Breathing and Focus Field Variation for NICMOS

1. Breathing and Focus Field Variation for NICMOS Deepashri Thatte Tommy Wiklind TIPS November 19, 2009

2. NICMOS PAM • Pupil Alignment Mechanism (PAM) for • adjusting focus. • Adjustable mirror moving ±10mm about • mechanical zero. • NIC1 and NIC2 share an intermediate • focus. Best focus for NIC3 is currently • beyond the maximum range setting of • the PAM. • PAM is also used to correct for misalign- • -ment between HST exit pupil and • NICMOS entrance pupil. NICMOS Instrument Handbook, 2009

3. NICMOS Focus Monitoring • Every two months for NIC1 and NIC2, every six months for NIC3. • The optimal focus position has not changed notably since 2002. • Focus is monitored in order to detect sudden and significant deviations from optimal position as well as long term trends.

4. Encircled energy method • Series of exposures of a crowded star field at different PAM settings. • The total number of counts in a fixed aperture is measured for several stars at each PAM position. • The PAM position corresponding to the maximum count rate is the best focus. • Note:- All the stars are normalized to the same flux level

5. Focus History

6. Factors affecting focus and image quality • Breathing • Focus Field Variation • PAM tilt

7. Breathing • Breathing:- Short term • variations in HST focus • on orbital time scale • Caused by thermal • expansion/contraction of • the HST optical telescope • assembly (OTA) as the • telescope warms up • during its orbital day and • cools down during orbital • night. • Also depends upon • orientation of the telescope.

8. HST Thermal Breathing Model The thermal model of the breathing effect relates the HST secondary mirror position to temperature variations. ∆SM ~ 0.7 (T – <T>) μm 10 microns of the breathing defocus at the HST Secondary mirror is equivalent to ~1 mm of PAM movement. ∆PAM = (1.171)(0.1)(∆SM) mm ( for NIC1) New PAM = old PAM (given by NPFOCUSP) + ∆PAM Di Nino, D., Makidon, R. B., Lallo, M. et al. ISR ACS 2008-03 Suchkov, A., Bergeron, L., Galas, G. ISR NICMOS 98-004, STScI

9. Breathing correction (a) No breathing correction (b) With breathing correction

10. Focus Field variation • The NICMOS foci vary spatially across the detector's field of view. • The magnitude of the effect for NIC2 and NIC3 is up to ~1.5mm in PAM space. • The focus determined for a star at a given position on the detector needs to be corrected for the FFV.

11. The curvature was determined by Suchkov & Galas by plotting PAM positions for best focus as a function of rows and columns separately. g(x,y)= a1x + a2y + a3x2 + a4y2 + a5 xy

12. f(z) = a0 exp (-[z + g(x,y)] –z0)/w)2 zo : PAM position for best focus at (0,0) w and a0 : width and amplitude z : PAM in mm f(z) : Normalized encircled energy • Fits to individual stars in regular • focus monitoring run. • (b) Corrected for focus field variation • (c) Corrected for focus field variation • and breathing.

13. NIC1 focus history

14. NIC2 focus history

15. NIC1 and NIC2 PAM

16. Variations in focus position for NIC1 & NIC2 • Not attributable to breathing, FFV or detector temperature • NIC1 and NIC2 variations correlated • Breathing and FFV only changes the average level • Amplitude of variations is large (a few tenths of mm in PAM space) • Time scale is months – year • The cause of these variations remains unexplained but • could be due to annual changes in solar input • Remains to be done: correlate focus variations with aft- • shroud temperature

17. Correction to PAM using aft-shroud temperature

18. SMOV4 focus test • NIC1 PAM 2.43 ± 0.07 mm • NIC2 PAM 0.77 ± 0.06 mm • Both NIC1 and NIC2 appear to have shifted approximately +0.6mm relative to the average focus position in the period 2002-2008. • Consistent with the secondary mirror move of +2.97 micron on July 20 2009. The SM move corresponds to +0.4mm for both NIC1 and NIC2.

19. SMOV4 PAM tilt results X-coma & y-coma were measured for all tilt positions. Total coma = (x2coma + y2coma)1/2

20. Acknowledgements Matt Lallo Pey-Lian Lim