IC 417 Resolution Worksheet

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STILL UNDER CONSTRUCTION .. PROCEED WITH CAUTION ...

Introduction

The spatial resolution of various instruments and missions is a very important thing for us to consider in the course of our work. We're using data that come from several different surveys, with different spatial resolution. This is most vividly seen by a comparison of WISE channel 1 and 2 with Spitzer GLIMPSE IRAC channel 1 and 2, which we can do in spades for IC417.

MY GOALS for you in doing this are to develop a sense of (1) what resolution mean and how it changes between telescopes, e.g., WISE vs. Spitzer vs. 2MASS resolution; and (2) understand what the challenges will be for us in matching across wavelengths.

For a general introduction, please start with the main text already on the wiki for Resolution. Please also 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 specific example will be stuff that we do as part of our project.

We will be using FinderChart at IRSA to retrieve images, but we will also use Goddard's Skyview to retrieve larger FITS images. You need a way to view and interact with FITS files. FinderChart (and its more generic friend IRSA Viewer) allow you to interact directly with the FITS files. You could also use DS9 which you can download here: DS9. (see FITS explanation below). Also, ds9 was the topic of a tutorial in NITARP tutorials.

Skyview basics and other things to note

Background information I'm assuming you know

If you need a refresher on mosaics, see What is a mosaic and why should I care?

If you need a refresher on angular measures on the sky, see this site from LCOGT.

A bit more information on FITS format is elsewhere on the wiki. The most important thing about FITS format vs. other image formats is that JPGs (and for that matter GIFs or PNGs) are "lossy compressed" files, which means that images in those formats actually LOSE INFORMATION, particularly in comparison 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 high 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 wikipedia page on lossy compression linked 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.

When you download the FITS files (from anywhere), the default filename is very likely 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 downloading images, you should rename the images straightaway to be something that you can understand and remember later on.

We will be using FinderChart and Goddard's Skyview. Detailed documentation for both of these is available at their respective websites.

Skyview

For Skyview, we will use the full Query form, not Quick View and not Non-Astronomer's page.

If, in the future, you need to find Skyview, 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.

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. It will make it seem as if nothing has happened when you submit your search request.

In Skyview, you can ask for more than one survey at the same time, but it uses the same 'common options' you specify on the query page. To select more than one that are not adjacent, hold down the command key while clicking. (That is, at least, on a mac. Your mileage may vary.)

Skyview will allow you to download both the JPG and the FITS file (click on "FITS" to download it). You want FITS, as per above. :)

FITS Viewers: ds9

You need software capable of reading FITS files. There is some information on using a variety of packages here.

As our first but certainly not last example of "astronomers 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 measure distances).

You might as well start to get comfortable with using ds9. It's free, and available for just about any platform. There are at least 2 tutorials on using ds9 developed by NITARP students on the wiki for doing some specific things - search in the wiki on ds9 - and more from the rest of the web, including some listed at the bottom of this page. Also, ds9 was the topic of a tutorial in NITARP tutorials.

For this worksheet, you need to be able to measure distances. Measuring distances in ds9 is basically creating a special 'region' that is a ruler, so you may find it clunky. From the menus on the top, select Region/Shape/Ruler. Click on one end of what you want to measure, then move to the other end and click again (or click-and-drag; you may need to experiment to see what your system wants). A line with arrows will be drawn connecting the two, along with the distance in text and dotted lines completing the triangle. By default, the distance will be in physical units (pixels of the image you are viewing), but by accessing the region's Get Information panel (top menu: Region/Get information; buttons in the middle of the ds9 screen: Region/Information), you can change both the endpoints and (more usefully) distance units to WCS so that the units will be in degrees, or minutes, or seconds.

In recent years, we have had some skittishness from Windows machines when running ds9. It may very well be that you will have an easier time using IRSA tools (see next) than ds9, although ds9 is (for the moment) ultimately more powerful.

ds9 Tutorials from Babar from 2012:

ds9 Tutorials from the official NITARP tutorial (Jan 2013):

FinderChart (and IRSA Viewer)

FinderChart and IRSA Viewer both use software that is called "Firefly", and both tools have a similar look-and-feel. In both cases, the search capability is integrated with the FITS viewer capability. (In Skyview, these capabilities are not integrated.) When FinderChart or IRSA Viewer give you images as a result of a search, you are looking at (and interacting with) the original FITS files. There is a toolbox on the top of both tools that can be used with the images. You can change color stretches and color tables, you can leave markers on the image, you can read in catalogs (and ds9 regions files), and you can measure distances.

