Frequently Asked Questions about HiRISE

General Interest
Why do we need another camera around Mars?

Mapping and Geometry
What is EQUIRECTANGULAR Map Projection?
What is POLAR STEREOGRAPHIC Map Projection?
What is Emission Angle?
What is Phase Angle?
What is Solar Incidence Angle?
What is Solar Longitude?
What Does “Longitude (East)” Mean?
What Does “Range to target site” Mean?
What Does “Original image scale range” Mean?

The Instrument and Imaging Modes
What is a CCD?
What is Binning?
What is TDI?

Color
What do RED, BG, and IR mean?
What are the BG and IR EDRs, and How are They Different from RED?
What does “false color” Mean?

Compression
What does “lossy compressed” Mean?
What is a “QuickLook” Image?

Data Processing
How Do You Process the Raw Data?
How Can I Process Raw HiRISE Data Myself?
How Long Does it Take for an Image to Get Processed?
What are the Black Boxes within Some Images?
Why are the Images Tilted with a Black Border around Them?
Can I get Unprojected (i.e. unrotated) Images?
How Long Does it Take for New HiRISE Images to be Released to the Public?

Stereo and Anaglyphs
What are Stereo Pairs and Anaglyphs?

PDS Data Products
How can I access the PDS HiRISE volumes?
What is the Difference between EDRs and RDRs?

Science Themes
What's a "Science Theme"?
What's the "Other" science theme?

Suggestions & HiWish
Can I make a HiRISE Target Suggestion?
My registration failed? What can I do?
Why can't I save my suggestion? The button is grayed-out!
I filled out the suggestion, but can't get to the next step. What's wrong?
Where Is the Face On Mars?
What are some strategies to improve my suggestion's chances?
How long might it take for my image to be taken and released?


What is EQUIRECTANGULAR Map Projection?
A map projection is an algorithm for mapping a three-dimensional object onto a two-dimensional surface. In this equirectangular projection, the meridians (vertical lines) and parallels (horizontal lines) are perfectly straight and equally spaced. We use the equirectangular projection when processing images whose center latitude is between 65° S and 65° N. For further information, see the USGA Map Projections page.

What is POLAR STEREOGRAPHIC Map Projection?
A map projection is an algorithm for mapping a three-dimensional object onto a two-dimensional surface. Polar stereographic projection is a special map projection used just for the North and South polar regions (areas north of 65° N and south of 65° S). In this projection, the directions are true only from the center point of projection (the pole). Scale increases away from the center point, and any straight line through the center point is a Great Circle (a circumference around the planet). For further information, see the USGA Map Projections page.

What is Emission Angle?
Measured from the center of the image, this is the angle between the HiRISE camera and a "normal" drawn perpendicular to the planet's surface. When HiRISE is looking straight down ("nadir"), the emission angle is 0°.

What is Phase Angle?
This is the angle between the sun, the surface, and the HiRISE camera at the time the picture was obtained.

What is Solar Incidence Angle?
Derived for the center of the image, this is the angle between the Sun and a "normal" drawn perpendicular to the planet's surface at the time the image was acquired. A higher incidence angle means that a person standing on the ground would see the sun lower toward the horizon.

What is Solar Longitude?
Commonly referred to as Ls (pronounced "L sub S"), this is the position of Mars relative to the Sun measured in degrees from the vernal equinox (start of northern Spring). This number is used as a measure of martian seasons: Northern Spring/Southern Autumn start at 0°, Northern Summer/Southern Winter start at 90°, Northern Autumn/Southern Spring start at 180°, and Northern Winter/Southern Summer begin at 270°. Click here to compute Ls on Mars for any date on Earth.

What Does “Longitude (East)” Mean?
Most organizations use East Longitude for Mars, which means that longitude is measured eastward from zero at Mars' prime meridian (centered on the feature known as Sinus Meridiani). But some groups do use West Longitude, so it's always important to specify East or West.

What is a CCD?
A CCD (Charge-Coupled Device) is a piece of silicon with electronics used to capture incoming light instead of a piece of film. CCDs are far more sensitive than film, and are thus superior for space photography. Your digital camera probably uses a CCD (though most cell phone and computer cameras use a different type of technology).

What is Binning?
Binning is a process that allows the charge from multiple adjacent pixels to be combined into one pixel. This increases the camera's light sensitivity and improves the signal-to-noise ratio. Binning occurs in the cross-scan and down-scan directions. "Bin 1" means that there has been no binning at all (because it means one pixel equals one pixel). "Bin 2" means that a 2x2 block of pixels were combined into one; this reduces the resolution by half, reduces the data volume by a factor of four, and increases the signal (i.e. light sensitivity) by a factor of four. Also, the signal-to-noise ratio is increased by a factor of two (if the same TDI setting is used for the binned and unbinned images).

