The only way to really determine which time of year is best for your area is to look at multiple images of the area and figure out which ones look best. (Or, you can ask me nicely—I’ve looked at every CTX image that’s ever been taken except the couple thousand that were just released, and it was my job for awhile to determine the best time(s) of year to image certain places.)

So, let’s say I want to make a mosaic of a place and I’ve determined that images taken around the B19 timeframe look the best. I would pick images from perhaps the B17–B22 range, all taken close in time to each other. I can also use 17–22 from any other mission phase [P/G/D/F]; the illumination conditions will be similar. But how do I go about browsing through the images in the first place?

My favourite way to explore the CTX dataset is a tool created by a group at Arizona State University called JMARS—Java Mission-planning and Analysis for Remote Sensing. The name is slightly misleading as it allows you to browse image datasets for many bodies in the solar system, not just Mars. The interface is relatively easy to use, and the JMARS website has many resources and tutorials to help you navigate the program.

I won’t go through how to use JMARS here as that could be an entire post on its own. Long story short, I use it to find the images that I want. Then I go to the USGS Map Projection on the Web (POW) site to download map-projected versions of the images. To minimize distortion, I like to use a sinusoidal map projection with a centre longitude matching that of the image. Conveniently, the CTX image names include that centre longitude, albeit in west-positive coordinates.

The next step is to bring the TIFF images downloaded from POW into Adobe Photoshop and align them. I’ve tried using the built-in Photomerge tool in Photoshop to do this with varied results, so I often end up making the mosaics by hand. Once the images are aligned, hopefully with good overlap on either side, I bust out the eraser tool. Channelling old-school methods of mosaicking, I erase along the seams to blend the images together. You don’t want to do this in a straight line though, or the seam will still be obvious. Erase around different features in the image for better blending.

Once the images are all aligned and blended, I usually add a bit of sharpening and contrast enhancement to help the features in the image stand out a bit more. Contrast enhancement is your friend! By now you should have a great mosaic that Photoshop may or may not choke on trying to produce (doing the Gale crater mosaic at full resolution was trying even on my 8-core Mac Pro tower a few years back).

So, I hope I’ve given you a start down the path of utilizing the amazing CTX dataset to create your own mosaics. If you’d like to see more CTX images, you can follow me on Twitter—I try to post a CTX image every day, but must admit it’s becoming a once every few days thing thanks to travel and research obligations. But, I’ll keep trying to get them out on a semi-regular basis!