Additional information about the LRO NAC images

The NAC images may be mirrored (flipped left to right), inverted (flipped upside down such that south is up rather than north being up), or rotated 180°. This is due to the way images are built up, row by row, by each NAC's single row of 5064 pixels. This type of CCD sensor is called a "push broom" style CCD sensor, and it works in the same way as a flatbed scanner for a computer. Whether or not the resulting image is mirrored, inverted, or rotated 180° depends on the LRO's orbital orientation ("front" end is pointed north or rotated 180º such that the "front" end is pointed south) and the LRO's flight direction (sailing north or south) above the lunar surface while the "push broom" style CCD sensor is being continuously read out in order to form an image, one 5064 pixel row after another.

The NAC images of the Apollo landing sites are categorized as "targets of opportunity," meaning that the LRO team tries to photograph the landing sites whenever conditions are favorable for obtaining images of the landing sites under specific lighting conditions such as when the sun is rising, setting, or at some other desired solar elevation. Sometimes the LRO is commanded point a bit to the left or right in order to photograph a landing site from an oblique viewpoint rather than when the LRO passes nearly directly over the landing site. Such images, in conjunction with a similar photograph obtained one orbit before or after when the LRO was looking straight down at the landing site, can then be computer processed in order to generate extremely accurate digital terrain models (DTMs) of the landing site and the surrounding terrain. Images taken at oblique angles will appear to be distorted mostly in width when compared to other images taken when the LRO was looking more or less directly straight down at the landing site.

The Slew angle value (data is in degrees) of the image's associated data record tells you the angle, relative to straight down (the LRO's Z axis), that the LRO was rotated about the LRO's X axis in order to look towards the left or right of its flight path over the lunar surface. But the Z axis gets rotated 180 degrees when the LRO's flight direction is changed from +X to -X. This leads to confusion when simply looking at the sign of the slew angle in order to determine whether the LRO was looking towards its left or right when imaging the lunar surface. The best method to determine whether the LRO was looking towards the left or right, relative to lunar north, is to look at the image's center longitude versus the LRO's longitude. If the image center longitude minus the LRO's longitude is a positive value, then the LRO was looking towards lunar east, if negative then the LRO was looking towards lunar west.

Virtually all of the NAC images are somewhat compressed along the vertical axis relative to the horizontal axis due to very slight overlap of each horizontal row of pixel data relative to the rows above and below. This slight row overlap is a necessary byproduct of the "push broom" CCD sensor design. The slight overlap assures that every part of the lunar surface passing under the LRO was actually imaged instead of there being very thin gaps of non-imaged lunar surface between each row of image data. Imagine the moving scan head in a flatbed scanner scanning in one row of image data, but that the scan head moves an entire inch before the scan head scans in the next row of image data. The result would be a gap of nearly an inch of missing information from the document being scanned. The LRO team similarly prevents gaps by commanding the CCDs in the NACs to be continuously read out "fast enough" to prevent missing gaps of information. Nevertheless, this results in some vertical image scale compression relative to the horizontal image scale. The image data for each image tells you what the image scaling is along each axis. For example, the data for an image taken when the LRO ideally is looking straight down at the lunar surface might specify the horizontal resolution as 0.50 meters (per pixel) and the vertical resolution as 0.55 meters (per pixel). Thus one would need to resample the image by stretching the vertical axis by 110% (0.55m divided by 0.50m and then multiplied by 100) in order to obtain an undistorted version of the image. Finally, note that resolution data for any LRO NAC image really is the calculated image scale (meters per pixel) of the image.

Free Windows software for viewing and converting the images to other image formats

Clicking on an image, below, brings up the image in ASU's implementation of the Zoomify image viewer which allows you to pan and zoom in on the image within your web browser. Shown to the left of the image in Zoomify is the image data plus links for downloading either the raw (EDR) or calibrated (CDR) versions of the image, or alternatively the multi-resolution and multi-layer PTIF version of the image.

The PTIF images have lossy compression since the images at each resolution use lossy JPEG compression. The PTIF images must be viewed in software which is capable of displaying multi-layer TIFF images. If you don't have PhotoShop or other graphics programs which can view PTIF images, then you can use the free GIMP software for viewing the PTIF images.

The raw and uncalibrated EDR images are 8-bit grayscale with file sizes up to 256MB. The calibrated CDR images are 16-bit grayscale with file sizes up to 512MB. Note that the EDR and CDR images are in NASA's Planetary Data System (PDS) image format. This requires image software which can understand NASA's PDS image format. Free software packages are available for viewing PDS format images and converting the images to other formats.

GIMP , if you install an additional plug-in, can directly load the 8-bit grayscale LRO EDR images directly into GIMP. Get the plug-in here: GIMP(*) PDS plug-in for loading NASA/ESA PDS/EDR format images and follow the instructions for installing the plug-in.

NASAView Image Display Software — Windows software for viewing PDS images and converting them to GIF or JPEG format.

XnView — Windows software which supports more than 400 graphics file formats and which can display and convert PDS images to a variety of other image formats including TIFF format. Download and install the XnView Standard version (JPEG2000). You will have to configure XnView's options so that it will recognize the PDS image format since, obviously, Windows and any installed graphics programs do not recognize this format. This is easily done by checking "Display all image file types" in XnView's configuration wizard on first launch or later under XnView's Options > General > Operations tab. Then simply right-click on a Planetary Data System image such as a downloaded LRO EDR or CDR image and select Convert in order to convert the image into another image format which your graphics programs can recognize.

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