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.
NICMOS Instrument Handbook, 2009
•PAM is also used to correct for misalign-ment between HST exit pupil and
NICMOS entrance pupil.
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
zo : PAM position for best focus at (0,0)
w and a0 : width and amplitude
z : PAM in mm
f(z) : Normalized encircled energy
(a) Fits to individual stars in regular
focus monitoring run.
(b) Corrected for focus field variation
(c) Corrected for focus field variation
and breathing.
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 aftshroud temperature
Correction to PAM using aft-shroud temperature
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.
Old values
PAM mm
New values
PAM mm
Best Focus for NIC1
1.8
2.3
Best Focus for NIC2
0.2
0.7
SMOV4 PAM tilt results
X-coma & y-coma were measured for all tilt positions.
Total coma = (x2coma + y2coma)1/2
Acknowledgements
Matt Lallo
Pey-Lian Lim