AST 4723 Lab 3: Data Archives and Image Viewers
Objective: The purpose of the lab this week is for you to learn how to acquire images from online archives, display images using a standard image viewer (ds9 in this case), and perform quick, first-look analysis of the images using IRAF. These skills will be directly relevant for the independent projects and subsequent labs. During this lab you will be downloading data from the HST archive and the ESO archive.
Getting Started:
1. Open a web browser from your desktop
2. Using cygwin connect to siesta or one of the other department computers (same procedure as in previous labs).
3. On this computer, change directories to /astro/data/siesta1/classes/ast4723/lab3. Create a new directory corresponding to your name or username and cd into this directory.
Lab Instructions:
1. Creating an account with the ESO Archive
The ESO archive contains data from the suite of ESO ground-based telescopes
(including the four VLT telescopes, the NTT, the MPG-ESO 2.2m, the ESO 3.6m, and the APEX submm telescope), and also hosts a copy of the HST archive. The first thing that you have to do to use the ESO archive is register as an ESO archive user.
a. Go to archive.eso.org. The top item on the left hand menu should say “ESO User Portal”. Click on this link and then “User Portal Login”. From this window, then click “I would like to create a new account.
b. Fill out the archive registration form. If given the option of choosing “Media Type”, select “FTP”. Once you have submitted the form, you should shortly receive an email confirming your account. We will return to the ESO archive later in this lab.
2. Downloading data from the ESO Archive
The ESO Archive contains data from all ESO observatories, including the VLT, NTT (New Technology Telescope), and several smaller telescopes. For this part of the lab, the goal will be to retrieve ESO images for a specific object that you will be assigned during lab. Note that each person/team will have a different object for which they will be retrieving data.
a. Go to the archive query form page on the ESO site
(http://archive.eso.org/eso/eso_archive_main.html) and perform a search for imaging of this target, requesting only images under the category “science”. b. Record in your notebook the instruments, exposure times, filters, and
observation dates for the longest exposure and the longest exposure with a VLT instrument (if any). If there are multiple observations with the same maximum exposure time, pick one of them. Note that the observation date can be found as part of the “Dataset ID”.
c. Select the frames with the longest exposure time (regardless of whether the data was taken with the VLT or another telescope), and request this data set. On the next screen you will be asked to select your output media. Choose “Transfer files yourself with ftp”. Note that on this page there is also the option to request calibration files – this will be relevant for your independent projects.
d. You will receive two email notifications – one indicating that your request has been received and a second notifying you when your data is ready for
retrieval. The wait for the ESO data set should be only a few minutes. While you are waiting, proceed with the next section of the lab. To retrieve the data:
i. Click on the ftp link near the end of the notification email. This will show you in your web browser all the files that you have requested.
ii. In your subdirectory under ast4723/lab3, type:
wget ftp://archive.eso.org/pub/archive/PATH_IN_EMAIL/NAME_OF_FILE where NAME_OF_FILE is a file listed in the web browser that you wish to retrieve. The command wget (short for web get) is a Linux command for retrieving files over the web. At this point you should have the ESO data downloaded. Record the name and size of the image file in your lab notebook, and check your disk usage to make sure that you haven’t exceeded your quota. 3. Downloading data from MAST
MAST is the Multimission Archive at STSCI, which contains data from a variety of NASA space missions. This is the preferred location for downloading HST data, and does not require registration to access the data. For this part of the lab, the goal will be to retrieve HST images for the specific object that you will be assigned during lab.
a. While waiting for the ESO data, go to the MAST web page (archive.stsci.edu) and perform a search for imaging data of the same object. If you do not select specific wavelengths, then the search will return all observations of this object by all missions in the archive.
a. You now want to answer a series of questions with your initial search:
i. With what telescopes, instruments, and filters has this telescope been observed?
ii. What years were the observations taken?
iii. Is any of the data proprietary? Data typically do not become public domain until about a year after it is taken. Data that are not yet public are designated by a shaded yellow box in the “Mark” column.
