8153( 3) - 01/14/00 15:06 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8153 Version: 3 Check-in Date: 19-Feb-1999 13:22:12 1.Proposal Title: Exploring the Luminosity Function near the, Limit of Hydrogen Burning in Globular Clusters ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Ivan King University of California - Berkeley CoI: Dr. Jay Anderson University of California N CoI: Dr. Giampaolo Piotto Universita di Padova N CoI: Dr. Carlton Pryor Rutgers, the State Univ. of New Jersey N CoI: Dr. Karl Gebhardt University of California, Santa Cruz N CoI: Prof. Adrienne Cool San Francisco State University N ------------------------------------------------------------------------------------ 5. Abstract This is a continuation of a program that has extended the color--magnitude diagram and luminosity function of NGC 6397 into the neighborhood of the hydrogen-burning limit (GO-6797, King et al.\ 1998). Accurate astrometry, over a baseline of a few years, allows an excellent proper-motion separation of faint cluster stars from the numerically dominant field stars. We propose to take second-epoch images of a second field in NGC 6397, so as to double the sparse numbers for that cluster, and to take second-epoch images of two rich fields near the center of M4, plus a more outlying field. This second cluster, with a different metallicity, will provide an even larger number of faint stars than NGC 6397. The resulting color--magnitude diagrams and luminosity functions will present an exciting challenge to theoreticians, as a test of both the mass--radius and mass--luminosity relations for the observable stars of lowest mass. ------------------------------------------------------------------------------------ 8153( 3) - 01/14/00 15:06 - [ 2] Observations Description ------------------------ All observations will be made with F814W, since proper-motion measurements do not require two-color observations to be repeated. In our previous work we found that 3 orbits (each divided into 3 exposures) gave good enough astrometry to derive accurate proper motions for our faintest stars. (We actually used 6 orbits, but only because two pointings were needed to cover our previous fields.) We will thus devote 3 orbits to each of our 4 fields. In each field, the 9 exposures will be made in 9 different dither positions. We have worked out a 9-way dither pattern that works simultaneously in the PC and the WF pixels. In order to maximize overlap with the first-epoch observations, we will require that each field be observed at the same orientation that was used in the first epoch. Our measuring methods go as follows. First we measure positions and magnitudes for all stars in each individual image by fitting a PSF obtained from the image itself. We then correct each position for the known distortion correction. Although the latter is not known to the accuracy of our final mean positions (which is better than 0.01 pixel), the nominal distortion correction makes the images geometrically flat enough for our differential astrometry techniques. These rely on measuring each star with respect to a tight reference net of surrounding stars that are all cluster members, so that they will have nearly identical motions, unlike the field stars. This net allows us to convert the measured positions of the star from each individual image to a common coordinate system that moves with the cluster mean motion. In this system we are able to compare the positions of each star at the two epochs, thus deriving a preliminary proper motion for it. For second-epoch positions we then iterate all the conversions, correcting each reference star for the internal proper motion that we have already measured for it, while deriving an improved proper motion for the target star that is in the middle of the net. Thus we are able to allow for the individual proper motions of the stars when using them in a reference net, but measure the proper motions of the stars one by one. Measuring at this level of care was sufficient to allow us to separate cluster stars from field stars in GO- 6797 (King et al.\ 1998), but we now attain a considerably higher accuracy, by correcting for the systematic errors that depend on the location of the star's center with respect to a pixel center. These are of complicated origin, but they arise basically from the severe degree of under-sampling, especially in the WF frames, and typically have an amplitude of about 0.05 pixel. We remove these errors almost totally, by (1) careful dithering, (2) examination of the distribution, over all measured stars, of the values of the fractional part of the measured position coordinates, and making corrections to flatten this distribution, and (3) examination of the residuals of individual measures of a star from its mean position, again as a function of pixel fraction, and deriving further corrections from these graphs. In addition to all this, we have worked out a way of using comparison with second-epoch images to compensate for inadequate dithering at the first epoch. Real Time Justification ----------------------- The requirement of repeating the previous orientations will restrict the scheduling of each visit to about a 30-day window. