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Mid-IR Observing Strategies
On this page we describe the principal observing and calibration issues that users of Gemini's mid-IR instruments should consider when planning their programs and writing their proposals. We recommend that proposers weigh the importance of each issue to their program's scientific goals and formulate their observing plans accordingly. For Michelle and T-ReCS, the principal concerns are the relatively small field of view, the need to chop and nod, and the restricted chopping amplitude. A secondary concern is astrometric accuracy.
Phase I proposals should briefly outline the observing plan and necessary special calibrations in order to justify the program's feasibility and the observing time request - remember that time must be included for any calibration observations not included in the baseline calibrations. Phase II programs should contain complete details on the observing sequences, including all calibrations.
See guidelines about:
- General observing issues
- Photometric Calibration
- PSF Calibration
A high-quality observation may also require supporting data for photometric calibration, point-spread function characterization, and so forth. Ideally, every type of calibration would be available for each science observation. This is difficult in practice because of the significant telescope time required to take all possible data, but thankfully only certain calibrations are usually needed to obtain a specific scientific result. For example, while a program to measure the spectral energy distribution of bright circumstellar disk star may require careful photometric calibrations and airmass corrections to obtain high accuracy, a program to image an extremely faint galaxy with expected S/N < 10 would probably not benefit from such an accurate calibration: most of the available telescope time is best spent integrating on the science target. In order to maximise the scientific return from limited time allocations, observing programs should contain only those calibrations that are needed to achieve the desired science.
For each issue, we briefly describe two cases for which the priority of the issue is "Low" and "High". Assigning a "Low" priority to a certain issue typically means that no special procedures are required or that baseline calibrations are acceptable. A "High" priority indicates that the issue needs careful attention and perhaps special observations. (Of course, the issues will likely have intermediate levels of priority for many programs.) Note that these "Low" and "High" priority labels are not reflected in the OT but are guidelines for creating either Phase I or Phase II files. One cannot simply ask for "High" priority astrometry, for example, and leave the details to the astronomer doing the observations at the telescope. If one wants more attention paid to a particular aspect of the calibrations one has to set up the associated observations in the phase II file and allow time for this in one's program.
It is also useful to provide details in NOTES in any phase II file to guide the astronomer who will be carrying out the queue observations. In general more detail of what is wanted and how the observations should be carried out is better. Another point to remember is that in queue observing a series of observations will not necessarily all be carried out on the same night or in any particular order, unless this was specifically requested in the original proposal. Particularly for band 3 programs it is very unlikely that all targets will be observed at optimum airmass or that all the observations of a given target will be done at once unless the observations are quite brief. We recommend that observers keep these factors in mind when creating a phase II file.
In addition to the issues discussed below, PIs need to consider the weather conditions required for the observations - see this page for some guidelines.
Following the discussion of individual issues, we present observing strategies for several example science programs. For each program we rank the priority of each observing issue and discuss a specific strategy to make the best use of telescope time to meet the important science goals. We feel that many programs can be developed based on these examples; if your program does not seem to fit any of the examples, you can always contact Gemini staff for advice.
The relative pointing requirements: centering of target in the instrument's field of view, dithering, etc.
Low: The target fits comfortably within the ~20 x 30 arcsec field of view. Example: A single point source, or a small nebula or galaxy. The pointing and placement of the target on the detector are not critical, so default pointings and chop/nod amplitudes are acceptable and mosaicking is not required.
High: The target size is comparable to or larger than the field of view, requiring more carefully planned pointing, dithering, and/or mosaicking patterns. If the target is to be put into the slit it will be imaged first and the telescope pointing will be adjusted to move it to the position corresponding to the slit location. Note that slit imaging is not possible with Michelle (the acquisition procedure relies on precise and repeatable positioning of optical elements within the instrument).
The nature of the source and the pointing requirements should be briefly described in the Phase I proposal in order to justify the observing time request. The precise positions and timings of required pointings should be defined in Phase II. For sources with high proper motions it is important to enter the proper motion in the phase II file, so that we can find the target object.
The default pointing accuracy is about 1 to 2 arcsec. Better accuracy can be achieved by offsetting or slewing from a nearby astrometric standard sufficiently bright to detect in a short mid-IR image (see the mid-IR astrometry page). In this case the PI must specify the astrometric standard and an optical reference star at phase II.
Low: The default astrometric precision is satisfactory. Example: Observation of a single point-source with well-determined coordinates.
