Mid-IR sky emission (PWV=2.3 mm and airmass=1.5, blue thin line) (*), telescope emission at two temperatures (black and red thick curves) with an emissivity of 5%, and a selection of CanariCam filters (green shadowed areas).
CanariCam is a mid-infrared (7.5 - 25 micron) imager with spectroscopic, coronagraphic, and polarimetric capabilities, which will be mounted at one of the Nasmyth foci of the GTC. It is designed to work as a diffraction-limited imager at 8 microns. The instrument uses a Raytheon 320x240 Si:As detector which covers a field of view of ~26"x19" on the sky. Most mechanism motors and optics are inside a cryostat which is cooled down to 28K using a He cryo-cooler system. Temperature control of the detector ensures that its optimum operating temperature (~9K) is stable in the mK range.
CanariCam is designed to reach the diffraction limit of the telescope at mid-IR wavelengths. However, to do this routinely the telescope will have to be equipped with a fast-guiding mode that will allow for fast tip-tilt correction. Currently fast guiding is not yet available and hence initially the very best image quality cannot be guaranteed.
The following table summarizes all observing modes available for
CanariCam. Only those modes that have been fully commissioned are
offered in the current semester.
| Imaging |
Spectroscopy* |
| Coronography** |
Polarimetry*** |
|
* Only low resolution spectroscopy at 10 micron is offered. Low
resolution spectroscopy at 20 micron and high resolution spectroscopy
are not offered. ** Not offered ***Not offered |
|
In this section, some general features of the mid-IR observing technique are reviewed. In most cases, practical examples and numbers used in CanariCam will be utilised to complement the explanation of the general concepts. This information is aimed particularly at those observers who are new to infrared astronomy.
The main difference between ground-based optical and infrared astronomy observations is that, at mid-IR wavelengths, the background is huge compared with the emission of any astronomical source. Besides, this background emission varies rather fast in time and in space. Nextv figure shows the Earth's atmospheric emission and the telescope emission (at 10 and 25 Celsius) in the CanariCam's wavelength operating range, assuming a telescope emissivity of 5%. Note that telescope emissivity can dominate over sky emission in the 10-micron wavelength range.
Mid-IR sky emission (PWV=2.3 mm
and airmass=1.5, blue thin line) (*), telescope emission at two temperatures
(black and red thick curves) with an emissivity of 5%, and a selection of
CanariCam filters (green shadowed areas).
(*) Note that the ORM's mid-IR atmospheric emission and transmission curves are not available. Hence, the sky emission and transmission curves shown here were downloaded from the Gemini Observatory mid-IR resources webpage and simulate the conditions at the Cerro Pachón Observatory. The curves were calculated using ATRAN models (Lord, 1992).
Some of the factors that contribute to the telescope emissivity are the spiders of the secondary mirror (M2) holding structure, the interspace between segments of the primary mirror (M1), the telescope tube metallic structures that are around M1, and the dust accumulated on the mirror surfaces.
To avoid or even eliminate some of these components:
Thanks to the pupil imaging engineering mode of CanariCam, we can glimpse at the GTC pupil. Next figure shows an image of the pupil taken on September 29th, 2010, where the lighter orange (yellow) represents the warm emission, while the darker orange represents the cooler emission. The "Rose Petal" Lyot mask was misaligned on purpose in order to show the secondary mirror spider structure. Also note that it is possible to see the emission from the space between the M1 segments.
Image of the pupil showing some of the warm components
of the telescope emission. In light orange we can see the emission from the
interspace between M1 segments and from the M2 support structure spider. The
"Rose Petal" Lyot mask within CanariCam was used.
The Earth's atmosphere not only emits strongly in the mid-IR, but also absorbs selectively the radiation from the celestial bodies, being almost transparent in only a few mid-IR observing windows. CanariCam is designed to work in two of such windows, the so called 10-micron (N) and 20-micron (Q) windows, respectively. Still, in these windows there are several telluric absorption lines that degrade the sensitivity of the observations. Some of the main contributors to the Earth's atmospheric absorptions are:
Next figures show the CanariCam filters (shadowed in green) and the sky transmission in the mid-IR(*) (in red). In both plots, all the absorption features mentioned above can be clearly distinguished.
