Nickel Spectrograph User's Manual: Nickelfocus

Nickelfocus

The principle of focusing the Nickel Spectrograph is simple. The spectrum of an emission line source is recorded at different focus positions using a narrow slit. Best focus is found where the line widths are at their minimum. The Nickel-specific focus program called "nickelfocus" is designed to assist the observer in finding the best focus for the Nickel Spectrograph.

Nickelfocus is written in the IDL language and run from within IDL on the computer zola. Nickelfocus operates on any emission line lamp spectrum, measuring selected lines to determine their full-width at half-maximum. The program provides a facility for interactively selecting lines to be measured. The program is fast and easy to use, allowing a large number of lines to be measured, resulting in accurate focusing.

Starting Nickelfocus

If you're going to run IDL from the zola console, or from a terminal hosted by zola, start by simply typing

idl

Once in IDL simply type

nickelfocus,[/hardcopy,center_line=center_line]

Nickelfocus takes the optional keywords /hardcopy and center_line and will prompt you for all the information it needs. The program assumes that you are focusing the Nickel spectrograph.

Keywords

/hardcopy Specifies that you want to print a hardcopy of the
final focus fit plot. The plot will be sent to the nickel telescope's
printer. The plot will also be saved in the file
on gorgon /data/idl_tmp/nickelfocus.ps until it
is overwritten the next time nickelfocus is run.

center_line=center_line Specifies the center row of the 10 row region that is averaged
together for identifying lines and measuring line widths. If no
center_line is set, the default is to find the center row of the
image and average the 10 rows around it.

Syntax Examples: nickelfocus
nickelfocus,/hardcopy
nickelfocus,center_line=100
nickelfocus,center_line=40,/hardcopy

Creating a Line List

Arc Line Accept/Reject Plot

To measure the width of the emission lines, nickelfocus needs to know their positions. This list can be interactively generated or read from a file.

The nickelfocus program will prompt if you want to create a new linelist file. If you respond yes to this query, you will then be prompted for an arc lamp file name. Enter the file name of the most recent arc lamp spectrum you have taken (e.g. scr.ccd). The program automatically identifies reasonably high signal-to-noise emission lines in the spectrum. Nickelfocus will then display a line profile of each identified line and query you if you wish to accept or reject each line. It is wise to accept only well separated, unsaturated, and unblended lines to get the best focus measurement. The selected lines are written to a file on zola,

/data/idl_tmp/linelist.nickel

and a plot is displayed showing the selected lines (the color diamond above the line shows the state of the line in the acceptance process: red is rejected, green accepted, white is current line being examined, blue is as yet undetermined). The linelist file is overwritten each time a new linelist is created. If you wish to save the file for future reference or use, copy it to a new file name.

Using an Existing Line List

An existing line list may be used, in which case the grating and X stage must be set so that the lines match the positions (to within a couple pixels) in the existing linelist.

To enter the existing linelist file, respond no to the 'Interactively create a new linelist file?' query. You will then be prompted for the linelist file name (please include the path to the file).

Measuring Focus

FWHM Plot

If you have created a new linelist, nickelfocus will use the first arc lamp file as the initial focus position. If you are using an existing linelist file, you will be prompted for a file name for the first focus images. Nickelfocus will measure the FWHM of each line, plot the FWHM across the spectrum, and print the median FWHM for the spectrum.

The program will then instruct you to move the collimator and take a new focus exposure.

Focus Plot

You will then be prompted for the next focus image name. The same calculations will be performed on this image. This process will repeat until at least three focus positions have been measured. Then a plot of median FWHM vs. collimator focus position will be displayed and a best fit parabola overlayed. A printout of the predicted best fit collimator position and FWHM will also be displayed. This parabolic fit will be updated with each additional focus position and file that is entered into nickelfocus.

Exiting nickelfocus

To exit nickelfocus, simply respond no to the 'Another focus position and file?' query.

Exiting IDL

To end an IDL session, type

exit

Helpful hints

A typical move of the collimator for focusing is 3000 counts.

If you wish to have the best focus at a specific region of the spectrum, select only lines (or a single line) near that region of the chip.

Elinor Gates, Bryant Grigsby. Last modified: Thu May 1 16:34:00 PDT 2008

User's Guide to the Nickel Spectrograph

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