Celestia Forums

Hi there. I'm a student, and my local observatory has charged me with making a computer model of the local cluster of galaxies. I've decided on using Celestia to facilitate this. However, I've encountered a couple of issues: most importantly, even the limiting magnitude at maximum, galaxies appear as tiny specks of light, while I'd need a more noticeable depiction, maybe a fixed sprite if the distance to a galaxy is greater than some value. Secondly, I'd like "go to" to go to the centre of each galaxy. Can this be done using scripting?

egasimus wrote:Hi there. I'm a student, and my local observatory has charged me with making a computer model of the local cluster of galaxies. I've decided on using Celestia to facilitate this. However, I've encountered a couple of issues: most importantly, even the limiting magnitude at maximum, galaxies appear as tiny specks of light, while I'd need a more noticeable depiction, maybe a fixed sprite if the distance to a galaxy is greater than some value. Secondly, I'd like "go to" to go to the centre of each galaxy. Can this be done using scripting?

You must activate the 'automag' feature (CTRL+Y) in order to see dim galaxies properly. This feature can also be activated via scripting. Since we have two scripting languages (LUA and cel scripting) you better say first which one you want to use.

Fridger

Well, what are the pros and cons of either? I have some programming experience so if you could give me a few pointers...

Cel scripts are simple linear lists of Celestia commands. The language has no loop structures, for example.

CelX scripts are written in Lua. Lua is a full-fledged object oriented embedded programming language. It can be difficult to understand by people who aren't familiar with modern programming practices.

Well, I'm quite familiar with programming.
What I'm thinking of making is creating an entirely new universe which contains only the galaxies of the Local Group, maybe with some fancier models than the ones currently present, and the ability to travel from galaxy to galaxy, and also to observe all galaxies from a distant point of view. Which always looks at the selected galaxy, putting it in the centre of the screen, and can rotate around it. Also, maybe a circle representing the supergalactic plane, and lines straight up and straight down to galaxies to represent their Y coordinate... something roughly like this, but more realistic:


Is that sort of thing possible?

egasimus wrote:Well, I'm quite familiar with programming.
What I'm thinking of making is creating an entirely new universe which contains only the galaxies of the Local Group, maybe with some fancier models than the ones currently present, and the ability to travel from galaxy to galaxy, and also to observe all galaxies from a distant point of view. Which always looks at the selected galaxy, putting it in the centre of the screen, and can rotate around it. Also, maybe a circle representing the supergalactic plane, and lines straight up and straight down to galaxies to represent their Y coordinate... something roughly like this, but more realistic:


Is that sort of thing possible?

Yes, that's easily possible.

My galaxies.dsc (= deepsky.dsc) database within the Celestia distribution contains ALL known galaxies of the local group (including dwarfs in a Volume <~ (10 Mpc)^3) (Note the name of the galaxy data file changed after Celestia version 1.5.x). The citation of the many scientific catalogs used, you find in the "boiler plate" of my galaxy data file in the Celestia 1.6 distribution @SVN:

galaxies.dsc wrote:# Revised NGC and IC Catalog, Wolfgang Steinicke, January 6, 2006
# http://www.ngcic.org/steinicke/default.htm
#
# Augmented by galaxies from Local Group volume (V <~ (10 Mpc)^3)
# Catalog of Neighboring Galaxies (Karachentsev+, 2004)
# http://cdsweb.u-strasbg.fr/viz-bin/Cat?J/AJ/127/2031
#
# Augmented by averages of distance entries from
# NED-1D: NASA/IPAC Extragalactic Database of Distances
# http://nedwww.ipac.caltech.edu/level5/NED1D/
#
# Augmented by
# NASA/IPAC Extragalactic Database (NED, batch)
# http://nedwww.ipac.caltech.edu/
#
# Augmented by
# Revised 3rd Reference Catalog of Bright Galaxies (RC3,VII/155)
# http://cdsweb.u-strasbg.fr/viz-bin/Cat?VII/155
#
# Augmented by
# Mark III Catalog of Galaxy Peculiar Velocities (Willick+ 1997,VII/198)
# http://cdsweb.u-strasbg.fr/viz-bin/Cat?VII/198#sRM2.18
#
# Augmented by distances from
# The SBF Survey of Galaxy Distances. IV.
# SBF Magnitudes, Colors, and Distances,
# J.L. Tonry et al., Astrophys J 546, 681 (2001)
#
# Augmented by distances from
# Compilation of "200 Brightest Galaxies"
# http://www.anzwers.org/free/universe/galax200.html
#
# Augmented by redshifts-> distances from
# VII/193 The CfA Redshift Catalogue, Version June 1995 (Huchra+ 1995)
# http://cdsweb.u-strasbg.fr/viz-bin/Cat?VII/193
#
# Using today's Hubble constant = 73.2 [km/sec/Mpc]
# (WMAP 2007, 3 years running, legacy archive)
#
# Abreviations for various distance methods used:
# SBF= SBF (Surface Brightness Fluctuations), T-F= Tully-Fischer
# V_cmb = rad. velocity in CMB frame & Hubble law
# cep=Cepheids, P=photometric, N(G)=planetary nebula (globular cluster) luminosity function
#
# Adapted for Celestia with Perl script: deepsky.pl Revision: 1.50
# Processed 2007-5-18 5 137 0 18:10:24 UTC
#
# by Dr. Fridger Schrempp, fridger.schrempp@desy.de
# ------------------------------------------------------