CanariCam's broad-band N and Q, narrow-band
20-micron and forbidden-line narrow-band 10-micron filters. Modelled Cerro Pachón's
atmospheric transmission (PWV=2.3 mm and airmass=1.5) is shown in red.
CanariCam's barrow-band Si and SiC filters
in the 10-micron window. Modelled Cerro Pachón's atmospheric transmission (PWV=2.3 mm
and airmass=1.5) is shown in red.
The presence of water vapor in the atmosphere has an important impact on the sky transmission and emissivity in the mid-IR, particularly in the 20 micron window. The larger the amount of water vapor, the lower the sky transmission and the higher the emissivity. The figure below shows the model sky transmission at an airmass of 1.5 for three different values of the PWV (2.3, 4.3 and 10 mm). These ATRAN models were obtained from the Gemini Observatory web site and correspond to the conditions of Cerro Pachon. They are very useful to understand how the mid-IR sky background changes with PWV.
Mid-IR sky transmission in the mid-IR at an airmass of 1.5 for three
values of PWV (red - 2.3 mm, blue - 4.3 mm and green 10 mm) based on
ATRAN models for Cerro Pachon.
The figure shows that the sky transmission in the 10 micron window decreases by ~5% between 2.3 and 10mm of PWV. However, the sky transmission in the 20 micron band decreases between ~30% in the bluest part and ~80% in the reddest part, when the PWV increases from 2.3 mm to 10 mm.
The effect of PWV on the sky emissivity is the opposite to its effect on the transmission, and it is shown in the figure below.
Mid-IR sky emissivity in the mid-IR at an
airmass of 1.5 for three values of PWV (red - 2.3 mm, blue - 4.3 mm and
green 10 mm) based on ATRAN models for Cerro Pachon.
The sky emissivity in the 10 micron window increases by ~5% when the PWV increases from 2.3 to 10 mm. The emissivity in the 20 micron window increases ~30% in the bluest part and ~80% in the reddest part, when the PWV increases from 2.3 mm to 10 mm. In the 20 micron window the width of the saturated water bands increases as the atmospheric water vapor content increases.
Hence, for practical purposes, the signal-to-noise ratio of a measurement is affected by high PWV through the combined effect of reduced tranparency of the atmosphere and its increased emissivity.
The PWV is highly variable and is one of the main parameters that define the infrared quality of a night. Based on its value, we can make the following rough classification:
A deep statistical study of the PWV at the ORM in the period 2001 - 2008 can be found in García-Lorenzo et al. 2010 . Figure 6 from García-Lorenzo et al. 2010 shows the percentage of nights that have a PWV < 3.0 mm. It can be seen that the worse period of the year regarding PWV is Autumn and that the best period is the end of Winter and beginning of Spring.
Monthly percentage of nights with
PWV <=3 for the period June 2001 - December 2008 (from García-Lorenzo et al. 2010).
The ubiquitous presence of mid-IR background forces instrument builders to locate all the opto-mechanical components and the detector under extreme cryogenic conditions. In the case of CanariCam, the motors that drive the opto-mechanical components work at a temperature of ~70K, while the nominal working temperature of the detector is about 9K.
Fast detector electronics is required to evacuate pixels before they become saturated by the massive arrival of background photons. Contrary to most optical detectors, mid-IR detectors are shutterless. The detector is continuously exposing and every few tens of milliseconds there is a readout and a detector frame is completed. The electronics takes care of co-adding detector frames into a buffer, until all co-added frames are saved into a disk (save-set). Typical frame exposure times for CanariCam are ~25 milliseconds, and co-added frames are saved typically every 10 seconds.
Still, in each single frame, we would normally see only background emission and detector noise, with no trace of the astronomical source at all. To be able to see the source of interest, a technique called "chopping and nodding" is used. Chopping consists of swinging the telescope secondary mirror at a typical frequency of a few Hz between the position of the source (on-source) and the nearby sky (off-source). Since the background seen by the instrument is different when the secondary mirror is oriented on-source and off-source, the telescope axes are actuated (nodded), typically every 30 seconds, to swap on- and off-source positions. The light of the astronomical source can be unveiled by a given combination of summations and subtractions of the chop-nod frames (see more details in the CanariCam User Manual and in the section Observing Strategy.
