Phase 2 constraints must agree with Phase 1 request
At Phase 1 specific observing conditions (lunar phase also known as Fractional Lunar Illumination or FLI, seeing, and transparency) were requested for each observing run. Your run was reviewed and allocated time with these specific conditions in mind.
At Phase 2, you can relax your constraints, but you cannot specify more stringent constraints. For example,
- if you requested Seeing = 1.0 at Phase 1, you can specify Seeing = 1.2 at Phase 2, but not Seeing = 0.8;
- if you requested Transparency = CLR at Phase 1, you can specify THN or THK at Phase 2, but not PHO.
The only exception to this is the case in which only a small subset of OBs need to be obtained under photometric conditions in order to provide an accurate flux calibration, such as for instance a deep field in which a network of bright flux reference objects can be set up from a shallow exposure. Such exceptions are accepted only if accurate flux calibration is needed for the scientific goals of the programme, and for an execution time not exceeding 20% of the total allocated time.
Also check the Service Mode guidelines on observing constraints.
Correspondence between Phase 1 and Phase 2 constraints:
- If you specified Seeing = 'n' at Phase 1, you must specify Seeing = 2.0 or greater at Phase 2 (1.4 for NACO).
- If you specified Lunar Phase = 'd' at Phase 1, you must specify FLI < 0.4 at Phase 2.
- If you specified Lunar Phase = 'g' at Phase 1, you must specify 0.4 ≤ FLI ≤ 0.7 and Moon Angular Distance = 120 or less at Phase 2.
- If you specified Lunar Phase = 'n' at Phase 1, you must specify FLI = 1.0 and Moon Angular Distance = 60 or less at Phase 2. Exceptions to this statement are applicable for infrared observations. If this is the case for your observations, please refer to the instrument-specific service mode rules for observing constraints for full details.
Note that FLI = 0.0 is interpreted as "moon below the horizon".
Important note on the moon distance constraint for optical wavelengths:
At the dry, low water vapour site of Paranal during clear or photometric nights we expect that the night sky brightness does not show a strong dependence of the lunar distance (Rayleigh scattering I ~ I_0 * (1+\cos2\theta) )
Measurements performed on Paranal in I-band during bright time have shown that the sky brightness is approximately constant for moon distances >~50deg. Hence, defining a moon distance constraint larger than 50 deg for I-band observations are unnecessarily limiting the time available for the execution. Observations at shorter wavelengths (B- to R-band) in grey and dark time are also not affected very much by the presence of the moon if the distance is >~50-60 deg, and often the sky is darker at 60-70 deg away from the moon than at 120 deg away, when the moon is low above the horizon (see Fig 5 of Patat 2004, Messenger Issue 118):
For observations in optical wavelengths it is advised to select moon distance up to 60-70 degrees. Selecting larger lunar distances may drastically reduce the gray/dark time periods in which the observations can be carried out.