QuickStart Observing Condition Constraints

Scheduling restrictions, or observing condition constraints, must be specified by the applicant for each QuickStart observation (or globally for groups of observations up to and including the entire programme). They fall into five categories and define the poorest conditions under which the observation should be executed. The constraints are image quality, sky background, sky transparency, cloud cover and air mass. Care should be taken in selecting appropriate conditions.

Each observing condition is characterised by a percentile that represents the frequency of occurrence of the specific property. In the tables that follow, if the characteristic extends across more than one percentile bin please select the largest appropriate percentile. For example, if your observation requires a 10µm image quality of 0.40 arcsec then select the 50%-ile bin. Similarly, you should select "any" as the sky transparency (water vapour) constraint when using Hokupa'a/QUIRC.

Caveat: the characteristics of the conditions described here are estimates based on our best-guess performance models of the telescope systems and site conditions. The values are subject to change. An important aspect of telescope commissioning will be to verify and, if necessary, update these models.

Image Quality

Explanation of table entries:

1. OSCIR entries are the estimated delivered image quality defined as the 50% encircled energy diameter in arcsec in the telescope focal plane. These values are expected to improve as thermal control of the facility is optimised. The 50% EED is equal to the full width at half maximum for a Gaussian profile.
2. The Hokupa'a/QUIRC and OSCIR entries assume use of one peripheral wavefront sensor (PWFS) for closed-loop primary mirror figure (aO) correction. The OSCIR entries further assume use of the same, or a different, PWFS for tip-tilt image motion compensation.  Current assumptions about PWFS detector performance correspond to a guide star R-band magnitude limit of 15-16 mag.
3. All values pertain to telescope pointing at zenith.

Sky Transparency (Cloud Cover)

Explanation of table entries:

1. The percentiles are based on long-term data for Mauna Kea and correspond to fractions of the usable time.
2. "Photometric"  - cloudless and capable of delivery stable flux.
3. "Patchy cloud" - relatively transparent patches amongst thicker cloud.

Sky Transparency (Water Vapour)

Explanation of table entries:

1. Mid-IR transparencies are characterised by the preciptable water vapour content (in mm) derived from the 225GHz zenith optical depth. The atmospheric absorption is strongly wavelength dependent as shown in model transmission spectra. Percentiles are based on long-term data for Mauna Kea.
2. "Any" means that the observation can be scheduled under any conditions.

Sky Background

For QuickStart, set the Sky Background percentile bin equal to that used for Sky Transparency (water vapour).

Air Mass

This constraint defines the maximum air mass, i.e. sec(zenith distance) = 1/cos(zd), at which the target should be observed. The air mass affects the sky transparency (e.g. the general atmospheric extinction as well as the depth and breadth of specific absorption bands due to atmospheric constituents), sky brightness and image quality. As a crude first approximation, the sky transparency, brightness and image quality each become poorer in proportion to the increase in air mass (e.g. sky brightness is twice as great at AM=2 than at AM=1). 

The maximum air mass should be specified in the technical justification section of your proposal.

Last update February 27, 2000; Phil Puxley