GSPC-II program

The bulk of the GSPC-II target fields coincides with those of the GSPC-I created to provide photometric calibrators for the first Guide Star Catalog. The GSPC-I consists of 1477 photoelectric sequences, with distribution determined by the survey plates used in the GSC. Specifically, there are 580 northern ( $\delta\geq 6^\circ$) sequences, centered on the $6^\circ$-grid (equinox 1855) of the original National Geographic Society - Palomar Observatory Sky Survey (POSS-I, Minkowski & Abell 1963) and the "Quick V'' survey (Lasker et al. 1990).

Figure 1: Distribution of the GSPC-II fields on the equatorial celestial sphere. Open circles refer to the sequences released with this paper, filled circles to the rest of the observed fields. $\rm RA = 0~h$ is to the left-hand side

There are 894 sequences at $\delta \leq 0^\circ$, corresponding to the fields of the SERC Survey and the Equatorial Extension thereto (Cannon 1984), taken on the standard $5^\circ$ grid of equinox 1950 coordinates. Five additional sequences were added to GSPC-I after its publication, for better coverage of fields N586, N594, N608, N611 and N613 at $\delta = +3^\circ$.

The northern program of the second generation Schmidt surveys includes all 894 fields of the POSS-II (Reid et al. 1991) in three bands: green (IIIa-J), red (IIIa-F), and near-IR (IV-N). The southern program consists of the Second Epoch Survey (AAO-R) and the equatorial counterpart, the SERC-ER, both (IIIa-F) surveys (Morgan et al. 1992).

While the AAO-R lies on the same grid pattern as the SERC and therefore does not require additional sequences, the POSS-II survey is placed on a $5^\circ$ grid of equinox 1950 coordinates. This results in a number of POSS-II plates being considerably shifted with respect to the original grid, and therefore poorly covered by the GSPC-I sequences. Enforcing the requirement that each photometric sequence lie within $2.7^\circ$ from the plate center, in order to avoid the area where the sensitivity of the plate is badly affected by vignetting, adds another 302 sequences to the list. All these additional fields bring the number of GSPC-II standard sequences to a total of 1780.

 

 

Table 1: Telescope usage and number of GSPC-II sequences collected (as of December 2000)

Telescope	Latitude	Size	# of Observed Fields			Total	Useful	Epoch
	( $\delta^{\circ}$)	(meters)	POSS-I	POSS-II	SERC-J	Runs	Nights	range
Kitt Peak	+31.9	0.9	323	146	34	17	63	1990-1999
Cerro Tololo	-30.0	0.9	177	13	488	21	111	1989-1998
ESO Dutch	-29.2	0.9	-	6	528	14	103	1991-1998
Wise	+30.6	1	226	-	2	24	36	1989-1991
Mt. Hopkins	+31.6	1.2	50	-	2	4	5	1994-1995
Mt. Laguna	+32.8	1	84	-	5	2	9	1990-1999
Lowell	+35.2	1	10	-	-	1	2	1990
Mt. Megantic	+45.4	1.6	6	-	-	3	1	1990
McDonald	+30.6	0.8	2	-	1	1	1	1990
JKT La Palma	+28.7	1	36	16	17	2	6	1998-1999

2.1 Observations

All of the GSPC-II data collected to date come from 10 different telescopes, for a total of 337 useful nights. Most of the northern CCD sequences were imaged at the Kitt Peak and Wise Observatories, whereas southern declinations have been mainly covered at the Cerro Tololo and ESO-La Silla observatories. Table 1 gives an up-to-date full account of all the telescopes employed, together with the epoch range in which GSPC-II observations were made. Figure 1 displays the position of the centers of all the observed sequences on the celestial sphere in equatorial coordinates. Although a considerable portion of northern data have been collected by the Wise Observatory (Ein Yahav, Israel), the reduction process revealed some deficiencies intrinsic to these data, particularly poor standard stars statistics, insufficient number of stars per sequence, absence of good sets of bias and flat field frames. For this reason, re-observations of all of the Wise sequences have been planned, with a current completion percentage of $\sim$88%. Globally, about $50\%$ of the GSPC-II fields has been observed from different telescopes/multiple nights, allowing a better control of their photometric quality. Finally, about $10\%$ of the fields remains still uncovered, either because they lack observations or because the existing data were rejected as non photometric.

Given the long time span of this survey project, many detectors have been replaced with new ones over the years. It would be too cumbersome to list here the characteristics of all the CCD cameras available at each telescope at the time each observing run occurred; it suffices to say that the size of the camera field was typically $3.5\times3.5$ arcmin for sequences at declinations <$-30^\circ$, $\sim$10 arcmin for $-30^\circ < \delta < 0^\circ$ and a combination of $\sim$10 and $\sim$23 arcmin for the northern hemisphere. As for image sampling, the pixel scale of the different CCDs ranged from $\sim$0.35''/px to $\sim$0.7''/px.

The original goal of the GSPC-II was to extend the photometry in the V and R bands to a limiting magnitude of $V\simeq 18$ with a precision of 0.05 magnitudes, to meet the HST mission requirements. Where resources were available, which is for a large part of the southern hemisphere and for a few northern fields, a B band was also included, and the limiting magnitude brought to $\simeq$20. The observation strategy consisted in taking CCD frames centered on the faintest star of the corresponding GSPC-I sequence, where the latter was available. When necessary, both long ($\sim$8-15 min) and short (<3 min) exposures were acquired in each field. Short exposures were chosen to be appropriate for the typical 14.5 mag target star. This serves the purpose of bridging the gap between the bright and faint end of the sequence and, at the same time, enables confirmation of the GSPC-I magnitude. For fields which did not have a GSPC-I counterpart, i.e., those corresponding to the POSS-II grid, the telescope was pointed to the nominal center of the corresponding survey plate, and the pointing adjusted in order to avoid bright stars.

Besides target fields, a pre-selected set of standard stars from the Landolt catalog (Landolt 1992) was regularly imaged through the night to estimate the extinction and the transformation to the standard photometric system, as explained in the following sections. Similarly, several dome, sky flats and bias frames were taken each night to accurately calibrate the instrumental response.