Instrument Science Report NICMOS-98-007
Possible Evidence for NICMOS Focus
“Precessional Breathing”
A. Suchkov
May 1, 1998
ABSTRACT
Combining the results of measurements of NICMOS focus and temperature (outside NICMOS dewar), we have found a correlation between detrended NICMOS focus and
detrended temperature. The correlation suggests that there is a thermally induced
(quasi)periodic component in NICMOS focus variation which has a time scale resembling
the HST precessional period. This component may thus be due to HST temperature variations caused by changing radiation flux from the Sun as the telescope attitude at the same
pointing (same target of the focus monitoring proposal) changes with respect to the Sun
because of precession. The mechanism of focus variation may be similar to the one causing the well-known HST orbital focus “breathing” except that in this case it may be associated mostly with light path variations in NICMOS fore optics. The data seem to be
consistent with some amount of “precessional breathing” in the Optical Telescope Assembly (OTA) as well. The amplitude of “precessional breathing” has been found to correspond to ~5 micron of the Secondary Mirror (SM) shift. This is to be compared to ~2
micron of the regular “orbital breathing” and ~5 micron of the actually adopted tolerance with respect to the HST long term focus drift.
1. Introduction
The HST focus is known to vary within ~ ± 2 µm , sometimes up to ± 4 µm , (in terms
of Secondary Mirror movement) on a time scale of one orbital period. These variations,
commonly referred to as focus breathing, are due to thermally induced mechanical tensions within the spacecraft as it moves on its orbit from night to day and back to night
(Hasan & Bely 1994). Also it has long been known that the actual focus variation pattern
is more complex than mere swings with the HST orbital period, and such things as the
spacecraft attitude with respect to the Sun must certainly be involved (see Hershey 1997).
Attitude change associated with telescope repointing is believed to cause focus variation
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on a time scale of a few orbits. Finally, there is a long term focus drift caused by OTA
shrinkage which results from water desoption in the Metering Truss (e.g., Suchkov &
Casertano 1997).
Focus variations have important implications for HST data quality, especially for
imaging and photometry. In order to adequately respond to those variations we need to
better understand and quantify them. Here we report on NICMOS focus variations occurring on a time scale of the HST precessional period which we have found from the results
of the NICMOS focus monitoring program and analysis of NICMOS temperature data.
2. Data
The focus data were obtained from the NICMOS focus monitoring program 7608. The
data used in this study cover the period from June 30, 1997, through December 17, 1997.
As described in Suchkov, Bergeron, and Galas (1997, 1998), three different techniques are
used to derive NICMOS focus: phase retrieval analysis, encircled energy measurements,
and plate scale measurements. We discuss here the results from phase retrieval for all three
NICMOS cameras. For camera 3 (NIC3) we also compare the phase retrieval results with
those based on plate scale measurements.
We have used NIMCOS temperatures measured at 10 different points outside dewar.
The temperature at the date of focus observation has been calculated by averaging the
measurements over the 1-day period preceding the observation. That period was chosen so
that it was small compared to the time between focus observations (typically two weeks)
but large enough to get the temperature variations due to orbital breathing and telescope
repointing smoothed out.
3. Results
Figure 1 displays the results of focus monitoring for camera 3 since the end of June
through mid-December, 1997. It shows that NIC3 focus was on uptrend in that time frame.
There is a marginal evidence that also camera 2 uptrended a bit over the same period of
time, and no regular pattern is seen for camera 1 (which may partially be due to larger
uncertainties in NIC1 focus measurements). Anyway, this behavior is consistent with the
idea that the NIC3 detector, originally displaced from its nominal position due to the
“dewar anomaly”, had been relaxing back toward the nominal position (see, e.g., Suchkov
et al 1998).
Along with the general uptrend, the phase retrieval results reveal substantial focus
variations on a time scale of a few weeks, with an amplitude of ~ 0.5 mm. The latter is
much larger than the typical scatter in NIC3 focus measurements, < 0.1 mm , thus these
focus excursions must be real. A clue to their nature can be found in the relationship
between focus and HST temperature.
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Figure 1: History of NICMOS camera 3 focus (mm of PAM space) from June through
December of 1997. Indicated are the correlation coefficient and the linear regression slope
(in mm per day).
The temperatures derived from measurements at different points outside the NICMOS
dewar are given in Figure 2. There is a good deal of similarity between the temperature
curves in Figure 2 and the focus curve in the upper panel of Figure 1, both showing an
uptrend and significant quasi-regular excursions from the trend line. The trends in these
two cases may be at least partially related. The general temperature increase from the summer period into the winter is likely due to increasing radiative flux from the Sun as the
Earth approaches its perihelion in winter. This makes the average HST temperature go up,
and the thermally induced mechanical stretching along the NICMOS optical pathway may
well be expected to affect focus position.