The commented Perl script of > 1200 lines that I wrote for generating the database, you find in the Celestia sources (-> src/tools/galaxies).

There is the option of including custom templates for individual galaxies. The rest I would do with a simple celx script.

Yet, I am sceptical about your sentence " ...maybe with some fancier models than the ones currently present", since the present galaxies are more "fancy" than they might look on first sight . Take for instance the MilkyWay itself: In the following screenshots, you see how well its arms are mapped out by the known locations of the pulsars (yellow & red squares):

Have a click for kingsize (important!)!

The green cross marks the sun's position in Orion's arm of the MW. The line from there to the center of the MW corresponds to zero galactic longitude.

The sideways views also confirm the thickness profile of our MilkyWay from the scatter of the pulsar locations! The yellow crosses mark the precise location of the galactic plane whence the perfect alignment of the Celestia MilkyWay becomes obvious...

Have a click for kingsize (important!)!

The second image shows sideways views of the alignment quality where the top and bottom images differ by a 90 degree rotation within the galactic plane. Look for the sun's position...

Finally, it takes some practice to achieve the correct display settings for galaxies (in terms of zoom factor and observer distance from a galaxy) ... See e.g.

viewtopic.php?f=2&t=13348

Fridger

Thanks a lot! (: I've had a look at the celx scripting documentation (here; is that the most recent version?). By "fancier", I meant maybe using some of those brightly coloured composite images. By the way, could I display a fixed-size sprite instead of an object when the distance to the object is too big? And be able to turn this on and off, of course.

And, how would I draw the Supergalactic plane?

egasimus wrote:By "fancier", I meant maybe using some of those brightly coloured composite images.

But these are typically photographs and only 2D rather than 3D. In addition, using photographs means you must enter them one-by-one as add-ons and align them properly (which is tedious and not easy) ...

Remember, the local group comprises quite a few galaxies which was the reason I rendered them entirely by computer!

The custom option I was talking of, refers to using custom templates instead of the generic ones (*.PNG) in the 'models' subdirectory of Celestia. The templates mark the dots where the code associates sprites of varying sizes and brightness later-on in the computer-based rendering.

They look typically like this (e.g. SBa.png for the SBa Hubble type):


Fridger

Sorry, I meant to say using something akin to those images. Can there be a custom template for every galaxy, and do the templates support differently coloured sprites? If so, those templates may be exactly what I need.

Nope, doesn't react to colors. I could try and hack this feature into Celestia, but certainly not in time.

egasimus wrote:Sorry, I meant to say using something akin to those images. Can there be a custom template for every galaxy, and do the templates support differently coloured sprites? If so, those templates may be exactly what I need.

No, the templates are insensitive to color, since they just serve as 2D bookkeeping (128x128) for the sprite positions. During the rendering step, a color table is scanned as function of the distance from the galaxy center, and the colors (RGB) & brightness values (Alpha) are then just entered for each GL_QUAD via the standard OpenGl glColor4f method

glColor4f(c.red(), c.green(), c.blue(), (4.0f*lightGain + 1.0f)*a);

This generic color profile I determined from actual true-color galaxy images from Hubble (M 76 etc). With > 10000 galaxies being rendered in Celestia, speed is an important requirement. Hence some simplifications are largely dictated by this. Also preparing and saving a dedicated color profile for each of the 10000+ galaxies is kind of a chore

Fridger