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NIRI Observing Strategies and Guidelines

This page attempts to bring together information that might affect decisions on observing strategy in various NIRI observing modes. It is not meant to be definitive or complete, and will evolve to reflect the prevalent observing procedures at Gemini. Information on OT-specific elements can be found on the NIRI OT tips and tricks web page.

Guiding:

NIRI requires the use of a peripheral wavefront sensor for both imaging and spectroscopy, and PWFS2 is preferred. PWFS2 can use fainter guide stars, it gives corrections at a higher frequency (200 Hz), it works better under windy or cloudy conditions, and it has a smaller arm which vignettes less of the science beam.

Calibrations:

Check that the baseline calibrations are sufficient for your project. Include the baseline calibrations and any additional required calibrations in the Phase II OT file, but remember that your program will be charged the time required to obtain anything beyond the baseline.

Flats are not taken for thermal-IR observations; they will need to be constructed from the sky images.

If you need photometry to better than 10%, you will need to add photometric standards to your observations to determine extinction.

If doing spectroscopy you will need to include a telluric standard before AND after each target in the Phase II,

Note that spectrophotometric standards are not done as part of the standard baseline calibration, but a telluric standard star is observed. The accuracy for which the flux density of a telluric standard is known is probably about 10%. More importantly, note that even if a spectrophotometric standard is used as a calibrator, variations in seeing and guiding as well as differences in how the objects are centered on the slit typically lead to +/-20% uncertainty in fluxes derived by ratioing the target spectrum by the standard spectrum, even in photometric conditions. If the project requires more accurate flux calibration than this, then the observer either should ensure that photometry is already available for his/her science targets or should secure such photometry as part of the science program.

f/6 JHK Broad-Band Imaging:

If the science target is extended, be sure to acquire additional sky frames. Dithers on the sky should be large enough to remove point sources when making sky images (5 to 10 arcsec works well). The peripheral wavefront sensors should be "frozen" when you move off to sky if the guide star is inaccessible at the sky location. This will make re-acquiring guiding efficient when returning to the target. The sky field can sometimes be chosen carefully to allow the PWFS to follow and guide on the offset sky field. Using multiple guide stars for different dither positions is supported in Phase II OT files, but has NOT been commissioned on the telescope.

If the target is not extended, the target images can also be used to construct the sky frame. Make sure dithers are much larger than the seeing (5 to 10 arcsec) so that point sources can be removed from the sky image.

Plan to reject the first exposure from each dither sequence following a configuration or exposure time change. Dark current instability will usually ruin the first image, so you should always take at least one more offset position than your S/N calculations demand. If multiple coadds are being used, you can include a single exposure (1 coadd) of the same exposure time before your offset iterator to save some time.

Exposure times are best kept in the range of 30 to 120 sec. Longer exposures are not advisable because the sky changes significantly between exposures, making sky subtraction difficult. If shorter exposures are needed, use several coadds to build up 30 sec or more before dithering. There is a fixed overhead of a few sec per image written, so many short exposures are much less efficient than coadding several short exposures prior to saving the image.

Avoid saturating objects in your field. Use shorter exposures and several coadds, if necessary. All exposure times longer than a couple of seconds will be background limited. Saturated stars leave residual images that persist for quite some time, and leave little ghost objects when dithering.

The read mode used should be "Medium Background," which results in a single read pair; this is sufficient because the recommended exposure times will result in background-limited frames.

f/6 JHK-Narrow-band Imaging:

Remember that most narrow-band filters are 1.5% wide, roughly 1/40 as wide as the broad-band filters. Exposure times and total integration times need to be much longer than for broad-band filters, although avoid saturating bright objects if possible.

The maximum recommended exposure time is about 300 seconds. Make sure the exposures are long enough to be background limited. This will not be possible in the "Medium Background" read mode; instead use the "Low Background" mode, which results in multiple read pairs before and after each exposure. To use this mode efficiently make sure that individual exposures are at least 44 sec. The only exception is when observing standards, for which the "Medium Backgound" (single read pair) read mode should be used.

Don't forget to include observations in a continuum (off line) filter, if needed; e.g. when mapping line emission on an extended source with both line and continuum.

Not all narrow-band filters have been received, and not all of those received can be installed at the same time! Please check the Filter List for availability.