What is TDI?
TDI (Time Delay Integration) is a method for increasing signal when there is fast relative motion between the camera and the object being imaged. We have four different TDI choices: 128, 64, 32, and 8 lines, so that we can image a spot up to 128 times and add up the signal. This is also the method we use to vary our exposure level, since our line time is fixed with altitude. With TDI an image is read one row at a time, from the bottom of the CCD to the top. As each row is read, signals are shifted. If we do not properly match our TDI readout rate (same as line time) to the velocity of the spacecraft, our images will be blurred.

What Does “Range to target site” Mean?
This is the distance between the HiRISE camera and the spot on Mars captured in the image, in kilometers.

What Does “Original image scale range” Mean?
"Original image scale range" is the initial pixel scale (in meters/pixel), which may vary if some CCDs are binned more than others. If a combination of bin 1 and bin 2 is used, and the bin 1 scale is 0.3 meters/pixel, then the original image scale range is 0.3-0.6 meters/pixel. Map-projected images have been re-sampled to either 0.25 m/px (bin 1), 0.5 m/px (bin 2), or 1.0 m/px (bin 4).

What do RED, BG, and IR mean?
These are shorthand titles for the different types of CCDs HiRISE has. The HiRISE camera has three different color filtered CCDs: red ("RED"), blue-green ("BG"), and near-infrared ("IR"). The wavelengths of these filters are as follows: RED: 570-830 nanometers BG: <580 nanometers IR: >790 nanometers

What are the BG and IR EDRs, and How are They Different from RED?
There are ten RED CCDs, two BG CCDs, and two IR CCDs. Combining the images taken by the three different color filters allows us to create "false" color images. The BG and IR CCDs are aligned with the center two RED CCDs, providing a two-CCD-wide color swath. This means that the images captured in the BG and IR products are aligned with the images captured in the RED4 and RED5 CCDs.

What does “false color” Mean?
"False" color means that the color you see in HiRISE images is not the "true" color human eyes would see on Mars. This is because the HiRISE camera views Mars in a different part of the spectrum than human eyes do. Nevertheless, false color imagery is extremely valuable because it illuminates the distinction between different materials and textures.

What does “lossy compressed” Mean?
Lossy compression reduces data volume, but with a loss in quality. Your digital camera probably uses lossy compression on the images it takes.

What is a “QuickLook” Image?
Quicklook products are full-resolution images that have undergone lossy compression (so they are much smaller in size). The lossy compression we use for these is very conservative, so the loss of image quality is not noticeable.

How Do You Process the Raw Data?
We have processing pipelines that perform a variety of procedures on raw HiRISE data. Basically, the images are radiometrically calibrated (to correct stripes, mismatch, and other camera artifacts), "stitched" (to mosaic together the image strips acquired by the individual CCDs), and geometrically projected (to map the image onto planetary coordinates). For more information on how we process our data, see: HiRISE_RDR_SIS (PDF)

How Can I Process Raw HiRISE Data Myself?
Anyone can download and process HiRISE data. You can do this using ISIS software, which is publicly available for free download. See the ISIS Web site for download information, processing instructions, and tutorials.

How Long Does it Take for an Image to Get Processed?
After an image is acquired by the HiRISE camera, it takes about 15 minutes to transmit it from the spacecraft to Earth, via the Deep Space Network (DSN). It can take a few hours for the data to be transmitted from the DSN to JPL, and JPL then sends the data to HiROC within an hour. Once an image is received by HiROC computers, it takes about two hours for our pipelines to process it. We then have to wait a week for the ephemerides (position of spacecraft relative to Mars) data to be constructed. After that, it takes about two hours to complete the remainder of the image processing.

What are the Black Boxes within Some Images?
These are known as data gaps. They are caused by interruptions in the data stream during transmission. Data are transmitted from the spacecraft to Earth through the DSN, and data can be lost along the way. For example, heavy wind or rain can disrupt DSN antennas, and this results in data gaps during transmission. Sometimes the data missing in gaps are recoverable simply by re-transmitting the data. Other times the missing data are gone for good. Because our compression technique (FELICS) operates on 20-line chunks of data, most gap lengths are multiples of 20 lines.

Why are the Images Tilted with a Black Border around Them?
Images are tilted as a result of being map-projected, which means that the camera coordinates are converted to map projection coordinates. This means that North is up in the image, and that every pixel is associated with Mars planetary coordinates. Because Mars rotates rapidly (like Earth), the original image are not quite aligned north-south.

