A future plan for the next-generation Subaru adaptive optics, is a system based on an adaptive secondary mirror. A ground-layer adaptive optics combined with a new wide-field multi-object infrared camera and spectrograph will be a main application of the adaptive secondary mirror. A preliminary simulation results show that the resolution achieved by the ground-layer adaptive optics is expected to be better than 0.2 arcsecond in the K-band over 15 arcminutes field-of-view. In this paper, the performance simulation is updated taking dependence on observation conditions, the zenith angle and the season, into account.
We describe measurements of both the vibration forces imparted by various types of observatory equipment, and the transmission of these forces through the soil, foundations and telescope pier. These are key uncertainties both in understanding how to mitigate vibration at existing observatories and for developing a vibration budget in the design of future observatories such as the Thirty Meter Telescope. Typical vibration surveys have measured only the resulting motion (acceleration); however, this depends on both the source and the system being excited (for example, isolating equipment results in less force being transmitted, but greater motion of the equipment itself). Instead, here we (a) apply a known force input to the pier from a shaker and measure the response at different locations, and (b) use isolator properties combined with measured acceleration to infer the forces applied by various equipment directly. The soil foundation and pier transmission can then be combined with a finite element model based vibration transmission analysis to estimate the optical consequences. Estimates of plausible source levels supports the development of a vibration budget for TMT that allocates allowable forces to the sources of vibration; this is described in a companion paper.
Hyper Suprime-Cam (HSC) is an 870 Mega pixel prime focus camera for the 8.2 m Subaru telescope. The wide field corrector delivers sharp image of 0.25 arc-sec FWHM in r-band over the entire 1.5 degree (in diameter) field of view. The collimation of the camera with respect to the optical axis of the primary mirror is realized by hexapod actuators whose mechanical accuracy is few microns. As a result, we expect to have seeing limited image most of the time. Expected median seeing is 0.67 arc-sec FWHM in i-band. The sensor is a p-ch fully depleted CCD of 200 micron thickness (2048 x 4096 15 μm square pixel) and we employ 116 of them to pave the 50 cm focal plane. Minimum interval between exposures is roughly 30 seconds including reading out arrays, transferring data to the control computer and saving them to the hard drive. HSC uniquely features the combination of large primary mirror, wide field of view, sharp image and high sensitivity especially in red. This enables accurate shape measurement of faint galaxies which is critical for planned weak lensing survey to probe the nature of dark energy. The system is being assembled now and will see the first light in August 2012.
A wide-field adaptive optics system based on an adaptive secondary mirror (ASM) is one of a future plan for
the next-generation Subaru adaptive optics system. The main application of ASM based AO will be a groundlayer
adaptive optics (GLAO) with field-of-view larger than 10 arc minutes. The high Strehl-ratio of on-source correction by high-order ASM (expected to be about 1000) and the reduction of emissivity are also attractive points. In this paper, we report a preliminary result of simulations for the these applications of ASM to study conceptual design of the next-generation wide-field Subaru adaptive optics.
A tip/tilt off-load function from AO188 deformable mirror mount to Subaru telescope infrared secondary mirror
has been implemented and tested. The function is effective to reduce the influence of strong background pattern
at thermal infrared wavelengths. We describe the function and report the test results in this paper.
The Subaru telescope provides a feature of auto guiding the telescope using the slit viewer (SV). The SV guide uses the
target star as a guide star. There are advantages to guide the target star directly. However, the guide accuracy was not
good. The guide star is located on the slit and some lights is vignetted by the slit. But the SV guide simply took the
centroid of the light to measure the position of the star without taking the vignetting of the slit into account. In 2006, we
improved the SV to detect center of gravity with the vignetting of the slit taken into account. It assumes a Gaussian
distribution of light except for the slit. By this improvement, the guiding accuracy of the telescope improved from 0.37
arcsec to less than 0.2 arcsec. The effect of the improvement was also confirmed with actual observations.
We present methodology of the autoguider (AG) and Shack-Hartmann (SH) sensing systems which will be used for a wide-field camera, Hyper Suprime-Cam (HSC), on the prime focus of the Subaru 8.2-m telescope. For both systems, stellar images are formed on the HSC science CCDs. Although light from AG stars must pass
through bandpass filters, we can obtain enough photons for AG stars brighter than mAB < 14 mag in any bandpass filter assumed in order to achieve accurate autoguiding. Spatial number density of such bright stars from the SDSS database requires an area
of about two 2k×4k CCDs for AG stars. The optics of SH system except for the imaging CCDs is located within the HSC filter unit.