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8153( 3) - 01/14/00 15:06 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8153( 3) - 01/14/00 15:06 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M4-CTR STELLAR CLUSTER, RA=16H 23M 37.76S +/- 0.01S, J2000 SURF(V) = 15.0+/-0.5 Globular Cluster DEC=-26D 31' 26.7" +/- 0.1" 2 M4-NEAR STELLAR CLUSTER, RA=16H 23M 40.54S +/- 0.01S, J2000 SURF(V) = 15.2+/-0.5 Globular Cluster DEC=-26D 30' 45.4" +/- 0.1" 3 M4-FAR STELLAR CLUSTER, RA=16H 23M 54.60S +/- 0.01S, J2000 SURF(V) = 16.0+/-0.5 Globular Cluster DEC=-26D 32' 24.3" +/- 0.1" 8153( 3) - 01/14/00 15:06 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 279.9D TO 280.0D On Hold Comments: Additional Comments: Orientation must match that of first-epoch exposures. Each visit is to a different field, and consists of 9 exposures in a dither pattern that samples every ninth of a pixel in both x and y, both for the WF and for the PC. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 M4-CTR WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 700 S Comments: First orbit. ------------------------------------------------------------------------------------------------------------------------------------ 2 M4-CTR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 20 S DITHER-LINE-SPACING=0.6693, DITHER-LINE-STEPS=4 Comments: Stepping 4.25 WF pixels x sqrt\{2\} gives even dither pattern in both WF and PC. ------------------------------------------------------------------------------------------------------------------------------------ 3 M4-CTR WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 700 S POS TARG 0.386, 0.336 Comments: End of first orbit. ------------------------------------------------------------------------------------------------------------------------------------ 4 M4-CTR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 700 S POS TARG 0.0125, -0.336 DITHER-LINE-SPACING=0.5989, DITHER-LINE-STEPS=3 Comments: Second orbit. ------------------------------------------------------------------------------------------------------------------------------------ 5 M4-CTR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 700 S POS TARG -0.374, 0.0249 DITHER-LINE-SPACING=0.5989, DITHER-LINE-STEPS=3 Comments: Orbit 3. ------------------------------------------------------------------------------------------------------------------------------------ 8153( 3) - 01/14/00 15:06 - [ 6] Visit: 02 Visit Priority: Visit Requirements: ORIENT 279.9D TO 280.0D On Hold Comments: Additional Comments: Orientation must match that of first-epoch exposures. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 M4-NEAR WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 700 S Comments: First orbit. ------------------------------------------------------------------------------------------------------------------------------------ 2 M4-NEAR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 20 S DITHER-LINE-SPACING=0.6693, DITHER-LINE-STEPS=4 Comments: Stepping 4.25 WF pixels x sqrt\{2\} gives even dither pattern in both WF and PC. ------------------------------------------------------------------------------------------------------------------------------------ 3 M4-NEAR WFPC2 IMAGE PC1-FIX F814W CR-SPLIT=NO 1 700 S POS TARG 0.386, 0.336 Comments: End of first orbit. ------------------------------------------------------------------------------------------------------------------------------------ 4 M4-NEAR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 700 S POS TARG 0.0125, -0.336 DITHER-LINE-SPACING=0.5989, DITHER-LINE-STEPS=3 Comments: Second orbit. ------------------------------------------------------------------------------------------------------------------------------------ 5 M4-NEAR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 700 S POS TARG -0.374, 0.0249 DITHER-LINE-SPACING=0.5989, DITHER-LINE-STEPS=3 Comments: Orbit 3. ------------------------------------------------------------------------------------------------------------------------------------ 8153( 3) - 01/14/00 15:06 - [ 7] Visit: 03 Visit Priority: Visit Requirements: ORIENT 280.0D TO 280.1D On Hold Comments: Additional Comments: Orientation must match that of first-epoch exposures. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 M4-FAR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 600 S DITHER-LINE-SPACING=0.7516, DITHER-LINE-STEPS=3 Comments: First orbit. ------------------------------------------------------------------------------------------------------------------------------------ 2 M4-FAR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 700 S POS TARG +0.02214, DITHER-LINE-SPACING=0.7516, +0.1882 DITHER-LINE-STEPS=3 Comments: Second orbit. ------------------------------------------------------------------------------------------------------------------------------------ 3 M4-FAR WFPC2 IMAGE PC1-FIX F814W DITHER-TYPE=LINE,CR-SPLIT=NO , 1 700 S POS TARG -0.02214, DITHER-LINE-SPACING=0.7516, -0.1882 DITHER-LINE-STEPS=3 Comments: Orbit 3. ------------------------------------------------------------------------------------------------------------------------------------ 8153( 3) - 01/14/00 15:06 - [ 8] Summary Form for Proposal 8153 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO DITHER-LINE-STEPS=4 DITHER-LINE-STEPS=3 DITHER-LINE-SPACING=0.5989 DITHER-LINE-SPACING=0.6693 DITHER-TYPE=LINE DITHER-LINE-SPACING=0.7516 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 279.9D TO 280.0D POS TARG 0.386, 0.336 POS TARG 0.0125, -0.336 POS TARG -0.374, 0.0249 ORIENT 280.0D TO 280.1D POS TARG +0.02214, +0.1882 POS TARG -0.02214, -0.1882 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M4-CTR M4-NEAR M4-FAR ------------------------------------------------------------------------------------------------------------------------------------