To illustrate the chopping technique and the importance of the background in the mid-IR, next figure shows (a) one on-source save-set, (b) one off-source save-set and (c) the subtraction of both. These are actual CanariCam data of a mid-IR standard star taken with the Si2-8.7 filter during the commissioning run held on September, 2010. In this case, the observation was done in chop mode (without nodding) with a chop frequency of 3 Hz. The detector frame time was 25 milliseconds, and 44 frames were coadded into each save-set. Therefore, each save-set has an exposure time of 1.1 seconds. In the on- and off-source save-sets there is an average of ~420000 ADU/pix. No trace of the star is seen at all in the on-source save-set. CanariCam has 16-bit analog-digital converters, which means that individual frames saturate at 65000 ADU. Hence, pixel wells were filled in each 25 ms frame up to ~9500 ADU, i.e. 14% of their capacity. The vertical stripes in frames (a) and (b) are the traces of the 16 detector readout channels. Once the off-source frame is subtracted from the on-source frame (c), we see the light from the star, which has a peak of 8000 ADU. Therefore the peak star light amounts only 2% of the light arriving from the background, and this is a rather bright star (of several Jy). Normally, science targets of interest are tens to hundreds of times fainter.
Single CanariCam save-set of a
standard star in chop mode.
Fortunately, when observing with CanariCam the user only has to worry about the total on-source exposure time. CanariCam software will take care of optimising the frame times and save times and of rejecting the appropriate number of frames while the secondary mirror is in motion between chop beams as well as while the telescope is in a nod transition.
Another important factor to bear in mind when preparing an observation with CanariCam@GTC is that at mid-IR wavelengths, the spatial resolution of the delivered images is no longer seeing-limited, but diffraction-limited. In next figure we see that the seeing decreases as ~ λ-0.2 while the diffraction limit of a telescopes grows as ~ λ/D.
Seeing and diffraction limits for
different sky conditions and telescope apertures.
The superior spatial resolution delivered by mid-IR instruments built for large aperture ground-based telescopes makes them very competitive when compared with space-based observatories. In space-based observatories, the sensitivity is higher due to the lack of background emission, but large apertures are technically very challenging. For instance, the typical FWHM of the MIPS@Spitzer PSF at 20 micron is ~5.5 arcsec, while CanariCam@GTC will deliver diffraction-limited images at 20 micron with a FWHM of 0.5 arcsec, i.e. a factor of 10 better than Spitzer.
The following table summarizes some basic parameters of the instrument.
| Spectral Range | 8-25 µm |
| Field of View | 25.6" X 19.2" |
| Plate Scale | 0.08", Nyquist sampled at 8 µm |
| Detector | See table below |
| Image Quality | EER80 < 2 pix (0.16") in imaging mode Diffraction limited at λ > 8 µm |
| Observing modes | |
| Imaging Spectroscopy Coronography Polarimetry |
Filters: (N,Q) and Narrow Band (18) Slit lenght: 19.2", width:0.17-1.04" R= 175,1313 (8-14 µm); 120,891 (16-26 µm) 10 micron window (8-14 µm) 10 micron window (8-14 µm) |
CanariCam's optical train consists of three powered mirrors and six flat mirrors, two of which are inserted into the beam when needed. The relationship of the powered mirrors and the various image planes is indicated in the figure below. The first powered mirror (transfer mirror M1) forms an image of the telescope entrance pupil and reimages the telescope focal plane. The second powered mirror (collimator M2) collimates the beam and forms a second image of the pupil. The third powered mirror (camera M3) images the telescope focal plane onto the detector. A flat mirror or one of several diffraction gratings is inserted near the final pupil image (p2) for imaging or spectroscopy. To improve operational efficiency, an additional flat mirror can be inserted just up-stream from the diffraction grating during spectroscopic observations to permit monitoring of the source location on the slit without the need of moving the grating turret from the grating to the mirror position. These and the remaining flat mirrors have been added to the system to fold the beam and permit packaging the system into a fairly compact layout. Aperture stops and coronagraphic masks are located at the telescope focal plane, rotating and fixed Lyot (pupil) stops and a selection of up to 26 discrete filters on a double filter wheel are located at or very near the first pupil image (p1), and a selection of spectroscopic slits are located at the first re-image of the telescope focal plane (i1). For polarimetry, rotatable half-wave plates can be inserted into the beam just up-stream from the telescope focal plane, and a Wollaston prism can be inserted into the collimated beam near p2. Finally, a lens doublet can be inserted between the first pupil and the first re-image of the telescope focal plane in order to view the pupil, and a lens doublet can be inserted near the telescope focal plane in order to view the outside (external) surface of the entrance window.
