IMACS Commissioning Report to the Magellan SAC Performance and Operation 2+ years after installation Alan Dressler and Tyson Hare, April 8, 2006 Capabilities Optical performance: Image quality and throughput Mechanical performance: Devices and Flexure Mosaic CCD Camera performance
IMACS Commissioning Report to the Magellan SACPerformance and Operation 2+ years after installation
Alan Dressler and Tyson Hare, April 8, 2006
Optical performance: Image quality and throughput
Mechanical performance: Devices and Flexure
Mosaic CCD Camera performance
Operating software performance
Telescope interface, including Shack-Hartmann
Multi-slit masks, design and production
Software observing tools
Summary of remaining issues and future work
Data reduction pipeline: COSMOS
Support Agreement: Tasks, responsibilities, personnel
1. IMACS has the largest field of an imaging spectrograph on any 8-m class telescope. IMACS f/2: 670 sq arcmin; IMACS f/4: 236 sq arcmin
Comparisons: VIMOS on VLT: 224 sq arcmin
DEIMOS on Keck 85 sq arcmin
GMOS: 30 sq arcmin
IMACS Cluster Building Survey - Dressler, Oemler, Gladders, Poggianti
IMACS Deep Survey -- McCarthy & Collaborators
4. Integral Field Unit (IFU) -- contributed by Durham University
0.20” sampling, two 1000-fiber bundles cover 5”x7” (f/2) or 4”x6” (f/4)
==> f/2, f4, means that full range of resolutions available
IFU observations may be mixed with mutli-slit observations -- IFU requires 3 of the 6 slots in the slitmask server.
GMOS -- similar unit supplied by Durham
VIMOS: 54” x 54” at 0.67” pixel (~6000 fibers)
5. Multi-Object Echelle
MOE consists of an echelle grating and cross-dispersing prism that mounts in one of the positions of the IMACS Disperser Server.
MOE provides a 2.5 pixel resolving power of R=21,000 (0.6” slit).
Each echelle spectrum is composed of 9 orders, from roughly 3400 Å to 9500 Å. Typically, 7 or 8 objects in the 15’x15’ field can be observed with full spectral coverage. An observation of RGB stars in Carina with the 480nm-780nm blocking filter (4 orders) covered 20 stars, and as many as 100 stars can be done in a single order.
Comparisons: ESI on Keck. Very similar specifications, but without multi-object capability
MOE spectrum of a red-giant star in the Carina dwarf galaxy, showing the detection of the challenging Eu II line.
6. Maryland-Magellan Tunable Filter (PI, Sylvain Veilleux, co-I, Ben Weiner)
SDL-Queensgate Fabry-Perot Etalon, 150-mm dia will enable narrow band (6-40Å) over fields ranging from 10-arcmin x 27 arcmin (full monochromatic spot diameter) to the full IMACS f/2 field. Wavelength coverage is from 5000-9200A.
Frequency-switching, synchronized with Mosaic CCD camera charge-shuffling enable wide-field surveys to unprecedented depths.
Ideal for deep redshift surveys (Ly), tracing dark matter with planetary nebulae, studying star formation and gas outflow in merging galaxies (including not only H, [O III], [S II], but also for fainter [O I], [Fe VII), [Fe X] over a wide range of redshifts.
Successful testing completed in Pasadena by Veilleux and Weiner in Feb ‘06. Pre-ship review conducted in March (passed). Will be shipped to Magellan soon and possible commissioning run in May. MMTF should be available in semester 07A for general proposals.
Comparison: None on an 8-m class telescope.
