2.4. Astrodata Tags

2.4.1. What are the Astrodata Tags?

The Astrodata Tags identify the data represented in the AstroData object. When a file on disk is opened with astrodata, the headers are inspected to identify which specific AstroData class needs to be loaded, AstroDataGmos, AstroDataNiri, etc. Based on the class the data is associated with, a list of “tags” will be defined. The tags will tell whether the file is a flatfield or a dark, if it is a raw dataset, or if it has been processed by the recipe system, if it is imaging or spectroscopy. The tags will tell the users and the system what that data is and also give some information about the processing status.

As a side note, the tags are used by DRAGONS Recipe System to match recipes and primitives to the data.

2.4.2. Using the Astrodata Tags

Try it yourself

Download the data package (Try it yourself) if you wish to follow along and run the examples. Then

$ cd <path>/ad_usermanual/playground
$ python

Before doing anything, you need to import astrodata and the Gemini instrument configuration package (gemini_instruments).

>>> import astrodata
>>> import gemini_instruments

Let us open a Gemini dataset and see what tags we get:

>>> ad = astrodata.open('../playdata/N20170609S0154.fits')
>>> ad.tags

The file we loaded is raw, GMOS North data. It is a 2D image and it is an acquisition image, not a science observation. The “UNPREPARED” tag indicates that the file has never been touched by the Recipe System which runs a “prepare” primitive as the first step of each recipe.

Let’s try another

>>> ad = astrodata.open('../playdata/N20170521S0925_forStack.fits')
>>> ad.tags

This file is a science GMOS North image. It has been processed by the Recipe System. The overscan level has been subtracted and the overscan section has been trimmed away. The tags do NOT include all the processing steps. Rather, at least from the time being, it focuses on steps that matter when associating calibrations.

The tags can be used when coding. For example:

>>> if 'GMOS' in ad.tags:
...   print('I am GMOS')
... else:
...   print('I am these instead:', ad.tags)


>>> if {'IMAGE', 'GMOS'}.issubset(ad.tags):
...   print('I am a GMOS Image.')

2.4.3. Using typewalk

In DRAGONS, there is a convenience tool that will list the Astrodata tags for all the FITS file in a directory.

To try it, from the shell, not Python, go to the “playdata” directory and run typewalk:

% cd <path>/ad_usermanual/playdata
% typewalk

directory:  /data/workspace/ad_usermanual/playdata
 N20170609S0154.fits ............... (ACQUISITION) (GEMINI) (GMOS) (IMAGE) (NORTH) (RAW) (SIDEREAL) (UNPREPARED)
 new154.fits ....................... (ACQUISITION) (GEMINI) (GMOS) (IMAGE) (NORTH) (RAW) (SIDEREAL) (UNPREPARED)
Done DataSpider.typewalk(..)

typewalk can be used to select specific data based on tags, and even create lists:

% typewalk --tags RAW
directory:  /data/workspace/ad_usermanual/playdata
 N20170609S0154.fits ............... (ACQUISITION) (GEMINI) (GMOS) (IMAGE) (NORTH) (RAW) (SIDEREAL) (UNPREPARED)
 new154.fits ....................... (ACQUISITION) (GEMINI) (GMOS) (IMAGE) (NORTH) (RAW) (SIDEREAL) (UNPREPARED)
Done DataSpider.typewalk(..)
% typewalk --tags RAW -o rawfiles.lis
% cat rawfiles.lis
# Auto-generated by typewalk, vv2.0 (beta)
# Written: Tue Mar  6 13:06:06 2018
# Qualifying types: RAW
# Qualifying logic: AND
# -----------------------

2.4.4. Creating New Astrodata Tags [Advanced Topic]

For proper and complete instructions on how to create Astrodata Tags and the AstroData class that hosts the tags, the reader is invited to refer to the Astrodata Programmer Manual. Here we provide a simple introduction that might help some readers better understand Astrodata Tags, or serve as a quick reference for those who have written Astrodata Tags in the past but need a little refresher.

The Astrodata Tags are defined in an AstroData class. The AstroData class specific to an instrument is located in a separate package, not in astrodata. For example, for Gemini instruments, all the various AstroData classes are contained in the gemini_instruments package.

An Astrodata Tag is a function within the instrument’s AstroData class. The tag function is distinguished from normal functions by applying the astro_data_tag() decorator to it. The tag function returns a astrodata.TagSet.

For example:

class AstroDataGmos(AstroDataGemini):
    def _tag_arc(self):
        if self.phu.get('OBSTYPE) == 'ARC':
            return TagSet(['ARC', 'CAL'])

The tag function looks at the headers and if the keyword “OBSTYPE” is set to “ARC”, the tags “ARC” and “CAL” (for calibration) will be assigned to the AstroData object.

A whole suite of such tag functions is needed to fully characterize all types of data an instrument can produce.

Tags are about what the dataset is, not it’s flavor. The Astrodata “descriptors” (see the section on Metadata and Headers) will describe the flavor. For example, tags will say that the data is an image, but the descriptor will say whether it is B-band or R-band. Tags are used for recipe and primitive selection. A way to understand the difference between a tag and a descriptor is in terms of the recipe that will be selected: A GMOS image will use the same recipe whether it’s a B-band or R-band image. However, a GMOS longslit spectrum will need a very different recipe. A bias is reduced differently from a science image, there should be a tag differentiating a bias from a science image. (There is for GMOS.)

For more information on adding to Astrodata, see the Astrodata Programmer Manual.