Preparation

LINK to hydrapro.tar file not found in the server

The best way to prepare for Hydra is to become familiar with the Hydra assignment program at home, and to come to Tololo a day in advance to see Hydra being used by the previous users.

Unlike most other instruments, Hydra requires that you come to the telescope with extremely accurate positions and a well planned program. We provide you with a simulator ("hydrasim") to do the assignments by hand or a Fortran program ("hydraassign") for a brain-dead way of doing the assignments.

See hydra software for both of these programs.

While "hydraassign" works well, you may want to check the assignments with "hydrasim" and maybe tweak up the final assignment.

You need to have sky positions if you want to do sky subtraction. You can add sky positions by hand using the "hydrasim" program. In addition, I have a Fortran program that will make circles of sky positions at selected radii. Contact me (Nick) if you want the program. Knut has an IDL program that also does this. You can get it as part of a tar file of other programs from http://www.ctio.noao.edu/~olsen/IDL/hydrapro.tar.

The hardest part of the preparation is the measurement of astrometric positions. If your data come from Mosaic II, then you probably already have derived good positions for your targets. Just make sure that you've checked the accuracy of the astrometry in the header against the USNO-A2, USNO-B, or other catalog. The IRAF MSCRED package has tools to do this. If your images have no astrometry in them, then you'll have to make it from scratch. The IRAF IMCOORDS package will get you started. Frank Valdes also has a web page with nice instructions, which were written with Mosaic data in mind, but which are helpful for other kinds of data.

The format of the coordinate files that are input into the Hydra programs are Fortran fixed format. You must follow the formats exactly (in terms of column number) and *do not* go beyond the columns noted. Don't be creative here. Some common gotcha's during assignment:

  1. Do not go beyond col=53. The space beyond this column will be used by the assignment programs to store the STATUS information. Any information written beyond col=53 will be misinterpreted by programs downstream. See the hydraassign page listed above for the *exact* format to use. Please also note that hydraassign is a Fortran program and has a maximum number of lines of data input. Presently it is 10000, which means that only 10,000 stars can be input into the program. If you exceed this, the remaining list will not be output to the *.hydra file.
     
  2. Do not use the same star number twice. This is very important. The GUI assigns the colored dots as a function of object number. If you have FOPS and object with the same number, the FOPS GUI dot will not appear.
    Do not have two entries in the table with exactly the same position. I have evidence that if you, say, have two sky positions with different object numbers but the same RA and dec, this will confuse the hydra program but not the hydrasim program. The hydra program will tell you that it can't perform the transformation, but will give you no other information as to what the heck is the matter. Believe me. Do not assign different numbers to the same objects. (Why would you do this? Maybe you want the FOPS stars to also be objects, so you just copy some of the objects to FOPS).
     
  3. Do not use the number 0 or negative integers.
     
  4. For assignments using "hydrasim", enter the RA of the object as the "SIDEREAL TIME" parameter unless you really know what ST you will be observing at. Leaving ST=0 will cause strange things if the RA is 12hrs in the simulator.
     
  5. There are 4 parts to your coordinate file (which I call a *.coo file): the field center (which may be an object of interest or a FOPS), your objects, the FOPS stars, and skies. These are denoted as "C", "F", "O", and "S".
     
  6. The FOPS stars should be between 10 and 15th magnitude. However, there should not be much more than 1 mag difference between the assigned FOPS otherwise you will not see the faint FOPS along with the bright FOPS. Also note that ideally you want to have fairly bright FOPS in preference to faint ones. In case of bad conditions (seeing, thin cloud, etc.) faint FOPS may not work. Play it safe and choose 10-13 mag. FOPS if they are available.
    There is a problem with the FOPS display. The FOPS display should show 12 boxes for the 12 FOPS signals. For some reason, our tv clips off the right hand boxes (FOPS 3,7,11. FOPS 0 is the S one, and the numbers go ccw). While you can guide with these FOPS, you can't really see them. I would strongly recommend you use the other FOPS which we can see.
     