7. GISMO - Gladders Image-Slicing Option for IMACS
High-multiplexing advantage in a 4’ x 4’ central field of IMACS:
Through a series of mirrors in the focal plane, and lenses above the plane, 16 central areas (each 2’ x 0.5’) are fully separated to utilize the entire 8K x 8K detector. 5-8 times multiplexing advantage, enabling for small field spectroscopy (HDF, HUDF, etc.), centers of galaxy clusters, globulars in nearby galaxies, very deep fields…
Progess: Optics now being delivered, mechanical design advancing. Available: semester 07B
Comparisons: no such device available elsewhere
Imaging -- little used, especially f/2. Problem with f/2 baffling
Centerfield slit-viewing spectroscopy: Enables traditional slit-viewing spectroscopy for brighter objects -- not necessary to use Mosaic CCD Camera for acquisition. Not much used, but Centerfield Guider has proven essential for on-axis Shack-Hartmann testing, and flux calibrations are routinely done with 7” hole in slit-viewing mode.
Centerfield acquisition also used for setups, particularly IFU.
LPD (Low-Dispersion Prism) -- very low dispersion mode, R~ 30-150 -- built by Scott Burles, commissioned in Jan ‘06. Very high multiplexing (thousands of spectra per exposure)
IMACS is the most versatile spectrograph on an 8-m telescope (on any telescope?). It has a wide range of dispersions, a unrivaled large-field capability for multi-object spectroscopy, and several “accessories” (IFU, MOE, MMTF, GISMO) -- two of which are unique.
All of the available modes have been used already, in just 2 years.
By the way, IMACS cost ~$6M, compared to ~$10M for DEIMOS and $11M ($91M for 8 instruments) for VIMOS.
IMACS receives a fraction of the support that DEIMOS and VIMOS do.
Imaging quality: f/4 camera
Design was for 0.15” FWHM central rad < 5’ degrading to 0.20” rad ~ 12’
The goal was to achieve ~0.30” images in “perfect” (0.25” FWHM) seeing.
Pinhole test in Pasadena: pinholes ~ 0.30” FWHM -- unable to confirm, but “unresolved,” a good sign.
Best image taken at telescope by Dave Osip, 0.45”-0.50” over entire f/4 field, 15’ x 15’
This suggests that the image was taken in ~0.45” seeing, with the camera performing within design specifications. Need a better seeing night to push this further.
Note: The telescope images also indicate that the collimator performs as expected, that is, it adds negligibly (~0.10”) to the image size.
Imaging quality: f/2 camera
The f/2 camera was designed to have image quality of 0.25” on axis, ~0.30” off axis (rms). It was meant to achieve ~0.4” images in “perfect” seeing. (The sampling of the camera is 0.20”/pixel.) Images of the pinhole mask (whose images are ~0.30”, smallest the laser could cut) show that the spec was met in the central (half-radius) part of the field, but that the images degrade (elongate) to ~0.45” in the very corners.
The best image has been taken by Dave Osip, who achieved ~0.50” FWHM in the central part of the field degrading to 0.70” FWHM in the outside.
Hartmann-mask tests have been taken by Osip and Phillips. These show a clear tilt of the focal plane that is the largest aberration, specific to the f/2 camera optics.
The f/2 camera design has been entered into ZEMAX by Marla Geha and will be used by Alan Uomoto to look for sensitivity of the f/2 optics (decenter, tilt) that could account for the test results.
A sensitive test, suggested by Shectman, is to measure to take out-of-focus images (doughnuts)
Dan Kelson has written some excellent software to do the analysis:
The size of the outer ring is a measure of focus,
The shape of the outer ring is a measure of astigmatism, and
The decentering of the secondary hole measures coma.
Imaging quality: f/2 camera
The problems with the pinhole images were noted in Pasadena, in the shop, but no resources were available to pursue it and there was some hope that proper illumination of IMACS by the telescope would improve the situation.
It is hoped that a simple cause can be found and corrected (such as moving one of the lens groups), but the problem may not be reparable through the optics. For example, a wedge in one of the two (strong) aspheres would not be correctable. However, significant improvement might be made just by tilting detector focal plane.
The effect of this on performance is bothersome, but not very significant.