Can I get Unprojected (i.e. unrotated) Images?
Yes, these are available in the PDS Extras directory. You can also create these products yourself by downloading the raw HiRISE data (known as EDRs) and processing them yourself. All HiRISE image processing is done using ISIS software, which anyone can download and use for free. Just go to the ISIS Web site for free software download and instructions for use.

What are Stereo Pairs and Anaglyphs?
An anaglyph (or "stereo image") is a three-dimensional image in which two images ("stereo pairs") of the same spot, taken from different angles, are superimposed. Viewing the anaglyph with a pair of red-and-blue (or red-and-cyan) 3D glasses allows the user's eyes to merge the two images, producing the visual effect of depth.

How can I access the PDS HiRISE volumes?
At this Web address: http://hirise-pds.lpl.arizona.edu/PDS/

What is the Difference between EDRs and RDRs?
EDRs (Experimental Data Record) are the raw, unprocessed data. RDRs (Reduced Data Record) have undergone a variety of processing steps to stitch together the CCDs, remove instrument noise and artifacts, and map project the image. Here are detailed descriptions of the EDR and RDR products:
HiRISE_EDR_SIS (PDF)
HiRISE_RDR_SIS (PDF)

How Long Does it Take for New HiRISE Images to be Released to the Public?
Every Wednesday we do our weekly release of roughly 5-10 images on the HiRISE Web site, so some images are released within weeks of acquisition. These images are selected by team scientists, who also write captions for web-released images. All the rest of our images are released every three months.

Can I make a HiRISE Target Suggestion?
Yes, please register for HiWish, login and you will be able to create target suggestions.

My registration failed? What can I do?
We streamlined the registration process after many people had trouble on day 1, so please try again (with a new username).

Why can't I save my suggestion? The button is grayed-out!
Please make sure you have provided four things: a title, a science rationale, a region of interest on the map (thick white rectangle with blue marker), and a science theme.

I filled out the suggestion, but can't get to the next step. What's wrong?
There should be an error message next to the field that needs to be changed. Typically, this is an invalid character in the title, or something of that nature.

Where Is the Face On Mars?
We have already looked at this in image PSP_003234_2210.

What are some strategies to improve my suggestion's chances?
First, make sure you justify the image in terms of small-scale features that might be seen, things like boulders or thin layers or dunes, not giant volcanoes or channels. We need to know how HiRISE's meter-scale resolution is necessary, especially if there are existing MOC, CTX, HSRC or other images of the area.
Second, choose an appropriate science theme. There are one-sentence descriptions below the map, and detailed descriptions via clicking on the science theme name.
Third, choose a location on Mars that isn't very popular--avoid regions with lots of other suggestions. Other suggestions are shown with white markers (when you are zoomed in on the map).

How long might it take for my image to be taken and released?
This is very difficult to predict. First, there's the caveat that we can't guarantee that we'll get to it. It depends on how highly a Science Theme Lead prioritizes it. They look at suggestions on a monthly basis. Uplink, downlink and validation are a matter of weeks. And, our PDS releases occur on a monthly basis. So in the absolute best case, it is a matter of months. Remember that you are competing against other suggestions and for the STL's prioritization.

What's a "Science Theme"?
We divide our images into different "science themes," to make the planning of our large collection of images more manageable. A different Co-Investigator (lead scientist) is in charge of each science theme, so it's a way of dividing up the workload and utilizing each scientist's special area of expertise. This also helps us capture images from different scientific areas, so we can make sure they're all represented.

What's the "Other" science theme?
This is mostly used for images we've already decided to take, for example, ride-alongs (image coordinations) with other instruments. Sometimes we just put them in there if we're not sure where else they belong.

Why do we need another camera around Mars?
HiRISE returns images of the Martian surface with higher resolution than ever seen before from an orbiter. This means we can see extraordinary detail in all kinds of surface features. Scientists all over the world are already using these images to understand many previously-unexplained phenomena on the Red Planet. We might also discover brand new types of features never seen before! The stereo and color capabilities will also allow us to explore Mars in 3D, and with compositional information. The ultra high resolution also makes HiRISE the perfect tool for investigating the safety of future landing sites for other missions, such as the Phoenix lander, MSL (Curiosity), or the Mars Science Laboratory. We’ve also done some searching for past Mars landers, both successful and not. But even without the higher resolution and added capabilities, additional cameras in Mars orbit are always valuable for imaging new terrains on Mars, and for monitoring the dynamic surface and atmosphere for activity and changes. For more information on how HiRISE images will further the scientific study of Mars, see the Science Themes page.