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Observing Strategies

General NIR Imaging Guidelines:

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 sensor 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. In some circumstances the sky field can 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.

For faint sources/long exposures, 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. Examples of how this is done are in the NIRI OT library (see, e.g., the Science Target Observation in "example imaging 3x3 mosaic offset to sky," in which a single observe has been inserted in the "Sequence" just ahead of the "NIRI Sequence").

f/6 JHK Broad-Band Imaging:

Integration times between dithers are best kept in the range of 30 to 60 sec (note that at H and K saturation on the sky background restricts individual exposures to such times). 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 separate short exposures are much less efficient than coadding several short exposures prior to writing the image.

Avoid saturating objects in your field when possible. 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. In order to be background limited, 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.

A few of the filters are not ideal. In particular, 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.

f/14 and f/32 1-2.5um imaging:

Because of the smaller pixel size at these f-numbers, much longer exposures are needed than at f/6 in order to be background limited; this may not be feasible for narrow band imaging. For faint objects one should use the low noise read mode, even for JHK imaging.

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.