C-WAYS Resolution Worksheet

Introduction

The spatial resolution of various instruments and missions is and will be a very important thing for us to consider in the course of assessing the literature studies of our regions, as well as doing our project with WISE and Spitzer data.

For a general introduction, please start with the main text already on the wiki for Resolution. Please look at the examples lower on that page, but you don't need to actually do the one that suggests that you go download data, etc. The skills you might have gained from that will be stuff that we will either do as part of this worksheet, or as part of our Summer visit.

We will be using Goddard's Skyview for this worksheet.

Each of you should be assigned as a primary person for a BRC target (since there are 5 core educators, and 3 BRCs, we have room for one more 'primary' -- mark legassie??). If you finish doing this for 'your' target, or want to continue exploration of one of the others, or another target entirely, please go ahead and do so!

Skyview basics and other things to note

We will be using Goddard's Skyview. There is documentation linked from that front page. We will use the full Query form, not the Non-Astronomer's page.

If, in the future, you need to find this, you will probably need to google "Goddard Skyview" as there is at least one other software package called Skyview (including one at IPAC that is mentioned more than once here in this wiki) that does something else entirely.

I WILL MAKE A YOUTUBE SCREENCAPTURE MOVIE TO GO HERE

Skyview pulls together some huge number of surveys in one place and makes them accessible to you in an easy, fast interface. It will resample and regrid and remosaic all sorts of surveys for you, from gamma rays to the radio. I don't know exactly if it conserves flux (e.g., if one can still do photometry off of the mosaics it provides); I would err on the side of caution and NOT use this for anything other than morphology, e.g., do science by eye with the mosaics, and you can use them for distance measurements, but don't do photometry on these mosaics.

Skyview will always spawn the same second window for the results. The first time you call it, it will spawn a second browser tab or window (depending on your local configuration), and then, if you don't close that second tab or window explicitly, the next search results will go into that same window, even if it's hidden below where you are currently working.

Skyview will give you a JPG right away, and allow you to download both the JPG and the FITS file (click on "FITS"). (Slightly more information on FITS format is elsewhere on the wiki.) JPGs (and for that matter GIFs or PNGs) are "lossy compressed" files, which means that it actually LOSES INFORMATION, certainly compared to the FITS file. JPGs are just fine for images you take of your kids with digital cameras - you rarely ever see evidence of the loss of information. (As an aside - you might see evidence of it if you take a picture of something with sharp contrast, or a sharp edge somewhere in the image. If you look at the jpeg up close, you will see 'ringing' of the sharp edge, which looks kind of like blurring. The wiki page on lossy compression above has an example of loss of information with pngs.)

So, what this means is: any time you are doing science, whether that is using your eye to see small details in the image, or measuring distances, or doing photometry, you always want to be using the FITS file, never a JPG, PNG, or GIF.

Therefore, you need software capable of reading FITS files. There is some information on using a variety of platforms here, but you might as well start to get comfortable with using ds9, since that's what we will be using later on in the project. There are at least 2 tutorials on using ds9 developed by NITARP students on the wiki for doing some specific things - search on ds9.

One last word of advice. When you go to download the FITS file, the default filename is related to the process id on the server, e.g., it won't mean anything to you 10 minutes after you download it. In the process of doing these exercises, you should rename the images straightaway to be something that you can understand later on.

Getting started - exploring POSS images

Go get a big mosaic, 5 deg, of your chosen region in DSS. DSS, which stands for "Digital Sky Survey", was an all-sky survey conducted using photographic plates at the Palomar Observatory. POSS is another abbreviation for this, e.g., Palomar Observatory Sky Survey. The images you are using, though, are electronic scans of those POSS plates, knitted together afterwards (hence, technically DSS rather than just plain POSS). There are two generations of these scans (DSS1 and DSS2), and two (often 3) colors -- red, blue, and IR. These are the original photographic bandpasses, not Johnson bands. Let Skyview use the default number of pixels (300).

Can you find tile boundaries in your large image? Find and note the ra/dec of a corner.

How many arcseconds/arcminutes/degrees are there per pixel in this image? (What do I mean by that? Most pixels are square, so rather than measuring the diagonal as you would a TV screen, measure along both sides; you ought to get the same number for both sides.) Calculate what you think it should be from size and number of pixels (watch your units!), then find the corresponding value in the FITS image header. In ds9, go to 'File' (at the top of the ds9 window, or the buttons in the top middle), and pick "view fits header" or "header". Make a note of what header keyword is used, and what units it's in.

Go back to Skyview and ask for a smaller image, 1 degree on a side, also with the default 300 px. How big are those pixels in arcseconds/arcminutes/degrees?

Go back to Skyview and ask for a much smaller image, 0.1 degree, still with the default 300 px. How big are those pixels -- what do I mean by pixels? What size are the individual pixels in the image as returned to you, and what size are the pixels you can see in the image itself by eye? You will need to zoom in, probably a lot. You will need to find a way to measure distances on images, and unfortunately, ds9 doesn't provide an easy way to do this. As our first but certainly not last example of "using whatever software you are most familiar with to do the job", you are more than welcome to use your own favorite FITS viewer (if yours has an easy way to do this). Otherwise, you will have to do this by hand. Note that as you move your mouse around on the image in ds9, it will give you an updated readout of the ra and dec in the top. You can change this from hh:mm:ss ddd:mm:ss format to decimal degrees for both ra and dec by picking from the "wcs" menu at the top, either 'degrees' or 'sexagesimal'. Make a note of the RA/Dec of the pixel corners and calculate the distance along the sides of a pixel as you see it in the image (as opposed to that in the FITS header). Technically, to be absolutely correct, because you are calculating distances on a sphere, in order to do this, you need to do spherical trigonometry. This matters because the angle subtended by 1 hour of RA on the celestial equator is much larger than that subtended by 1 hour of RA near the celestial pole. However, over these relatively small distances, it should be fine to simply subtract the ra and dec to get a reasonable estimate of the size of the pixels.

OK, returning to my question above - What size are the individual pixels in the image as returned to you, and what size are the pixels you can see in the image itself by eye? Skyview did exactly what you asked it to do, and gave you an image 300 pixels across. What is the native resolution of the DSS image?

The original POSS spatial resolution was set by the seeing at Palomar that night, plus the size of the silver grains. When it got scanned, during the digitization process, the resolution becomes the size of the pixels you see there.

Now, let's be careful. Normally, to 'believe' a detection of anything, astronomers require that it be seen in at least 2 pixels. If something is seen in just 1 pixel, it's hard to tell if it's a single hot pixel, or a cosmic ray, or a real detection. Thus, spatial resolution is most frequently quoted as certainly more than 1 pixel, often approaching 2 pixels. The quoted resolution of the DSS is 1.7 arcsec per pixel. How does this match with what you calculated above?