1. ACS PSF Variations Over The Field of View & ACS Coronagraph Status
John Krist
TIPS Meeting
August 21, 2003

2. Brown plot

3. Causes of PSF Variations
Focus
Small changes vary peak/wing light distribution, but doesn’t change FWHM much
Astigmatism
Makes core elliptical
Coma
Shifts core towards one side
Charge diffusion
Blurs PSF, altering FWHM

4. Charge Diffusion (Thinned, backside-illuminated CCD)
Illuminated surface
Electrode
Electrode
Electrode
Back surface

5. 47 Tuc
47 Tuc
ACS/WFC
F550M

6. WFC F550W PSF Variations

7. WFC F550M PSF Characteristics

8. WFC Astigmatism & Coma

9. WFC F550M Focus & Blur

10. Derived ACS CCD Thicknesses
Results from Jeremy Walsh & collaborators (ST-ECF)

11. Thickness vs. Blur

12. WFC Charge Diffusion Kernels
Best Case
Worst Case
F435W
F550M
F814W

13. Blur Variations with Wavelength

14. Tiny Tim PSF Field Dependence

15. Small Aperture Variations
85%-106%
variation!

16. ACS/WFC PSF Field Variations
PSF FWHM is largely defined by charge diffusion, which varies with CCD thickness
PSF asymmetries caused by astigmatism & coma interacting with defocus
Focus variations are dominated by CCD warping and offsets
Photometry using a small (r<4 pix) aperture will be sensitive to charge diffusion variations (up to 15% error for r=1 pix)
Similar variations will be seen in WFC3

17. WFC Flat fields
Geometric distortion
effects removed

18. Spot position

19. Spot motion trend

20. Current Coronagraph Issues
Cannot take on-orbit flats in most filters
Earth flats taken each week in F330W to monitor spot position
Latest spot position uploaded to HST as a USE OFFSET offset
Pipeline coronagraphic flats are ground flats and will be replaced by dummy flats
Custom flats must be made for each visit and filter – will replace dummy flats