HDI operations manual

Michael Richmond
Oct 16, 2017



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 ; specifically, a 231-84-1-142 deep depletion chip, serial number 20110426, with astronomy process and a broadband anti-reflective coating (thanks to Alice Reinheimer of e2V for this information). 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 are roughly shown below. Look in the Technical Notes for more information.

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

      1-amp            35 sec              1.3             7.3 

You can find descriptions of technical tests of HDI in a series of Technical Notes. If you have material to contribute to this archive, please contact Michael Richmond.

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.

Please do not issue the "shutdown" command. If you do, you will leave the camera in a frozen state from which the only solution is manual intervention by people who are off-site. Don't do it.

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). When the user issues this command, the current image will continue to count down to the end of its exposure time, but will not read out; any future images in a sequence will not be taken. 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


       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 first one, fitsconv.csh, is "safe" -- it will not modify data values at all -- but limited: it only works on images read through a single amplifier. The subsequent ones will divide all pixel values by 2, but can be used on either 1-amp or 4-amp images.

fitsconv.csh: for 1-amp only

This routine supplied by Douglas Arion of Carthage College in Nov, 2016. Thanks!

This script is designed to work on a set of raw HDI images at all once. To use it,

  1. create a directory and 'cd' into the directory
  2. place all raw HDI images into this directory
  3. copy the fitsconv.csh script into this directory:
               cp ~/Carthage/fitsconv.csh .
  4. run the script
    The script will create a new subdirectory called Converted and place converted, simple FITS images into this subdirectory. The converted images will have the same names as the original raw images. The raw images will not be modified.

hdi_to_simple: for 1-amp or 4-amp images
The data values in the simplified images created by the following scripts will be 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, and then type a single command:


followed by


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. Also, the changes made to your environment library variables by heainit may cause other programs to fail to run properly. You should probably use this shell only for image-conversion work, and use other terminal shells for your other work.

A script to help pick the best focus position

Originally written as HDI Tech Note 16

In order to determine the best focus position for the telescope, one takes a series of images, changing the focus position by some small stepsize each time. There is, alas, no special focus routine which automatically changes the focus position of the telescope, or which waits for the user to do so, so that images of stars at multiple focus positions appear on a single frame.

Ron Probst provides the following suggestion for acquiring a set of good focus frames:

      - Decide what to do: e.g. 32200-32400 by steps of 50, so 5 exposures
      - Set focus to starting point. Set jog to desired step size.
      - Set "etime" to 10 sec and write appropriate "comment"
      - Do "go 5" to take a series of 5, with only one cleaning cycle at start.
      - As first exposure is reading out, jog focus to next value for second exposure.
      - Repeat through series of 5 (in this case) exposures.
      - Transfer images to emerald and examine with imexam.

We have created a piece of software which may help you to analyze the resulting images and pick the best focus position.

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):

How to shut down the camera

Shutting down the camera should NOT be an ordinary action. The only situations which require it are

In case you are facing one of these situations, here's the procedure:

  1. issue a command to turn off the CCD voltages first:
  2. now you can remove power from the electronics safely via:
             power stargrasp off

If possible, before turning the power off to the computer hdiserver, contact Peter Onaka's team (who built HDI) and ask them to stop the software (virtual machines) on hdiserver. You can send E-mail to Sidik Isani at isani at ifa dot hawaii dot edu. If there's no time, because a storm is coming, then just follow the shutdown procedures and hope for the best. It's possible that when re-starting after an emergency shutdown, the computer hdiserver may not initialize properly, which may cause HDI not to work. The solution is to contact Peter Onaka's team and wait for them to fix the problem.

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

After HDI was returned to KPNO in late 2014, measurements indicate that the response to light may be non-linear above roughly 30,000 counts per pixel. See HDI Tech Note 8 for the details.

We recommend flatfield exposures with mean values less than 30,000 counts.

You can read a chronological list of flatfield exposure times at

The table below shows only the most recent reported values.

Kaitlin Schmidt and Ekta Shah of RIT acquired dome flatfield exposures during Jan, 2016. Be careful to verify that the telescope is aligned properly with the dome-flat spot! The measurements are as follows.

Jan 2016

Filter Lamp setting Exptime (sec) Mean counts
U High 100% 20 23,000
B Low 100% 28 27,000
V Low 100% 12 25,000
R Low 100% 8 27,000
I Low 100% 6 23,000
6580 High 50% 16 26,000
6740 High 50% 14 26,000
Gunn Z Low 100% 15 25,000

Thanks to Douglas and Follette for the Gunn Z value, added 6/20/2016

In Oct, 2017, Allyn Smith checked the flatfield lamps. His measurements are shown below, but his executive summary was the low intensity lights do require more time now than previously, but the high intensity lights seem to be functioning at about the same level.

Oct 2017

Filter Lamp setting Exptime (sec) Mean counts
Harris V Low 80% 12 13,000
Harris R Low 80% 10 17,000
Harris I Low 80% 10 20,000
Wash M Low 80% 16 16,000
Wash C High 75% 9 18,000
Wash 51 = DDO 51 High 75% 5 20,000
Hα (6580) High 50% 20 25,000
OIII High 90% 12 24,000
SDSS u High 90% 75 21,000
SDSS g Low 80% 17 15,000
SDSS r Low 80% 10 15,000
SDSS i Low 80% 10 15,000
SDSS z Low 80% 16 14,000