All queue-mode observations must have observing condition constraints specified by the proposer that describe the minimum (i.e. poorest) conditions under which the observation should be executed. Classical programmes must also specify the minimum acceptable conditions and, optionally, a backup programme able to take advantage of poorer conditions. The observing condition constraints must be specified in the Phase I proposal to avoid loading the queue entirely with one type of conditions (e.g. best image quality).

The constraints are divided into five categories (if appropriate, values for Mauna Kea and Cerro Pachon are given separately):

• Image quality
• Sky transparency (cloud cover)
• Sky transparency (water vapour content)
• Sky background
• Air mass (zenith distance)

The first four of these are specified in the Phase I (initial) proposal; the fifth may be specified in the phase II Observing Tool




• Old constraint sets

Some of the specific properties corresponding to these categories are wavelength dependent and are not relevant for all observations. For example, the sky background at visible wavelengths is dominated by the lunar phase and moon-to-target angle, whereas at mid-IR wavelengths the combination of cloud cover and water vapour condition affect the background and its variability. For image quality, sky transparency and background we have chosen to represent the variation in these conditions (which is deterministic in the case of visible sky brightness, statistical in the case of quantities such as water vapour column) by a percentile representing the frequency of occurrence of the specific property. Observing constraints are specified in terms of these percentiles e.g. (best) 20%-ile, 50%-ile (better than median) etc.

This page provides a translation between the frequency of occurrence and the specific value for the relevant property as well as further information and guidance on their use by observers. The emphasis is on providing observers with these constraints in meaningful units (and corresponding to those used in the integration time calculators) as well as indicating their likelihood.

These examples illustrate some of the factors that a user might take into account when selecting their observing condition constraints:

1. Example - NIRI spectroscopy of an extended object
2. Example - NIRI imaging of structure within an extended object

More information about how observing conditions affect data quality is also available for the near-IR and mid-IR. In addition, (large ascii) files containing model high resolution spectra of the sky emission and transmission are available for a range of airmasses and water vapor columns are available for the near-IR and mid-IR.

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Image Quality (non-AO) - MK and CP

Example interpretation of the table An image at K of a target at zenith with a bright guide star in the Peripheral Wavefront Sensor would be expected to have a diameter at 50% encircled energy of no more than 0.35 arcsec 20% of the time and no more than 0.55 arcsec 70% of the time.

Please read the following essential notes:

• Definition of values
  Numerical values in the constraint columns are the current measured delivered image quality, defined as the 50% encircled energy diameter in arcsec, in the telescope focal plane at the specified wavelengths. The 50% EED is equal to the full width at half maximum for a Gaussian profile. Note that the relevant parameter here is image quality and not simply seeing, that is, a wind speed distribution and the telescope performance (e.g. windshake, servo and wavefront sensor characteristics) have been incorporated into the analysis. The model was adapted by Mark Chun from original Mathematica calculations by Charles Jenkins (see also Jenkins 1998, MNRAS, 294, 69) with a subsequent correction (in August 2002) by Phil Puxley to the extant seeing distribution.


• Dependence on Telescope Elevation
  The values in the above table apply to the telescope pointing at zenith. The performance degradation away from the zenith can be approximated crudely as (air mass)^0.6 in the visible and short wavelength infrared. The integration time calculators take into account the dependence of image quality on wavelength (by interpolation) and airmass when calculating signal-to-noise ratios. The exponent is lower and variable in the 10µm and 20µm windows; values being used in the integration time calculators at these wavelengths are uncertain and may be updated. If your program requires a certain absolute image quality (e.g. for resolving objects at small separations) you should consider the possible elevations at which your observations could be executed when deciding upon image quality constraints.


• Wavefront sensors
  Use of wavefront sensors for image motion compensation (fast guiding) is required. For most wavelengths, values for peripheral and on-instrument WFSs are given (see each instrument's guiding options pages for details of available wavefront sensors). In all cases the use of a wavefront sensor for closed-loop primary mirror figure (aO) correction is assumed.


• Mid-infrared
  Image quality at Gemini South has been measured to be within ~10%(~20%)(~50%) of the theoretical diffraction limit 20%(70%)(85%) of the time in the 10 µm atmospheric window and within 10% of the diffraction limit 85% of the time in the 20 µm atmospheric window. The diffraction-limited FWHM is 0.31" at 11.7µm and 0.49" at 18.3 µm, the central wavelengths of the filters in which the percentiles have been evaluated. Similar values have been found at Gemini North. Note that there is little difference between the 20%-ile and 70%-ile image quality bins in the N band and no difference between 20%ile, 70%ile, and 85%ile in the Q band. PIs may wish to take this into account before requesting IQ20 conditions given their low frequency of occurrence. This table was updated in March 2007.


• IQ = "Any"
  This means that the observation can be scheduled under any image quality conditions. At optical and near-IR wavelength the image quality distribution has a long (non-Gaussian) tail. The values quoted are typical of the poorest conditions.


• Adaptive Optics
  Estimates of the AO-corrected image quality are described on the Altair pages.

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Sky Transparency (Cloud Cover) - MK and CP

Explanation of table entries:

1. The percentiles, which are based on long-term data for Mauna Kea, correspond to percentages of the time which have that transparency.


