[X] My Assistant

[GBAE001]

Hey all,

I'm just starting to get into space processing as a hobby and need some guidance. I've been making MSL/MER panoramas and have run against a wall in terms of vignettes that appear in each image. I also wanted to make this thread general with the interest of also informing other newbies of this issue and how to correct it. I decided to make a little write-up in case other newbies don't know what I mean, but if I make any mistakes let me know and I'll update the original post. Here's one of my attempts, which should highlight the problem: https://www.flickr.com/photos/gbae/33257950...ublic/lightbox/ and https://www.flickr.com/photos/gbae/33112785...ublic/lightbox/

Background

When dealing with spacecraft images and photography in general, often dust/speck accumulate on the lens. A photograph or several are often taken of a flat subject, such as the twilight sky, so that the values of this "flatfield" can be divided through an algorithm against any other image to produce a "clean" image. This is especially useful for mosaics as it can become quite noticeable, as shown above. MER/MSL have occasional "twilight flats" taken of the twilight sky to use as flats.

The Problem

My example is from Sol 2318 from MSL's right MastCam. Here is an example of a hill with a clear vignette a speck in the center: https://mars.jpl.nasa.gov/msl-raw-images/ms...875C00_DXXX.jpg

Here's a twilight flat from Sol 2317, the same vignetting and speck also visible: https://mars.jpl.nasa.gov/msl-raw-images/ms...869D01_DXXX.jpg

First issue: the flat isn't even the same size as the image! The flat looks further out and has black bars around the side, assuredly from the MastCam case. Is it simply cropped for the first image or is there lens distortion between the images?

What I'm trying to get at is: how do you people use flatfields in this manner? Especially for MER/MSL? I don't have photoshop, I have GIMP 2.10. But I do have AstroImageJ, which is a subset of ImageJ for astrophotgraphy. I have fiddled around with the flatfield functionality in the program with no success, as its documentation is both sparse and goes over my head. Here's an image of what I found, under Process > Data Reduction Facility (chosen paths are from myself trying to play around with the software): https://imgur.com/a/XMxF22M

I'd appreciate any guidance both on how to use flatfields in MER/MSL processing both in the programs I have and those I don't so the thread can be of good use to anyone else with my issues.

Best regards, Gabe.

[elakdawalla]

GBAE001, thanks very much for making this discussion a general one and sharing what you've done so far -- that'll be helpful for other people trying to work out the same problems for themselves in the future.

I usually deal with the CCD crud with a judicious application of the Photoshop healing brush, but haven't tried to fix vignetting with anything other than trial and error with Photoshop filters, so I'm looking forward to watching this conversation.

[mcaplinger]

Terminology: vignetting is a global shading across the field unrelated to the little bits of crud on the focal plane. https://en.wikipedia.org/wiki/Vignetting