High: Astrometric precision to better than the PSF width is required. Example: Imaging of a complex galactic nucleus for the purpose of identifying infrared counterparts to radio sources.
If special astrometric calibrations are required, the desired accuracy should be clearly stated and discussed in the Phase I proposal in order to demonstrate the feasibility of the program.
Chopping and Nodding:
The settings of the chop and nod amplitude and direction. Note that at present chop throws are restricted to 15" or less. It is not yet known whether larger chop throws are safe (i.e., are not likely to damage the chopping mechanisms or the secondary mirror).
Low: The target is sufficiently isolated that the default chopping settings, 15" amplitude at PA = 0 (N-S) are satisfactory. Note that for beam-switched chopping and nodding (i.e, nodding a distance of one chop throw in the direction of the chop, the sky on each side of the target should be free of known sources of emission. Note that when using a small chop amplitude in this mode, the positive and two negative images of the target will be on the array; however, as guiding is only available in one of the chop beams (see this discussion), the image quality of the negative images will usually be significantly degraded.
High: The target is in a crowded field, requiring careful selection of chop and nod amplitudes and angles. If the number of neighboring sources is small, careful selection of the chop angle may be all that's required (chopping at any position angle is straightforward). In extreme cases, Gemini could be consulted to determine possible strategies.
A characteristic of the Raytheon SBRC arrays used in Michelle and T-ReCS (and the Subaru mid-IR instrument, COMICS) are level drop phenomena (a.k.a the "hammer effect") where the channel of the detector where the target is located has a depressed response for the part of the detector that is read-out after a bright object (see Sako et al. 2003, PASP 115 1407 [ADS abstract] for a detailed description and examples). On the difference images from chopping this shows up as a negative streak above the target on the image, or as a depressed response in every channel of the array at the same row as the bright source. In the other chop position the negative image of the target may also cause a similar effect. If the chopping for such a target is chosen such that the negative images are along a detector row or column from the positive image in the difference images, the result will be two or three interacting streaks. An example of the effect is shown here for an 8 Jy (at 12 microns) Cohen standard imaged with Michelle in the semi-broad N' filter.
By chopping at an angle, rather than along the rows or the columns the streak from the negative beam will not interact with that from the positive beam. This is important when searching for low-level structure around a bright target. In addition if it is known that there is low-level structure in the N-S or E-W directions in a target, it would be useful to rotate the array so that this emission will be oriented at roughly 45 degrees and be out of the region of the streaks as much as possible. Alternatively, the chop angle can be set to be along the short axis of the array and with an amplitude of 15 arcseconds. Then, as long as the object is smaller than 3 arcseconds in radius, the negative images will be off the array.
The hammer effect is always present at a very small fraction of the peak flux (~0.1% in the four-point sampling mode used with T-ReCS). If the noise level is below that value, then the effect becomes visible. The exact ratio varies with the frame rate, flux incident on the detector, etc., so it is not easy to give a specific value for the flux density at which the hammer effect is seen. However, for sources brighter than 10 Jy or so, and/or when accurate knowledge of low-level PSF structure is important to the goals of the programme, the chopping should be set up as described above to minimise the impact of the hammer effect. The threshold is higher for the Qa filter and in polarimetry mode (where the reduced throughput of the waveplates diminishes the flux reaching the detector).
The basic observing strategy for targets in crowded fields should be noted in the Phase I proposal, in order to justify the feasibility of the proposed observations. The precise chop and nod settings for each target should be defined in Phase II.
The required precision of the photometric calibration.
Low: The science goal does not require photometric calibration precision better than ~10%. Example: preliminary mapping of a field to identify new sources. In this case the baseline calibrations provide sufficient accuracy.
High: The science goal requires photometric calibration better than ~10%. Example: Multi-wavelength imaging of a T-Tauri star in order to measure the spectral energy distribution precisely and search for weak silicate emission or absorption. In this case the baseline calibration is insufficient and special standard star and airmass correction measurements should be requested.
The basic photometric calibration requirements should be described in the Phase I proposal and extra time requested for any standards beyond the baseline calibrations. The precise timings of calibrations should be defined in Phase II.
PSF calibration considerations for mid-IR data are somewhat different than at shorter wavelengths because in many atmospheric conditions the image quality will be close to diffraction-limited. Therefore, the resultant image quality is less sensitive to seeing and more sensitive to factors such as the optical quality of the telescope and the performance of the chopping and tip-tilt fast guiding.
