Canaricam

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.

General Information:

• Mid-IR Observing
• Instrument Features
• Instrument Detector
• Observing Modes
• Imaging
• Filters
• Spectroscopy
• Gratings
• Slits
• Wavelength calibration
• Coronography
• Polarimetry
• Overheads, Data Structure and Entrance Windows
• Observing Strategy
• Calibrations
• Data Reduction
• Exposure Time Calculator (external link)
• Commissioning Data (new window)
• Guaranteed Time - Reserved Targets (new window)
• Support Astronomers at GTC
• Useful Documents
• More Information (external links)
• Acknowledgements

Mid-IR Observing

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:

• The GTC secondary mirror (telescope pupil) is undersized, i.e. a small fraction of the primary mirror is vignetted so that no thermal emission from structures outside the primary mirror enter CanariCam.
• M2 also has retractable baffles that cover sky holes (e.g. the Cassegrain hole) when we observe with optical instruments, but leave them open to the sky when we observe in the mid-IR.
• CanariCam has Lyot masks that follow the shape of the GTC pupil. These prevent the thermal emission from the secondary mirror support spiders from reaching the detector.
• M1 is designed so that the physical separation between segments is kept to a minimum of 3 mm.
• GTC mirrors are cleaned regularly with CO2 snow to avoid accumulation of dust, which may be particularly important during the Summer, when the frequency of Saharian dust storms increases.
• GTC mirrors are re-aluminised when the aluminium coating starts to degrade, which ensures maximum reflectivity (minimum emissivity).

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:

• Ozone, with an important absorption feature at 9.6 µm.
• CO2, whose strong absorption at ~ 15 µm defines the separation between the 10 and 20 µm observing windows.
• Water vapour, which shows several bands in the 10 µm window and strongly dominates the 20 µm window.

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:

• PWV < 1.5 mm -- Good to excellent conditions to observe both at 10 and 20 µm.
• 1.5 < PWV < 3.0 mm -- Good conditions to observe at 10 µm and average to poor conditions at 20 µm.
• 3.0 < PWV < 5.0 mm -- Poor conditions to observe at 10 µm and very bad at 20 µm.
• PWV > 5.0 mm -- Very bad conditions both at 10 and 20 µm.

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.