Observing Constraints

During the night, selection of targets to be observed depends on current conditions, especially seeing, sky transparency, sky brightness (Moon phase) and distance to the Moon. Night observing schedules will be adjusted accordingly, if weather changes. For each individual target they are set in the Phase 2, but the strictest ones must be well defined already in the Proposal.

In their proposals, the PIs should explicitly give the following information:

the maximum value of seeing, at which observation may be performed, in arcsec.
Air mass
the maximum air mass/zenithal distance (or minimum elevation) at which a target may be observed. The recommended value, set by default, is $ \sec z =2$ (30$ ^\circ$ over the horizon), but PIs may request a lower limit, between 20$ ^\circ$-30$ ^\circ$ ( $ \sec z\simeq2$-$ 3$). Note, however, that the Atmospheric Dispersion Corrector will not work properly at $ \sec z>2$. Note also that constraining airmass $ <$2 in the proposal is not allowed.
Sky transparency
the sky clearness and magnitude drop due to clouds, defined as a number between 0 (totally cloudy) and 1 (clear).
Moon phase/sky brightness
: the acceptable brightness of the sky, coming from the illumination and elevation of the Moon - “dark”, “gray” or “dark+gray”. “Dark” is always 0-3 days from the new moon. Between 4-11 days, “gray” time is defined when the Moon is over the horizon. HSC runs are not allocated during bright time.
Moon distance
the minimum separation from the Moon, at which the target can be observed, in degrees. Only distances larger than 30 deg are allowed (to avoid significant contamination with stray light). For “dark” time, 30 deg is set by default, and cannot be changed by the PI. For “gray” time, it is the PI who sets the constraint.
Time constraints
window of time when the PI wants the observations to be performed. There are no particular limits for its length, and multiple windows are allowed. It is allowed to put several targets per window, or define several windows for one object. Note that the chance for the OBs to be observed becomes higher by specifying the time windows as relaxed as possible.

Note, that if a highly-ranked proposal is intended for restrictive conditions (for example seeing $ <0.6''$) that are not currently met ($ \sim0.8''$), this proposal will not be executed at such moment, and a lower-ranked one that allows for given conditions ($ <1''$) will be chosen. Relaxing the constraints increases the probability of having observations done. Defining different constraints for different targets is allowed.

In order to help to choose the optimum observing constraints for your project, that will increase the probability of execution and still allow for the science you intend to make, please familiarize yourself with the Statistical Information for Observing Condition Constraints in:


Since the semester S17A, time-critical observations may be proposed. If the PI requires the targets to be observed in a specific time, please provide sufficient information, such as: date(s), time, and duration of the time windows. Use the Box 13 of the Application Form, or add comments in the target list (see Sec. 2.2). Note that the chance for the OBs to be executed may increase by specifying the time windows as relaxed as possible. If during the proposed time window the environmental conditions (seeing, transparency, etc.) are not met, the observations will not be carried out.