2. "Photometric" - cloudless and capable of delivery of stable flux.


3. "Cloudless" - for photometry accurate to a few percent in the mid-IR, careful attention must be paid to regular observations of suitable standard stars. The former 20%-ile ("low sky noise") bin was removed from this table on 2006 May 25 as it proved impractical to measure.


4. "Patchy cloud or extended thin cirrus" - relatively transparent patches, sometimes amongst thicker cloud resulting in some loss in transmission and variability, or a more uniform covering of thin cloud (usually cirrus). For the purpose of integration time calculation it is assumed that clearer patches have a transmission that is poorer by 0.3 mag than the nominal atmospheric extinction and that extended cirrus attenuates by no more that 0.3 mag.


5. "Cloudy" - cloud cover over essentially the whole sky. For the purpose of integration time calculation it is assumed that the transmission is poorer by 2 mag (T~15%) than the nominal atmospheric extinction and is variable. The increase in background makes these conditions unusable at thermal infrared wavelengths, except perhaps for some very bright targets in high resolution spectroscopic modes. Stable guiding can be difficult in these conditions.


6. "Usable" - any conditions under which the telescope is open. The increase in background makes these conditions unusable at thermal infrared wavelengths. For the purpose of integration time calculation it is assumed that the transmission is poorer by 3 mag (T~6%) than the nominal atmospheric extinction. Stable guiding can be difficult or impossible in these conditions.

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Sky Transparency (Water Vapour)

 

Explanation of table entries:

1. In the integration time calculators the optical transparency is derived from model transmission spectra. See "Sky Transparency (cloud cover)" for a related constraint.


2. The atmospheric transmission is strongly wavelength dependent as shown in model transmission spectra. With the exception of water vapour the major absorbing species at infrared wavelengths (e.g/., CO, CO₂, N₂O, CH₄, O₃) have absorbing columns that depend only on airmass. However, the water vapor abundance above Mauna Kea and Cerro Pachon can vary by over an order of magnitude, even for clear skies. At many wavelengths in the near- and mid-IR transparencies are strongly dependent on the precipitable water vapour.The CSO 225 GHz optical depth, "tau", is used to determine the current PWV conditions on MK while on CP the water vapour column is estimated from the sky background in T-ReCS N and Q band images. A new water vapour monitor is in the process of being installed at Gemini South..


3. Percentiles for Mauna Kea are based on long-term data. For Cerro Pachon, conditions are assumed to be similar to La Silla, where statistics indicate that the PWV is approximately twice that of Cerro Paranal. Therefore, the percentiles given are based on 1992-1994 data from the ESO/VLT site at Cerro Paranal, multiplied by 2. Note that the percentiles are based on year-round data, but the PWV during Chilean summer (Jan-Mar) is generally double that of the rest of the year, as shown in the Paranal data and the 1999 data from the future ALMA site. The Mauna Kea data do not show a strong seasonal variation. 


4. "Any" means that the observation can be scheduled under any conditions.

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Sky Background

 

Explanation of table entries:

1. The near-infrared J- and H-band backgrounds are dominated by OH airglow lines. The K-band background has significant contributions from both OH and thermal emission from CH₄ and H₂O. Within the integration time calculator the OH background is assumed to be constant and only a function of airmass (once the sun is sufficiently below the horizon) even though the OH emission is known to vary during the night (more details).


2. The near-IR 3-5 um and mid-IR 8-25 um backgrounds are determined by the (wavelength) dependent thermal emission from numerous atmospheric species including water vapour. Note that cloud cover conditions (which are defined in the Sky Transparency constraint) contribute to the infrared sky brightness, but are not part of this constraint). 


3. All values pertain to the zenith and 50%-ile H₂O, and do not include telescope and instrument emission.


4. Optical background values originate from a Monte Carlo simulation of the sky brightness using a model which includes scattered moonlight and zodiacal light, and pertains to high ecliptic latitude. The sky colour is different between constraint bins. Crudely speaking, the moon is below the horizon during about one half of queue-mode hours. These tabulated values are used to scale an empirical sky spectrum within the integration time calculator.


5. "Any" means that the observation can be scheduled under any (clear) sky background conditions.

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Air Mass

This constraint defines the maximum air mass [= 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 such as H₂O, CO, and CO₂), sky brightness and image quality. As a crude first approximation, the sky transparency decreases and brightness increases in proportion to the increase in air mass (e.g. sky brightness is twice as great at air mass = 2 than at air mass = 1) and the image quality degrades as (air mass)^0.6.

The airmass constraint is not used at phase I, although it can/should be entered in the integration time calculators to make an accurate estimate of observing time and to show how the expected signal-to-noise for an observation varies with elevation. Elevation constraints can be inserted at phase II; note that the phase II default is no elevation constraint. An example of an observation that would use these constraints is one using GMOS that needs to restrict the hour angles so that the position angle of the slit(s) is close to the parallactic angle. Targets with no elevation constraints will be observed at airmasses < 2.0.

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Constraint sets used previously

These pages are no longer maintained but are available for completeness:

• Pre-August 2002
• August 2002 - August 2004