Most remarkable is however a strong coupling between focus and temperature deviations from the respective trend lines. Figures 3 to 5 show temperature residuals plotted
against detrended focus from phase retrieval (focus residuals) for all three cameras. Significant correlation between the residuals is obvious, suggesting, firstly, that both focus and
temperature excursions are real and, secondly, that they are related. A likely interpretation
of this correlation is that the HST temperature variation on a time scale of a few weeks
induces focus change similar to focus breathing induced by temperature variation on the
orbital time scale. Both temperature and focus variations appear to be quasi-periodic, with
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a pattern consistent with three complete cycles over a period of 130 to 150 days. These
numbers yield the cycle period coinciding with the HST synodic precession period to
within ± 3 days. Although the data sampling and the data time coverage are not sufficient
to yield a reliable estimate for the cycle period, this coincidence looks encouraging. The
explanation of the correlation between the focus and temperature residuals is then rather
straightforward: at the same pointing, the change in the HST attitude with respect to the
Sun caused by precession should cause the HST thermal balance, hence average temperature, to oscillate with the synodic precession period. These temperature variations appear
to impact NICMOS focus position in the same way as the orbital breathing except that the
mostly affected part of the light path in this case seems to be associated with NICMOS
fore optics rather than Optical Telescope Assembly (OTA). As pointed out by Matt Lallo,
the scatter in NICMOS focus residuals is about twice as large as that in the OTA focus
residuals as derived from WFPC2 data, which rules out OTA as the main source of NICMOS focus oscillations. However some contribution from OTA cannot be excluded. It
would be consistent with the fact that focus from plate scale appears to have very little of
the excursions from the trend line (Figure 1) seen in focus results from phase retrieval. As
discussed in Suchkov et al. 1998, focus from plate scale is sensitive mostly to detector
motion with respect to the NICMOS Field Divider Assembly (FDA) whereas phase
retrieval provides absolute focus position which depends on the entire optical path. Lack
of large focus excursions in the plate scale measurements suggests that some fraction of
quasi-periodic focus variations seen in phase retrieval results may be related to OTA as
well. Some evidence for small-amplitude OTA precessional breathing has been found by
John Hershey (private communication). He noticed that subtraction of a 56 day period
function with half-amplitude of ~1 µm from raw WFPC2 focus data results in a slight
reduction in focus residuals.
Given the above interpretation, we will be referring to quasi-periodic focus variations
seen in Figure 1 as “precessional” NICMOS focus breathing. As seen in Figures 3 to 5, the
amplitude of precessional breathing ranges from ~ 0.4 mm (NIC2 data) to ~ 0.6 mm
(NIC3 data) in PAM space. If focus variation is expressed in terms of Secondary Mirror
displacement required to produce a given amount of focus shift,
∆focus (mm)
∆SM = --------------------------------- µm ,
0.11 × G i
where G i = 1.171, 1.212, and 1.083 for NIC1, NIC2, and NIC3, respectively, (Burrows & Krist, 1997), then the above numbers translate to 3.0 µm and 5 µm in SM space,
respectively. Comparing the latter numbers with the orbital breathing amplitude, ~ 2 µm ,
(see, e.g., Suchkov and Casertano, 1997), we conclude that “precessional” breathing is
about twice as “deep” as orbital breathing. One may also compare these number with the
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actually adopted 5 µm tolerance with respect to HST focus drift caused by the OTA
shrinkage.
4. Acknowledgments
I would like to thank John Basinski for providing the engineering temperature data. It
was a pleasure discussing the results with John as well as with Alex Storrs, Wayne Kinzel,
John Hershey, and Eddie Bergeron. Matt Lallo provided valuable comments, having indicated, in particular, that comparison of focus residuals from NICMOS and WFPC2 data
argues against OTA as the main contributor to NICMOS precessional focus breathing.
5. References
Burrows, C.J. & Krist, J.E. 1997, Memorandum, March 19, 1997, ‘‘NICMOS Focus’’
Hasan, H. & Bely P. 1994, in: The Restoration of HST Images and Spectra II,
eds. R.J. Hanish & R.L. White (Baltimore: STScI), p.157
Hershey, J.L. 1997, Document: SESD-97-01
Suchkov, A., Bergeron, L., & Galas, G. 1997, in: 1997 HST Calibration Workshop,
p. 308.
Suchkov, A., Bergeron, L., & Galas, G. 1998, ISR NICMOS-98-004
Suchkov, A. & Casertano, S. 1997, ISR WFPC2-97-001
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Figure 2: History of NICMOS temperature outside dewar from June through December
of 1997. Indicated are the correlation coefficient, the area where temperature was measured, and the specification of that area.
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Figure 3: Correlation between temperature residuals and NIC1 focus residuals. Indicated
are the correlation coefficient, the area where temperature was measured, and the specification of that area.
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Figure 4: Correlation between temperature residuals and NIC2 focus residuals. The annotations are the same as in Figure 3.
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Figure 5: Correlation between temperature residuals and NIC3 focus residuals. The annotations are the same as in Figure 3.
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