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Near Infrared Camera and Multi-Object Spectrometer Instrument Handbook for Cycle 17 > Chapter 4: Imaging > 4.4 Image Quality

4.4 Image Quality
4.4.1
The high image quality of NICMOS is summarized by the Stroll ratio of the PSF, which is defined as the ratio of the observed-to-perfect PSF peak fluxes. Table 4.5 lists the Stroll ratios for representative filters in all three NICMOS cameras (courtesy of John Krist, STScI). The ratio is very high, between 0.8 and 0.9 for NICMOS images, at all wavelengths and in all Cameras at their optimal focus. For more information see Seconder et al. 2008 "Domains of Observability in the Near_Infrared with HST/NICMOS and AO Augmented Large Ground-Based Telescopes"
For NIC3, the quoted Strehl ratio is for optimal focus measurements obtained during the Cycle 7 & 7N campaigns.
4.4.2
The changes in dewar geometry leading to the degraded focus in NIC3 have also affected NIC1 and NIC2. By measuring the PSFs of stars at a series of PAM positions it was determined that the optimal focus for NIC1 occurred for a PAM position of ~ +1.8 mm and the optimal focus for NIC2 at ~ 0.2 mm after the installation of NCS in 2002. This difference is significant enough that NIC1 and NIC2 are not considered to be parfocal. However, the NIC1 and NIC2 foci are still sufficiently close that the intermediate focus position between the two cameras, NIC1-2, has been defined for simultaneous observations. The compromise focus position has been chosen to share the waveforms error equally between NIC1 and NIC2. The image degradation induced by this compromise focus is smaller than a few percent in each camera, and negligible for most purposes. Most users will find this focus sufficient to reach their scientific goals when using both cameras. Additionally, a separate PAM position at the optimal focus is defined and maintained for each camera.
The encircled energy profiles for NIC1 and NIC2 at representative wavelengths are shown in Figures 4.7 through 4.11.
Figure 4.7: Encircled Energy for NIC1, F110W.
Figure 4.8: Encircled Energy for NIC1, F160W.
Figure 4.9: Encircled Energy for NIC2, F110W.
Figure 4.10: Encircled Energy for NIC2, F160W.
Figure 4.11: Encircled Energy for NIC2, F222M.
Vignetting in NIC1 and NIC2
The lateral shifts of the NICMOS dewar have resulted in vignetting in all three cameras. The primary source of the vignetting is a slight misalignment of the FDA mask. Relatively small losses in throughput are observed at the bottom ~ 15 rows of both NIC1 and NIC2 as shown in Figure 4.12, but a more substantial vignetting is seen in NIC3.
Figure 4.12 shows the column plot of the ratio of an in-flight flat field to a pre-launch flat field for NIC1 and NIC2 for the F110W filter. The approximately 10% decrease seen near the bottom of the detector (left in the figure) demonstrates that vignetting has reduced the throughput. The decrease in throughput in NIC3 is due to movement of the vignetting edge for the bottom 10-15 rows and is more dramatic.
Figure 4.12: Vignetting in all three NICMOS Cameras as a function of row number.
4.4.3
NIC3 has suffered the largest shift in focus due to dewar deformation of about –12 mm in PAM space during the final stages of cryogen exhaustion. Early measurements after the installation of NCS in April 2002 confirm this number. This focus shift is outside the range that can be compensated with the PAM (maximum shift is –9.5 mm).
During Cycle 7 and 7N, NIC3 was operated in optimal focus during two special observing campaigns of 2–3 weeks each, in January and June 1998, when the HST secondary mirror was moved to recover optimal focus. Outside those two periods, NIC3 was operated at best internal focus, namely with the PAM at –9.5 mm.
The typical FWHM of NIC3 images (both at optimal and best internal focus) is ~1.3 pixels, with small variations between different wavelengths (Storrs, NICMOS ISR-98-006); the fractional flux within the first Airy ring ranges from 43% in J to 49% in H to 58% in K with NIC3 in optimal focus. The size of the Airy ring for NIC3 PSF has been calculated for oversampled TinyTim PSFs.
Figure 4.13: NIC3 encircled energy at 1.6 microns at optimum focus (PAM=–12 mm, solid line) and at the best achievable focus (PAM=–9.5 mm, dashed line) with the HST secondary mirror at nominal position.
 
