MMIRS Recommendations

Imaging

The recommended detector setup for imaging is ramp=1.475 sec, gain=2.68 (low gain, high dynamic range). Ramp 1.475 sec is recommended to retain the maximum possible dynamic range and avoid saturating 2MASS stars which many PIs use for calibration purposes.

Maximum exposures per filter in typical sky conditions

Dithering patterns: MMIRS images need to be dithered for sky subtraction. A good default dither pattern for small sources is a random 30×30 arcsec dither. If your sources are small, you can just put “30” in the Dither box in the target creation dialog, and you’ll get the 30 arcsec random dither. Approximately one minute is needed to maintain wavefront correction at any given sky location. So if you request (for example) 20x20sec exposures in Ks, and a 30 arcsec dither, observers will typically take 3x20sec exposures, dither to the next position, and so on.

If you design your own dither catalog, we recommend designing it in such a way that the telescope stays in each position for one minute, e.g., 3×20 sec exposures in Ks, or 4×15 sec exposures in H, before moving to another dithering position.

Dithering for extended objects: For objects that cover a large fraction of the MMIRS field of view, such as imaging a SN in a nearby galaxy, it is necessary to use one of: a large dither pattern, a very large dither pattern that is bigger than the field of view, or a separate offset sky target. Such large dithers or offset targets will typically take more overhead to execute, so budget for that in your time allocation. The telescope allows dithers as large as 1 degree to be executed as offsets in the dithering catalog. See MMIRS dither catalogs for more information about dithers. Custom dither catalogs can be made as a text file by the PI, and must be emailed to a MMTO staff scientist prior to the observing run so that they can be transferred to the observing computer.

Longslit

The recommended gain setting is 0.95 (low) with ramp=4.426 sec. Dither size options are typically 5, 7 (default), 10, 15, 20, 30, 60, 120, 210″. Maximum exposure times are 300 seconds. Use an even number of exposures.

Use the “long” slit. The “short” slit is not recommended – it has no advantages in practice and the data are more difficult to reduce.

See below for recommended combinations of grism and filter for both longslit and mask spectroscopy.

Please note that finding charts are required for MMIRS longslit spectroscopy to aid the observers in placing the target onto the slit. The target location should be visible in the chart and indicated with tickmarks or similar. Charts are required for MMIRS longslit, are useful but not required for imaging, and not required for slitmasks. Please read Finding charts for MMIRS for advice on making finding charts, with examples.

The observers align the target onto the slit by taking short direct images. If your target isn’t visible in a roughly 15-sec MMIRS exposure (is fainter than about mag=18), you should: specify an offset star RA/Dec in your catalog entry, request the PA that puts both offset star and target onto the slit, and clearly mark the offset star on the finding chart. The offset star should be clearly localizable and not overexposed on the chart. The star will be centered on the slit, so it needs to be <3 arcmin from your target, and preferably closer than 1.5 arcmin.

Masks

The recommended gain setting is 0.95 (low) with ramp=4.426 sec. Typical dither patterns are size/pattern: 1.8″-1.4″ ABA’B’ (recommended), 1.6″-1.2″ ABA’B’, 2.0″-1.6″ ABA’B’. Maximum exposure times are 300 seconds. A typical two hour block would consist of 300 second exposures with 24 visits.

MMIRS slitmasks are now designed using the MMIRSMask program, which runs in your web browser; the older xfitmask program is no longer used. MMIRSMask is similar to the BinoMask designer for Binospec, with a few important differences. See tips for MMIRS slit mask design for more information.

Be advised that only certain combinations of grism and filter are supported by the MMIRS data reduction pipeline. The H grism is obsolete; please choose H3000. The recommended and most used modes are grism/filter: J/zJ, HK/HK3, H3000/H, K3000/Kspec. We strongly suggest using one of these combinations unless there is a compelling reason not to. If you have questions about the modes, please contact an MMT instrument scientist.

GrismFilterSp. Res. (R, 0.4″ slit)Wavelength (um)Supported
JJ22001.15-1.35deprecated
JzJ24000.95-1.50yes
HH23001.50-1.80deprecated
HHK3 [*]23001.25-2.15deprecated
HKHK3 [*]14001.25-2.34yes
H3000H30001.50-1.80yes
K3000Kspec30001.90-2.45yes
HKzJ [**]16000.95-1.50no
HKY [**]16000.95-1.10no

[*] With the H or HK grism, use the HK3 filter rather than the older HK filter.

[**] Only use HK+zJ or HK+Y if you are absolutely sure and have your own data pipeline.