For this worksheet, you need to be able to measure distances. Click on the ruler icon, then click and drag in the image to measure a distance. Click on the layers icon to bring up a pop-up that specifies the units for the length of the vector you have drawn in degrees, arcminutes, or arcseconds.

Notes on Distances

You can also measure distances by hand by comparing pixel coordinates. Note that as you move your mouse around on the image in any of these FITS viewers, it will give you an updated readout of the ra and dec near the top. You can change this from hh:mm:ss ddd:mm:ss format to decimal degrees for both ra and dec -- for ds9, you do this by picking from the "wcs" menu at the top, either 'degrees' or 'sexagesimal'. Make a note of the RA/Dec from which you want to measure a distance, and the RA/Dec of the end point of the distance measure.

No matter how, exactly, you do this, WATCH YOUR UNITS. RA by default is in hours, not degrees. Dec by default IS in degrees.

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 relatively small distances, it should be mostly fine to simply subtract the RA and Dec to get a reasonable estimate of the distance BUT WATCH YOUR UNITS because RA by default is in hours:min of time:sec of time, not deg:arcmin:arcsec.

The spherical trig does make a difference, though. See this excerpt from someone's class notes with some really nice graphics and explanations of why you need to do this, and how to do it right. (hint: For the distances we'll consider here, you need a cosine of the declination. I won't make you do the full spherical trig for distances more than a degree.) For the ambitious, anticipating skills you'll need downstream from this worksheet, try programming a spreadsheet to do this for you, given two RA,Dec position pairs. NB: Be sure to watch your units on the Dec-- some cosine functions want radians, and some take degrees. (Bonus: how much of a difference does it make if you leave out the cos(dec) term?)


Getting started: what sizes do we expect?

Googling to get what you need is ok!

Let's start by calibrating our expectations.

  • Q1.1: What approximate angular size is the Moon?
  • Q1.2: What approximate angular size is Jupiter?
  • Q1.3: What approximate angular size is Proxima Centauri? It is a M5.5 Ve, and so its radius is about 0.15 Rsun. Its parallax is 774.25 milliarcsec.
  • Q1.4: Put our Sun, with a Kuiper Belt, at the distance of Proxima Centauri. What angular size would the Sun be? The Kuiper Belt? In reality, the circumstellar disk surface brightness is much, much fainter than the central star, but for purposes of this example, let's ignore that. Take the solar radius as 7e5 km and the KB as 6e9 km.
  • Q1.5: The disk around beta Pictoris is about 1650 AU in radius. What angular size would that be? (Again, though, the brightnesses are so different, in order to see the disk at all, you have to block out the brightness of the central star and integrate for a long time.)
  • Q1.6: IC417 is about 2 kpc away. Put a star/disk system just like Beta Pictoris in IC417. What size would it be, ignoring issues of surface brightness and contrast with the star?

THE POINT OF DOING THIS: will we see any disks or rings around our stars using our data?

FinderChart: Obtaining images of our region

We are studying a square that is ~1 deg on a side, centered on 5:28:00 34:30:00. (bonus: what is that in decimal degrees?)

Go to FinderChart and ask it for degree-size images at that coordinate for POSS, 2MASS, WISE, and IRAS. (There aren't any SDSS data.) Turn off catalog searching because otherwise it gets just too confusing.

Q2.1 : For the images that it returns, what is the size of each pixel for each survey? Try at least one image from each of the surveys. Three of these four surveys were electronic from the start; the original POSS was photographs, so the 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, this gets mapped into the pixels you see in the images.

Q2.2 : 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.

Note that, specifically because FinderChart is grabbing things from the original survey, sometimes you "run off the edge" of a stored tile. It's bad for 2MASS -- the tiles are very tiny, and IRSA's been working on better ones for quite some time. But, even for WISE, the edge of the tile affects the southernmost portion of our images. The power of Skyview is that it mosaics the tiles together. That is also the danger.

Q2.3 : Go to Skyview. The four most important parameter choices Skyview gives you are:

  • center position
  • survey (wavelength)
  • image size in pixels
  • image size in degrees

Skyview will happily and without complaint or warning resample and regrid the pixels to whatever scale you want. What do you need to do to get 'native pixel' resolution out of Skyview? You should have the information from earlier questions to figure out how many pixels you need to cover ~1 deg on a side, centered on 5:28:00 34:30:00, so go and do the math, and ask Skyview to give you a full-sized 1 deg image. Note that you can request more than one survey at a time, but Skyview will use the same parameters for each of them.

Note that it is happily tiling WISE data, but struggles with background matching between tiles in 2MASS. What I get back looks like a patchwork quilt. This is part of why IRSA is working on bigger 2MASS mosaics. Any day now ...

