HDI operations manual

Michael Richmond
Oct 19, 2013
Oct 29, 2013
Oct 30, 2013
Nov 02, 2013
Nov 03, 2013
Nov 05, 2013
Nov 12, 2013
Jan 22, 2014 (after installation of vmhdi-1.0)
Jan 24, 2014
Mar 4, 2014
Apr 20, 2014

Contents:


Introduction

In October, 2013, the main instrument for the WIYN 0.9-m telescope was switched from the imaging camera called S2KB to a new camera called the Half Degree Imager (or HDI for short). The two cameras are similar in many ways, but differ in two main respects:

HDI is built around the e2V 231-84 CCD chip . You can look at a quick comparison of the properties of S2KB and HDI to get an idea for the differences between them.

The properties of HDI have not been fully measured yet. Preliminary values are shown below. As values are measured, I'll fill in the numbers below.

      mode          readout time       gain (e-/ADU)    readnoise (e-)
    ----------------------------------------------------------------------
      4-amp            9 sec               1.3             7-8

      1-amp            35 sec              1.3              9  
    ----------------------------------------------------------------------


Basic web interface

The most direct way to operate HDI is to use a web browser to connect to a simple command-line interface. Go to

Your browser should display a bunch of text in a variety of colors, something like this:

Most of the screen contains a mixture of green and yellow text: these are messages printed by the camera controller as it goes through the steps of reading out the chip, saving the data to disk, and so forth. Down at the bottom of the screen, the text in the grey background provides status information on the condition of HDI: next to dev1 is the temperature of the CCD, for example. The single line of text with a blue background shows the current image information: in this case, the most recent file had type DARK and an exposure time of 1000 seconds.

Near the bottom of the screen, between the grey CCD status information and the blue image information, is a single blank line. In this space, the user can type commands. One can use the up-arrow and down-arrow keys to recall past commands quickly, and the left-arrow and right-arrow keys to move the cursor within a line of text.

One of the most important commands is help. In response, the system will list all the commands:

Observers will typically need only a few of these commands.


Taking an exposure

When the user types go to take an exposure, the camera will go through a series of steps: cleaning the CCD, opening the shutter, waiting for the desired time, closing the shutter, reading out the CCD, saving the image on the HDI data archive.

During these procedures, a window will pop up in the web command display.

This window will show the progress of each task as it executes. During this time, the user will be unable to type new commands.

In order to abort an exposure or sequence of exposures, the user can type "Control-C" (hold Control key, type "c" at the same time). After an abort, the camera may print error messages; if so, follow the steps described in Troubleshooting to recover.

Images are assigned names with a format like



    c6584t0023o00.fits

       in which

         'c'           is always the first letter
         '6584'        are final 4 digits of Julian Date at UT = 00:00
         't'           follows the Julian Date
         '0023'        is a four-digit index of images taken this day
         'o'           encodes "type" of image:  'b' = bias
                                                 'd' = dark
                                                 'o' = object (also for flats)
         '00'          always follows the "type"
         '.fits'       is the file extension

Each image is placed into a separate directory in the data archive. The name of the directory is the same as the first 11 characters of the image's file name. Thus, the image in the example above,



    file  c6584t0023o00.fits    is in directory   c6584t0023o   
          -----------

One can access an image in the data archive by its URL; in our example above, we would go to

After all exposure tasks have completed, the user can look at the image and download it. To do so, he should switch to the web image browser and quick display and click on the date string to refresh the list of images. In the example below, the date string is 20131020, in bold white font.


Web image browser and quick display

All the images taken by HDI are automatically saved in a computer dedicated to the camera, which serves as a data archive. The user can access images in this archive through a web browser.

The browser should show a display like this:

The basic structure of this display is threefold:

dataset at the top
In the example above, you can see that the dataset being displayed include images taken on 20131019 = UT 2013 Oct 19. In smaller text is a list of all recent datasets. Clicking on any one of the items will switch to the dataset for the given date.

Clicking on the current date string (20131019 in the example above) will refresh the list. It's a good idea to do this regularly in order to see the latest images.

list of images on the left
In the example above, there are eight images. A small amount of information about each image is displayed in this list. The "comment" area will contain whatever the user has provided using the comment command.