  7. In the hydra and "hydraassign" program, there is a subtle difference between "move" and "assign". Move moves the active fiber to EXACTLY the xy position of the cursor, while "assign" moves the fiber to the object position you click on. If you are in doubt of what the fiber is pointing to, just look at the "Field Display Tool" (this pops up when you select SETUP on the toolbar) which tells you the name of the fiber and the object it is pointing to.
     
  8. Similarly, there can be confusion in the "drag and drop" method of assigning. To active a fiber, just left-click on it. Now keep the left button down and drag the cursor to the object. While you are dragging, the cursor changes to a "circle with a dot in it." If you release the left button on the graphic symbol of the object, the fiber is assigned to the object position. However, if you drop it far enough away from the object, the fiber is assigned to a "RANDOM SKY" position. So make sure your fiber is assigned to an object and not sky. Again, by clicking on the fiber you can see the assignment in the "Field Display Tool". Or if you look real closely at the GUI under zoom, you can see the fiber is not quite on top of the object.
    You can also assign fibers by:
    1. left click on the fiber
    2. right click to get menu. Chose "Assign (move to fiber)"
    3. move the cursor to the object.
    4. left click over the object.
       
  9.  GSC coords are not good enough for Hydra. They may be good enough for space data, but for the more wide-field applications like Hydra, the errors in rotation and distortion are too large for accurate astrometric positions. You should tie your system to the USNO-2.0 catalog (or the latest version). This catalog by Dave Monet is tied to the TYCHO and USNO ACT astrometric systems. Your astrometric grid should be based on this, or an equivalent (FK5 for instance) system. A convenient access to the USNO-2.0 that I use is VizieR.
    The USNO-2.0 is pretty complete to O=21 and E=20, so for many stellar projects, all your stars are in the USNO-2.0.
     
  10. The fibers are 2.0 arcseconds, so you need coords good to 0.3 arcseconds. The measured rms error of the fiber positions is about 0.3 arcseconds.
     
  11. Here is what I do to quickly assign fibers if I am not using the "hydraassign" program:
    1. Make a *.coo file with field center, fops, objects, and skies.
    2. Load the file into hydra using the "Setup" button on the toolbar and select the "Setup Field" tool
    3. Enter your *.coo file.
    4. Once it is loaded, enter a reasonable value for "How long to you plan to observe?" and "Anticipated sidereal time at midpoint" in the "Field configuration tool". This is important.
    5. Drag and drop the objects. This has been described as the world's slowest video game. Note that all fibers that are not active are marked by a black dot in the GUI. The color of the dot can only be seen at higher zoom.
    6. Assign at least 3 FOPS and ~8 skies. They should be spread evenly around the field you are actively observing. The more sky fibers you have, the better the sky subtraction. Knut recommends 20.
    7. Save the field. This gets written to the fields directory and whatever other directory you want.
    8. If you want to clear the screen of the fiber assignments, use: File: Run Script: configparkall. See the following web page for the description of the script commands.
       
  12. The guts of the assignment is controlled by the "concentricities" file. This is defined in the ".fiborg" source file as $ECCENFILE in the $FIBINITS directory. This file tells Hydra what fibers are active, which are locked, which are FOPS, etc. From time to time we change this file if fibers become bad or get fixed. Presently we are using the file called "concentricitieslarge" which activates only the large fibers. Go the "Overview" page if you want the latest concentricities file.
     
  13. Note that on the engineering night prior to each run, we update the rotation of the Hydra field. Occasionally, this means that some of your fields may not be able to configure all of the fibers. Also, bear in mind that if you are observing at a sidereal time very different from the one at which the assignment was made, the differences in atmospheric refraction may prevent you from configuring all of the fibers.
     
  14. Summary of preparation.
    1. Download the "hydrasim" program and become familiar with using the Hydra interface. Make sure you are using the correct concentricities file.
    2. Measure astrometric coordinates for your objects and place them in the appropriate format for input into the Hydra program. Verify that "hydrasim" can read this object file.
    3. Make some assignment files.