1. WFPC2 1-Gyro Test Results
Matt McMaster
Marco Sirianni

2. The Test
Program was initially designed for ACS/HRC using the TGM original program test as a template.
11077 was converted to WFPC2/PC by Matt McMaster maintaining an observational strategy very close to the original.
Attention was focused on the PC because the undersampling was less severe than for the WFs WFPC2/PC mas (ACS/HRC mas)
The program consisted of multiple exposures of two rich star clusters: NGC 5904 and NGC 6341.
Two sets of observation were taken, one on Jan 29 in 2-gyro mode and the other on Jan 31 in 1-gyro mode.
Target
RA (2000)
Dec (2000)
Guide Star Brightness
Exptime
(Sec)
Number of
Images
NGC 5904
V≈14
10,100,400 28
NGC 6341
V≈13 and 14
140

3. The Analysis
The software developed for the TGM test was modified to be used with WFPC2 data.
The FWHM of the PSF for stars with S/N > 10 was calculated by fitting a gaussian profile to the stellar light profile
Resulted in tens to hundreds of measurement per image depending on exposure time and richness of the cluster
All of the measurements in each image were averaged to produce a mean PSF for the observation
2-Gyro
1-Gyro

4. Results 2- Gyro 1- Gyro PSF FWHM distribution Mean : 1.67 pix ± 0.16
(unit: PC pixel -all data included)
2- Gyro
Mean : pix ± 0.16
Min : 1.13
Max : 1.99
1- Gyro
Mean : pix ± 0.14
Min : 1.09
Max : 1.89
The average PSF FWHM is remarkably similar indicating no obvious
degradation in performance in 1-Gyro mode

5. Results – (cont.)
As done for the TGM test, the dependence of the FWHM
on exposure time, guide star brightness, and position on
the sky was analyzed:
There is no difference between images taken with a V=13 and V=14 guide star
There are small differences in the PSF width due to the exposure time duration.
Data from NGC 6341 only

6. Results - (cont.)
2 – GYRO
Exptime FWHM
10 sec ± 0.03
100 sec ± 0.03
400 sec ± 0.04
1 – GYRO
Exptime FWHM
10 sec ± 0.02
100 sec ± 0.04
400 sec ± 0.05
Such differences were also noted during the original 2-Gyro on orbit verification test with the ACS/HRC.

7. Results – (Cont.)
The average PSF width in NGC 5904 data is systematically smaller than in NGC 6341, but there is no dependency on the gyro guiding configuration.
Target
1 Gyro
2 Gyro
NGC 5904
1.41 ±0.04
1.46 ±0.07
NGC 6341
1.68 ±0.01
1.71 ±0.01
A similar result was found for the clusters observed in the original 2-gyro on-orbit test with HRC

8. Conclusions There is no evidence that the quality of the science
data obtained in 1-gyro
mode is any worse than
in 2-gyro mode.
NGC 6341 /PC
1-Gyro mode
F555W -100 sec