A new star catalog
is always a cause for celebration. Anyone who does more than dabble
at astronomy visualization realizes that it is getting ahold of the
data that is the sine qua non of serious work.
And yet getting the
data is just the first of your many challenges. Very soon after downloading
the data you will realize that one needs to have done some advanced
study in astrophysics and/or have the services and advice of an
astrophysicist or two in order to get anywhere. Without one or the other or both, the would-be visualizer will soon feel that he or she is lost in the forest on a moonless night.
But getting the data
is the first necessary step, so it was with great pleasure that I
read about the newly announced star survey with the catchy title of
The second data release of the INT Photometric Hα Survey of the
Northern Galactic Plane (IPHAS DR2)
And
what is in this fabulous new
survey?
The INT/WFC
Photometric Hα Survey of the Northern Galactic Plane (IPHAS) is a
1800 deg2 imaging survey covering Galactic latitudes |b| < 5° and
longitudes ℓ = 30°–215° in the r, i, and Hα filters using the
Wide Field Camera (WFC) on the 2.5-m Isaac Newton Telescope (INT) in
La Palma. We present the first quality-controlled and globally
calibrated source catalogue derived from the survey, providing
single-epoch photometry for 219 million unique sources across 92 per
cent of the footprint. The observations were carried out between 2003
and 2012 at a median seeing of 1.1 arcsec (sampled at 0.33 arcsec
pixel−1) and to a mean 5σ depth of 21.2 (r), 20.0 (i), and 20.3
(Hα) in the Vega magnitude system.
And so forth.
There are
219,000,000 unique sources in this survey. That is quite a bit more
than the 10K star Yale Bright Star Catalog that we all grew up on. (1)
And yet there are
certain similarities when you look a bit closer. Certain subtle indications that suggest that the compilers of this fabulous expanse of data are adepts of the esoteric knowledge of astrophysics and that this data is intended for other members of this elite group.
In order to brace you for the tasks to come, should you decide to take on the challenge of visualizing this august data set, this post will present some first principles that are important for someone from the world of 3D as they step into the galaxy of creative decisions to be made and the arcane knowledge to master before the images latent in this esoteric compendium will be revealed.
I myself was first introduced to this knowledge during my brief but rewarding tenure on the Digital Galaxy Project portion of the rebuild of the
Hayden Planetarium at the American Museum of Natural History in New
York City. Everyone on that project was a dedicated idealist with stars in their eyes.(2)
Because, you see,
this dataset, like most astronomy datasets, is not intended to be for
computer animation people. It is first and foremost to help
scientists in the field, both those associated with the survey itself
and those in the larger field of astrophysics, do their research. If
people want to use it to “visualize the data” or make interesting
pictures, that is all well and good, but that purpose is much further
down the list of intended uses.
If we want to
visualize the night sky what information would we ideally want?
Well, we might want the 3D position of the stars, the brightness of
the star, the color and presumably the type of star, as well as
information about other objects in the sky that are not stars.
This is a short list, there are actually many other things one might
want to know for visualization purposes, but lets stop here.
Ironically, a normal star survey or catalog does not generally have
any of this information. At least not directly.
That is because what
scientists are looking for in a star survey are the facts as they
were observed. They may also be interested in your interpretations
of the facts, but first and foremost they are interested in the data
and how it was collected.
That means that
instead of the 3D position of the “star” which we do not observe
directly, except in a few exceptional cases, what the star catalog
has is the position of the star as observed from earth on an
imaginary 2D spherical coordinate system, a virtual sphere around the
earth. In other words, the catalog does not say where the star is,
but rather what direction it was observed to be from earth. The
survey does not contain how bright the star was, or even if it was a
star, but rather the spectra and intensity of the energy source as
observed with the specific instrument. From that spectra, and from
other information about the source of energy, one can deduce the type
of star it probably is, but that is an interpretation of the data,
not the data itself.
The star catalog or
survey will not report the “color” as one normally thinks of such
things, whether in RGB values, or CIE or other systems, but rather
what was observed by the specific instrument or instruments used to
collect the information. From the information in the catalog and the
operating characteristics of the sensor one can derive spectra and
from that a handy “color” in the visible bands if one so desires.
Nor is there any
particular guarantee that the catalog is comprehensive, in some
normal sense of the word. The authors of the catalog are not
guaranteeing you that this survey contains all possible “stars”
in the sky, or part of the sky. Instead what they are reporting here
is the information about what they observed, and often some measure
or estimate of how complete it is. The famous Messier catalog is not
a complete list of anything, except the list of objects that Messier
did not want to be bothered with anymore in his search for comets.
In a similar
fashion, the data does not tell you how bright the star actually is,
if it is a star, but tells you the intensity of the source as
observed from earth. Again, an astrophysicist has various techniques
to convert the apparent magnitude of the source to the absolute
magnitude, if that is what you need.
The new star catalog
has 219 million unique sources which may or may not be stars, and a
variety of information about each, including some derived information
(e.g. a probability that the specific source is indeed a star).
But it won't tell
you anything about ISM (interstellar media), H2 regions and so forth.
When downloading the data, you can choose to download prepared datasets of the survey, or you can choose which data fields you want to review. There are 99 different fields, and a dozen or so different formats you can get them in. Just getting the data will probably take several days.
This post is the tip
of the iceberg of what you need to know to do visualization from
astronomy datasets but we will reserve these other topics for later
posts.
The announcement can
be found here
The database can be
found here
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Notes
1. The Yale Bright
Star Catalog is a famous dataset of the 10,000 or so brightest stars
visible from earth. These are the stars that one could typically see
from earth on a clear night with the naked eye from somewhere on the
planet. It is a famous catalog in part because it was widely
distributed even in days before the Internet or the Arpanet.
2. There were many,
many people on this project which was part of a much larger project
to rebuild the Hayden Planetarium and the entire north side of the
American Museum of Natural History. Many people thought we were from Mars. The cast of characters included such luminaries as Dennis Davison, Neil deGrasse Tyson,
Carter Emmart, Frank Summers, Aram Friedman, Loretta Skeddle, Julio
Morano, Benjy Benjamin, Ron Drimmel and many others. And of course, your humble author, was also briefly a consultant on this project.
