CephC-LABS Resolution Worksheet - Revision history

Rebull: /* Comparing resolution of various surveys, and why it matters */

2017-04-13T16:18:25Z

Comparing resolution of various surveys, and why it matters

Rebull: /* Pulling it all together */

2017-02-28T18:55:09Z

Pulling it all together

Rebull at 21:54, 23 February 2017

2017-02-23T21:54:28Z

Rebull: /* Notes on Distances */

2017-02-23T21:34:52Z

Notes on Distances

Rebull at 21:29, 23 February 2017

2017-02-23T21:29:01Z

Rebull: /* Pulling it all together */

2017-02-09T20:14:21Z

Pulling it all together

Rebull: /* Pulling it all together */

2017-02-09T18:25:08Z

Pulling it all together

Rebull: /* Comparing resolution of various surveys, and why it matters */

2017-02-09T18:24:12Z

Comparing resolution of various surveys, and why it matters

Rebull: /* Comparing resolution of various surveys, and why it matters */

2017-02-09T17:59:00Z

Comparing resolution of various surveys, and why it matters

Rebull: /* Obtaining images of our region, and resolution of various surveys */

2017-02-09T17:53:19Z

Obtaining images of our region, and resolution of various surveys

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 =Comparing resolution of various surveys, and why it matters=
-'''Q3.1 :''' You should have found that the various surveys had very different resolutions. Take the large images you obtained from Skyview (pro: big, con: patchwork) or from FinderChart (pro: know good data, con: tiny footprint), and create a three-color image using bands of your choice. FinderChart will let you create 3-color images for each survey with a single click ([https://www.youtube.com/watch?v=m0l-uWMjFrI 3-min video from IRSA YouTube Channel on making 3-color images].) For ds9, you need to tell it, "Ok, I want to make a 3-color image now" (Frame/rgb) and then you can load in each plane separately (in the pop-up, pick the color plane, then do File/open. Change the color plane and go back to file/open, etc.). (I don't remember if that is in the Tutorial or not, but I think it is.)  IRSA Viewer also lets you make 3-color images, including from images you have on disk, so you can use your Skyview images in IRSA Viewer.  See if you can create a 3-color image where the difference in resolution between the images you used for the color planes is evident. (You should read in your highest spatial resolution image first if you are using IRSA tools.) What bands did you use, and what did you notice about your resultant 3-color image?
+'''Q3.1 :''' You should have found that the various surveys had very different resolutions. Take the large images you obtained from Skyview (pro: big, con: patchwork) or from FinderChart (pro: know good data, con: tiny footprint), and create a three-color image using bands of your choice. FinderChart will let you create 3-color images for each survey with a single click ([https://youtu.be/wmP6fy7Fhg4 video on making 3-color images].) For ds9, you need to tell it, "Ok, I want to make a 3-color image now" (Frame/rgb) and then you can load in each plane separately (in the pop-up, pick the color plane, then do File/open. Change the color plane and go back to file/open, etc.). (I don't remember if that is in the Tutorial or not, but I think it is.)  IRSA Viewer also lets you make 3-color images, including from images you have on disk, so you can use your Skyview images in IRSA Viewer.  See if you can create a 3-color image where the difference in resolution between the images you used for the color planes is evident. (You should read in your highest spatial resolution image first if you are using IRSA tools.) What bands did you use, and what did you notice about your resultant 3-color image?
 '''Aside''': You can also blink images in FinderChart or ds9, which sometimes is more helpful for seeing differences than a 3-color image -- in order to do this in ds9, 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. (That command 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.) To scroll through the whole stack, pick 'next' or 'previous', or go ahead and blink them. You can configure the length of time spent on each frame. 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.

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 # Is is possible that the sources seen as individual with Herschel will break into pieces when viewed with IRAC?
 # Is there any guarantee that a single source seen with Herschel is really a single object?
 =Postscript on the resolution issues: Slight improvements are sometimes possible=

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 =Next steps directly related to these skills and issues=
-For our project, we have a list of "things in which we are interested" which consists of (a) objects we (well, Rob Gutermuth) identified via IR colors as possible YSOs; (b) objects that we (well, Scott Wolk and I) identified from their X-ray properties as possible YSOs; (c) objects that we (well, I) identified from their light curves as possible variables (which are likely but not guaranteed by any means to be YSOs); (d) objects from the literature that someone identified as interesting -- Halpha stars and the SCUBA sources. This list of sources is inhomogeneous in that sources come from many different observations. Our path forward:
+Last year's team started with a list of "things in which we are interested" which consisted of (a) objects we (well, Rob Gutermuth) identified via IR colors as possible YSOs; (b) objects that we (well, Scott Wolk and I) identified from their X-ray properties as possible YSOs; (c) objects that we (well, I) identified from their light curves as possible variables (which are likely but not guaranteed by any means to be YSOs); (d) objects from the literature that someone identified as interesting -- Halpha stars and the SCUBA sources. This list of sources is inhomogeneous in that sources come from many different observations.
-* Assemble list of objects from SCUBA. Make sure that we have matched them appropriately in the catalog. There may be some we can't do anything about until we have the Herschel data to link us between 24 um and 450/850 um. (Will use FinderChart for this.)
-* Take net list of "things in which we are interested" (e.g., candidate YSOs) and pass it to FinderChart for POSS/SDSS/2MASS/WISE; take notes on which look like point sources and which look like artifacts or non-point-sources.
+This year, we are starting from the long wavelengths. That is, starting with the SCUBA (450 and 800 um) sources, and adding to them sources seen at any of the long-wavelength bands we have: 24, 70, 100, 160, 250, 300, and 500 um. Some of them will be the same sources as last year's team worked with. Some will be new!
-* If at all possible, also look at these objects (those surviving prior step) in GLIMPSE and IPHAS using ds9. (This will be a little harder than FinderChart in that it has a little steeper learning curve to make it work smoothly.)  Take notes on which look like point sources and which may be breaking into pieces in the higher-resolution images.
+Our path forward:

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 No matter how, exactly, you do this, WATCH YOUR UNITS. '''RA by default is in hours, not degrees. Dec by default IS in degrees.''' How do you convert between hours and degrees? (Hint: there are 24 hours of RA ...and 360 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.
+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. For quick and dirty purposes, 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 [http://spiff.rit.edu/classes/phys301/lectures/precession/precession.html#sep 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 in most cases.) 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 in Ceph C does it make if you leave out the cos(dec) term? Is that going to get worse or better if we move closer to the north celestial pole?)

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 You can also measure distances in FinderChart (or IRSA Viewer). For this Resolution 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.
-FinderChart and IRSA Viewer also let you retrieve and overlay catalogs. Skyview doesn't let you do that at all.
+Images retrieved via FinderChart or IRSA Viewer are coming from the original archives corresponding to each survey, so they are basically guaranteed to be unresampled images, so they are OK for doing detailed science, including photometry.  There is an excruciatingly simple way of guesstimating photometry within FinderChart or IRSA Viewer, but it's nowhere near accurate enough for scientific analysis.
-Images retrieved via FinderChart or IRSA Viewer are coming from the original archives corresponding to each survey, so they are basically guaranteed to be unresampled images, so they are OK for doing detailed science, including photometry.
+FinderChart and IRSA Viewer also let you retrieve and overlay catalogs. Skyview doesn't let you do that at all. On the other hand, FinderChart and IRSA Viewer are limited to the tiles currently publicly stored here at IRSA; most of the time, you won't notice the tiles, but the 2MASS tiles are crazy small (that's a technical term) so if you ask for any reasonable amount of sky at all, you'll get a tiny 2MASS image surrounded by black, blank sky. In order to get big 2MASS images, you need to get them some other way, like from Skyview.
 Click on "prepare download" to get the FITS (or the pngs, or a pdf, or the html for that matter).

@@ Line 169: / Line 169: @@
 Questions to be sure you know the answer to (for reference, I guess these are Q4.x):
 # How can you get access to data using Skyview? Using FinderChart? When would you use one vs. the other?
-# Just because you have resampled an image to really tiny pixels, does it add information to the image? (Will you laugh at CSI and Law & Order and their compatriots when they wave a magic wand over an image?)
+# Just because you have resampled an image to really tiny pixels, does it add information to the image? (Will you laugh at CSI and Law & Order and their compatriots when they wave a magic wand over an image? "Computer, ENHANCE!")
 # Will we see disks or rings in our data?
 # How does the spatial resolution compare among 2MASS, WISE, Spitzer/IRAC, Spitzer/MIPS, Herschel/PACS, Herschel/SPIRE compare?