To study properties of cold dark matter (CDM), which can only be observed through its gravitational interaction
with galaxies, spatially resolved spectra at least to the K-band are desirable. We started designing a spectrograph
which observes multiple targets spatially resolved in a telescope field of view fed with multi-object adaptive
optics (MOAO). The current design either places field lenses on the telescope field of view to image the pupil
onto steering mirrors, or uses a single set of field lens to deliver beams to pick-off arms. The steering mirror on
the pupil image tilts and selects a sub-field from each of the telescope field of view physically split by the field
lenses. This allows cheaper and more robust construction of a method to select the target fields with a limitation
in selections of the target fields. On the other hand, the pick-off arm implementation allows more flexibility
in assigning targets to fields of the integral field units (IFUs) especially when targets are clustered. The IFU
arranges spatial elements of each of sub-field of view to be fed into the spectrograph. If enough pixels are afforded,
using microlens arrays, which image pupils of spatial elements onto the object plane of the spectrograph is ideal
in robustness. Otherwise, an image slicer is to be located to arrange the sub-field of view onto the entrance slit.
The instrument should be built as modules to allow expeditious scientific results.
The Subaru Telescope has been operated smoothly for eight years after its first light. With the advent of instruments with high spatial resolution such as the adaptive optics, elongation of images has been noticed towards specific azimuth (AZ) and elevation (EL). With accelerometers with high time resolution, we detected vibrations of the telescope and could attribute the elongation of images to the vibrations. The detected vibrations are at 3.6 Hz and at 7-9 Hz in AZ direction and at 5-6 Hz in EL direction. Image motion due to these vibrations is 0.4 arcsec peak-to-peak at maximum, which is not negligible compared to image motion of 0.063 arcsec rms in quiescent state. The motion, which can not be canceled with the auto guider, results in elongation of images. The 3.6 Hz vibration in AZ direction is only excited while culmination EL of above 80 degrees. The 7-9 Hz vibration in AZ direction and the 5-6 Hz vibration in EL direction are excited by periodic errors in incremental encoders which are used to measure velocity of telescope rotation. We investigated possibilities to reduce the vibrations with tuning control loops of the AZ and EL axes.
The Subaru telescope had its astronomical first light in January 1999 and has been stably operated since the common use started in December 2000. The telescope is mounted on an alt-azimuth structure. The structure of 550 tons is supported by six hydrostatic oil pads which lift the structure by 50 microns. The azimuth (Az) and elevation (El) axes are driven by direct-drive linear motors, ensuring very smooth pointing and tracking operations. The Az rail consists of eight circular arc pieces. They were installed in January 1997 with a peak-to-peak level of within 0.1mm. However at a later time, vertical undulations of the Az rail were found to be more than 0.2 mm peak-to-peak at some locations where the telescope structure in the rest position applies load. Open-loop tracking accuracy of the telescope, which was about 2 arcsec RMS on the sky, was found to be due to the undulations of the Az rail. We made a table to correct telescope pointings due to the undulations. It has made open-loop tracking accuracy better than 0.2arcsec RMS. Since then, we have been monitoring the flatness of the Az rail. So far the undulations have not changed.
Multi-integral-field spectrographs for near-infrared observations require a large number of complex cryogenic mechanisms to select source images in the telescope field of view which are then re-format on the spectrograph entrance slit. Source selection can be achieved in several ways, but the two methods most adequate for large fields and cryogenic environment are positioning of an optical element in the telescope field to pick off the source image, or steering a mirror located in a pupil image to deflect the light from a source into relay optics. The first solution permits high flexibility in source selection at the cost of large mechanical travels. The second solution limits the source selection to one per pre-defined sub-field, but gets by with small mirror tilts. Higher flexibility can be regained for the second solution by assigning different sub-field sizes to the steering mirrors in the central and in the peripheral areas of the field of view. We present a solution for a cryogenic steering mirror unit with a mirror diameter of about 20 mm and tilt angles of a few degrees, appropriate for source selection in a 1 arc minute field of an 8 m class telescope. The gimbaled mirror can be tilted about two perpendicular axes in the tangential plane of the mirror apex. The mirror is driven by two Nanomotors, and the motor strokes are measured by LVDTs. Motors and sensors are specified for operation at LHe temperatures.
KMOS is a cryogenic multi-object near-infrared spectrograph for the VLT. It will be equipped with about 20 deployable integral field units (IFUs) which can be positioned anywhere in the 7.2 arcmin diameter field o the VLT Nasmyth focus by a cryogenic robot. We describe IFUs using micro lens arrays and optical fibers to arrange the two-dimensional fields from the IFUs on the spectrograph entrance slit. Each micro-lens array is mounted in a spider arm which also houses the pre-optics with a cold stop. The spider arms are positioned by a cryogenic robot which is built around the image plane. For the IFUs, two solutions are considered: monolithic mirco-lens arrays with fibers attached to the back where the entrance pupil is imaged, and tapered fibers with integrated lenses which are bundled together to form a lens array. The flexibility of optical fibers relaxes boundary conditions for integration of the instrument components. On the other hand, FRD and geometric characteristics of optical fibers leads to higher AΩ accepted by the spectrograph. Conceptual design of the instrument is presented as well as advantages and disadvantages of the fiber IFUs.
