HOW-TO Schedule Astro-WISE compliant observations¶
The Astro-WISE system is a database driven environment for the reduction of astronomical image data. The system is capable of processing data from many different Wide-Field cameras. Currently, the system is using data from
- OmegaCAM imager at the VLT survey telescope
- WFI, the Wide-Field Imager at the 2.2m ESO telescope, La Silla
- WFC the Wide Field Camera at the INT, La Palma
- MDM8K at the MDM observatory
The Astro-WISE system was primarily created for the reduction of OmegaCAM data. This is reflected in the design of the system, and has consequences for any observations to be scheduled for instruments other than OmegaCAM itself.
This chapter provides guidelines for scheduling observations that are in agreement with the data model and data requirements of the Astro-WISE system. These guidelines not only facilitate easy processing by the Astro-WISE system, but also allow a smooth insertion of this data into the database (i.e. ingestion). It is, therefore, strongly advised to adhere as much as possible to the guidelines provided in this chapter. A short summary of the guidelines is given in Table 1. If you plan to use the Astro-WISE system for the reduction of data from instruments other than the ones mentioned above it is advised to contact the Astro-WISE group in Groningen.
Data requirements¶
In the following subsections, short descriptions are given of the minimum requirements one has to meet to smoothly work with the Astro-WISE system.
Read noise and bias¶
In the OmegaCAM calibration plan, the read noise is determined as part of the daily health check procedures. For the Astro-WISE system, this means that there is a processing step devoted to deriving this read noise. This step in the data reduction can not be skipped, because the read noise is a necessary ingredient in creating the master bias.
For deriving the read noise and the master bias, bias exposures are required. For the read noise, exactly two bias exposures are required, for the master bias at least five (the OmegaCAM calibration plan even specifies as many as ten). Please, make sure that at least once during the observing run read noise observations are taken (otherwise the Astro-WISE system would have to fall back on pre-fab values of the read noise which might not always be the most appropriate). To easily distinguish between these sets of bias images in preparation for the ingest, it is advisable to use the different entries for the OBJECT header key word as given in Table 1.
Dome flats and twilight flats¶
In the Astro-WISE system, the science data is flat-fielded with a master flat that has been derived from dome flats and/or twilight flats. Optionally, one can, in addition, use flats derived from the nightsky to construct the master flat.
For deriving a master dome flat, the system requires at least five exposures for every filter in which science observations are taken. The same requirements are in place for deriving a master twilight flat. To easily distinguish between the two sets of flat-field images in preparation for the ingest, it is advisable to use the different entries for the OBJECT header key word as given in Table 1
Photometric standard fields¶
This section concerns itself with observations for establishing a zeropoint for the night.
A description of the contents of the most recent standard star catalog in the Astro-WISE system can be found here: HOW-TO Photometric reference catalog and standard extinction curve
There it is also described how to do photometric calibration based on other standard stars which requires ingestion of their standard magnitudes in the Astro-WISE system.
In order to be able to derive the zeropoint for the night with the Astro-WISE system, it is necessary to observe one of the prescribed photometric standard fields once in the middle of every night, for every filter in which science observations are taken during that night (no dithering is required, one exposure will suffice).
Table 1: Data requirements for Astro-WISE compliant processing. All entries in the table are mandatory, except for the last one; the extinction measurement is optional. The first and second columns give the purpose and type of the data. The minimum observing frequency and number of exposures per observation are given in the third and fourth columns. The last column gives the recommended header value of the OBJECT key word.
| Purpose | Data type | Frequency | # Exps. | OBJECT |
|---|---|---|---|---|
| readnoise | bias | \(\ge\)1/run | \(=\)2 | BIAS, READNOISE |
| bias | bias | \(\ge\)1/night | \(\ge\)5 | BIAS |
| dome | flatfield | \(\ge\)1/run | \(\ge\)5/filter | FLAT, DOME |
| twilight | flatfield | \(\ge\)1/night | \(\ge\)5/filter | FLAT, SKY |
| photom | science | \(=\)1/night | \(=\)1/filter | STD, ZEROPOINT |
| science | optional | \(=\)2/filter | STD, EXTINCTION |
Photometric monitoring¶
The OmegaCAM calibration plan contains observations of a field near the South Equatorial Pole for the purpose of monitoring the stability of the atmosphere. The selection of the field is currently in progress, but the field will most likely be near ra=51.4286deg, dec=89.0426deg. Currently, such photometric monitoring observations are not mandatory for successful data reduction with the Astro-WISE system. Furthermore, the Astro-WISE system facilitates the derivation of the extinction using two standard star fields observed at different airmasses.
Gain¶
Gains have already been determined for all instruments currently supported by the Astro-WISE system. Optionally, one can measure the gain using a very specific serie of dome flat measurements.
Notes on specific instruments¶
WFI@ESO2.2m¶
No instrument specifics at present.
MDM8K@MDM2.4m¶
Make sure that all the output from the instrument is in Multi-Extension FITS format, and that both the calibration and science data are binned to 1024x2048 pixels.
Standard tiling and pixelation of the sky¶
A preliminary tiling and pixel grid scheme has been defined for OmegaCAM (1 square degree FOV). It involves 22717 Square degree fields for the Southern hemisphere. A main aim is to facilitate the stacking/co-adding/differencing of images taken at different epochs/bands/projects even when combining data from different Wide-field-imager data in the Astro-WISE system Therefore it might be prudent to use the pointings of the tiling grid in observations with any instrument. Please refer to this white paper on Tiling the sky for more details:
http://www.astro.rug.nl/ omegacam/dataReduction/Tilingpaper.html
Viewing observations already in the Astro-WISE system¶
To get an overview of the pointings of observations already present in the Astro-WISEdatabase one can use the ObsView module.
Usage of ObsViewer is explained in HOW-TO Visualize observational pointings with ObsViewer (OLD).
To get more details on the observations one can use the DBViewer. Usage of the DBViewer is explained in HOW-TO DBViewer: plotting and accessing (external) image- and source-catalogs