@@ Line 146: / Line 146: @@
 '''Q3.2 :''' Include some GLIMPSE images (IRAC 1 and 2) in your resolution experiments. (You can get them from me or from the Contributed Products from GLIMPSE in the [http://sha.ipac.caltech.edu/applications/Spitzer/SHA/#id=SearchByPosition&RequestClass=ServerRequest&DoSearch=true&SearchByPosition.field.radius=0.3333333334&UserTargetWorldPt=346.40118;62.54626;EQ_J2000&TargetPanel.field.targetName=J230536.28+623246.5&SimpleTargetPanel.field.resolvedBy=nedthensimbad&MoreOptions.field.prodtype=aor,pbcd,supermosaic,inventory&InventorySearch.radius=0.3333333334&shortDesc=Position&isBookmarkAble=true&isDrillDownRoot=true&isSearchResult=true Spitzer Heritage Archive]. (Be a little careful because this search will give you more images than you really need; you want the tile GLM_11150+0235* -- look where the blue circle (your target) falls on the image.) Here the PSF is distinctly triangular, even for isolated sources.
-'''Q3.3 :''' For each of the following sources, can you find the counterparts between 2MASS and WISE (ch 1 and 2)? Bonus points if you give me the coordinate-based names from WISE and 2MASS '''and''' GLIMPSE. (Hint: catalog search in FinderChart.) (If you want, regions file of these sources: [[file:section3sourcescepc.reg.txt]] - save that actual file (not the wiki link) to disk and change filename to remove the .txt extension, leaving .reg for use in ds9 or FinderChart/IRSA Viewer)
+'''Q3.3 :''' For each of the following sources, can you find the counterparts between 2MASS and WISE? What about Spitzer? Bonus points if you give me the coordinate-based names from WISE, 2MASS, GLIMPSE and Spitzer (SEIP). (Hint: catalog search in FinderChart.) (If you want, regions file of these sources: [[file:section3sourcescepc.reg.txt]] - save that actual file (not the wiki link) to disk and change filename to remove the .txt extension, leaving .reg for use in ds9 or FinderChart/IRSA Viewer) Do these objects have counterparts in any of the Herschel data?
 Easy :
-*Source #1 at 23:05:36.28 +62:32:46.5 (the Halpha source from Mikami et al.)
+*Source #1 at 23:05:36.28 +62:32:46.5 (an Halpha source from Mikami et al.)
-*Source #2 at 23:06:38.24 +62:32:04.1 (the Halpha source from Witham et al.)
+*Source #2 at 23:06:38.24 +62:32:04.1 (an Halpha source from Witham et al.)
 A bit harder:
 *Source #3 at 23:05:55.14 +62:32:59.9 (ostensibly a match to a SCUBA source. Bonus points (HARD): why should we be worried about the WISE counterpart?)

@@ Line 149: / Line 149: @@
 Easy :
-*Source at 23:05:36.28 +62:32:46.5 (the Halpha source from Mikami et al.)
+*Source #1 at 23:05:36.28 +62:32:46.5 (the Halpha source from Mikami et al.)
-*Source at 23:06:38.24 +62:32:04.1 (the Halpha source from Witham et al.)
+*Source #2 at 23:06:38.24 +62:32:04.1 (the Halpha source from Witham et al.)
 A bit harder:
-*Source at 23:05:55.14 +62:32:59.9 (ostensibly a match to a SCUBA source. Bonus points (HARD): why should we be worried about the WISE counterpart?)
+*Source #3 at 23:05:55.14 +62:32:59.9 (ostensibly a match to a SCUBA source. Bonus points (HARD): why should we be worried about the WISE counterpart?)
-*Source at 23:05:45.81 +62:30:21.7 (this one pops up as having a close neighbor in the catalog. Do you see why?)
+*Source #4 at 23:05:45.81 +62:30:21.7 (this one pops up as having a close neighbor in the catalog. Do you see why?)
 Hard:
-*Source at 23:05:10.77 +62:27:18.7 (why do I have this in the "hard" bin?)
+*Source #5 at 23:05:10.77 +62:27:18.7 (why do I have this in the "hard" bin?)
-*Source at 23:05:12.44 +62:29:41.9 (why do I have this in the "hard" bin? Hint: you need GLIMPSE.)
+*Source #6 at 23:05:12.44 +62:29:41.9 (why do I have this in the "hard" bin? Hint: you need GLIMPSE.)
 Really hard:
-*23:06:29.75 +62:26:11.7 (why do I have this in the "hard" bin? Hint: you need SDSS)
+*Source #7 at 23:06:29.75 +62:26:11.7 (why do I have this in the "hard" bin? Hint: you need SDSS)

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 '''Q2.5 :''' As I say above, Skyview will happily and without complaint or warning resample and regrid the pixels to whatever scale you want.  Ask Skyview for a small patch of sky with far more pixels than you would get if you used native pixel resolution. Load this oversampled image in the same FITS viewer as your native pixel resolution image and compare them.  Look at sizes of sources, amplitude of background variations, etc. Did that massive resampling add any information to the image?
-'''Q2.6 :''' For our project, we will need Herschel data. The Herschel tiles are LARGE and our object is kind of on the edge of them. The easiest way to access the Herschel images is via [http://irsa.ipac.caltech.edu/data/Herschel/HHLI/overview.html HHLI], the Herschel High Level Images. Go and pull the PACS and SPIRE data for our region. If you don't have enough memory on your computer to display them (or enough network bandwidth to easily download them), get my cropped versions. What is the size of the pixels in the images? What is the native resolution of the images?
+'''Q2.6 :''' For our project, we will need Herschel data. The Herschel tiles are LARGE and our object is kind of on the edge of them. The easiest way to access the Herschel images is via [http://irsa.ipac.caltech.edu/data/Herschel/HHLI/overview.html HHLI], the Herschel High Level Images. Go and pull the PACS and SPIRE data for our region; note that you will retrieve several tiles and you'll need to load them in to find the ones that have the most data on our region. If you don't have enough memory on your computer to display them (or enough network bandwidth to easily download them), get my cropped versions. What is the size of the pixels in the images? What is the native resolution of the images?
 =Comparing resolution of various surveys, and why it matters=