1.1 File formats
So far, this course has been primarily interested in producing visually pleasing images. In Week 6 you learned how to edit your images, making adjustments to brightness, contrast and colour balance to produce the best effect.
Files for this type of image are usually saved in a format such as .jpg or .png, which are suitable for display on a computer screen. The COAST images that you have saved so far are in the .png format.
In this week’s activities you will be making scientific measurements from your images and this involves saving them in a different format which contains more information.
Why might images saved after editing not be suitable for scientific analysis?
Adjustments such as contrast and brightness made to the images could change the results of any measurements made from them. In order to make scientific measurements, and especially in order to compare results taken from different images, it is important to look at raw data, before any adjustments for visual effect have been made.
Another reason for using a standard scientific format is so that astronomers can compare results from images taken using different telescopes and different imaging system.
What information about an image might be useful to astronomers when exchanging images? Think about all the things that you specify when requesting an image.
As well as the raw data, it would be useful to know the celestial coordinates, the date and time that the image was taken, the exposure time, and what filter was used.
For this reason, astronomers use an astronomical data format called FITS (Flexible Image Transport System). This format allows astronomical imaging software to store information about the image together with the raw data making up the image in a single file, known as a FITS file. When exchanging images, all the information is kept with the image in this single file which saves having to keep separate notes of filters, coordinates and exposure times. If you receive a FITS file from another astronomer it is possible, by looking in the FITS header, to find out all the information about when it was taken, where the telescope was pointing and all the exposure and filter details.
Another consideration for making scientific measurements is the quality and amount of detail in the numerical values. Digital images are made up of individual pixels, with the brightness of each pixel represented by a number. Most frequently, zero represents pitch black and the highest numbers on the scale represent white, with numbers in between denoting different levels of grey.
In normal image files such as .jpg or .png each pixel is represented by an 8-bit number which can represent one of 256 different brightness levels. FITS images store the brightness of each pixel as a 16-bit number, which allows a much wider range of 65 536 different brightness levels. This is known as the bit depth of the image.
FITS files have a bit depth of 16. Being able to cover such a wide range of brightness levels is especially important for astronomical images, which typically have very bright objects such as stars (represented by high values in each pixel) against a very dark background (represented by pixels with very low values).
FITS files are also uncompressed, meaning that a full 16 bit value is saved for each pixel in the image. This can result in much larger files than compressed images such as .jpg files, but it means that all of the information is available for scientific measurements.
For all of these reasons, most astronomical image processing software is designed to work with FITS image files, including the software that you will use to measure the brightness of your variable star.
Fortunately, COAST has the option to save images in this FITS format, which is ideal for the processing that you are about to do.