T-ReCS Observing Strategies

Introduction

In addition to computing required integration times, a proposal writer should consider other details of an observation related to the telescope and instrument. For 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.

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 high-redshift 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 use telescope time most efficiently, observing programs should contain only those calibrations that are needed to achieve the desired science.

On this page we describe the principal observing and calibration issues that users of T-ReCS 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. 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. Phase II programs should contain complete details on the observing sequences, including any special calibrations.

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.) We often give a specific example, and briefly discuss what information on each issue should be included in the Phase I and Phase II proposals.

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" quality 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 setup the associated observations in the phase II file and allow time for this in ones 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 it is unlikely that a long series of observations will be carried out in any particular order, unless one has a band 1 program. 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. One needs to keep these possibilities in mind when creating a phase II file.

Following the 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 one of the examples, then perhaps Gemini staff should be contacted for advice.

Observing Issues:

• Pointing:
  
  The relative pointing requirements: centering of target in the T-ReCS field of view, dithering, etc.
  
  
  • Low: The target fits comfortably within the ~20x25 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 dither patterns 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.
  
  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 PM in the phase II file, so that we can find the target object.
  
  
• Astrometry:
  
  The precision of the absolute astrometry as indicated in the WCS included in each FITS header. We expect the default pointing accuracy to be 1 to 2 arcsec. Better accuracy can be achieved by using a USER 1 star (see T-ReCS Astrometry).
  
  
  • Low: The default astrometric precision is satisfactory. Example: Observation of a single point-source with well-determined coordinates.
  • High: Astrometric precision to a fraction of the PSF-width is required. Example: Imaging of 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 chopper throws are restricted to 15" or less. 
  
  
  • 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 chopper throw in the direction of the chop, the sky on each side of the target should be free of known sources of emission. For nodding perpendicular to the chopper throw [note: this is not yet well commissioned] one only needs to be sure that the sky on one side of the target be free of known sources of emission. Note that when using a small chop in this mode, all four images of the target will be on the array. This increases the s/n, in principle by as much as sqrt(2); however, as guiding is only available in one of the chopper beams, the image quality of the other beam will usually be degraded, resulting in a lower s/n measurement in that beam and not as large an improvement in s/n.

    For brighter sources (more than 10 Jy) it is better to chop at 45 degrees because a bright source causes low-level "streaks" along the rows and columns. By chopping at an angle rather than along the rows or the columns the "streaks" from the negative beam will not interact with those from the positive beam. This is particularly important when searching for low-level structure around a bright target. In addition if one knows 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.

  • 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.
  
  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.
  
  
• Photometric Calibration:
  
  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 calibration provides 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, including names of special standard stars and integration times, if any, should be described in the Phase I proposal in order to justify the observing time request. The precise timings of calibrations should be defined in Phase II.
  
  
• PSF Calibration:
  
  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 may provide an approximate characterization of the point-spread-function. 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 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 or stars 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" along the rows and columns which distorts the PSF at low levels.

  
  Any special PSF calibration requirements, including names of PSF-reference stars and integration times, should be described in the Phase I proposal in order to justify the proposal's feasibility and the observing time request. The precise sequence of science target and PSF-reference star observations should be defined in Phase II.
  
  
• Flat Fielding:
  
  NOTE : Currently flat fielding techniques are still in the development and test phase for T-ReCS. The detector response appears to be intrinsicly fairly flat over the field of view, and such attempts as we have made to apply flat field frames have increased the noise without improving the image flatness significantly. For the moment we are NOT taking flat field observations for T-ReCS.

Example Observing Strategies

1. Simple Imaging
   • 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, the flat fields if needed can be taken from a library, and astrometry can be derived from the approximate (1") WCS in the FITS header and by registering the images with radio or NIR data.
   
   
2. Mosaics
   • 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 T-ReCS detector. In this case the position of each sub-field must be specified, with due consideration of the desired overlap. The chopping limitation of about 15" must also be considered; it is generally difficult to make mosaics of large objects because one cannot chop entirely off-source.
   
   
3. Deep Imaging of faint targets
   • Example: A two-hour integration on a high-redshift galaxy in order to measure the N-band flux.
   • Pointing: High
   • Astrometry: High
   • Chop/Nod: Low
   • Photometry: High
   • PSF : Low
   Faint targets require long integration times with minimal interruptions. If the final S/N is < 50 accurate flat fields are not a priority. If better than baseline calibration is required, special calibrations should be requested. In addition, the astrometry can be well determined by using a USER 1 star.
   
   
4. 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.

   Note that the best PSF accuracy requires rather large overheads which are charged to the program since they are in excess of standard baseline calibrations.

Last update 2006 March 13; James Radomski