Throughput of IMACS, including telescope (M1,M2,M3,ADC)
Throughput of IMACS is lower than anticipated, peaks at ~21% for spectroscopy
(peak) Expected Actual
M1 x M2 x M3 x ADC: 0.60 0.62 IMACS optics (20 air-glass) 0.80 0.70
IMACS grism, grating, filter 0.75 0.60
Mosaic CCD Camera 0.85 0.84
throughput 0.31 0.22
MAGIC IMACS f/2 direct filter CCD
B 27.35 26.85 -0.77 0.20 0.59 0.78 V 27.29 26.82 -0.47 0.24 0.69 0.80 R 27.52 27.12 -0.40 0.22 0.60 0.87 I 26.83 26.35 -0.47 0.14 0.85 0.38
Assume telescope, CCD same, remove ADC (5% loss) ==> IMACS optics are 0.70 (30%, not 20% loss)
(0.89 x 0.82 x 0.89 x 0.95)
Note: LDSS-3 reaches 30% with similar type grisms!
These filters have been scanned by Gabriel Martin
Tyson Hare and Dressler are planning a laser test that will measure the IMACS collimator, f/2, and f/4 cameras, but even if something is found, throughput can only rise by 3-4% -- CCD and grisms?
First problem made it difficult to diagnose second problem, and together they made it very difficult to make and hold good images.
Second problem seems to have been (mostly? completely?) removed by controlling the tilt of the secondary mirror as Baade is moved in elevation. (Extensive work by Osip and Perez.) Images at IMACS track images with guiders at Clay very well over the last year.
Remaining problems with Shack-Hartmann correction using IMACS off-axis guiders
Clardy has developed a mature program for designing multislit masks. The program has been distributed over the Consortium and upgrades are made available on a regular basis. Thoroughly documented.
Clardy program also writes the SMF files that drive the laser cutter. and Some refinements may still need to be made in order to facilitate cutting N > 2000 object masks and reducing machine failure (or allow the slit mask cutting to resume at an arbitrary point, simply.
In general, the Convergent Prima laser milling machine has performed brilliantly -- quality of slits is very high. Realignment of the optics was needed in mid-2005 -- company sent a technician. New components ordered to allow Magellan personnel to do this.
Ongoing problem with maintaining a good air supply. Action?
Cutting procedure fails at some point every 4th the 5th mask or so. Needs to be restarted in order to complete. Seems to be due to problem in the way the autofocus feature works. Still trying to diagnose the problem or make a better workaround.
Excellent interface made to allow uploading files on the web (Phillips, Osip), and a good bookkeeping system is also on the web.
submission of masks: Things are better, but few observers meet the “month-in-advance” requirement. More typical is 2-3 weeks. This remains a major problem for Magellan operation!
Very poor preparation for observing tools at beginning of commissioning. The situation has been rectified slowly, thanks to the efforts of Magellan staff, who have put almost all this together.
Of particular note: Excellent mask-alignment tool by Phillips -- fast, accurate, simple -- many compliments
A complete data reduction pipeline for mutli-slit observations has been written, distributed, and documented (not extensive, but usable) by Gus Oemler. Difficulty of use: moderate. Efficiency: excellent. Quality of result: good. Oemler has been responsive to questions and suggestions.
Dan Kelson and Greg have taken responsibility to “keep” and improve the software, interfacing with users as problems arise and modifying the code has required.
11. Support Agreement & other requirements
Facility Instrument Support Agreement has been written using template provided by Matt Johns. Reviewed and modified by Dressler, Hare, Perez, Phillips, Thompson, Martin, & Osip on April 13. Agreement covers responsibilities, documentation, communication between Magellan and IMACS team, setups, procedures, maintenance, repair, etc. Up-to-date IMACS User Manual from Dressler. Extensive documentation of IMACS systems from Hare. Spare parts are on site. List of continuing issues has been made and prioritized.
Report to the Council that IMACS has finished its commissioning period and is now considered a Facilities Instrument, as described in the Facilities Instrumentation Procedure. Some new capabilities are still under development, and there are still some problems, but the Magellan staff and IMACS team commit to continue to work these issues with present level of effort or greater.