Change page style: 

NIFS Observing Strategies

This page brings together information that might affect decisions on observing strategy in various NIFS observing modes, and provides guidelines/tips to maximize observing efficiency and avoid some common errors. The other NIFS web pages also give information about what to expect from the instrument. For information about actually setting up observations in the Observing Tool, please see the Observation Preparation section of these pages.

Here we describe briefly several factors that users may wish to take into account when designing their observing sequences:

Guiding

The type of guiding configuration you choose for your program has a significant impact on the quality of the data you will obtain. To assess the best strategy for your program you ought to have an estimate of: 1. the spatial profile for your source, 2. its brightness, 3. its proximity to bright point-sources.

With these details in hand please read: adaptive optics at GEMINI and guiding options with NIFS. To get (1) a feel for the possible corrections as a function of observed wavelength, and (2) a rough estimate of the relative importance of guiding star brightness and its proximity to your science target, you may find useful: those theoretical estimates of Strehl corrections obtained with Natural Guide Star (NGS) adaptive optics system

The Altair (AO) field of view is not a concentric circle around the NIFS field - it is a little off-centre and egg-shaped. If your guide star is close to the edge of the Altair field of view this may cause slight obscuration of the guide star, which can affect guiding. To gave some additional margin of uncertainty, you may wish to change the position angle to put the guide star a little more inside the accessible field for Altair.

  • If your source is large and extended, you might wish to set the acquisition for this source to use a blind offset. It is important that the star from which you can do the blind-offset is in the same guiding configuration as your science target.
  • If you choose SFO open loop guiding:

    • Please read this first: SFO open loop guiding .
    • If your target is extended it needs to be brighter than 16.5 mag in R. If it is a point source you can use this method for a source as faint as 18.5 mag. However, in that case you should have a back up guiding strategy, e.g. LGS + P1 or natural seeing.
    • In general SFO re-tuning takes less time than an acquisition but it depends of the magnitude of the SFO star and the magnitude of the galaxy nucleus. To minimize overheads plan two hour observation blocks with a SFO re-tuning in the middle. This works best for SFO star brighter than 12 mag and nucleus brighter than ~15.5 mag. Better corrections may be obtained by re-tuning every ~ 45 minutes.

    Read Modes and Use of Multiple Coadds

    In the OT the read mode is automatically set based on the length of the individual exposures. The OT will not dynamically change the read mode, but the template observations will have the read mode (and exposure time) set based on the source brightness. In particular:

  • IF K<=9 THEN Read Mode = Bright Object, Exposure Time=10
  • IF 9 < K<=13 THEN Read Mode = Medium Object, Exposure Time=80
  • IF K > 13 THEN Read Mode = Faint Object, Exposure Time=600

  • NIFS supports coadding which involves taking multiple exposures and writing the average to a single FITS file. The benefit is avoiding the overhead associated with writing multiple FITS files (8 seconds per file). The downside is that cosmic rays from each exposure will be inextricably merged, and of course, coadded exposures may not be dithered. Coadding is therefore most useful for programs which must take many short exposures to detect faint features without saturating bright objects.

    Non standard central wavelengths

    We have noticed that the grating is somewhat less reliable when using non-default central wavelengths. This is a feature of the mechanism control. For this reason, if the nominal wavelength range does suffice, it is recommended that you use it.

    Observing Conditions

    Dry observing conditions are typically only required when measuring spectral features in the wings of the atmospheric transmission windows. Please refer to this page for relevant plots at different wavelengths.

    Fainter guide stars may require gray (SB80) or even dark (SB50) sky brightness conditions. Note, however, that constraining the sky brightness will reduce the probability of completing such observations as gray/dark time occurs less frequently and is in high demand because of competing optical programs which are sensitive to sky brightness.

    Telluric

    Given the strength and variability of sky lines in the IR it is important that telluric standards are observed at most every 2 hours, and preferably after each 1.5 hours of observations. In most cases, the time spent on tellurics are not charged to the program but to the partner country. However if more than one telluric is required for each 2 hours of observing time, those additional tellurics are charged to the program.

