User's Guide to the Nickel Spectrograph


Table of Contents


Introduction
Hardware Overview
Aperture Wheel
Filter Wheel
Collimator
Grisms
CCD Stage
Flexure
Detector Characteristics
Software Overview
CCD User Interface
Motor Controller
Nickelfocus
Observing Hints
Calibration Lamp Spectra
Checklists

Mt. Hamilton Homepage

Focusing the Nickel Spectrograph

There are two programs available for focusing the Nickel Spectrograph: nickelfocus and nickelfocusp. Both programs are written in the IDL language and run from within IDL. IDL can be run from zola or noir.

Nickelfocus uses emission line source spectra taken with a narrow long slit to help the observer determine the best collimator position for the narrowest lines.

Nickelfocusp uses emission line source spectra taken with a pinhole to help the observer determine the best collimator position for the narrowest lines and best spatial dimension focus. At this time, we recommend using nickelfocusp instead of nickelfocus.



Nickelfocusp

If you're going to run IDL from zola or noir, start by simply typing

idl

Once in IDL simply type

nickelfocusp,[dir=dir,files=files]

Nickelfocusp takes the optional keywords dir and files and will prompt you for all the information it needs.


Keywords

dir=dir Specifies a directory for the data files if they aren't in
the default directory for the data or scratch image directories.
files=files Specifies a file that lists the file names of the focus images.
Syntax Examples: nickelfocusp
nickelfocusp,dir='/workspace/'
nickelfocusp,files='file.list'
nickelfocusp,dir='./',files='focusimages.list'


Creating a Line List

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

The nickelfocusp 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.fits). The program automatically identifies reasonably high signal-to-noise emission lines in the spectrum. Nickelfocusp will then display a contour plot of each identified line and query you if you wish to accept or reject each line. It is wise to accept only well separated and unblended lines to get the best focus measurement (also, take care not to select cosmic rays). The selected lines are written to a file on the host computer to,

/data/idl_tmp/linelistp.nickel
and a plot is displayed showing the selected lines (the color of the the line shows the state of the line in the acceptance process: red is rejected, green accepted, blue is current line being examined, white 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.

Arc Line Accept/Reject Plot

Do not accept Cosmic Rays or Blended Lines for focusing. Left: Cosmic Ray, Right: Blended Line


Using an Existing Line List

An existing line list may be used, in which case the grating and tilt (red side) or grism and x stage (blue side) and CCD readout window must be set so that the lines match the positions (to within a few 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

If you have created a new linelist, nickelfocusp 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 image. Nickelfocusp will measure the FWHM of each line in X and Y, as well as the peak flux, and print the mean X FWHM, Y FWHM, and Peak flux for the spectral lines.

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 plots of X FWHM, Y FWHM, and normalized Peak flux vs. collimator focus position will be displayed and a best fit parabola overlayed. A printout of the predicted best fit collimator position and FWHM (or peak flux) will also be displayed. These parabolic fits will be updated with each additional focus position and file that is entered into nickelfocusp.

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

FWHM Plot


Exiting nickelfocusp

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



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 | Keywords | Creating a Line List | Using an Existing Line List | Measuring Focus | Exiting nickelfocus | Exiting IDL | Helpful Hints


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 zola /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 500 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.


Support Astronomers (sa@ucolick.org)
Last modified: Tue Jul 17 18:58:26 PDT 2012