We present a system for the exchange and handling of cold field masks in LUCIFER, the near infrared camera and spectrograph for the LBT. Inside the LUCIFER cryostat, 10 field-stop and long-slit masks, and 23 multi-slit masks are stored in a stationary and an exchangeable cabinet respectively. With LUCIFER at operating temperature, the exchangeable cabinet with its multi-slit
masks can be transferred from the LUCIFER cryostat to an auxiliary cryostat, and a second cabinet harboring the newly made, pre-cooled masks can be transferred back to LUCIFER from a second
auxiliary cryostat. Inside LUCIFER, a robot transports the individual masks from their storage position in the cabinet to the focal plane and inserts them in a mask mount where they are centered on two pins. The position accuracy of the masks in the focal plane is anticipated to be better than ± 10 μm. A mechanism which locks the masks in their cabinets and releases only the one connected to the transport robot permits mask exchange in arbitrary
orientation of the cryostat.
MIRTOS, Mid-IR Test Observation System, is a high spatial resolution mid IR (MIR) camera for the Subaru Telescope. It consists of two IR imagers. One is for MIR bands with a Si:As array with 320 by 240 pixels. It has 21 by 16 arcsec field of view (FOV) with a pixel scale of 0.067 arcsec. It also images the pupil of the telescope. The other is a near IR camera. A 256 by 256 InSb array with 0.028 arcsec/pixel is used to image 7 by 7 arcsec FOV at one corner of the MIR FOV. We apply Shift-and-Add (SAA) technique; a technique that shifts images detecting the displacements and adds them to cancel seeing. However it is often difficult to shift and add MIR images using a reference within because of low sensitivity in MIR for short exposure time. We solve this problem utilizing NIR images taken simultaneously as position references. We call this method two-wavelength shift-and-add (TWSAA). In this paper we show result from the test observations. 1) Pupil image was taken. It shows hot structures around the secondary mirror that are now planned to be covered by reflecting plates to direct the beam to the sky. 2) Correlation of motion between MIR peak position and NIR centroid position shows that NIR images can be used as TWSAA reference for MIR observations. 3) On a standard star and the core NGC 1068, SAA method was applied to reconstruct images. Resulting images show higher spatial resolution than previous observations.
We are constructing Mid-IR Test Observation System (MIRTOS) as an IR imaging system to evaluate and monitor the performance of the Japanese National Large Telescope, SUBARU. The system combines two cameras. One of the camera is for near-IR (NIR) and the other for mid-IR (MIR). They capture images simultaneously at the rate fast enough to freeze the seeing. Simultaneous NIR images are useful not only for evaluation of the image quality of the telescope but for two-wavelength shift-and-add that preserves diffraction-limited angular resolution of MIR images for a long integration. The system also has telescope emissivity mapping mode that images the telescope entrance pupil in MIR. For the MIR camera, a Santa Barbara Research Center (SBRC) Si:As IBS array with 320 by 240 pixels is used with pixel scale of 0.067 arcsec/pixel that takes enough samples to make diffraction limited images at 8 microns. For the NIR camera, an SBRC InSb array with 256 by 256 is used. Pixel scale is 0.028 arcsec/pixel. It is optimized to detect position of the brightest speckle in images at the wavelength of 2.2 microns with wide enough field of view to image a reference point source. In the emissivity mapping mode, a temperature controlled black body is inserted just outside the dewar window as an absolute calibration source for the telescope emissivity. In this paper we will present detailed design of MIRTOS.
With the advent of ground-based 8-meter telescopes and large format detector arrays, the mid-IR (MIR) astronomy is expected to produce images with higher angular resolution and improved sensitivity than currently available. TO obtain diffraction limited images, it is necessary to compensate for atmospheric seeing. We proposed a scheme in which, by detecting the near-IR (NIR) speckle positions of a reference star, for which we have higher sensitivity than in the MIR, we superpose the MIR images taken simultaneously by shifting its position to cancel the effects of seeing. We call this two-wavelength shift-and-add (TWSAA) method. In this paper, we describe an observation carried out in the fall of 1995 to test the concept. A NIR camera was built with 256 by 256 InSb array detector. With a dichroic splitter, a pair of images of a single start at two wavelengths were formed on two halves of the array. We obtained 126 pairs of images each integrated for 0.1 seconds simultaneously at the K- and L-bands. With the TWSAA method using the K-band images as the reference, the peak signal to noise ratio of the L-band integrated image was enhanced by a factor of 5.7 compared to simple addition.
The infrared instrumentation plan for the Subaru telescope is described. Four approved infrared instruments and one test observation system are now in the construction phase. They are coronagraph imager using adaptive optics (CIAO), cooled mid- infrared camera and spectrograph (COMICS), infrared camera and spectrograph (IRCS), OH-airglow suppressor spectrograph (OHS) and mid-infrared test observation system (MIRTOS). Their performance goals and construction schedules are summarized. The plan for procurement and evaluation of infrared arrays required by these instruments is briefly described.