Difference between revisions of "Measuring Resolutions (2023)"

From CoolWiki
Jump to navigationJump to search
(Created page with "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 fi...")
 
m
 
(19 intermediate revisions by the same user not shown)
Line 1: Line 1:
−
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.  
+
 
 +
=Goals=
 +
 
 +
'''Read the entire page below before you try to fill out this table!'''
 +
 
 +
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 https://youtu.be/Y5DEo6amN44 but it is totally silent because I’m in my mom’s room at the nursing facility and she is dozing.
 +
 
 +
 
  
 
{| class="wikitable"
 
{| class="wikitable"
 
|-
 
|-
−
! Data source !! Wavelengths !! Reported spatial resolution !! Empirical spatial resolution
+
! Data source !! Wavelengths !! Reported spatial resolution !! Empirical spatial resolution !! Where you find the images
 +
|-
 +
| PanSTARRS || 0.48-0.96 um || || || https://ps1images.stsci.edu/cgi-bin/ps1cutouts
 
|-
 
|-
−
| PanSTARRS || 0.48-0.96 um || ||
+
| IPHAS || 0.62-0.77 um || || || https://www.iphas.org/images/
 
|-
 
|-
−
| Gaia || 0.622-0.777 um || || ''(no images released so can't do this)''
+
| Gaia || 0.622-0.777 um || || ''(no images released so can't do this)'' || (don't exist)
 
|-
 
|-
−
| 2MASS || 1.2-2.2 um || ||
+
| 2MASS || 1.2-2.2 um || || || Finder Chart or IRSA Viewer (use 6 deg img if IRSA Viewer)
 
|-
 
|-
−
| WISE || 3.5-22 um || ||
+
| WISE || 3.5-22 um || || || Finder Chart or IRSA Viewer
 
|-
 
|-
−
| Spitzer/IRAC || 3.5-8 um || ||
+
| Spitzer/IRAC || 3.5-8 um || || || Finder Chart or IRSA Viewer, but some data sets only IRSA Viewer
 
|-
 
|-
−
| Spitzer/MIPS || 24 um || ||
+
| Spitzer/MIPS || 24 um || || || Finder Chart or IRSA Viewer
 
|-
 
|-
−
| Herschel/PACS || 70-160 um || ||
+
| Herschel/PACS || 70-160 um || || || IRSA Viewer
 
|-
 
|-
−
| Herschel/SPIRE || 250-500 um || ||
+
| Herschel/SPIRE || 250-500 um || || || IRSA Viewer
 
|-
 
|-
−
| Akari/IRC || 9-18 um || || ''(no images released so can't do this)''
+
| Akari/IRC || 9-18 um || ''(~5 arcsec; FIS is 1-1.5 ARCMIN)'' || ''(no images released so can't do this)'' || (aren't accessible; ones in IRSA tools are FIS, not IRC)
 
|-
 
|-
−
| MSX || 7.8-21 um || ||
+
| MSX || 7.8-21 um || ''(harder than i thought it would be; it's 12-20 arcsec)'' || ''(varies a lot over the bands; you can just pick 1 or 2)'' || IRSA Viewer
 
|-
 
|-
−
| IRAS || 12-100 um || ||
+
| IRAS || 12-100 um || || || Finder Chart or IRSA Viewer
 
|}
 
|}
  
−
* 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)
+
* 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), and attempt the questions in "Backing out to the bigger picture"
−
* 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).
+
* 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), and answer the questions in "Backing out to the bigger picture"
−
* Exceeds expectations: fill out as many cells as you can!  
+
* Exceeds expectations: fill out as many cells as you can, and answer the questions in "Backing out to the bigger picture", and come up with some questions of your own! :)
 +
 
 +
'''Read the entire page below before you try to fill out this table!'''
  
 
=Background motivation=
 
=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 .
+
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.  
 
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.  
Line 42: Line 55:
 
In order to fill out that table, you need to explore both documentation and the images themselves.  
 
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. (arcseconds per pixel to be precise!) If you are finding numbers in degrees or arcMINUTES per pixel, convert the units! 60 arcseconds to an arcminute, and 60 arcminutes to a degree.
  
 +
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 at least the three core data sets (2MASS, Spitzer/IRAC, WISE)? (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.
  
−
Q1.1 : 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.  
+
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?
  
−
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.)
+
The IAU-compliant names of sources are based on positions. Many catalogs and papers 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”?
  
−
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?  
+
Sh 2-187: Why is it that the "blue clusters" are easier to find in Spitzer data than WISE data?
  
−
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.
+
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?
  
−
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.
+
How is all of this going to affect our ability to match sources across wavelengths from optical to 24 microns?
 +
 
 +
= 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 and use 6 degree, not 6x.
 +
 
 +
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/
  
−
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=
 
=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.")
+
 
 
*[[FITS format]]
 
*[[FITS format]]
 
*[[Finding FITS files]]
 
*[[Finding FITS files]]
 
*[[Viewing FITS files]]  
 
*[[Viewing FITS files]]  
−
*[[All ds9 information in one place]]
 
 
*[[Overview of measuring distances]]
 
*[[Overview of measuring distances]]

Latest revision as of 15:51, 16 May 2023

Goals

Read the entire page below before you try to fill out this table!

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 https://youtu.be/Y5DEo6amN44 but it is totally silent because I’m in my mom’s room at the nursing facility and she is dozing.


Data source Wavelengths Reported spatial resolution Empirical spatial resolution Where you find the images
PanSTARRS 0.48-0.96 um https://ps1images.stsci.edu/cgi-bin/ps1cutouts
IPHAS 0.62-0.77 um https://www.iphas.org/images/
Gaia 0.622-0.777 um (no images released so can't do this) (don't exist)
2MASS 1.2-2.2 um Finder Chart or IRSA Viewer (use 6 deg img if IRSA Viewer)
WISE 3.5-22 um Finder Chart or IRSA Viewer
Spitzer/IRAC 3.5-8 um Finder Chart or IRSA Viewer, but some data sets only IRSA Viewer
Spitzer/MIPS 24 um Finder Chart or IRSA Viewer
Herschel/PACS 70-160 um IRSA Viewer
Herschel/SPIRE 250-500 um IRSA Viewer
Akari/IRC 9-18 um (~5 arcsec; FIS is 1-1.5 ARCMIN) (no images released so can't do this) (aren't accessible; ones in IRSA tools are FIS, not IRC)
MSX 7.8-21 um (harder than i thought it would be; it's 12-20 arcsec) (varies a lot over the bands; you can just pick 1 or 2) IRSA Viewer
IRAS 12-100 um Finder Chart or IRSA Viewer
  • 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), and attempt the questions in "Backing out to the bigger picture"
  • 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), and answer the questions in "Backing out to the bigger picture"
  • Exceeds expectations: fill out as many cells as you can, and answer the questions in "Backing out to the bigger picture", and come up with some questions of your own! :)

Read the entire page below before you try to fill out this table!

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. (arcseconds per pixel to be precise!) If you are finding numbers in degrees or arcMINUTES per pixel, convert the units! 60 arcseconds to an arcminute, and 60 arcminutes to a degree.

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 at least the three core data sets (2MASS, Spitzer/IRAC, WISE)? (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 catalogs and papers 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?

How is all of this going to affect our ability to match sources across wavelengths from optical to 24 microns?

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 and use 6 degree, not 6x.

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

Retrieved from "https://vmcoolwiki.ipac.caltech.edu/index.php?title=Measuring_Resolutions_(2023)&oldid=13888"