Measuring Resolutions (2023)
Since spatial resolution issues are going to be important for our work, let’s explore what spatial resolutions we have in our data. The first goal of this worksheet is to fill out the following table. The second goal is to apply that knowledge to two of our objects (see below). I have made a movie with sort of QuickStart guidance.
| Data source | Wavelengths | Reported spatial resolution | Empirical spatial resolution |
|---|---|---|---|
| PanSTARRS | 0.48-0.96 um | ||
| Gaia | 0.622-0.777 um | (no images released so can't do this) | |
| 2MASS | 1.2-2.2 um | ||
| WISE | 3.5-22 um | ||
| Spitzer/IRAC | 3.5-8 um | ||
| Spitzer/MIPS | 24 um | ||
| Herschel/PACS | 70-160 um | ||
| Herschel/SPIRE | 250-500 um | ||
| Akari/IRC | 9-18 um | (no images released so can't do this) | |
| MSX | 7.8-21 um | ||
| IRAS | 12-100 um |
- Minimum expectations: look up the reported resolution for, and empirically find the spatial resolution of, 2MASS, Spitzer/IRAC, and WISE. (columns 3 and 4 for for three rows)
- Meets expectations: look up the reported resolution for everything (all of column 3) and empirically find the spatial resolution of 2MASS, Spitzer/IRAC, and WISE (three rows).
- Exceeds expectations: fill out as many cells as you can!
Background motivation
Use Finder Chart to pull up images of Sh 2-192 and Sh 2-187 over 20 arcminutes. Click on the '3color' button to make a color image at the end of each row. For both of these targets, zoom in a bit and really start to look at the point sources (the stars, as opposed to the diffuse fluffy stuff) in each of these images. Are they the same apparent size in each of the images? Are there the same numbers of sources in each of the images? (Some of the differences are due to resolution and some of this is astrophysics!) Look at the blue cluster (one of our stated goals) in Sh 2-187. Does it look different across the bands in Finder Chart? This is why we are thinking about these issues of spatial resolution .
You may wish to have both a Finder Chart session and an IRSA Viewer session on these targets to pull all the images you need for this work. Some of the images you need above (depending on how much you want to do) aren't available in IRSA tools.
Getting started on filling out that table
In order to fill out that table, you need to explore both documentation and the images themselves.
Q1.1 : Retrieve images of our area. For the images that it returns, what is the size of each pixel for each survey? (Option #1 to do this: Make the image big enough in your view of it that you can see pixels, and measure the size of it using ruler tools (not a real ruler). Option #2 to do this: look in the FITS header and find a useful keyword.) Try at least one image from each of the surveys.
Q1.2 : You will need to Google for this one. What is the original native pixel size for these surveys? Finder Chart gives you images that come straight from the original surveys, so they should match the original native pixel size for each survey.
Q1.4 : Did you do the calculations right? Here's how to check. Look at the sources in the 2MASS image you retrieved from Finder Chart (which you know is native px size) and compare it to the sources in the image you retrieved from Skyview. Have you lost information? (To see what this looks like, try to make it lose information deliberately by asking for much larger pixels.)
Q1.5 : Skyview attempts to knit tiles together, but sometimes you can see the original tile boundaries, and it looks like a patchwork quilt. Do you see this here?
Q1.6 : For at least one frame from each of a few of the surveys we picked, from either your Finder Chart or Skyview images (assuming you are confident you have native pixel resolution), go and measure the sizes of 3 to 5 ‘typical’ isolated point sources in these images. What kinds of sizes are you getting for each survey? (It is going to be hard to find 'typical' in IRAS; do what you can.) Changing the color table/stretch is useful for telling if the image is slightly asymmetric (implying a barely resolved companion) or saturated or other things.
Skyview won't give you Spitzer images, because Spitzer isn't an all-sky survey. But there are lots of large images available at IRSA from Spitzer. SEIP = Spitzer Enhanced Imaging Products, but this too works in tiles, and the request you give Finder Chart or IRSA Viewer may run off the edges of some of those tiles. There are data there, just not in the tile that the IRSA tools may be pulling for you. To find individual sources in regions off the tile it gives you, ask for a smaller region.
Q1.7: The IAU-compliant names of sources are based on positions. Many of the catalogs and papers that we have list some sort of unique ID within the survey, but its ‘real’ name is the position-based name, which is typically included in the catalogs if not all the journal articles (the journal articles are supposed to use position-based names, but they don’t always). People often assign and use internal source IDs in papers because it’s easier to say “source 346” in conversations with collaborators rather than the full phone number that might look like 18033652-2423108. But, why is it that IRAS sources are given as, e.g., "IRAS 18006-2422" and 2MASS sources are given as, e.g., “2MASS 18033652-2423108”?
Relevant topics from the rest of the wiki
(e.g., these are the "Lego bricks" to go investigate in order to build this "Lego kit.")