    This link will help you find the right telluric given the coordinates of your source and the duration of the on-target observation.

    Please check if your telluric is a double or if it may lead to saturation. NIFS's response becomes non-linear (~6%) at 38,000 and saturated at 48,000 ADU. For non-AO (P2) observations, the integrated flux is fairly similar to the AO integrated flux, but the peak flux on the detector at the peak of the PSF is easily a factor 10 lower. So with AO, the bright limit for NIFS is around K=5.
    For P2, it is more like K=3. There are neutral density filters which can be used to bring down the flux if needed. They are not used very often, and their spectral signature is not well caracterised, so additional calibrations may be needed for those cases.
    For P2 tellurics, it is recommended that you slightly increase the exposure time to compensate for the lower peak flux, but not longer than 30s. Going longer than 30s on tellurics (either AO or P2) is not efficient, and usually not needed - you can usually find a brighter hot star, perhaps by going to A1-2 type instead of A0.

    Please use the integration time calculator (ITC) or the table below page to estimate the required exposure times for tellurics.

    Estimated Exposure Times for Tellurics
    Exposure Time per Frame (sec) Observing Conditions
    IQ85/CC70 IQ85/CC50 IQ70/CC70 IQ70/CC50 IQ20/CC70 IQ20/CC50
    5.3 K = 6.5 - 5 mag K = 7.5 - 5 mag K = 8 - 5 mag K = 8 - 5 mag K = 8.5 - 5.5 mag K = 8.5 - 5.5 mag
    10 K = 7.5 - 6 mag K = 8.5 - 7 mag K = 8.5 - 7 mag K = 9.0 - 7 mag K = 9.5 - 7.5 mag K = 9.5 - 7.5 mag
    15 K = 8 - 6.5 mag K = 9 - 7 mag K = 9 - 7 mag K = 10 - 7.5 mag K = 10 - 7.5 mag K = 10 - 7.5 mag
    20 K = 8.5 - 7 mag K = 9 - 7 mag K = 9.5 - 8 mag K = 10 - 8.5 mag K = 10 - 8.5 mag K = 10.5 - 8.5 mag

    Maximum Exposure Times and Saturation

    The maximum exposure time you should use is set by several constraints:

    (1) Saturation on the sky
    (2) Saturation on the object
    (3) Sky variations during a dither sequence

  • Individual exposures longer than 900 seconds are not recommended due to sky variations on those time scales.

  • Offsetting (Nodding, Dithering)

    Small dithers e.g. 0.3" are encouraged to account for variations in spaxel sensitivity.

    Infrared observations generally require background subtraction. Here are some guidelines:

    • For objects that fill the NIFS FOV, off-source sky observations are required. Depending on your guiding configuration and the size of your source you may have to set the guiding to "freeze" at sky position outside the guiding range of your guiding configuration.
    • Generally it is recommended to spend the same amount of time on object and on sky for best sky subtraction, and to have each science frame adjacent to a sky frame.
    • Cross-talk when observing bright sources H < 8 with fainter structure (e.g. observations of young stellar jets):
      • "Cross-talk" is a smearing of flux along the IFU image slice that has the bright stellar PSF. This shows up as a brighter line of flux in the +/- y direction in the NIFS data cubes. Relatively speaking this is a small effect. However, it can decrease the S/N on faint emission if that emission happens to be aligned with this effect. The recommendation for such cases is that you rotate the NIFS field so that the jets are perpendicular and don't align with this flux smearing effect.

    Understanding your PSF

    • There are several methods to estimate your PSF, several of those have been employed in refereed publications. Here we suggest the observational PSF that will facilitate the ideal PSF determination
      • Distance between guide and PSF star should be the same as the distance between the guide and your Science target +/- 3"
      • Magnitude of guide for the PSF observations should be the same as the magnitude of the guide for your Science +/- 0.5
      • Position Angle PSF-Guide to PSF should be the same as position angle of Science target-guide to Science target +/- 20degrees.
      • The declination of your PSF should be the same as that of the Science target +/- 5 degrees.
      • You strive to achieve the same SNR for your PSF observation as you will for your Science star.

    Last Modified: A. Petric 2015July08