For seeing-limited imaging, the FoV with LUCI is large enough that there are often multiple 2MASS stars in the images and this should yield a photometric accuracy of ~±.0.1m. All 2MASS point sources are easily visible in a typical 60s broadband exposure with LUCI. With in-field calibrators are not sensitive to variations in transparency while observing.
On truly photometric nights, it is possible to improve somewhat on this uncertainty, though it requires multiple observations of photometric standards, spanning the night to show photometric stability as well as airmass to derive an extinction correction. A compilation of NIR photometric standards is available on the Calar Alto Observatory website.
Telluric standards are bright targets used for spectroscopic calibration. These are typically spectral type A0V and/or G2V stars near (i.e., within a specified difference in airmass and angular separation from) a science target. In the near-infrared, the atmosphere is not perfectly transparent across all wavelengths. Various molecules in the atmosphere, primarily water, also have strong rotational and vibrational transitions that create significant absorption at certain wavelengths. Correcting for this absorption is what a telluric standard is used for.
Telluric stars are usually very bright to minimize the amount of time you spend taking data on the calibrators. As such, we recommend taking exposures where the source is visible in the acquisition sequence through crossed filters (usually OH1060+z or HeI+z) so the source does not saturate on the detector. The filters need to be uncrossed for the slit exposure, however, to get enough sky flux on the detector to see where the slit is located. The telluric scripts also will automatically cross and uncross particular combinations of filters as needed to prevent the telluric star from saturating the detector while still allowing for sufficient sky flux to get through the slit so its position can be identified.
Example observation are prepared in the OT along with lists of suitable G2V and A0V candidate telluric calibrators. Other resources for determining suitable Tellurics are:
Telluric stars are aligned on the masks from the Longslit tab on the RTD, whether this is through a longslit or a MOS mask. When observing a telluric star through MOS masks, you must know which slit is the first one the telluric should be observed through, or subsequent offsets will not place the telluric through other slit lets. The telluric script produced by Scriptor will likely put the star near the center of the detector, so the offset to place the source on the first slit can be large.
While perfect collimation is not really needed for spectroscopic observations of the telluric stars and they are bright enough to get high SNR in a minimum exposure time, we still recommend integrating for 5×3.0s in order to average over some atmospheric and guiding residuals. These should be set as the defaults.