Low: The science goal does not require detailed knowledge of the point-spread function. Examples: preliminary mapping of a new field; imaging of a point source solely for photometric measurements. In this case, the baseline photometric calibration data provide an approximate characterization of the point-spread function if needed. Due to variations in the PSF caused by changes in telescope image quality, guiding performance, seeing and wind speed variations, and other factors, the baseline calibration and science target PSF's can be significantly different. Our experience indicates that the PSF is often reasonably stable and so the baseline photometric calibration actually also provides a fairly good PSF.
High: The science goal requires highly accurate knowledge of the PSF. Example: Imaging of a T-Tauri star to detect faint extended emission from a disk, for which the PSF must be accurately measured to permit subtraction of the central source and/or deconvolution of the data. In this case, PSF-reference stars close to the target should be selected and periodic PSF measurements planned. Ideally, the science target and PSF-reference guide star magnitudes should be similar to ensure consistent guiding performance.
For targets of intermediate brightness, the PSF star(s) should be chosen to also be of intermediate brightness (of order 10 Jy at N-band, somewhat brighter for Q-band) so that a good PSF is obtained in a short time. Very bright sources cause some low-level streaking, the "hammer effect" along the rows and columns which distorts the PSF at low levels.
Any special PSF calibration requirements should be described in the Phase I proposal in order to justify the proposal's feasibility and the observing time request, and extra time added for these calibrations as appropriate. The precise sequence of science target and PSF-reference star observations should be defined in Phase II.
Imaging flat-fields can in principle be derived from observations of the sky at different airmasses. However we have been unable to find a satisfactory method of creating imaging flats that does not also significantly increase the noise level in the images. As the detector response appears to be intrinsically fairly flat over the field of view we therefore do not recommend that PIs attempt flat-fielding of Michelle and T-ReCS images.
For spectroscopy with Michelle, flat field frames are taken by looking at a surface inside Michelle's calibration unit (previously, the primary mirror instrument cover was used). These have been found to be quite stable and reproducible for low resolution spectroscopy, but the fringing appears to vary somewhat with time in the higher resolution modes. Flat and bias observations are taken for each science observation. Flats and bias frames are not taken for spectroscopy with T-ReCS.
Example Science Programs
- Example : Initial "reconnaissance" imaging of small targets.
- Pointing: Low
- Astrometry: Low
- Chop/Nod: Low
- Photometry: Low
- PSF : Low
The Simple Imaging strategy is suitable for quick snapshots of a large number of targets, or of a few targets in many filters, when the science goal is to determine approximate source positions, fluxes, and morphologies. In this case the photometric and PSF calibrations can be derived from the baseline calibrations and approximate astrometry obtained by registering the images with radio or NIR data.
- Example : Initial "reconnaissance" imaging of large targets.
- Pointing: High
- Astrometry: Low
- Chop/Nod: Low
- Photometry: Low
- PSF : Low
This is similar to Simple Imaging, except that the field is larger than the detector. In this case the position of each sub-field must be specified, with due consideration of the desired overlap. The chopping limitation of 15" must also be considered; it is generally difficult to make mosaics of large objects because one cannot chop entirely off-source.
Deep Imaging of faint targets
- Example: A two-hour integration on a faint extragalactic supernova in order to measure the N-band flux with minimal contribution from surrounding, unrelated emission.
- Pointing: High
- Astrometry: High
- Chop/Nod: Low
- Photometry: High
- PSF : Low
Faint targets require long integration times with minimal interruptions. If better than baseline photometric calibration is required, additional photometric standard observations should be requested. In addition, the astrometry can be well determined by using a pointing offset ("user 1") star.
High angular resolution imaging
- Example : Imaging of T-Tauri star to detect extended disk emission.
- Pointing: Low
- Astrometry: Low
- Chop/Nod: Low
- Photometry: Low
- PSF : High
In this case the photometric accuracy and astrometry requirements are low because the star's total flux and position are already well known. The PSF characterization is critical because the goal is to separate faint disk emission from the bright, point-like stellar photosphere. The optimal integration times and number of cycles are dependent on the system stability and target brightness. One should go no longer than about 30 minutes between PSF observations if the highest accuracy is required (no longer than 2 hours in the Q band).
Note that the best PSF accuracy entails frequent calibration images which are charged to the program since they are in excess of standard baseline calibrations. These overheads must be accounted for in the original time request.
Last update 2007 May 22; Jim De Buizer