The focus position as well as plots, images, and the Table below are calculated using the data from a targeted NICMOS focus monitoring calibration program. Since the focus data reduction and analysis techniques are being constantly improved, and the results from the early data sets are being re-computed each time an improvement is made, the reader may find slight differences between the currrent and previous versions of the focus history plot/table.
The NICMOS cameras were designed to share a common focus whose position can be adjusted using the Pupil Alignment Mechanism, or PAM. PAM's main part is an adjustable mirror which can be moved within +/- 10 mm about its zero position, thus allowing fine tuning of the actual location of the focus. It was hoped that, whatever changes to the HST optical path may happen the focus can always be brought back to the position of the detectors. Unfortunately, the unforeseen deformation of the NICMOS dewar has caused large mechanical distortions within NICMOS, which resulted in loss of a common focus for the three cameras. Worst of all, the camera 3 detector was pushed way out of the range within which the PAM can adjust the focus position. Soon after the Servicing Mission, this detector required a PAM position of about -17 mm to be in focus, which was far beyond the reach of the PAM.
The ongoing deformation processes in NICMOS keep changing the location of the detectors with respect to the PAM zero position. This change is shown in the plots displayed below. The abscissa is the day of observation (day number starting January 1st, 1997). The ordinate gives the position that the PAM should have to get the focus at the location of the detector ( Implied PAM Position, often referred to as " the focus position in the PAM space"). It is obtained using a method known as "phase retrieval" The method involves PSF fitting and is based on minimization of the rms wavefront error as PSF fitting on a chosen stellar image is performed. For camera 3, also the plate scale measurements are used to deduce the change in the focus position. The corresponding numbers are given in the Table. The images display a stellar field (open cluster NGC 3603) taken at a PAM position that brings the focus as close as possible to the detector position. In the case of Camera 3, this is, of course, on the extreme end of the PAM range. The image of NGC 3606 is shown at the PAM position of about -8 mm. Much better image quality is achieved at the currently nominal PAM setting of -9.5 mm.
The present NICMOS focus update includes focus measurements made after refocussing of the HST secondary mirror On January 12, 1998 (day 377 since January 1, 1997), the HST secondary mirror was moved to perform refocussing that would place NICMOS3 into focus. That was the start of the 3-week NIC 3 campaign. As seen from the focus history Table and plots, the goal of refocussing was successfully achieved. The focus position for camera 3 was found to be -9.76 +/- 0.24 mm in PAM space. This is well within the NIC 3 campaign focus tolerance of 1 mm at the nominal PAM setting of -9.5 mm. The focus measurements showed that the focus shift resulting from OTA refocussing was consistent with expected 2.5 mm PAM change.
Note that secondary mirror refocussing and the focus change induced by the FOM (see below) do not change the plate scale, and hence the "plate scale method" continues to track the internal dewar motions only. It no longer gives, in an absolute sense, the correct value of the focus position.
The large change in focus noted in the previous two focus measurements, which was of the same amplitude in all three cameras, was traced to the use of a new Field Offset Mirror (FOM) position. This mirror was moved to remove vignetting in camera 3, and for NIC3 prime observations is now the default. The change was made to support the NIC3 campaign. Due to the structure of the focus montitoring proposal, the same FOM offset was also applied to NIC1 and NIC2. Normal prime observations in NIC1 and NIC2 do not have the offset FOM position. Indicated in the focus plots are estimates of the FOM-corrected focus position for NIC1 and NIC2 only (NIC3 prime observations do have the new FOM position). The actual focus position during the execution of the focus monitoring proposal is also shown (upper points in NIC1 and NIC2 plots). The new FOM position gives a shift in focus position by approximately 1.5 mm, although the exact value is rather uncertain.
After NIC3 campaign, the HST secondary mirror has been reset to its nominal position which differs by only +0.5 micron from the pre-NIC3 campaign setting. The data from the focus monitoring program obtained on February 1st, 1998, right after refocussing of the secondary mirror, suggest that focus in all three cameras has moved back to the expected positions.
The Table and the focus history plot below contain FOM correction of -1.5 mm applied to all focus results for NIC1 and NIC2 obtained during the period day 351 through day 397. Prior and after that time frame FOM position for these two cameras is nominal. The actual focus in NIC1 and NIC2 observed under off-nominal FOM setting is also shown in the focus history plot (data points above the curve).