Top: Unfolded optical layout of CanariCam indicating relationship of powered components to image and pupil planes. Bottom: 3D-view of the optical layout.
The following table summarizes some basic parameters of the detector.
| Detector | Raytheon CRC-774 320 x 240 Si:As IBC |
| Fiel of View | 25.6"x19.2" |
| Physical Pixel Size | 50 x 50 μm (0.08arcsec) |
| Full-well Capacity | 1.0e7 or 3.0e7 electrons (high or low gain) |
| Array Readnoise | Deep well: 3.6 ADU or 2060 e- Shallow well: 4.6 ADU or 828 e- |
| Dark Current | ≤ 100 electrons/sec @ T = 6 K |
| Dwell depth | 14106 - 41106 e- |
| Quantum Yield | >40% at peak Number of Output Channels : 16 |
Note: At the time of factory acceptance, only two dead pixels were identified in the detector, with coordinates (207,82) and (208,82).
A plot of the quantum efficiency and an image of the detector can be found below.
Quantum Efficiency of CanariCam detector
To date, the Raytheon device remains the largest format mid-IR array in astronomical use. However, this device displays several unwanted effects when exposed to a very bright source. Below we discuss these effects as well as readout methods used to reduce them.
Each contiguous group of 20 vertical columns is read out through one of the 16 output channels (i.e., columns 1-20 are readout through channel 1, columns 21-40 are readout through channel 2, etc.). In addition, the first pixel read out in each channel is the lower-left one and the last is the upper right one.
Note: The most approprate readout mode for each observing condition is to be determined during the on-sky commissioning of the instrument.
In the following figure we show an example of the cross talk features seen in the CanariCam detector in the S1R3 readout mode. The image was taken in the lab with a 3-spot mask in the ArIII filter. The depth of the cross-talk (dark horizontal strips crossing the 2 brightest spots) has a dependency on frame time, but does not depend on the separation from the source. In any case these features are always < 1% of the source peak.
Cross talk features seen in the CanariCam detector in the S1R3 readout mode
Imaging with CanariCam is diffraction-limited at 8 micron over the full 26"x19" field of view. The detector plate scale (0.08 arcsec/pixel) ensures the diffraction-limited Point Spread Function is Nyquist sampled at 8 micron. During factory acceptance testing, image distortion accross the FOV has been proved to be less than 2 pixels center-to-corner.
CanariCam FiltersCanariCam mounts 6 medium-band Silicate filters and 12 narrow, medium and broad band filters that were specified by the VISIR filter consortium. Filters were provided by the University of Reading.
A very slight, but noticeable, difference among some of the filters in the optical beam direction after passage through the filters. This results in a slight shift in the location of an astronomical image as one changes filters.
