FORS1 Photometric System Characterization

February-March 2007

1. Introduction

These observations are aimed at fully charactering FORS1 photometric system (UBVRI zero-points, color-terms and extinction coefficients) for the Tektronix 2048x2048 pixel CCD (24µm px). To this aim, the two Landolt/Stetson photometric fields SA95 and SA101 will be observed on a number of epochs and at different airmasses during February and March 2007. Different epochs will cover different areas of the two field and will be obtained also with a set of position angles. In order not to affect the normal calibration activities, the OBs will include also the usual pointing currently adopted by the Calibration Plan.

Obviously, these observations have to be executed before FORS1 detector is upgraded.

2. Selected Photometric Fields

The two fields are observable within the VLT pointing limits for the full February-March time interval both at low and high airmass.
The selection has been operated taking into account the presence of bright stars, Landolt and Stetson stars.

For more details on the selection, have a look here. Different symbols have the following meaning:

blue squares: Landolt stars faint enough in U to be in the linear regime after 10 sec
red circles with crosses: Stetson stars that may bleed after 10 sec
green circles: Stetson stars in the linear regime after 10 sec with some filters missing
magenta circles: Stetson stars in the linear regime after 10 sec with data for BVRI

The following table contains links to the pointing patterns. The first row in each file contains the coordinates of the original Calibration Plan field.

SA95

SA101

PG1633

The position of each pointing with respect to the ordinary Calibration Plan pointing is shown here for SA95, SA101 and PG1633. In these maps (20' x 20'), the boxes indicate the FORS field of view (6'.8 x 6'.8). North is always at the top and East to the left. The red boxes mark the Calibration Plan pointings. The two positions within each pointing are traced with the same color.
Finding charts to each of the 3 pointings (including 2 offsets) are accessible from the following table. In all finding charts, the red box refers to the Calibration Plan field.

SA95_P1	SA101_P1	PG1633_P1
SA95_P2	SA101_P2	PG1633_P2
SA95_P3	SA101_P3	PG1633_P3

The offset patterns for each pointing are available here. All OBs include the relevant finding chart.

3. Observing Strategy and OB naming

The observing strategy is simple and it is outlined in the following list:

For each field there are 12 OBs (3 pointings x 4 ROT angles)
OBs for SA101 have to be observed during evenigh twilight (exceptions are expected for the second half of March)
OBs for PG1633 have to be observed during morning twilight
As a general rule, each time a evening SA101 OB is executed, this has to be accompanied by the execution of one morning PG1633 OB.
Any given new position is observed at 4 position angles differing by 90 degrees.
In addition to the 12+12 evening/morning twilight OBs, there are 12 OBs for field SA101 which should be observed during the night, several hours after the twilight observation of the same field, i.e. at a significantly higher airmass.

In order to allow for an easy selection of the OBs, they have been named as follows:

PHOTEST_EVENING_x	x=1,2,...,12	Evening twilight observations of SA101 (two offsets)	22 min
PHOTEST_NIGHT_x	x=1,2,...,12	Night time observations of SA101 (one offset)	16 min
PHOTEST_MORNING_x	x=1,2,...,12	Morning twilight observations of PG1633 (two offsets)	22 min

Ideally, when one PHOTEST_EVENING_x OB is executed, this should be followed by a PHOTEST_NIGHT_x when this has reached a significantly different airmass. Finally, a PHOTEST_MORNING_x OB should be executed during morning twilight.

OBs must be executed following the ascending numbering order.

3. Run ID and Queue

All OBs have been prepared under the engineering account 60.A-9203(A) and they have been included in a specific queue, called PHOTEST_FORS1. USD will take care of maintaining this queue as OBs are executed.

4. Hourly Airmass Tables

In order to facilitate the NA when running the observations, the following table provides direct links to hourly airmass tables for the various OB sets at different dates during February and March 2007.

DATE	EVENING	NIGHT	MORNING
2007-02-01	X	X	X
2007-02-15	X	X	X
2007-03-01	X	X	X
2007-03-15	X	X	X
2007-03-31	X	X	X

5. Relevant documents

More info can be found in the following documents:

Last Modified: Mon Mar 12 16:56:48 CET 2007 - FPA