Breathing and Focus Field Variation for NICMOS

Deepashri Thatte

Tommy Wiklind

TIPS November 19, 2009

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

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.

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

Focus History

Factors affecting focus and image quality

• Breathing

• Focus Field Variation

• PAM tilt

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.

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

Breathing correction

(a) No breathing correction (b) With breathing correction

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.

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

f(z) = a0 exp (-[z + g(x,y)] –z0)/w)2

(a) Fits to individual stars in regular focus monitoring run.(b) Corrected for focus field variation(c) Corrected for focus field variation and breathing.

zo : PAM position for best focus at (0,0)w and a0 : width and amplitude z : PAM in mmf(z) : Normalized encircled energy

NIC1 focus history

No correction FFV only Breathing Breathing & FFV

2.19 ±0.31 2.30 ±0.31 2.21± 0.41 2.32 ±0.40

NIC2 focus history

No correction FFV only Breathing Breathing & FFV

0.37 ±0.46 0.35 ±0.45 0.33± 0.55 0.31±0.54

NIC1 and NIC2 PAM

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

Correction to PAM using aft-shroud temperature

SMOV4 focus test

Old values

PAM mm

New values

PAM mm

Best Focus for NIC1 1.8 2.3

Best Focus for NIC2 0.2 0.7

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.

SMOV4 PAM tilt results

X-coma & y-coma were measured for all tilt positions.

Total coma = (x2coma + y2

coma)1/2

Acknowledgements

Matt Lallo

Pey-Lian Lim