If I zoom in, I see a reddish mottled pattern. Is that sensor noise, background radiation, or something else?
Similarly, if I zoom out to 30.39 angular degrees, there's a similar-ish green mottled pattern that's very prominent. What's that? It looks like it's got linear features, which might be a stitching artefact of some sort, maybe?
> If I zoom in, I see a reddish mottled pattern. Is that sensor noise, background radiation, or something else?
I assume you mean the pattern that is distributed across the general image? That's noise in one of the three filter images used to make the color image. 2MASS imaged in the J, H, and Ks filters (likely mapped to blue, green, and red, respectively). It could be that the choice of stretch is emphasizing the noise in that filter a bit more. Alternately, the noise there could be a bit higher (at the long wavelength end, Ks starts to cover a part of the spectrum where there's appreciable emission from a ~300K blackbody, i.e., the general temperature of things Earth).
> Similarly, if I zoom out to 30.39 angular degrees, there's a similar-ish green mottled pattern that's very prominent. What's that? It looks like it's got linear features, which might be a stitching artefact of some sort, maybe?
Perhaps regions where the image was made using only the H band from the 2MASS survey?
I'm not an astronomer, but I believe the "mottled red" features are distant objects that are redshifted. Note that they're almost all red no matter where you zoom in. That's the evidence that space is expanding in a nutshell.
Distant objects do tend to look redder because of redshift, but the mottled pattern is a bit too regular to all be distant galaxies. It's more likely to be the noise in the data used for the red channel (likely the infrared Ks filter).
Similarly, if I zoom out to 30.39 angular degrees, there's a similar-ish green mottled pattern that's very prominent. What's that? It looks like it's got linear features, which might be a stitching artefact of some sort, maybe?