- 2004.11.15
Recently two of my students were selected to participate in the
Teacher Observing Program at Kitt Peak National Observatory
through a program sponsored by the National Optical Astronomy
Observatory in Tucson, Arizona. This involved making a proposal for the
use of a 36" telescope (for you non-astronomers, that's bigger than
what most amateurs own but on the small end of professional-class
scopes). My students suggested observing BL
Lac, a blazar or active galactic nucleus, and various globular
clusters to observe RR Lyrae
variables. A brief overview of our experience is posted in Teacher
Leaders in Research Based Science Education.
The purpose of this article is to describe the technical problems we
encountered and solved along the way, provide assistance to others who
may face similar problems, and make comments about the state of
advanced scientific software for the Macintosh.
The Task
When using professional-class telescopes, astronomers are interested
in isolating the true signal of an object being imaged from the noise
or interference in the telescope and camera that can obscure the thing
you're trying to measure. Telescopes use CCD cameras similar to those
used in digital cameras, but they require careful calibration to remove
sources of unwanted signal.
Calibration of the camera involves taking "bias frames" to show how
electrical signals used to read the camera's picture may provide false
spots in the image, and "flat frames," which are pictures of blank
white spots on the observatory wall, to show how dust in the optical
path may cast shadows on the image, making parts of the image
artificially brighter or dimmer than it would otherwise appear.
In practice, a user must apply these corrections to each picture
taken to remove these unwanted signals so the remaining image is pure
signal with no static, so to speak. Then measurements of brightness and
other characteristics can be taken that are scientifically
meaningful.
An excellent tutorial on the details of this process is
CCD Observing.
Using the camera provided us with a collection of dark frames, bias
frames, and "raw" images of our various target objects. Our task was to
combine these into a single calibrated image and then take
measurements.
All of the images generated by this program are in FITS format. FITS
is a file format that preserves the original data as read out on the
chip in the camera. Unlike JPEGs or other formats, altering the
appearance of a FITS file does not alter the original data - just how
it is displayed. This is pretty much a universal standard among
professional astronomers.
All of our computers were Mac laptops. We had access to the
following pieces of software. Links are included to the sources users
can get to these. The limitations of each piece of software are
noted.
NIH Image:
This freeware program for Mac OS 9 can open FITS files, but it
requires customized Pascal-like macros to do basic astronomical
functions such as brightness counts (adding up the counts detected in
each pixel of a star's image). The macros available for us to use (from
the Teacher Leaders in Research Based Education project - not available
for download) would not open the images provided by the telescope we
used.
Image
Processing: This program from the Hands-On Universe project
could open FITS files we'd collected in other places (if you ignored
some user warnings, etc.) Unfortunately it would not open images we
took ourselves at the Kitt Peak telescope.
Image J: This
Java-based successor to NIH Image opens everything just fine, but it
doesn't yet have the specialized macros needed by amateur and
professional astronomers to perform measurements. I got a copy of a
beta set of macros under development, but it isn't functional yet. Like
NIH Image, most plugins and macros available on the Web are aimed at
medical applications.
IRAF: This Unix-based program is what is used by most
professional astronomers to analyze images. It is very powerful,
difficult to use, and uses many arcane text-based commands similar to
Unix commands. Installing it is not a simple task, but it is doable. An
experienced power user who encounters no problems can finish the
installation in about two hours. I encountered problems, so it took
perhaps three days of off-and-on work to solve all the issues before
IRAF became functional. This was only possible at all because of the
very thorough and well-written set of instructions at Marcos Huerta's
website.
This points out a common problem with science software for the Mac.
Many professional astronomers use Macs because they need easy access to
Unix. I've met several who use them for exactly that reason. These
programs represent the low end of the spectrum (such as NIH Image)
where programs are available, but in some limited way or for
specialized educational purposes. At the other end is the high-powered
IRAF, which does anything but has a steep learning curve.
There isn't anything in the middle for Mac users equivalent to a
program like Maxim DL
for the PC, for example, which is what most amateur astronomers are
using for this kind of work these days.
Mac users wanting to work with Maxim DL have no choice but to use
Virtual PC, which is pretty much an advertisement for getting a real PC
- just functional enough to run real PC software, but slow enough to be
annoying to the point of wanting a real machine.
This is why I say Mac astronomical image processing software is
polarized. There are the low-end educational versions (great at what
they are designed for, but limited) and the high-end (IRAF, which, by
the way, doesn't run in Windows at all), but there's nothing in
between.
There are a number of programs that show promise but are not
complete packages. The most useful of these is Keith's
Image Stacker, which can be used to combine bias and dark
frames as well as "stack" images to enhance contrast. You cannot take
measurements such as brightness counts with KIS, however, and my
experience was it could not open my FITS files for technical reasons
I'll explain in a separate article. For what it does, it is really
useful, but it doesn't do exactly what I need, which is sort of in a
gray area between amateur and professional needs.
There has been an ongoing discussion
thread about these and other astronomical issues sponsored by
the Applelust site. If you are interested in astronomy and use a Mac,
you should subscribe and read the astronomy related articles posted at
Applelust.
In a more technical treatment on
our space academy website, I detail how I installed IRAF using
Huerta's instructions and help files, and then how I managed to convert
the images from the format we got them in into a format our older
software at school could read as well as how we generated some
preliminary data from the images.
is a longtime Mac user. He was using digital sensors on Apple II computers in the 1980's and has networked computers in his classroom since before the internet existed. In 2006 he was selected at the California Computer Using Educator's teacher of the year. His students have used NASA space probes and regularly participate in piloting new materials for NASA. He is the author of two books and numerous articles and scientific papers. He currently teaches astronomy and physics in California, where he lives with his twin sons, Jony and Ben.< And there's still a Mac G3 in his classroom which finds occasional use.
