How HiRISE Works

 

Lesson One: Camera Basics

 

 

 

Usually when we think of a photograph we think of an image taken all at one time, like this photo of Yosemite.  It is made up of 443 columns and 674 rows of pixels, meaning the camera captured almost 300,000 pixels at once.  Typical digital cameras today can capture 4-8 million pixels in all at once.

With HiRISE, we will often take pictures with more than a billion (yes, that is with a "B"!) pixels.  Each HiRISE pixel only covers about 0.1 square meters (1 square foot) but we want to examine areas that are more than 72 square kilometers (or 28 square miles) in a single picture.  We just can't build electronics that can capture that many pixels all at once.  Instead, we use the "push broom" method.

 

A Push Broom Camera?

 

A push broom comes in handy when sweeping a long hallway, since you can make long sweeps to pick up an entire swath of dust.  Likewise, HiRISE doesn't take a single image of an entire scene all at once, but instead builds up a picture by "sweeping" a swath of Mars.  As the MRO spacecraft speeds over the surface of Mars, the camera builds up the picture by grabbing one row of pixels at a time.  This means HiRISE has to deal with no more than about 20,000 pixels at a time.

 

When sweeping a hallway, the width of the swath you clean with each push of the broom just depends on how wide the broom is.  How far you push the broom controls how much hallway gets cleaned with each stroke.

For HiRISE, the width of the image is controlled by how many detectors we use (we can have images that are 10 detectors wide with 2000 pixels in each detector).  But the length of the image just depends on how many rows of pixels we choose to collect.  We choose this to balance how much ground we cover versus how much data we can store on the spacecraft, how much data we can radio back that day, and how hot the electronics would get.  The camera's internal memory can hold just over a billion pixels.  The data then get transferred to the spacecraft's memory (which acts like a hard drive) and then is sent back to Earth via a radio link. 

 

 

 

 

 

The Inner Workings of HiRISE

 

This drawing shows how light (shown by the yellow arrows) enters the front of the camera, is gathered by the 50 centimeter (20 inch) diameter 'primary mirror' and then is sent by a series of additional mirrors to be focused onto the detectors.


 

 

 

 


Below is the actual HiRISE mirror sitting in the laboratory before installation into the camera. The mirror is 50 cm (about 20 in) in diameter. The hole in the middle allows light that has reflected off the secondary mirror to pass through to the detectors that sit behind the mirror.

 

 

 

The HiRISE 'focal plane array', shown here in the laboratory, consists of 14 linear CCDs, each sitting behind a filter.  Most of the CCDs use a red filter that just allows red light to enter the detector.  In the very center of the camera, there are four extra CCDs, two each behind blue-green and infrared filters.  Therefore, the central 20% (2/10) of each image will be obtained in 3 different filters, allowing image-processing specialists back on Earth to reconstruct the central part of each image in color.  A simulation of a HiRISE image is shown on this page.

 

 

 

HiRISE Works Well With the Other MRO Instruments

 

While HiRISE is obtaining a very high-resolution image of a narrow swath of Mars' surface, the other MRO instruments will be busy as well.  The CTX imager will provide a wider-angle view of the same region to help place the HiRISE image into the context of its surroundings.  The CRISM multi-spectral imager will obtain simultaneous images in the near-infrared spectral region to help identify the surface composition.

 

To learn about the resolution of HiRISE images, move on to Lesson Two.

 

Back to HiRISE Learning Page.