You are in: Instruments > MIR > Ground-Based MIR Observing

Introduction to Mid-IR Observing from the Ground

Compared to observing from space platforms such as Spitzer, observing at mid-IR wavelengths from large ground-based telescopes on good sites brings two principal benefits. First the image quality from such sites is usually diffraction limited (thus scaling with the telescope diameter) and therefore on Gemini, for example, is higher by more than an order of magnitude than on mid-IR space telescopes such as Spitzer. Second, ground-based telescopes, and Gemini North in particular, offer the possibility of observing at higher spectral resolution than do mid-IR instruments on current space telescopes. However, because of the bright thermal background, ground-based mid-IR observations are less sensitive than those from cooled space-borne telescopes, and in fact pose great challenges. This page outlines the principal techniques employed to alleviate the problems that arise in carrying out ground-based mid-IR imaging/photometry and spectroscopy. This presentation, given by Pat Roche of Oxford University at the 2005 UK Gemini Support Group mid-IR workshop, also contains useful background information on mid-IR observing.

Dealing with the thermal background: chopping and nodding

All Michelle, T-ReCS and TEXES observations are made in the thermal infrared, where the atmosphere and telescope are bright sources of radiation. Performing mid-IR observations from the ground is analogous to making optical observations during daytime on a telescope with luminous (although also reflecting) surfaces. The sky background in the best parts of the N band window (near 11.5 microns) is roughly 0 mag per square arcsec and it is brighter still in the Q band (17 to 25 microns).

The sky brightness changes on short timescales and over small distances on the sky, and often these changes are much larger than those due to Poisson statistics (i.e., >> sqrt(N), where N is the background in photons/sec). To alleviate the effects of this when performing imaging and low-resolution spectroscopy, a nearby position on the sky is observed frequently by moving the secondary mirror at a frequency of a few Hz (the telescope is said to chop between the target position and an adjacent sky position), and the pairs of images are subtracted. Moving the secondary mirror results in the telescope being seen by the detector in slightly different ways in the two secondary mirror positions. Since the telescope glows strongly at mid-IR wavelengths the subtraction leaves a residual radiative offset, which is much less than the telescope and sky brightnesses but is still significant. To get rid of (most of) this offset the entire telescope is moved or nodded typically about twice per minute. Normally the nod is set to be the same amplitude and direction as the chop, so that the science target switches chop positions between the two nod positions. Thus a pair of subtracted chop/nod observations produces a positive image of the target in the center which corresponds to half of the exposure time, and two displaced negative images of the target, each of which corresponds to 1/4 of the exposure time. If the chop and nod are large enough the negative images are off of the array and are not seen. Currently the maximum chop throw at Gemini is 15 arcsec.

The level of background radiation sets the frequency at which the detector is read out to avoid saturation (typically about every 20 milli-seconds) and the frequency at which chopping is needed (typically around 3 Hz); chopping needs to be faster when using filters that are more sensitive to atmospheric changes (see below). The result of the chop/nod procedure is to remove most of the background and leave the residual astronomical signal. There is a more detailed discussion of the technique at this link.

In stable observing conditions, the background can be removed remarkably well. If, however, the atmosphere is changing over the course of a few minutes (e.g., when there are clouds) then the cancellation of a chop/nod cycle will be poor. This usually results in an offset in the sky background away from zero, which can be either positive or negative depending on how the sky is varying. The Gemini IRAF "midir" package provides tools to identify and reject frames with high residual background that would otherwise compromise "good" observations in the same sequence.

Currently Gemini only uses one of its peripheral wavefront sensors for tip/tilt guiding. Because the sensor has a very small field of view, autoguiding is only active at one of the chop positions. The image quality in the unguided position is almost always much worse than in the guided position and the unguided images should generally not be considered useful for science. When the nod distance is equal to the chop throw, half of the measurement time is spent on the single guided image and 1/4 of the time on each of the two unguided images. Compared to an ideal chop system with autoguiding at both chop positions, Gemini measurements are lower S/N by a factor of sqrt(3/2).