Continuing divergence between phase retrieval/encircled energy results and plate scale results for camera 3 seems to indicate that recent focus change is induced by a process not related to detector motion with respect to the Field Divider Assembly because the part of the focus change caused by that motion is well tracked by plate scale change.
The attached focus history plot displays, for camera 3, the regular phase retrieval and plate scale measurements. Additionally the results of phase retrieval corrected for the new FOM setting (in effect since day 351) and the secondary mirror move on January 12, 1998 (day 377) are presented. This is convenient for comparison of the focus behavior before and after day 377 in terms of detector motion. The plot shows that there were no dramatic changes in the camera 3 focus over the last few months except for those caused by the new FOM tilt and the secondary mirror move.
The large error in the phase retrieval result for camera 1 is explained by the fact that there were only parallel observations with this camera, which provided only one star suitable for phase retrieval. That star was very faint and yielded less reliable phase retrieval results than usual. The encircled energy measurements however were adequate. Proposal 7901 is designed in such a way that camera 1 and camera 2 alternate as prime cameras each other visit. Hence next time camera 1 executes as prime while camera 2 will observe in parallel.
We have slightly improved the coefficients used to reduce focus to the detector center and recomputed focus from phase retrieval for all visits since day 160. Comparison with the previous results shows that the change caused by new focus centering is quite small.
On June 4th, 1998, the HST Secondary Mirror (SM) was reset to place NIC3 in focus for the second NIC3 campaign. The commanded SM move of 16.6 microns away from the Primary Mirror was expected to result in NIC3 best focus at the nominal NIC3 setting of -9.5 mm in PAM space. The post-move focus results (see the focus history table and plot) showed that the NIC3 focus moved in the desired direction, although a slight offset from the aimed PAM position remained. The subsequent analysis which involved the attitude breathing model revealed however that the focus measurements were probably impacted by an unusually large breathing anomaly which occured during the post-move focus sweep. This anomaly may account for the offset seen in the last focus results.
The NICMOS focus data obtained on August 6, 1998, show that NIC3 focus continues drifting away from its nominal setting. At FOM tilt = 0, it would have a value which is below the level at the end of the SMOV in late May last year.
Camera 2 focus remains pretty much unchanged, close to the nominal PAM setting of 0.69 mm. During the visit on August 6, camera 1 observed in parallel with camera 2, and its field of view happens to have no star suitable for phase retrieval analysis.
The updated summary of the NICMOS focus history is given in the Table below and in the attached focus history plot. Note that, in the plot, focus during NIC3 campaigns is corrected for the Secondary Mirror move while the Table contains the actual uncorrected values.
This is NICMOS focus update as of September 4, 1998. Analysis of the data shows that focus in camera 3 may have started to recover from a downward trend seen since the end of May. Focus in that camera may have been impacted by extended periods of low Sun-angle HST pointings we had during this summer. Low angles seem to result in lower NICMOS fore-optics temperature. The latter is known to correlate with focus position, so low temperatures may have caused more negative focus. However, it is not clear if this is the only or even main cause of the recently observed focus behavior.
The measurements for cameras 1 and 2 have yielded a focus position very close to nominal. With fewer focus measurements for these cameras, it is hard to compare recent focus systematics in these cameras with those we have seen in camera 3.
This is NICMOS focus update as of October 28, 1998. The latest results suggestthat focus in all three camera remains stable.
Starting November 16, the focus observations will be conducted twice a week to closely monitor any changes that may occur during the NICMOS warm-up period. Since the NICMOS operating temperatures in the cryocooler era will be higher, the focus results during the warm-up are expected to provide information pertinent to instrument's future operational environment.
This plot shows the "best focus" position of NICMOS cameras as a function of time. The dash-dotted line is the current nominal PAM setting. The line at -9.5 mm for camera 3 is the current NIC3 nominal PAM setting, and the end of the useable PAM focus range. The different methods used to derive the focus position are indicated by different symbols as shown in the plot legend. Phase-retrieval gives a result in absolute PAM units. As the cold-well moves forward and back relative to the cold-mask, the effective plate-scale of the detector changes. The change in plate-scale between epochs is proportional to the motion of the cold-well. The scale changes are related to absolute PAM space by scaling them to measures from phase-retrieval at a few of the epochs.
These are images of NGC3603 obtained at the nearest PAM setting to the best focus: