Started by Sebastian Jester

More info

See dvo links on IPPwiki front page, and in particular DVO Challenge for examples of dvo command usage, and ​dvo.pdf for background (even though dvo.pdf is quite outdated, it's the only place where you can read about dvo's design principles...).

dvo concepts

Skycells, Objects, measurements, averages, photcodes, CATDIRs

dvo has a spatial index

dvo stores data in subdirectories of a CATDIR that correspond to different parts of the sky. This naturally allows comparatively fast spatial indexing.

Correspondingly, dvo operations are typically done on a given <code>region</code> of the sky.

Objects, measurements, averages

dvo uses two types of tables

• Measurement tables contain all individual measurements from different epochs and filters in a giant table. There is originally no notion of which measurements correspond to the same astronomical object, and which to different ones.

• Average tables know which measurements belong to the same objects, and record the average value, scatter, and error in the mean of the same object over different epochs, for each filter. They also record the proper motion and parrallax (if it has been determined).

Both sets of tables, the links between them, and derived quantities are created or computed by a set of commands outside of dvo (<code>addstar, relphot, relastro</code>). Once you start dvo, everything you need should already be there...

photcodes

A photcode is an internal identifier of a photometric system. dvo distinguishes between different kinds of photcodes, depending on which kind of information is accessible in addition to the photometric catalog, and how the photometric data were computed. dvo allows conversion and cross-calibration between different systems.

• ref (reference) photcodes are for external catalogs. Typically, only the photometric data and a (rough) epoch are available, but it is not possible to access the original images or instrumental magnitudes.
• dep (dependent) photcodes are for measurements where all input data are accessible, i.e. images and instrumental magnitudes, zero points, air mass, observing time etc. Measurements are reported in dep photcodes. There can be multiple dep photcodes for a given bandpass, eg. in PS1, every chip/filter combination (300 of them) gets its own dep photcode.
• average photcodes are for derived quantities such as averages of dep measurements, where there isn't an image that can be identified as the origin of the photometric quantity. These are meant to be generic filters, e.g. the (abstract) average PS1 r-band filter.

Existing dep and ref photcodes can be averaged into average ones.

Installing dvo

dvo comes as part of IPP - see Installation-related links on IPPwiki main page. Talk to your local IPP guru about where it's installed. We'll also be aiming to make a standalone dvo-only tarball for use on laptops.

In Heidelberg, see /IPP/README on the IPP cluster (and read all of it!)

Using dvo

Startup

dvo -D CATDIR /path/to/catdir

e.g. in Heidelberg:

dvo -D CATDIR /IPP/data/GPC1/flatcorr_20081029/catdir.flatcorr

Getting help

Most dvo commands have a help text that can be read with

help command

The dvo shell allows tab completion of commands and filenames.

Most commands will also com/explain what they expect if called without arguments.

Setting the <code>region</code>

Set the region for plotting and plot a coordinate grid:

region 314.7428 37.1718 3 cgrid

Set the region for catalog extraction:

skyregion 310 320 33 40

Loading data

Show which images are present in the current skyregion:

images

Extracting average quantities into vectors (similar to those in sm, or arrays in IDL or C etc.) is done via <code>avextract</code>; from the currently defined region by giving the <code>-region</code> argument:

avextract -region RA DEC g r i y [... takes a while for big regions with lots of measurements ...]

NB: From looking at the source code, the syntax is supposed to be:

-region # take ra,dec limits from currently defined plotting 'region' -skyregion ramin ramax decmin decmax # use ra,dec limits as given no -region or -skyregion command # use previously defined skyregion

However, with the <code>-region</code> setting, the output list does not actually seem to be affected by changing the <code>region</code>. I.e. better use the <code>skyregion</code> variant!

Also note that there is only ever one active skyregion, ie. <code>avextract -skyregion</code> changes the skyregion in the same way as the <code>skyregion</code> command. You can see the currently active one with the

skyregion

command.

In addition to the quantities themselves, there is both an uncertainty and scatter; I think, usually the scatter is reported in the <code>:err</code> version of quantities; for RA and DEC, the error is in dRA, dDEC (see <code>avextract --help fields</code>:

avextract -region RA dRA DEC dDEC g g:err r r:err i i:err y y:err

Note there have been problems with the calculations of some scatter/error columns, so not all of them may be there.

To see which vectors you've loaded, say

vectors

There are two more <code>extract</code> commands:

• <code>mextract</code> extracts measurements; however, one can't compute colours from these (e.g.) because they are returned without any notion of which entry in the vector belongs to which object
• <code>mmextract</code> extracts pairs of measurements belonging to the same object

Searching

Restricting output of avextract etc. to interesting objects

The <code>avextract</code>, <code>mextract</code> and <code>mmextract</code> commands allow restricting the rows that are returned by adding a SQL-like <code>where</code> clause:

avextract -region RA DEC g r i y where g-r < 0

Find out which images contain a given ra, dec

gimages 314.7428 37.1718 -region

Plotting and computing

To plot two vectors against each other, say

limits g r box plot g r

To compute and plot new vectors, say

set gr = g - r clear limits gr r box plot gr r

To plot things in sky coordinates, use <code>cplot</code>:

cplot RA DEC

Things we want to know/do

M31 / local group

• Load M31 etc. data into dvo

• Pull out star/galaxy separation flag, restrict sample to stars
• Filter on magnitude errors
• Dump ra dec mag magerr stargalflag into ASCII file for customized analysis