Q2.4 : Now, let's be careful. Normally, to 'believe' a detection of anything, astronomers require that it be seen in more than 1 pixel. 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, if cited without a "per pixel", is most frequently quoted as certainly more than 1 pixel, often ~2 pixels. What this physically means, in essence, is BOTH the following two questions: (1) "How many pixels have to be affected before I believe it is a real detection?" and (2) "How close do two sources have to be before I can no longer distinguish them as two individual sources?" (Real life numbers: the quoted resolution of IRAC is ~2 arcsec, but the native pixel size is 1.2 arcsec, and mosaics often have the pixels resampled to be 0.6 arcsec.) The quoted resolution of the DSS is 1.7 arcsec per pixel (or about 2 arcsec, depending on the photographic plate). For at least one frame from each of the four surveys we picked, from either your FinderChart or Skyview images (assuming you are confident you have native pixel resolution), go and measure the sizes of '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.)

These numbers are what people mean when they quote the 'resolution of a survey'. 

In order to quickly get a gut-level understanding of this, you can stack them up in ds9. Load them all into ds9.  In order to do this, either use the command line (ds9 *fits) or start ds9, then do file/open and find the first image; do frame/new then file/open and load the second image, etc. If you used the command line trick, you will load all the images into individual tiles, in alphabetical order (which is most likely not wavelength order!). If you did them one-by-one, you will have them virtually in a stack, in the order you loaded them. To see all of them at once, click on 'frame' then 'tile.' To get it back to one at a time (in a virtual stack), pick 'single.' To line them up on the sky, pick from the top "frame" menu/match/frame/wcs to match them in terms of area on the sky. To scroll through the whole stack, pick 'next' or 'previous'. You can also blink them. You can change the ordering - explore the menu options on the top "Frame" menu.  In the 'single' frame case, the image you are looking at is the active one; in the 'tile' view, the one with the blue outline is the active one. Click on the tile to make it the active one. You may occasionally leave behind a green circle; this is a "region", and they are ultimately very helpful, but at this point, often very annoying. To make it go away, pick it, and hit backspace or delete on your keyboard.

In any case, pick any of the images as primary, and go to the 'frame' menu at the top; go down to "match", pick "frame" again, and then pick "WCS". That means, "align all the images I have loaded in ds9 to be North up, all on the same spatial scale as the image I have selected when I initiate this command." (WCS stands for world coordinate system, meaning that there is information about the ra, dec, and mapping of pixels to ra and dec in the FITS header. ds9 and many other tools are capable of reading that information and translating it in real time to ra and dec under your mouse as you move.) What is the area covered by your image from IRAS (in square degrees!)? WISE? 2MASS? POSS?

How big is the 'resolution element' here, for each of these channels?

Because this is relatively easy to do - get a 300 px image in native resolution for several channels at once - get HD 787 and the 2nd position example above (IRAS 03275+3020 = 3h30m35.90s +30d30m24.0s). Which HD 787 images are saturated?

Q3.4 : For a laugh -- go back and try COBE too, though you will probably NOT want native pixel resolution for that; ask it to get you the same area as your IRAS image. How big are the COBE pixels in arcseconds/arcminutes/degrees?

Q3.5 : For further thought: HD 787 is on the de la Reza source list, implying that they thought it had a measurable IRAS flux at all four bands. Look at those images. Do you see a source? Just because you don't see it by eye, doesn't mean it's not there, but it might make one .. look into the catalog data quality flags. Make a note somewhere that we will have to do this, not just for IRAS, but WISE and 2MASS too...

Q3.6 : So, knowing what you do now, why is it that IRAS sources are given as, e.g., "IRAS 21391+5802" and 2MASS sources are given as, e.g., "2MASS 21402612+5814243" ?

Finder Chart IRSA tool

Skyview is, as you see, very useful for getting your bearings, and investigating the morphology of the general area of the region. But, we will be worrying about individual objects, all over the sky, and moreover, Skyview doesn't let you search on a list of targets. And downloading individual images one at a time is .. annoying. It would sure be nice to search for a whole bunch of targets at once, and be able to download all the images at once, and, even better, be able to interact with the catalogs at the same time. Guess what? We have a tool for that! :)

FinderChart is a web-based tool housed at IRSA. It is, in some ways, a very much pared down version of Skyview, which only serves relatively tiny images from only a few surveys... but only in native pixel resolution. I use it far more often than Skyview, in general, because I am very often investigating the properties of individual sources, or trying to understand what I see in Spitzer or WISE. These are also guaranteed to be unresampled images, so they are OK for doing detailed science. On the other hand, we shouldn't need to do photometry on 2MASS or WISE or even Spitzer images, but it is still useful to, e.g., not worry about seams between tiles.

