1. Multi-Wavelength Imaging Using Multi-Observatory Data
Zolt Levay Space Telescope Science Institute

2. Color composites Fields of view Registration “Politics”
Why would we want to combine images from various observatories?
To make pretty pictures, of course.
But also to make illustrations that convey some information about the subjects.

3. Image Registration Rigorous, data/wcs-based (IRAF, IDL, etc.)
Visual/manual (Photoshop)
It’s likely best to geometrically register images rigorously in analysis software (IRAF, IDL, etc.). If you know how to do that.
However, given the range of tools in Photoshop, it’s quite easy to register images geometrically, as long as there are identifiable/matchable features, the pixel scales are not grossly different, and there is minimal additional distortion. A convenient method is to use multiple layers, invert the top layer and set its opacity to 50%. Using the free transform tools (Edit > Free Transform or CMD/CTRL-T), one can rotate, resize and distort either image to match the other. If the images are identical, the registered combination will be a uniform gray. Otherwise, features will be apparent but they will overlap.
If there are not enough features to simply register visually, it can be useful to include

6. Helix Nebula
STScI-PRC04-32 STScI-PRC03-11
HST ACS
NOAO 0.9m Mosaic I
H-alpha (F658N, red), [O III] (F502N, blue), plus an average of those in the green channel
This image of the Helix Nebula is an example of combining images at different scales and fields of view.
ACS image scaled to 1/2 pixel scale: 0”.1/px
KPNO images scaled to same resolution
Individual grayscale images were merged using a mask and then color-combined.

7. Helix Nebula
HST ACS (left), KPNO 0.9m (right)
resolution comparison

8. Helix Nebula
HST/ACS, KPNO 0.9m
H-alpha
Mask used to blend the

10. Here’s a somewhat extreme example
Here’s a somewhat extreme example. The background, gray image is a mosaic of images of the Aristarchus crater neighborhood on the Moon from the Clementine mission. The (false) color images are two fields from Hubble’s ACS/HRC (constructed from ratios in different filters to emphasize mineralogy) in the same area. The Clementine images were made (or at least mapped) with the line of sight perpendicular to the surface. However, because of Hubble’s view from the Earth, and Aristarchus’s location toward the limb of the Moon, the HST images must be distorted for them to match. In this case (because I don’t know how to perform a rigorous remapping), I used Distort and Warp features of the Photoshop’s Free Transform to match them by eye.

11. Extended-Regime Images
Selected bands/layers vs. meta-composite
Color assignment
Resolution discrepancies
Registration

12. Kepler’s Supernova Remnant
STScI-PRC04-29
Hubble, Chandra, Spitzer
4-color composite from a single band of each

13. Chandra 0.3-6keV

14. Spitzer MIPS 24, 70, IRAC 4.5, 8

15. HST F502N, F550M, F658N, F660N, F673N

17. Kepler’s Supernova Remnant
STScI-PRC04-29
Hubble, Chandra, Spitzer
4-color composite from a single band of each

18. Galaxy Cluster Abell 2125
STScI-PRC04-02
Observed by four observatories:
VLA, Radio
KPNO, Optical (Oxygen emission)
HST, Optical, V (cyan), I (red)
Issues:
Registration
Color assignment, >3 channels
Too much brightness, saturation, dilution of color separation

19. N132D SNR
HST/optical CXO/x-ray
Heritage /4/05 STScI-PRC05-30
Mostly optical (HST/ACS) with light contribution of x-ray (Chandra)

20. It’s not necessary to include each individual image equally.
In the case of N132D, we (Lisa, mostly) produced a composite from the HST/ACS data that was quite striking, with a very dense, rich background of colorful stars and ghostly wisps of nebulosity. There was also a composite of the Chandra data that was quite compelling and colorful. A straight composite of these two proved somewhat overwhelming, however, so we rendered the x-ray image in monochrome, layered in Screen mode at a lowered opacity (70%).
Admittedly, this minimizes the x-ray contribution at the expense of the optical image, but this was a Hubble release, after all.