NTT Imaging Calibration Plan
(Minimal Version)
Authors: Dave Silva & Gautier Mathys
Document: LSO-PLA-ESO-40400-0001
Release: 0.2/1997-Jun-02
(Minimal Version)
Authors: Dave Silva & Gautier Mathys
Document: LSO-PLA-ESO-40400-0001
Release: 0.2/1997-Jun-02
This document describes the minimal necessary calibration plan for NTT imaging programs using SUSI2 and the EMMI imaging modes BIMG and RILD. A more complete SUSI2 plan is described by Comeron (1997), which also provides a complete description of the purpose of each kind of calibration data. General Remarks --------------- (1) All calibration data described here will be taken using the following "standard CCD configuration": Readout speed: normal Windowing: none Binning: none Exposure Time: > 10 secs to minimize shutter timing effects EMMI red arm imaging (RILD) will be taken in dual-port "fast" mode. (2) All calibration data described in this document will be acquired, named, tracked, processed, and archived via standard Data Flow System procedures and according to standard Data Flow System protocols. A set of calibration OBs will be developed jointly by DMD and the NTT Team. (3) All programs which require "non-standard" calibration data will have to acquire and process that data independently. "Non-standard" data is defined to be any data which is windowed or binned and/or uses special filters or read-out speeds. If non-standard science observations are made, non-standard calibration data is required. Non-standard night-time calibration activity must be accomplished within the time allocated to each program by the OPC. Time required to complete on-standard daytime calibration activity will typically be absorbed by ESO. (4) Classical observers will be asked to adhere to this plan, although they will not be required to adhere to it. (5) It is assumed that the NTT Team will supply the following items at least monthly as part of their normal preventive maintenance activities: shutter timing correction detector gain detector read-noise detector linearity bad/hot pixel masks (6) It would clearly take too much time to provide adequate calibration for all NTT imaging modes (let alone all imaging and spectroscopy modes) on all nights. This is especially true if the nightly observation of photometric standard stars is required. The nightly observing schedule must take this into account. In general, only one (1) imaging mode should be supported per night. The obvious exception to this rule is when observing starts with either BIMG or RILD but the delivered image size later falls below 0.5 arcsecs. In that instance, if a high priority SUSI program is in the queue, that program should be executed. (7) This plan will be in effect through the end of P60. The imposition of a calibration plan for P61 and beyond is currently at the discretion of the NTT Team. Required Calibration Data ------------------------- Dark Frames: acquired monthly as part of normal NTT preventive maintenance activities Bias Frames: what: nine (9) frames per day when: TBD by NTT Team Low count level flatfields: acquired monthly as part of normal NTT preventive maintenance activities Dome Flats: level: >= 20K ADU but in linear range number: three (3) per filter used during previous night when: in early morning *after* a night of observations *but* before the filters are changed who: NTT daytime operator Comment: During most service observing nights, the number of filters used will be constrained to four (4) filters. Twilight Flats (UBVRI only): level: >= 10K ADU but in linear range number: five (5) per twilight period Comments: During each twilight period, acquire set of twilight flat frames for one (1) filter. Rotate through UBVRI. Twilights should be acquired regardless of whether one of these filters was used on this specific night. Photometric Standards (UBVRI only): General Characteristics ----------------------- Standard fields will typically be drawn from Landolt lists. A list of appropriate OBs will be developed jointly by DMD and the NTT Team. There will be a minimum of two (2) standard stars per field. Color range: -0.2 <= B-V <= 1.5 -> try to achieve every night. OBs will be defined for these standard fields. During service observing, a maximum of four (4) filters per night should be used. The photometric flux calibration of narrowband filters is the responsibility of the users. We will *not* provide the photometric transformation coefficients on a nightly basis. If time permits, coefficients will be computed once per period and published. In general, however, users are expected to compute their own transformations based on data provided to them. Execution Options ----------------- >>>> Option A: Official NTT Policy The following policy was established by J. Spyromilio in his memo to the DG describing the general NTT calibration plan: For each filter used during every clear night, observe 20 - 30 standards at a variety of airmasses, regardless of whether programs being executed that night require photometric calibration. For efficiency, these standards should be grouped into 5 - 7 standard fields which contain as many standards as possible. Note that it is the number of standard stars not standard fields that is important. Advantages: All archived data has associated photometric calibration for nights when data was acquired. Calibration is of varying precision. Disadvantages: Injects large calibration overhead into schedule (1 - 3 hours per night every night imaging occurs). >>>> Option B: A Modest Proposal from Silva The following strategy was adopted by the WIYN Queue Observing Experiment. Postulates: (a) Standard star exposure times are short relative to CCD readout times. (b) It is impossible to assess how photometric a night really was until the standards are reduced. (c) If one takes many standard stars and many deep science exposures on a night which really was not photometric enough, all the standard data was wasted and must be thrown away and the science data must be calibrated in a different way. Strategy: On a night which appears photometric, observe many standard stars (30 - 40) while also acquiring short exposures (5 - 10 mins) of all fields of programs which requested accurate photometric calibrations. Repeat on different nights as necessary until reduction of standard stars demonstrates that user specifications for photometric accuracy have been achieved for all service observing programs. Acquire deep exposures of science targets on other nights. Apply zeropoint from calibrated shallow exposures. On all other clear or semi-clear night, observe one (1) standard field through each filter used that night. Estimate instrumental zeropoints for these nights using this one set of instrumental magnitudes and the extinction and color terms derived on the "calibration nights". Advantages: Uses much less telescope time at night than Option A. More definitively calibrates programs which need accurate photometric calibration. Disadvantages: Archived data which was not directly calibrated have on average less accurate instrumental zeropoints. Unless otherwise directed by the NTT Team, Option A will be executed.