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.
Starting from Period 96, we differentiate between seeing constraints at Phase 1 and image quality constraints at Phase 2 (with the exception of AO instruments and VLTI, see the observing conditions definitions for more details). With the help of the Exposure Time Calculators (ETCs) the requested Phase 1 V-band seeing at zenith can be transformed into the desired image quality at the observed wavelength and airmass for each instrument.
Duirng Phase 2 preparation (and review only), 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 an image quality that corresponds to a V-band seeing at zenith of 1.2 at Phase 2, but not an image quality equivalent to 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.