The CO 2-0 filter is not well-centered on the CO band; it includes considerable continuum shortward of the 2-0 band head. Also, the "CH4 short" and "CH4 long" filters have considerable overlap and "CH4 long," intended to sample the continuum shortward of the methane absorption in T dwarfs, extends far into the T dwarf methane absorption band. It may be preferable to substitute the "H-continuum" filter for "CH4 short." If you contemplate using any of these filters, check the filter profiles carefully.

Thermal Imaging (L and M bands):

Because of the short times in which the background will saturate the array, L-band observations are only possible using the f/14 and f/32 cameras, or the f/6 camera using a subarray. At M-band, only the f/32 camera can be used full-frame, or the f/14 camera with a subarray. You may specify subarrays for the thermal bands and we will adjust them to smaller subarrays as necessary, given the background at the time of observations (but let us know if doing so would compromise your science).

There is a few second overhead for each image written. To keep efficiency reasonable, please use exposure times as long as possible without saturating, and coadd many exposures to total 30-60 seconds before dithering.

Spectroscopy:

Like narrow-band imaging, 1.0-2.5um spectroscopy of faint sources should use the "Low Background" read mode. Keep the exposure time longer than 44 sec and shorter than a few minutes. Plan to throw out the first image of a sequence. Bright standard star observations are much shorter and should use the "Medium Background" read mode.

Spectroscopy in the L band can be done with exposures of 1-3 seconds and thus in the "Medium Background" read mode (the minimum recommended exposure time is 2.7 sec for this mode). However, for higher efficiency or for bright standards it should be done in the "High Background" mode (minimum recommended exposure time is 0.9 sec).

Spectroscopy in the M band must be done in the "High Background" read mode. Use of the 768x768 subarray will result in greater efficiency, and no spectral information is lost, because the excluded portions of the array are outside the bandpass of the blocking filter. Flats are not possible with the M grism and either the full array or the 768x768 subarray, because of saturation. Therefore it is important to place the target and the standard star in the same rows of the array. Unfortunately, use of the 512x1024 subarray does not gain anmy improvement in readout time and is equivalent to using the full array.

Spectroscopic observations should nod the target along the slit for sky and detector defect removal. Smaller nods more accurately retain the target in the slit than do large nods. Although the slit is 50-90 arcsec long it is not necessary to use all of it on small targets. For a point source nods of 3 arcsec are ample.

Bright targets (J<21.5, H<20.5, K<20.7) which give S/N>3 in 60s imaging integrations in good (IQ=70, CC=50) conditions can be acquired directly. The target will be centered in the slit by placing a near-IR image of the target at the center of the slit, before introducing the appropriate slit, blocking filter and grism. Fainter objects can be acquired via accurate user-supplied blind offsets from a nearby bright object. In this mode the bright reference (User1) star will be centered in the slit, and then the blind offsets will be applied to shift the science target into the slit.

The current f/6 slits and grisms are designed for use at f/6. Generally speaking, higher spectral resolution cannot be achieved using the f/14 camera and the present set of grisms, and the wavelength coverage at f/14 will be severely reduced. The f/6 camera should therefore be used in almost all cases.

Slitless spectroscopy may be useful for some programs. The background will be much higher, of course. Some observations may use narrow-band filters in conjunction with slitless grism spectroscopy to good advantage. This mode has not been tested yet.

Polarimetry:

Polarimetric observations are not yet available. Note that spectropolarimetry will not be possible with NIRI because the Wollaston prism and the grisms are in the same wheel.

Coronography:

Coronagraphic observations are not yet available.

Observing Overheads:

Current estimates of observing overheads using NIRI with either Altair or the peripheral wavefront sensors can be found on the NIRI Overheads Page.

For automated sequences of exposures (dither patterns) the estimated on-source efficiency is ~75% (i.e. 25% of the elapsed time is used for telescope offsetting, WFS re-acquisition, detector readout etc). This estimate is realistic for 30 sec and longer exposures at each dither position. Very short exposures (t<5 sec) will have much lower efficiency because of the fixed overhead per image (approximately 3 to 5 seconds per frame at present). Multiple coadds should be used for exposure times less than about 30 sec to keep the overheads reasonable and the efficiency high.

Note that spectroscopic observations longer than ~1 hour have an additional overhead associated with recentering the object in the slit due to flexure between the peripheral wavefront sensors and the detector.

Observations in different filters that are structured in the OT as separate obseravations (rather than in a NIRI iterator) take no additional setup time.

All overheads should be included in the time requested for each observation, however, only the overheads actually used will be charged to the program.

Last update 2007 May 9; Andrew Stephens, Tom Geballe & Joe Jensen