This week we continued with HPF and Laser Frequency Comb (LFC) commissioning. We made a lot forward progress in getting spectra with the comb and had our official “first light”. Below are a few pictures that might help visualize some of what transpired.
In the last week we have had a few updates for two of our instruments. For VIRUS we are up to 29 working spectrographs. We actually have several more units but some of the oldest and slightly mis-behaving units have been sent back to Austin for realignment and recommissioning. For HPF we are thrilled to announce that a NIST laser comb has been installed in the calibration room in the HET basement. This allows us to send a picket fence of spectral features through a separate fiber next to the science fibers. During the data reduction and analysis of the HPF spectra they can look at the position of these pickets and determine how the instrument might be subtly moving and correct for it. So far it seems like it is working great.
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This week the big changes to the telescope have been the removal of the Prime Focus Instrument Platform (PFIP) work platform and the arrival of 3 more VIRUS units. The PFIP work platform, as the name suggests, allowed us to drop people off to work on the PFIP. We will still be able to work on the PFIP but now we have to do it from the JLG worker-lift. The work platform covered up a number of mirrors (about 10% of the light collecting area) as can be seen in the images below taken with our pupil viewing camera.
With the delivery of 3 more VIRUS units we now have 28 working VIRUS units or 56 spectrographs or more than 12,000 fibers on the sky!
The exciting news this week has been the arrival of the Habitable Planet Finder (HPF). This is the first of our new high resolution instruments and an instrument well suited to working in bright moon conditions. The HPF was designed and built by our Penn State partners and arrived on the 16th.
This instrument is designed for extremely high precision spectroscopy capable of detecting the reflex motion of stars as small earth sized planets go around them. To achieve that precision the instrument is housed in our temperature controlled basement at the HET inside a temperature controlled room inside a large temperature controlled vacuum chamber. All of these efforts allow them to control the temperature of the optics of the instrument at a level of 0.001 degrees Celsius.
After very carefully cleaning the enclosure that will house the vacuum chambered instrument the HPF team
was able to open their instrument and after a very through inspection proudly announced that they have just as many pieces of glass as they did in the assembly lab at Penn State (an optics joke). After a few final checks and the inclusion of their single moving part inside of the spectrograph they sealed up the vacuum chamber which, if things continue to go very well, may remain sealed for several years to come. The process of pumping the vacuum out of the large chamber took the rest of the weekend.
In the coming days and weeks the HPF team will monitor its stability, install the laser metrology system and get the systems ready for on-sky commissioning.
Quite an exhausting and exciting week at the HET!
This week we are pleased to announce that a new VIRUS unit was installed in side two of the VIRUS enclosure. This brings us to 22 VIRUS units or 44 spectrographs. We also took a little time in the last engineering run to add on some valves to the vacuum fittings which will allow us to cold pump on the VIRUS units which takes far less time to do than to warm up and then repump which was our older methodology. Keeping 22 VIRUS units going is starting to be a little easier but still takes a lot of management.
In addition to the work on VIRUS, we have also installed in the coherent fiber bundles for the HPF. These coherent fiber bundles will be used to setup stars on HPF science fibers. HPF will arrive in the coming weeks and we are very excited to get our first high resolution instrument on sky in the coming months.
We are in another 3 week science period but our big news this week is that we have been able to bring a few more VIRUS Units on-line which brings our total number of active units up to 21 units. Recall that a VIRUS unit is made up of 2 spectrographs so we now have 42 spectrographs on-line. These newest units are located in VIRUS enclosure 2 which means that we are now making use of both of the “saddle bags” which were installed at the HET as part of the Wide Field Upgrade. This makes the VIRUS closer to being ready for the main part of the HETDEX survey. As we add more Spectrograph units the system continues to get more complex with multiplexers and timing systems.
In some recent very clear nights we were able to observe 10 HETDEX shots (what we call a pointing for the survey) in a single night. This is a new record and our setup times are now typically below 5 minutes when moving from one shot to another. Further small milestones….
We have now observed one month out of four in the 2017-3 period. The month started off pretty slow with lots of bad weather in August but we have had some good clear nights. We are still conducting 7 days of engineering around full moon, at least until we get our first bright time instrument. So far we have collected 52.9 hours of charged time and completed 21.8% of the TAC allocated time. We have started collecting the first LRS2 Guaranteed Time Observations (GTO) allocated by the HET board for the instrument team as well as the first time allocated by the HET board for the HETDEX experiment. The latter includes time on well studied fields to make sure that our Lyman alpha emitters detected not really other types of emission sources and to better quantify our sensitivity/throughput.
We have completed the 2017-2 trimester (April – July). It was a very successful. The HET Board allowed us to transition from 2 weeks of science operations per lunation to 3 weeks of science operations. We also completed a large commissioning effort of 27 hours of VIRUS exposures on well studied EGS fields. Below are some statistics from the trimester: 138.5 hours of acceptable high priority exposures and 40.5 hours of low priority filler exposures. The average overhead for LRS2 per requested visit was 7.9 minutes which is nearly a factor of two better than the overheads with LRS before the Wide Field Upgrade. The average visit length was 18 minutes long and the longest completed visit was 120 minutes. The completion rate for priority 0 targets was 97.2%, for priority 1 targets was 98.1%, for priority 2 targets was 90.1% and for priority 3 targets was 96.4%.
For the new 2017-3 trimester (August – November) we will continue with the 3 weeks of science per lunation. The HET board has approved the first HETDEX time as well as Guaranteed Time Observing (GTO) for the commissioning team of LRS2. In addition, we are expecting to see the arrival of the Habitable-zone Planet Finder (HPF) spectrograph.
This week marked the end of another dark run and thus the end of another science period. We are now also three quarters of the way through the current science period labeled 17-1. As such we are starting to see a larger number of filler Priority 4 targets being observed. In this last dark run we observed 374 acceptable observations for a total of 38 hours and 122 Priority 4 targets for a total of 22 hours. We saw 6% of the time lost due to problems. This is a fairly respectable number and fairly typical of the mature HET before the takedown.
We are now three quarters of the way through the current 17-1 period. The Telescope Allocation Committees have begun meeting and the new allocations are being submitted for the 17-2 period which begins April 1. For this period the same suite of instruments will be available for the PIs: LRS2-B and LRS2-R and the two weeks around dark time will also be fully committed to science observing.
Despite some pretty poor weather this past week we are finished with the bright time engineering run and ready to start science over the 14 days around new moon. In this past engineering run we found some problems with the wave-front sensor software and implemented a new forward model of the hexapod motion. This new model allows for us to compensate for an unintended very small rotation that is induced by hexapod motion over the course of a trajectory. The impact of this rotation was to cause the field to rotate by several arcseconds over a trajectory. Our new forward model eliminates all of the hexapod induced rotation but allowed us to see a much smaller Azimuth dependent rotation term. This much smaller term will be addressed after this next science run, during the next full moon. Overall, a good improvement to the telescope’s operation.