In most chop systems there is an additional factor of ~1.5 loss in S/N from the ideal due to overheads taking up roughly the same amount of time as the exposures. The overheads are associated with motion and settling of the secondary mirror, reacquiring guiding after each chop cycle, and reading out the array. See each instrument's Performance and Use page for more details.

Overall, compared to staring and nodding (as for typical near-IR observations) the loss in S/N due to the production of three images, single beam guiding, and overheads is more than a factor of two. We chop only because we have to.

Stare/nod medium- and high-resolution spectroscopy:

In spectroscopic mode the background per pixel on the detector is much lower, both because the light is being dispersed and because the slit limits the field of view. One often can chop and nod more slowly than for imaging observations, but tests have shown that the same chop/nod method used for imaging must be used for low resolution spectroscopy. At the higher spectral resolutions available with Michelle (but not T-ReCS) sky fluctuations are low enough that stare/nod, with its inherently higher efficiency, may be used to advantage. At these resolutions and for short exposures Michelle is read-noise limited at wavelengths where the earth's atmosphere is transparent, and there is a considerable advantage in taking as long exposures as possible at those wavelengths. However, if portions of the spectral intervals to be observed contain telluric absorption lines, the sky emission in those lines may saturate the array if the exposure is too long, resulting in no useful information close to the line wavelengths (while improved S/N is obtained at other wavelengths in the intervals). Those who wish to propose to use Michelle at the higher resolutions are encouraged to get in touch with Gemini staff to discuss this issue.

Choice of filters

As is evident from plots of the atmospheric transmission at 7-25 µm , the transmissions of the different Michelle and T-ReCS filters are affected differently by water vapor, ozone, methane, CO2, and other atmospheric gases. Some of the filters are located in very clean portions of the transmission spectrum and others are not. While some science programs require the use of many filters, others need only one or two and are interested mainly in sensitivity. The transmissions of some filters vary strongly with airmass, while others do so only slightly.

We have put together a table of transmittances and relative signal-to-noise ratios for many of the Michelle and T-ReCS filters, as functions of airmass and water column (as given by ATRAN). The following general conclusions can be drawn.

The cleanest filters in the 10um window are Si-2, Si-5 and N'. Si-2 is the most sensitive filter for detecting sources whose mid-IR spectral energy distributions fall off rapidly with increasing wavelength (e.g., stellar photospheres). For objects with flatter spectra and for highly reddened objects, N' is preferred as it has nearly identical transmittance to Si-5 but has more than twice the bandwidth.
By far the cleanest 20um filter is Qa, although it doesn't compare to any of the better 10um filters.
Filters whose transmittances are highly dependent on water vapor are Si-1 and all Q filters. Even when the water column is low their transmittances are not high, even at the zenith. Thus, in general measurements using these filters should be made close to the meridian (i.e., near the lowest airmass) whenever possible and they should not be made on wet nights.
The Si-3 and Si-4 filters are affected strongly by ozone, but only slightly by water vapor. Measurements with them should be made close to the meridian whenever possible.

Mid-IR seeing

The seeing at N and Q is usually much more stable than that at shorter wavelengths, being commonly around 0.4 arcseconds at N and 0.6 arcseconds at Q (the diffraction limits are ~0.3 arcsec at N and ~0.6 arcsec at Q). In conditions of good seeing one or two (and sometimes even three) diffraction rings around a point source may be seen. Typical strehls for good conditions are 0.6 for the narrowband filters in the N-band window and 0.9 for the Qa filter at longer wavelengths. Only occasionally is the seeing much worse than this at N-band or Q-band, although seeing of 1 arcsecond or more is not unknown in the N-band. When this happens the seeing tends to be extremely poor at shorter wavelengths.

Last update 24 Aug, 2006; T. Geballe and R. Mason, based on previous work by K. Volk