[X] My Assistant

[Astroboy]

Hey gang, I finally discovered an easy way to align images of various celestial targets taken at slightly different times! I used a plugin for ImageJ called bUnwarpJ. I'm really pleased with most of the results I've gotten so far. I've noticed that the receding limb sometimes doesn't warp as well as the approaching limb, though. That's something I still need to figure out, along with mosaicking and the issue I'm about to bring up.

Anyway, to celebrate performing this pretty rudimentary image processing task for the first time, I tried aligning Dawn images taken through filters 2, 7 and 8 (corresponding roughly to green, red and blue) using .img files I had converted to .gif through NasaView. The thing is, my resulting color images keep coming out looking incredibly garish. One attempt ended up with a bright pink Ceres.

I've noticed a lot of raw NASA images, calibrated or uncalibrated, vary incredibly wildly with respect to brightness, and even across images taken through the same filter with the same exposure. How am I supposed to rectify this, or even begin to know what to do? This has been driving me crazy for a while. I know there's no such thing as "true" color, that color is subjective, etc., but I've always assumed that a given combination of three frames taken through three different filters can be mixed in a specific way that is scientifically useful, predictable, and proper or something.

I guess what I'm asking is, how do you guys decide on how much of each filter to use when mixing color?

[Astroboy]

Once again, I know there's no valid color in digital imaging. What I'm asking is when you're given a repeating series of images of a target taken through multiple filters, with each filter always using the same exposure, and you'd like to process three differently filtered images into one color image, what do you do when images taken through the same filter have extremely inconsistent brightness levels that can't be explained by varying exposure? Are there any references as to how a frame taken through a certain filter with a certain exposure should look, without these brightness fluctuations? I want to know how you guys choose the specific levels of each filter you choose when processing images, whether or not they include ultraviolet or infrared or whatever. I'm not interested in duplicating the exact kind of color my eye would see, I just want to be like you guys.

[mcaplinger]

what do you do when images taken through the same filter have extremely inconsistent brightness levels that can't be explained by varying exposure?

If the target illumination conditions (solar distance, phase, etc) are the same, then the only way the images can have different levels is if there's something else going on, like different gain settings (what a typical consumer digital camera calls "ISO".) You have to know all of these settings to get images that can be directly compared.

Our cameras on MRO, LRO, MSL and Juno don't have gain settings, but some of our other cameras do (e.g. our cameras on OREx).

The point of some radiometrically-corrected PDS products is to have images that have such variations removed somehow by appropriate scaling.