Because NIC3 undersamples the PSF, the degradation of image quality with the PAM mirror at –9.5 mm was found to be relatively small compared to the optimal-focus image quality. Figure 4.13 shows the encircled energy for NIC3 at 1.6 microns for both the optimal focus (PAM=–12 mm at the time of the measurement) and the best focus (PAM=–9.5 mm). Both curves report simulations obtained with TinyTim PSFs convolved with the NIC3 pixel response, but the results are very similar to the actual observations. The loss in the peak flux is around 20% and the loss in encircled energy beyond one pixel radius (0.2") is no more than 10–15%. Given the minimal loss of performance with the slight out-of-focus operations, NIC3 will be operated without moving the HST secondary mirror and will be offered “as is” in future Cycles.
Some observers may consider obtaining NIC3 parallel observations, while NIC1 and NIC2 are used for the primary science observations. At the NIC1-2 best focus, the image quality of NIC3 is obviously degraded, with a PSF FWHM over 3 times larger than the NIC3 best focus PSF FWHM. Images are “donut” shaped and, therefore, are not useful for scientific purposes.
Vignetting in NIC3
In addition to focus degradation, NIC3 is also affected by vignetting from two sources. The first is a cold vignetting, due to the lateral shift of the FDA mask, similar to the vignetting affecting NIC1 and NIC2. The second is due to a warm bulkhead edge, which produces elevated thermal background and degraded image quality over the bottom 25% of the detector (bottom ~60 rows).
The warm vignetting was successfully removed during early 1998 by moving the FOM to a position Y=+16 arcsec, producing a corresponding translation in the NIC3 field-of-view. This translation removed the warm vignetting and slightly improved focus. The price was the introduction of a mild astigmatism, which is, however, below the λ/14 criterion for image quality (except at J, where the wavefront error is λ/10).
The only vignetting left in NIC3 with the FOM at Y=+16 arcsec is the one produced by the FDA and affects the bottom 10–15 rows of the detector, in the same manner as NIC1 and NIC2. NIC3 is now operated with the FOM at the +16 arcsec position as default. The loss in throughput is shown in Figure 4.14.
Figure 4.14: NIC3 Vignetting as a function of FOM offset.
4.4.4
NICMOS provides full Nyquist sampling beyond 1 μm and ~1.7 μm in NIC1 and NIC2, respectively. In addition to all the properties of diffraction-limited imaging, the NICMOS point spread function (PSF) has a few ‘off-nominal’ characteristics. These are mostly induced by the thermal stress suffered by the dewar early in the instrument’s life.
Each NICMOS camera has a cold mask located at the entrance to the dewar that is designed to block thermal emission from the OTA pupil obstructions. The NIC2 cold mask also serves as the Lyot stop for the coronagraph. Due to the thermal stress suffered by the NICMOS dewar, the cold masks are slightly misaligned relative to the OTA.
Because of the cold mask misalignment, the diffraction pattern is not symmetric. NICMOS images of point sources show slightly elliptical diffraction rings and the diffraction spikes show alternating light and dark bands and asymmetries. They are caused by unequal offsets between the corresponding pairs of spider diagonals.
Since Cycle 7, the foci for NIC1 and NIC2 have moved slightly in the negative direction, and are therefore somewhat closer together. This improves the quality of the images taken at the NIC1-2 intermediate focus. The focus for NIC3 remains beyond the range of the pupil alignment mechanism (PAM), but has moved in the positive direction, thus improving the image quality over Cycle 7. Figure 4.15 shows the primary PSF for NIC1 at the optimal PAM setting.
Figure 4.15: NIC1 PSF at the optimal PAM setting. The left image is a cross section of the primary PSF pictured to the right.
4.4.5
Coma and astigmatism in the NICMOS cameras are generally small, with the wavefront error typically less than 0.05 μm, that is, less than 5% of the wavelength at 1 μm. The mean values of coma and astigmatism measured in Cycle 11 along the detector’s x- and y-coordinates, are given in Table 4.6, expressed as wavefront errors. In NIC3, the astigmatism along the detector’s x-axis increased to ~5% and became more unstable after the nominal FOM y-tilt was done in December of 1997. It had been changed from 0 to 16 arcsec in order to reduce the significant vignetting in this camera. With regard to the temporal behavior of NICMOS aberrations, the y-coma in all three cameras had been gradually increasing by ~2–5% during NICMOS operations throughout the 1997–1998 lifetime period (Suchkov, NICMOS ISR 99-003).
x-coma, μm
0.0032 0.002
0.015 0.012
0.026 0.006
0.016 0.012
y-coma, μm
0.022 0.005
–0.044 0.0081
–0.040 0.011
x-astigmatism, μm
0.006 0.003
0.003 0.02
0.003 0.016
0.062 0.020
y-astigmatism, μm
0.024 0.004
0.024 0.004
0.027 0.013
0.040 0.019
4.4.6
The PSF is at least to some extent a function of position in the NICMOS field of view. Preliminary data indicate that this effect is small (less than ~6% on the PSF FWHM) and that only a small degradation will be observed. Movement of the FOM, on the other hand, has been shown to have a greater effect on the PSF quality. The NICMOS team is developing software to account for this field dependence.
4.4.7
The NICMOS PSF suffers from small temporal variations induced by the HST breathing and by variable shifts of the instrument’s cold masks (for a review of this topic see Krist et al. 1998, PASP 110, 1046).
The HST focus position is known to oscillate with a period of one HST orbit. The focus changes are attributed to the contraction/expansion of the OTA due to thermal variations during an orbital period. These short term focus variations are usually referred to as “OTA breathing”, “HST breathing”, “focus breathing”, or simply “breathing”. Breathing affects all data obtained with all instruments onboard HST.
Thermally induced HST focus variations also depend on the thermal history of the telescope. For example, after a telescope slew, the telescope temperature variation exhibits the regular orbital component plus a component associated with the change in telescope attitude. The focus changes due to telescope attitude are complicated functions of Sun angle and telescope roll. More information and models can be found on the “HST Thermal Focus Modeling” Web site at URL:
http://www.stsci.edu/hst/observatory/focus/thermal.html.
The telescope attitude also appears to affect the temperature of the NICMOS fore-optics, which are outside the dewar. A noticeable oscillatory pattern about the NICMOS focus trend lines was found to correlate with temperature variations of the fore-optics. It has not been fully investigated whether or not the correlation of the fore-optics temperature with NICMOS focus changes is an additional focus change, or only reflects the OTA focus change.
Another source of temporal variation for the PSF is the “wiggling” of the cold masks on orbital timescales. This causes asymmetries in the PSFs and residuals in PSF subtracted images.
HST breathing and cold mask “wiggling” produce variations of 5% to 10% on the FWHM of the NIC2 PSFs on typical timescales of one orbit.

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