b. Pick the longest exposure observation taken with HST WFPC2 or ACS
cameras. Mark this selection and click “Submit marked data for retrieval from STSDAS”.
c. On the next screen you will be asked to fill out assorted information for data retrieval:
i. For the username, type “anonymous” and for the password type your departmental email address (e.g. [email protected]).
ii. Under deliver options, select “STAGE”.
iii. Under “Science Files Requested”, uncheck all boxes, and at the bottom under “File Extensions Requested” highlight “DRZ”. The DRZ files are the final reduced images from HST. If you’re curious to see what they look like before cosmic ray removal, also request the “FLT” files. iv. Once you submit your request, you will receive an email with
information about the files that you have requested. You will later receive a second email once your data is ready for retrieval from the archive.
v. Requests to the HST archive take a while to process (the data is reprocessed into final images every time it is requested), and you may or may not receive your data before the end of lab. Once you do receive this email, follow the instructions in the second email to download the data into your directory under lab 3 on siesta1. Place it in a subdirectory called HST_TARGETNAME. In the event that you do not receive the data before the end of lab, you should connect to siesta some time in the next few days (requests only stay active for a few days) to download and view the data.
4. The DS9 Image Viewer
Once you have images downloaded, you need a way to look at them. The program ds9 is the most commonly used image viewer in astronomy, and the aim of this next section is to learn how to effectively use it.
a. From within the same directory, type “ds9 &”. Recall that the “&” tells the computer that you want the program to run in the background so that you can still type at the command line. A ds9 graphical window should pop up on your screen.
b. From the ds9 menus, open the files that you just downloaded. Note that it may end in .gz (meaning that the file is zipped) so you’ll need to modify the
extensions in the “Filter” box to list files that end in gz. If this doesn’t make sense to you yet, ask us.
c. Once you have the file open in your viewer, spend some time getting used to ds9. Start by browsing through the reference manual (accessed from the “help” menu in the upper right) and exploring the different menus. d. Next, try to do the following things within the image viewer
1. Try changing the scaling. Recall that last year we discussed that the way an image appears on the screen is a function of range of pixel values (flux values) spanning the range between black and white, and the relation between pixel value and intensity on the screen (linear, log, etc). Record how the image looks for the following settings:
i. zscale and linear ii. minmax and linear iii. 99% and squared
2. Slide the mouse around the image while holding down the right mouse button. This changes the image stretch. When you are done, go to the color button and then click on “grey”. This will restore the image to the original stretch.
3. While under the color menu, test out all the different color schemes. Record which color schemes you find best for viewing objects in the image. Note that the “invert” button works in conjunction with any other color map. Compare the “grey” and inverted grey color maps – with which version can you see faint objects more clearly?
4. To move around the image, click with the middle mouse button on the region where you would like to center the image. Try centering on several different objects in this fashion.
5. Zoom to fit the entire image in the window, and then zoom in by a factor of 8 magnification. Also look in the panner window (upper left) to see how the magnification changes there.
6. For two locations in the image (preferably at the locations of objects), record the information listed below. Note that for the latter three items you will need to access the WCS menu
x,y location pixel value
RA,DEC in J2000 coordinates
Latitude and longitude in ecliptic coordinates
7. Identify four interesting objects in the image; draw circles around two of them and boxes around the other two. This can be done by selecting the correct region shape from under the regions menu, and clicking the left mouse button at the location where you want the region to appear. 8. Double click on each region that you have selected. A new window
will pop up. In this window enter a brief comment about the object. When you are done, save the regions to a file called test.reg.
9. Overlay a coordinate grid on the image. Edit the coordinate grid parameters so that the coordinate grid type is “publication”, with exterior axes.
10. Print the image, with the coordinate grid overlaid and the region files displayed, to a file called ds9.ps
e. OK…time to learn a bit about the image. Look at the image header
(information that is recorded at the start of a fits image, and viewable from within ds9). Record the following information. Some of this information you already know from the archive, but the header will confirm that that
information is correct.