Go get a 20 arcminute patch of sky using FinderChart of HD 787. Ask it for DSS, 2MASS, WISE, and IRAS. (The default is to look for SDSS too, but there isn't any Sloan coverage.) When it brings up the results, what it is showing you are the actual FITS images, not jpegs or pngs. Click on the toolbar icon to bring up the toolbox full of things you can do to/with the images, either all at once, or individually. As shown, they all cover the same patch of sky; zooming in on one zooms them all in. Changing the color table is useful for telling if the image is slightly asymmetric (implying a barely resolved companion) or saturated or other things. Click on "prepare download" to get the FITS (or the pngs, or a pdf, or the html for that matter). Do it again for IRAS 03275+3020 = 3h30m35.90s +30d30m24.0s. For one of them, check and convince yourself that these are native pixel scale. Load them into ds9 with one of your other images from above. Use ds9's tiles view and snap them all to the WCS coordinates.

FinderChart also lets you retrieve and overlay catalogs. With IRAS 03275+3020 = 3h30m35.90s +30d30m24.0s loaded in, go and get the IRAS Point Source Catalog (PSC) and overlay it - you probably want to change the search parameters such that it doesn't get sources over the whole 20x20arcmin patch, but more like 5 arcmin. The Faint Source Catalog (FSC) is exactly that - a fainter reprocessing of the IRAS images. (IRAS was the only thing we had for a really long time...) Go and get that one too. How well does it match sources in the IRAS image? The WISE image? (Kind of amazing to me anyway that it did as well as it did!) Can you find an IRAS source that broke into pieces with WISE? You can do the same thing for WISE and 2MASS catalogs, but there are a LOT more sources (which you can see in the images). Downstream, we will use Gator to request catalog entries around each source in our source list all at once. We will learn what all the columns mean in the catalogs, and decide which sources to keep and believe, and which to not believe. The IRAS PSC sources in this region are bright at different IRAS bands, and are believable (or not) flux densities at different bands - you need to learn about the data quality flags to understand which sources the catalog itself believes are real.

You may also wish to try the 3rd example above: 3h41m58.60s +31d48m21.0s - there should be only one PSC source at this location, and it breaks into many pieces with WISE. THIS is the kind of thing I'm worried about with the sources in our target list!

Pulling it all together

Recall that THE POINT of doing this is to start to develop a sense of (1) what resolution to believe from any given telescope (and to what extent you can trust Skyview); and (2) how this changes between telescopes, e.g., IRAS vs. WISE vs. 2MASS resolution and what this means at a gut level. Is this starting to make more sense?

Basic questions to be sure you know the answer to:

  • How can you get access to data using Skyview? Using FinderChart? When would you use one vs. the other?
  • Will we see disks or rings in our data?
  • What is the spatial resolution of IRAS? WISE?
  • Is is possible that the sources seen as individual with IRAS will break into pieces when viewed with WISE?
  • Is there any guarantee that a single source seen with WISE is really a single object?

Postscript: Slight improvements are sometimes possible

By this point, I've hammered into you things about the native resolution from these various surveys. You should have a gut-level understanding now that you can't get more information out of the image than was recorded by it in the first place.

However.

I have swept some things under the rug. IRAS data was so interesting, and it was going to be so long before astronomers got any more data in those wavelengths on that scale, that very clever people got to work on how to get even more information out of IRAS data. Imagine those big IRAS pixels scanning over a patch of warm sky. The next time the spacecraft scans that same patch of sky, the pixels are offset a little bit from where it was on the last pass, and consequently the fluxes it measures are just a little different. Same for the next scan, and the next. If you have lots of scans over the same region, you can recover a little bit of the information on a slightly higher (better) spatial resolution. This page has some general information on the specific application of this method to IRAS, called "Hi-Res", along with example pictures. It uses the Maximum Correlation Method (MCM; H.H. Aumann, J.W. Fowler and M. Melnyk, 1990, AJ, 99, 1674). It is computationally expensive (meaning it takes a while to run), and requires lots of individual tweaking and customization, so it has not been run (blindly) over the whole sky. The degree of improvement is related to the number of scans; as for WISE, the number of passes is a function of the ecliptic latitude, so just running Hi-Res doesn't get you a specific improved resolution. Hi-Res got famous in the context of IRAS. People are developing ways to run this kind of algorithm on WISE and even Spitzer data, but we're not going to try and use it, as there are no particularly user-friendly interfaces to it (at least at the level we would need), and the incremental benefit we'd gain from this probably outweighs the work it would take to get there.

In the context of our project, we won't need to care about any of this, but I thought I should be complete in case anyone cares! :)

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