| ID # | Name | λcentral (µm) | Δλ (µm) | Transmission (plot) |
Transmission (ASCII file) |
| 1 | Si-1 | 7.8 | 1.1 | >60% Si-1 |
Si-1.txt |
| 2 | PAH1(+ArIII) | 8.60 |
0.43 |
>70% PAH1+ArIII_ref1 |
PAH1+ArIII_ref1.txt |
| 3 | Si-2 | 8.7 |
1.1 |
>60% Si-2 |
Si-2.txt |
| 4 | ArIII(+PAH1) | 8.99 |
0.13 |
>70% ArIII+PAH1_ref2 |
ArIII+PAH1_ref2.txt |
| 5 | Si-3 | 9.8 |
1.0 |
>60% Si-3 |
Si-3.txt |
| 6 | Si-4 | 10.3 |
0.9 |
>60% Si-4 |
Si-4.txt |
| 7 | N | 10.36 |
5.2 |
>80% N_band |
N_band.txt |
| 8 | SIV | 10.52 |
0.16 |
>70% SIV |
SIV.txt |
| 9 | PAH2 | 11.30 |
0.60 |
>70% PAH2 |
PAH2.txt |
| 10 | Si-5 | 11.6 |
0.9 |
>60% Si-5 |
Si-5.txt |
| 11 | SiC | 11.75 |
2.5 |
>80% SiC |
SiC.txt |
| 12 | Si-6 | 12.5 |
0.7 |
>60% Si-6 |
Si-6.txt |
| 13 | NeII | 12.81 |
0.2 |
>70% NeII |
NeII.txt |
| 14 | NeII_ref2 | 13.10 |
0.2 |
>70% NeII_ref2 |
NeII_ref2.txt |
| 15 | QH2 | 17.0 |
0.4 |
>60% QH2 |
QH2.txt |
| 16 | Q1 | 17.65 |
0.9 |
>70% Q1 |
Q1.txt |
| 17 | Q4 | 20.5 |
1.0 |
>70% Q4 |
Q4.txt |
| 18 | Q-W | 20.9 |
8.8 |
>60% Q_wide |
Q_wide.txt |
| 19 | Q8 | 24.5 |
0.8 |
>60% Q8 |
Q8.txt |
Two low resolution gratings are provided, one spanning 7.4-13.5 μm and the other spanning 15.7-25.3 μm in a single setting. A high resolution grating is provided in each of these atmospheric windows, with the gratings being adjustable to center on the detector any wavelength within that window. Long Slits (19" length) of 9 different widths located in the slit wheel can be selected depending on the wavelength, resolution and conditions of the observation.
GratingsThe gratings are optimized for use in first order, although use in other orders is permitted. When using the high-resolution gratings, the grating may be positioned to center any wavelength within the operating range on the detector array. Multiple settings of the high-resolution gratings are needed to span the entire 10 or 20 μm atmospheric window. When using either of the low-resolution gratings, a single setting places the entire operational wavelength range for that grating on the detector array.
Values shown in table are predicted, not measured.
| Characteristics | 10 µm LR | 20 µm LR | 10 µm HR | 20 µm HR |
| Central λc (µm) | 10.5 | 20.5 | 10.5 | 20.5 |
| Blaze λ (µm) | 9.87 | 19.96 | 10.08 | 20.18 |
| Blaze Angle (°) | 2.6 | 4.0 | 16.4 | 20.8 |
| Lines/mm | 9.1 | 6.1 | 56.0 | 35.2 |
| ΔλDet (µ,m) | 6 | 9 | 0.98 | 1.54 |
| Angle of incidence (°) | 14.8 | 15.7 | 29.5 | 33.7 |
| Angle diffraction (°) at λcenter | 9.2 | 8.3 | 5.5 | 9.7 |
| R=λ/Δλ | 175 | 175 | 1300 | 890 |
| Smallest Resolvable Δλ (µm) | 0.06 | 0.17 | 0.008 | 0.023 |
Slits with different widths will be available to match the wavelength, conditions and resolution. Typically for each grating there will be 2 slits available corresponding to the resolution and two times the resolution at the maximum wavelength (1 and 2 λ/D)
| Slit Width [arcsec] | Slit Width [pixels] | GTC Diffraction Limited λcentral [micron] |
| 0.17 | 2.08 | 7.0 |
| 0.20 | 2.45 | 8.3 |
| 0.23 | 2.85 | 9.5 |
| 0.26 | 3.25 | 10.7 |
| 0.36 | 4.31 | 14.9 |
| 0.41 | 5.01 | 16.9 |
| 0.45 | 5.70 | 18.6 |
| 0.52 | 6.50 | 21.5 |
| 1.04 | 13.0 | seeing limited |
There are two possible ways of wavelength calibrating CanariCam spectra. One can use a polystyrene film located in one fo the filter wheels, which has very well defined absorption features. Most users, though would prefer to use sky lines to do their wavelength calibration.
The figures below illustrate an example of a low-res spectrum taken with CanariCam at 10 micron with the polystyrene film.
Spectrum of the polystyrene film taken with CanariCam 10 micron low-res grating in the lab and the same film. The wavelength range is 7.3 micron to 13.3 micron, left to right.
The coronograph is designed to suppress the emission in the PSF wings of a bright source so that very faint companions or extended emission (e.g. from a disk) can be detected. It consists of a hard-edged, low-reflectivity mask of 0.83 arcsec in radius and a rotating Lyot stop resembling the pupil of the telescope. Coronography plate scale is the same as that for the imaging mode.
