Measuring Resolutions (2023)

Goals

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
IPHAS	0.62-0.77 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		(varies a lot over the bands; you can just pick 1 or 2)
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 (column four, 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.

What does "spatial resolution" mean? How close together can two sources in the image be before your eye or even the computer can no longer distinguish them as two separate sources? If the spatial resolution is poor, then two sources don't have to be very close together before the telescope sees them as just one source. The spatial resolution of a telescope is set in part by the size of the mirror and the wavelength of light being used. The combination of these things also tends to set the size of the pixels used in the camera. If your telescope+wavelength tells you that you are expecting a resolution of ~4 arcseconds, then there is no point in paying for a detector with 0.3 arcsecond pixels -- usually astronomers need a source to affect at minimum 2 pixels, preferably at least 3, before we believe a detection. So a spatial resolution of ~4 arcsec and 0.3 arcsec px would be ~13 pixels for a source, and that's just not necessary. (This is also why I'm having you explore pixel sizes as well as spatial resolution below.)

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.

Column three is all documentation, most of which can be found at IRSA. Google!! :) "spatial resolution of 2MASS", etc. You should be finding numbers in units of arcseconds.

Column four means you need to start pulling images. Finder Chart. IRSA Viewer. Sh 2-192 and Sh 2-187 over 20 arcminutes.

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.

For at least one frame from each of the surveys you want to work with, go and measure the sizes of 3 to 5 ‘typical’ isolated point sources in these images. That's your empirical spatial resolution for the table above. 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; it is also going to change the apparent size of the sources! (It is going to be hard to find 'typical' in IRAS if you want to work with those data; do what you can.)

Backing out to the bigger picture again

Do you notice any trends of resolution with wavelength?

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”?

Sh 2-187: Why is it that the "blue clusters" are easier to find in Spitzer data than WISE data?

Sh 2-192: Why is it that the source right at the position of Sh 2-192 gets suddenly much larger at 22 um? Is that resolution or astrophysics?

Hints and Tips

Sh 2-187 has Spitzer data in SEIP but not GLIMPSE. Sh 2-192 has data in GLIMPSE but not SEIP (so you need to go to IRSA Viewer to find it).

Both of them have tiles too small in 2MASS to cover the whole 20 arcmin. If you want a bigger 2MASS tile, you need to go to IRSA Viewer.

Align and lock by position in Finder Chart is on by default, but it isn't in IRSA Viewer.

PanSTARRS images: https://ps1images.stsci.edu/cgi-bin/ps1cutouts

IPHAS images: https://www.iphas.org/images/

Relevant topics from the rest of the wiki

• FITS format
• Finding FITS files
• Viewing FITS files
• Overview of measuring distances