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
- Ks: 20 sec
- H: 15 sec
- J: 60 sec
- Y: >120 sec
Dithering patterns: Approximately one minute is needed to maintain wavefront correction at any given sky location. Therefore, we recommend designing dithering catalogs 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. We recommend a random 30×30 arcsec dithering pattern as a standard for (quasi-)point source objects.
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. 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.
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.
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 The H grism is obsolete; please choose H3000.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 : 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.
| Grism | Filter | Sp. Res. (R, 0.4″ slit) | Wavelength (um) | Supported |
| J | J | 2200 | 1.15-1.35 | deprecated |
| J | zJ | 2400 | 0.95-1.50 | yes |
| H | H | 2300 | 1.50-1.80 | deprecated |
| H | HK3 [*] | 2300 | 1.25-2.15 | deprecated |
| HK | HK3 [*] | 1400 | 1.25-2.34 | yes |
| H3000 | H | 3000 | 1.50-1.80 | yes |
| K3000 | Kspec | 3000 | 1.90-2.45 | yes |
| HK | zJ [**] | 1600 | 0.95-1.50 | no |
| HK | Y [**] | 1600 | 0.95-1.10 | no |
[*] 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.