Although the field of view is the full imaging field of view (except for that portion of the field covered by the occulting mask), superior results will be obtained for the central ~10arcsec centered on the occulted point source.
Based on simulations, it is expected that at 1 arcsec from the primary, the intensity of a bright point source in the coronagraphic mode will be at least a factor of 10 lower than it is in normal imaging mode.
The figure below shows one of the Lyot masks used for coronography. The image was taken in the lab as part of the factory acceptance testing of CanariCam The Lyot stop wheel can be rotated to ensure the Lyot mask follows the pupil as the telescope follows a star on the sky.
Lyot masks used for coronography.
CanariCam permits dual-beam polarimetry in the 10 micron window by insertion of half-wave plates, a field mask and a Wollaston prism.
The half-wave plate can be rotated to four position angles (0°,22.5°, 45° and 67.5°), which results in the rotation (relative to the Wollaston prism and detector) of the plane of polarization. The Wollaston prism separates the two planes of polarization (the ordinary and extraordinary rays), which are subsequently imaged on the detector. The focal-plane mask prevents overlap of the orthogonally polarized images for extended sources. The retardance/efficiency versus wavelength, and the laboratory transmission of the combined HWP and Wollaston was measured to be 83% at 10.5 micron.
The field of view is reduced by a factor of ~2 through the inclusion of a polarimetry mask in the aperture wheel. This is required so that the orthogonally polarised light (o and e rays) from the source (sky) do not overlap onto each other.
As there is some chromatic birefringence, the 'slots' of the polarimetry mask in CanariCam are slightly oversized. The useful exposed field of view is 320x25 pixels per slot per polarised beam, corresponding to 25.6"x2". A total of >2.5 slots can be cleanly used, hence providing a field of view of 25.6"x5" in imaging polarimetry mode.
Sckech of how dual-beam imaging polarimetry works. The light any object of the sky, which can or cannot be polarized, passes through the HWP. The HWP polarizes the beam linearly in one direction. Then the polarization mask selects only half of the FOV. Finally the beam is split in ordinary and extraordinary rays (O and E rays).
Image polarimetry of a black body (laboratory ambient temperature) taken with CanariCam. The light has passed through the HWP, polarimetric mask and Wollaston prism. Narrow and very dark stips are due to the mask slots being oversized.
Several types of window material, such as KBr, are highly transmissive at mid-infrared wavelengths and strong enough to serve as a pressure bulkhead, but they have moderate drawbacks when used as windows; e.g., they are easy to scratch and/or hygroscopic, or only transmissive over limited wavelength regions.
To permit the observer to select the appropriate window for a given combination of scientific requirements and observing conditions, three science entrance windows are installed.
The available window materials are:
At 10 micron the ZnSe transmission is 94%, and for KRS-5 the throughput is >66% for all wavelengths >6 micron.
Window tranmissions measured using an infrared FTS spectrograph. KBr and KRS-5 factory transmission curves at 20 microns can be seen in the CanariCam User Manual.
In-depth information of the observing strategy is given in the CanariCam User Manual and also in the Gemini mid-IR resources web page. A summary of the main features is given in this page.
Mid-IR sources normally are over 4 orders of magnitude fainter than the background emission. Time variations in the noise associated with the detector electronics, and telescope and sky background make imperative the use of a quasi-real time subtraction of the background contribution. The technique used to achieve an accurate background subtraction is the combination of chopping the telescope secondary mirror and nodding the telescope (chop-nod). The chop-nod technique ensures the removal of the radiative offset, a background emission that remains after subtraction of chopping on- and off-source images, due to the fact that the background seen by CanariCam detector is not the same in both orientations of the telescope's secondary mirror.
The chopping profile is defined by the chop angle and and chop throw. The chop angle is the orientation of the chop direction on the sky. The chop throw is the angular separation between the on-source and off-source positions on the sky.
General case for the chop profile.
It is possible to chop off the detector or to chop on the detector. In the former case the source will not be present in the off-source image, while in the later case the source will appear in the off-source images. There is not advantage on using one or the other method regarding the final signal-to-noise ration achieved on the target (see details in the CanariCam User Manual. However, chopping on the detector is more recommended for compact targets, because with this technique only 1/4 of the array FOV can be used. Chopping off the detector is normally recommended in the case of extended sources.
