Instructions
OBSTEMP deals with "when" a target is observed – namely setting the observational constraints required to optimally extract scientific information from the observation.
Registered WASP users can also access the
OBSTEMP form on WASP, similar to the one hosted at ING for phase-1 proposal preparation.
These constraints represent “worst-case†observing parameters – targets will be observed under these conditions or better. Note, however, that a worst case represents an scenario when all worst-case observational constraints meet simultaneously.
Applicants also should note also that the probability of observing targets with highly restrictive OBSTEMP codes may be quite low.
Seeing
Seeing is provided by a DIMM monitor located nearby the WHT, and it is measured as the full-width half-maximum (FWHM) of the brightness profile of a star if located at airmass 1.00. General guidance for MOS observers is to request seeing not much worse than the diameter of the fibres (1.3"). mIFU and LIFU observations are more tolerant of poorer seeing. During winter nights, the seeing is worse than 1.3" on a substantial fraction of the nights. An instrument-level limit of seeing <3" is adopted due to degradation of guiding quality in poorer seeing.
Transparency
This component defines the minimum transparency required for the observation. In the absence of cloud and Saharan dust, the V-band transparency (i.e. the fraction of top-of-the-atmosphere light reaching the WHT) is ~ 0.88, and likely scales as (0.88)^airmass (i.e. T ~ 0.83 at airmass 1.5, T~ 0.77 at airmass 2).
Extinction due to Saharan dust rarely exceeds a few tenths of a magnitude (i.e. reduces transparency by ≲ a few tenths) and varies little with position on the sky. Thin cloud, reducing transparency by a few tenths, can be patchy, so only an all-sky average can be predicted (based on the guide-star signal from previous observations).
A survey-wide limit on transparency (T > 0.4) is adopted to avoid various performance degradations such as guidance on attenuated guide stars, delivery of science data with significantly reduced SNR and increased risk of precipitation on the telescope optics.
Elevation angle
Users may set the minimum elevation angle (and thus airmass) that their targets should be observed with. An instrument-wide limit of airmass < 3 is set due to several effects degrading the quality of data at higher airmass: increasing differential refraction; higher extinction; worse light pollution.
Please remember to check for the visibility of your target using STARALT when placing an elevation limit.
Moon distance
Users may specify the minimum angular distance between the moon and the target. Observations under low moon distance may suffer from sky background light gradients across the field, as well as higher levels of reflected light causing spurious signals within the focal plane.
An instrument-wide lower limit of 30 degrees ensures sky brightness gradients and scattered light effects are kept to a minimum. This limit applies to any observations set with "unconstrained" (note that when requesting dark sky brightness, moon distance must be always set to "unconstrained").
Sky brightness
The maximum V-band surface brightness of the sky (mag / sq. arcsec) required to observe the target.
The dark-of-moon sky brightness varies by a few tenths of a magnitude depending on ecliptic and galactic latitude and phase of the solar cycle. With the moon up, the sky can be up to ~4 mag brighter than dark-of-moon, depending on lunar phase, elevation and angular distance from the target. Science observations are carried out mainly during astronomical night, but those OBs tolerating moonlit skies can also use the latest bit of evening twilight, or the earliest bit of morning twilight – the only relevant criterion is sky surface brightness. No observations will be carried out when the sky brightness exceeds that at full moon.
