The Polarized State of Astronomical Image Processing Software for the Mac

- 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.