Figure below shows the case of chopping off the detector. Different background contributions are represented by different background colors in the detector. The radiative offse is represented by a triangular section on the detector. The radiative offset in the second step (second row of detectors) is approximately the same but with oposite sign. By adding up On(A)-Off(A)+On(B)-Off(B) exposures the radiative offset contribution is removed.
Case of Chopping off the detector.
Figure below shows the case of chopping on the detector with nodding perpendicular to the chopping direction. In the represented case chopping is along the vertical dimension of the detector and nodding along the horizontal one. Only 1/4 of the array FOV can be used.
Case of chopping on the detector with nodding perpendicular to the chopping direction.
Based on the experience acquired during CanariCam on-sky commissioning, the following overheads should be taken into account during proposal preparation.
In all cases, the difference between the on-source and elapsed time is due to:
Besides the overheads associated to each observing mode, the following overheads should be taken into account for every required standard (see Section Calibrations below), assuming an on-source time of 0.5 mininutes:
In the case of spectroscopic observations, wavelength calibration by means of the Polystyrene plate has an extra overhead of 5 minutes.
We recommend the following link to the Gemini web site to search for standards close to a given Right Ascension and Declination:Mid-IR standard star selection tool.
In the following lines, we give some recommendations on the type of calibrations that may be required for each type of program (please refer to Section Overheads for information on the overheads to be taken into account for calibration standards).
CanariCam data is delivered in standard multi-extension FITS (MEF) file format. Each extension contains the 320x240 pixel image, as well as specific headers relevant to those extensions. The zeroth header is a general header, containing greater information about the full data file.
In chop and chop-nod modes, there are 2 chop positions per saveset, and M savesets, meaning that each image is therefore [320,240,2,M]. Almost all science data are taken in chop-nod mode, and each extension contains the savesets for a single nod position. CanariCam can be used in the nod sequences ABAB or ABBA, with the difference being only an insignificant change in overheads. However, it is essential for a minimum number of AB pairs to be observed to permit adequate correction for radiative offset.
The exposure time can be a confusing term when applied to mid-IR observations, there are four exposure time commonly expressed:
It is important to note that only the exposure time is input by the user (either in the ETC or in the Phase-2 form). All other parameters are set automatically.
Detailed information on how to use the software to reduce data for the different CanariCam observing modes can be found in the CanariCam User Manual.
Gemini Observatory staff has made an excellent work by providing observers with thorough information and useful tools to prepare their mid-IR observations. We encourage prospective CanariCam observers to consult the Mid-IR resources page at the Gemini Observatory web site. Two of these tools are:
Such tools can be used during proposal preparation to search for standard stars close to your favourite science target and to have a rough estimate of their fluxes. Note that some of the T-ReCS filters (e.g. Si filter) coincide with CanariCam filters.
The CanariCam Exposure Time Calculater (ETC) has been based on a similar tool for the T-ReCS instrument at GEMINI. However, at the time of writing its adaptation to CanariCam has not been completed yet and therefore the ETC still contains some details related to T-ReCS, and some of the assistance pages do not exist. Moreover, the ETC runs on a server at the University of Florida. In spite of these shortcomings, the key output provided by the ETC is reasonably accurate as far as we have been able to verify from commissioning data. We are working on correcting this situation, but in the meantime, in case of any doubt you can contact the CanariCam contact person at GRANTECAN, or your contact support astronomer. The CanariCam ETC is available here (external link).
This web page and the CanariCam User Manual makes extensive use of the detailed information provided by the Gemini mid-IR team. We are deeply grateful to the Gemini Observatory staff for sharing with us their work and for allowing us to publish part of their mid-IR resources in our web site. The key people associated with those activities are Drs. Rachel Mason, Kevin Volk, and Scott Fisher. We also use sections of the Starlink POLPACK software, written and documented by Berry & Gledhill. Much of the IRAF script work for the software was written by, or under the direction of, Kathleen Labrie, also at Gemini.
We have made use of the atmospheric transmission models generated using the ATRAN modelling software (Lord, S.D. 1992, NASA Technical Memor. 103957) and published in the Gemini Observatory web site.