Exposure time Filter
Image dimensions (how many pixels in each direction?) Right ascension
Declination Equinox Airmass
f. Next, for comparison we’re going to use ds9 to download an image of the same field from the digitized sky survey and compare it with the ESO image.
1. Under the analysis window, go to the “Image Server Menu” and
request a DSS image of the same field. This will appear in a new frame buffer within your image viewer. Note that under the “Frame” menu you have the option of displaying the images side-by-side (tile) or one at a time (single). You can then move between these images with either the “blink” or “next/previous” buttons. Also save a copy of the dss image to disk under the name dss.fits
2. In your original image zoom in to look at a region of interest. Once you’re there, align the two images based upon the world coordinate system (WCS) coordinates. You should be able to immediately see the difference in data quality between the DSS data (Palomar 200” data with photographic plates) and the modern ground-based data (likely taken under better seeing conditions than the Palomar data).
g. Once you have retrieved the HST data from the MAST archive, display it in a new frame within ds9. If you are unable to download it during lab, then this and the next step should be done as homework prior to next Tuesday. The resolution differences between the data sets should be striking. This
comparison illustrates the greatest advantage of ACS. What advantage can you see to the ground-based data?
h. Finally, align all three images and save jpg images of each frame to files called vlt.jpg, dss.jpg, and hst.jpg.
5. Using IRAF for Quick Look Analysis
Now that you are have acquired the data, it is time to perform a bit of quantitative analysis on the image from within IRAF. The ultimate aim from this part of the lab is to compute the mean FWHM of the PSF for your ESO image to assess the image quality. We will delve into IRAF in greater detail next week. For now a brief introduction to a few commands will suffice.
The relevant IRAF commands that you will need for this part are: display, imexam, imstat, imarith, average, and help. If you have not previously used IRAF, then there are a few preliminary steps that you will need to perform (listed below). If you already have an iraf account, skip these steps.
1. Setting up IRAF. To set up IRAF do the following.
% cd
% mkdir iraf % cd iraf % mkiraf
-- creating a new uparm directory
Terminal types: xgterm,xterm,gterm,vt640,vt100,etc. Enter terminal type: xterm
You are now ready to run iraf. 2. Logging into IRAF
% cd iraf % cl
The command “cl” enters you into the IRAF environment. You always want to start IRAF from within your “iraf” directory. Otherwise not all of the
commands will work. When you enter iraf, your command prompt will look like:
cl>
When you are ready to exit IRAF, type “logout”.
Alternately, you can log into PyRAF, which is the more modern python-based version. To do so, you must still complete step 1., but to login you type the command “pyraf”. When you enter PyRAF, your command prompt will look like:
-->
When you are ready to exit PyRAF, type “.exit”. 3. Displaying an image from within IRAF
The command display enables you to display an image from within iraf. To use it, you should move to the directory containing the image that you wish to display. If you simply type “display” at the command line, IRAF will prompt you for additional information that it needs. Use this command to display the ESO image in your ds9 viewer.
In IRAF, as in Linux, there are help pages describing every command. To access the IRAF help pages the command is help rather than man. You likely do not need to read the help page for display, but will need to do so for some of the other commands.
4. Image statistics
Use the command imstat to measure the mean, median, mode, and standard deviation of the pixel values in the ESO image.
5. Quickly measuring object photometry
Using the command imexam, measure the following information for 10 stars in your image:
x,y position
Direct and Moffat FWHM (the imexam help page describes these) Flux
When you measure this information, redirect the output to a file. This can be done using either standard Linux redirection or editing parameters for the command imexam.
6. FWHM
At this point you should have a file containing the FWHM values for 10 stars. Edit this file to remove any outliers with particularly large FWHM values (i.e. galaxies) or cases where the PSF could not be measured. Next, using either a text editor or linux commands, make two new files that each contain only a
single column corresponding the direct and Moffat PSF. Use the iraf command average to compute the mean and standard deviation for both of these two PSF measurements. Record this information in your lab notebook. This is the end of the lab. If you do not have time to complete the above exercises during the lab period, you will be expected to finish them either as homework or during the next lab period.