The very first column, filesetID, is the name of a directory holding information on each image. Clicking on this column will take the user to another web page -- see the discussion below.

quick-look JPEG on the right
In the example above, the image c6584t0245o, is being shown. The camera controller converts each image into JPEG format for quick-look purposes. Images are displayed with a rainbow colormap to highlight faint details.

At the bottom of the JPEG image, a single line of text provides some information on statistics of the image. The median value is listed with text like this:

            16249 e- sky or bg
        
Even though this text states "e-", the units of this value are ADU, not electrons.

If one clicks on the filesetID column of the data archive browser, one will be taken to a web page like the following:

One sees again the same three-part display:

This time, the image link contains the full file name of a FITS image: in the example above, c6584t0037o00.fits . If the user clicks on this link, a window will pop up asking if the user would like to

Note that all raw HDI files are not simple FITS images, but are instead multi-extension FITS files (MEFs). Working with MEFs is a bit more complicated than with simple FITS images. It might help to read the NOAO guide to working with mosaic CCD data.


Web status graphs

A few of the properties of HDI are logged continuously and displayed on a set of graphs. The user can see these graphs by clicking on the word Graphs in the upper-right corner of any of the HDI web pages.

Below is an example of these graphs; click on the image for a full-sized version.

The four graphs show

The pressure values on the y-axis are in torr. Any values below 500 micro-torr are probably not very precise -- don't worry if you see spikes or dips, as long as their values remain below 500 micro-torr.


Saving your images to emerald

By default, all HDI images are stored on a dedicated computer which is connected directly to HDI. It is that computer which serves the images and other information via the web interface described above.

Most observers will need to copy the data from this datastore to the computer emerald in order to examine it in detail, and in order to transfer it to their home institution or a laptop. There are two ways to do this.

  1. As described above, if one clicks on the filesetID of an image in the browser quick-look display, one can download a copy of the image to the "/home/36inch/Downloads" directory on emerald. This is easy to do for an image or two, but not the best way to deal with many tens or hundreds of images.
  2. One can use the automatic scripts described below to transfer a large number of files at once, or automatically copy each new image as it is acquired.


Converting HDI images to simple 16-bit integer FITS files

HDI saves raw images in FITS files, but they aren't the simplest type of FITS files. Every raw HDI image consists of a header unit, and then at least one extension:

Some image processing packages will properly handle complex FITS files, but others will not; moreover, properly displaying all the data in a multi-amp image can be a little tricky, as the orientation of the data may change from amplifier to amplifier.

If you would like to create simplified FITS images for quick-look purposes during your run, you may use the scripts described below.

The data values in the simplified images have been modified from their raw values. Don't use them for science unless you really know what you are doing.

The first step is to start the Bourne Shell: type


        bash
   

which will set up a number of environment variables so that the scripts below will run properly. This may change your current directory, so it may be necessary to cd back to your previous directory before you continue.


Known 'features' (aka annoyances)

HDI is still not a mature instrument. Some of the software is in a state of flux. Experienced observers who have used S2KB may note the following (as of Jan 22, 2014):


Errors and troubleshooting

These are known failure modes of HDI, and the procedures to recover from each.

  1. communications failure with HDI controller


Flatfield lamp and exposure guide

On Jan 21, 2014, RIT students Sravani Vaddi and Kayla Emerson experimented with the flatfield lamps to figure out the appropriate lamp settings and exposure times to yield a decent signal in each of the common filters. Their results are shown below; their goal was to find combinations which would yield around 25,000 counts in an image.

Filter Lamp setting Exptime (sec) Mean counts
U High 100% 13 24,000
B Low 100% 25 25,000
V Low 100% 12 25,000
R Low 100% 8 26,000
I Low 100% 8 27,000
 
Hα (6580) High 50% 11 24,000
Hα+40 (6620) High 50% 11 25,000
Hα+80 (6660) High 50% 11 25,000
Hα+120 (6700) High 50% 11 27,000
Hα+160 (6700) High 50% 11 26,000