Guidelines for service/queuing observations

1. Contact point and reference astronomer

Each TAC program with approved service/queuing observations has a reference TNG astronomer (hereafter RefAst) who handles all the communication with the program P.I. or his/her delegate (hereafter HomeAst). The names of the contact astronomers are given in the on-line schedule. To make sure that observations are not delayed or downgraded we strongly encourage early contacts with the RefAst.

The RefAst is in charge of

  • providing the user-name and password to access to the FLEX interactive web system. Using this the HomeAst can directly submit/modify/update the observing blocks as well as checking if a given observing block has been executed

  • helping the HomeAst in the preparation of the observing blocks

  • sending one or more CD-ROM with all the data pertinent to the program at the end of the running semester or once all the observations have been executed.
Moreover, the RefAst is generally available to communicate about matters concerning the program.

2. Observing blocks

The queued observations are organized in observing blocks (OBs) which consist of one or more exposures of a given object with a given (fixed) instrument configuration. In particular circumstances, blocks can be "linked" together, in which cases all the linked OBs must be executed within the same night. OBs are executed depending on the priority grade given by the TAC (A, B or C), on the given constraints (moon, sky transparency, seeing, airmass, data to be taken at specific dates/times), and on the total time necessary to complete the observing block or group of linked OBs.

Please note that the constraint on seeing cannot be strictly guaranteed in all cases, because the atmospheric turbulence may vary significantly on relatively short time scales. For this reason frames with overall image quality up to 20% above the requested limit may be considered as correctly executed OBs. Also note that, in case of unexpected changes of weather conditions, observing blocks executed above the 80% of the full integration could be considered as correctly completed.

To increase the efficiency of the system as well as the probability that a given observation is executed, we recommend minimizing the constraints and organize your strategy in order to split the observations into relatively short blocks. The total time of each observing block should not exceed 60 minutes. The same limit applies to the total time of linked blocks. Although longer blocks may be executed, TNG cannot however guarantee that they will be completed. If an OB (or linked group of OBs) longer than 60 minutes is interrupted by meteorological reasons after 1 hour it may be nevertheless considered completed.

3. Times and overheads

Besides the exposure time, each executed observing blocks has an associated overhead which is detracted from the allocated time and includes:

  • Telescope pointing and guide-star acquisition : 5 minutes

  • Centering object into the slit for spectroscopic observations: 5 minutes. This time could become much longer in case of direct centering on very faint objects (see Section 4 below)

  • Change of instrumental setup: 1 minute (alternative to the previous two, see below)

  • CCD read-out times for optical instruments (see the dedicated time calculator)

  • NICS overheads (see the dedicated document)

  • Change of instruments: 6 minutes for instruments within the same Nasmyth focus, 25 minutes for change of Nasmyth. This overhead is included only when the change of instrument during the same night is explicitly requested by the program.
In case several observing blocks are executed with the same instruments on a given object and telescope position, the telescope overheads (pointing and centering in slit, if any) are added only to the first OB, while the others will only include the overhead for the change of instrumental setup (if any).

Example: imaging U (100sec), V (60sec), R (60sec), I (100sec) and spectroscopy LRB (600sec), LRR (600sec) of a star with DOLORES reading in all cases the complete 2048x2048 image.

Total Integration time 25.3 min
Pointing and guide-star acquisition 5.0 min
Set object into slit 5.0 min
5 changes of instrument setup 5.0 min
6 CCD readouts 7.1 min
Total overheads 22.1 min

Overheads for observations with the adaptive optics module (AdOpt) could be much larger. For more information please contact the instrument responsible.

4. Standard stars

TNG offers a frame of one standard star per filter and one for each spectroscopic setup. Any other observation of standard star(s), either photometric or spectroscopic, must be explicitly requested with the appropriate OB(s) which must be linked to the target OB(s) in case the standard observation must be taken within the same night. The time used for standard star observations is part of the time allocated to the proposal and is counted in the same way as on-target observations.

5. Spectroscopic observations of faint objects

Centering the object in the slit is a relatively fast operation the source is bright enough to be clearly visible (i.e. with a signal-to-noise ratio larger than 10) on a 60sec pre-image. Programs requiring spectroscopy of fainter targets can be executed within the above mentioned overheads only when their positions relative to a nearby (ideally within less than 1 arcmin), brighter "pivot" source is known. The observations will be then performed by aligning the slit along the object-pivot line, i.e. by rotating the instrument to a pre-determined position angle, and centering the pivot source in the slit with the due Y-offset from the center. To this purpose, the HomeAst must clearly provide the value of slit position angle and Y-offset and, ideally, include them in the finding chart as in the example below