Nickel Spectrograph User's Manual: Observing Hints

Observing Hints

Filling Dewar with LN₂

It is the observer's responsibility to fill CCD-9's dewar with LN₂ twice daily, once before observing begins in the evening and again before leaving the dome in the morning.

Carefully insert the funnel into the hole in the top of dewar #9.
Put on safety glasses and cryogloves (stored at the safety station next to the dome door).
Carefully pour LN₂ from the 10 liter dewar into the small thermos.
Carefully pour LN₂ from the small thermos into the funnel.
Repeat previous 2 steps until LN₂ is overflowing from the hole in the top of the dewar. It typically takes 1.5 to 2.5 thermoses of LN₂ to fill dewar #9.

Setting Wavelength Range

Setting the desired wavelength range is accomplished by moving the CCD X-Stage.

Determine what your central wavelength will be based on the equation listed for the grism. Move CCD Stage to the desired position, having first made sure that the locking screws are backed out.
Put the diagonal mirror into the light path ( see POCO's Diag. button).
Turn on the desired arc lamp(s).
Take a short exposure (e.g. 10 seconds) with a narrow slit to check central wavelength and coverage.
If not at the desired central wavelength, move CCD x-stage a small amount and take another exposure, iterating until you are satisfied with the spectral coverage.
If there is no reason to move the x-stage during observations (i.e., a single setup is all that is required for the science data), tighten the locking screws for the x-stage and disable the CCD stage in the Motor Control GUI.

Spectrograph Focus

Focusing the spectrograph is accomplished using the nickelfocus or nickelfocusp program, which has you take arc lamp exposures with a narrow slit at multiple collimator focus positions.

Make sure spectrograph wavelength coverage is properly set for the desired grism.
Insert narrowest available slit for nickelfocus or the pinhole for nickelfocusp.
Take arc lamp exposure, making sure you have reasonable signal-to-noise for most lines of interest and that lines are not saturated.
Run IDL nickelfocus or nickelfocusp program, moving collimator and taking new arc lamp exposures as directed.
When done, set collimator focus to best position calculated by nickelfocus or nickelfocusp.

Arc Lamp Calibrations

There are two arc lamps available for calibrating Nickel spectra: HgCd and Ne. HgCd is good for the blue end calibration while Ne is good for the red end of the spectrum. See Calibration Lamp Spectra for sample Nickel spectra with line identifications.

The HgCd lamp needs about 2 minutes to warm up so that the Hg and Cd lines are at full strength. It also has argon in it, so for the first few seconds after the lamp is turned on these lines are visible, but quickly disappear.

Focusing Guider on Slit

Focusing the guider camera on the slit is important for target acquition and autoguiding. This involves illuminating the slit with a light source and moving a set of trombone mirrors to adjust focus.

Start the guider software if it isn't already running.
Select a narrow slit aperture (any slit will do, but best focus is usually easier to determine with a narrow slit).
Stow the rainscreen and open the mirror cover.
Pull the diagonal mirror out of the light path.
Turn on the flat field lamps to provide a reasonable light source.
Move the trombone mirrors until the sharpest focus on the slit on the guide camera is achieved.

Dome Flats

Dome flats can be done using the windscreen as the flat field screen. Due to flexure in the instrument, we recommend taking dome flat fields at each position of the sky where data was taken (typically right before or after the data is acquired).

Move the windscreen up in front of the telescope (you can use the dome web cameras to evaluate if the screen is up high enough, or visually inspect it yourself by going into the dome).
Turn on the flat field lamps, setting an appropriate brightness level.
Take flats, making sure the diagonal mirror is out of the way.

igetnick - Multiple Exposure Script

Igetnick is a script to take multiple exposures with the current exposure configuration. The script runs from the user account on noir.

Syntax:
igetnick NumExposures PauseTime sel=Num

NumExposures is the number of exposures you wish to take with the current exposure parameters.
PauseTime is an optional parameter and is the time (in seconds) between the end of one exposure and the beginning of the next.
sel=Num is an optional parameter and is the number of the currently selected setup in the data-taking software. If no setup is specified, igetnick assumes selection 0 (e.g. sel=0). Warning: The setup selected in igetnick must match the current setup number in the data-taking software. If it does not, there will be a mismatch of parameters and exposures may fail, have the wrong exposure time, readout parameters, etc.

Examples:

Take 10 exposures with no pause between exposures:
igetnick 10

Take 3 exposures with a 10 second pause between exposures:
igetnick 3 10

Take 5 exposure with selected setup number 2:
igetnick 5 sel=2

Target Acquisition

Target Acquisition is done by identifying the field on the Guide Camera with the diagonal mirror in the light path, which gives a wider field of view, steering the telescope to place your target on the proper fiducial. You then move the diagonal mirror out of way, giving you an image of the slit and (if all goes well) your science target on the slit. Due to flexure, it is likely that you will need to fine tune the position of your target on the slit. If the target is bright enough, you can autoguide off the slit, otherwise you will have to find a guide star in the limited field of view of the guider.

If your target is too faint to see on the guide camera, you can use the direct imaging mode of the spectrograph to center the object on the slit.

Prepare for direct imaging but leave the slit aperture in position. Take a short exposure and determine the pixel position of the center of the slit.
Move the aperture wheel to the open position.
Take a short exposure of your field and measure distance between the actual position of the target and the center of the slit (direct imaging is ~2 arcsec/pixel with the 110mm lens).
Use the Offset Telescope function of POCO to move the telescope to center the object.
Take another exposure to be sure the object is on the proper pixel. If not, repeat the previous two steps until satisfied with the object position.
Insert the aperture and grism.
Start autoguiding on a field star.
Take data.

Bias and Dark Frames

While the spectrograph is very light tight when the shutter is closed, there is a fair amount of stray light in the Nickel dome during the daytime. Thus, for biases and darks we recommend closing the manual dark slide before taking exposures.

CCD-9 does have bias structure, so we do recommend taking zero second exposure length bias frames so that structure can be removed during data processing.

There is very little dark current with CCD-9, so it should not be necessary to take long dark frames. However, should you choose to do this, be sure that all lights are off in the dome, the mirror cover and manual dark slide are closed before starting your exposures.

If you will be taking long darks at the end of the night and wish to head to bed while the darks are being taken, be sure to do all the end of night tasks first (e.g. park the telescope, close mirror cover and dome, fill dewar, etc), then use the igetnick script to take your darks while you get some sleep. Be sure to notify the staff (mh-techs@ucolick.org) that darks are in progress and at what time the darks will be completed.

Minimum Exposure Times

For dark exposures the minimum exposure time is 0.0 seconds, enabling observers to take zero second bias frames for calibration if desired.

While technically there is no minimum exposure time for the Nickel Spectrograph open shutter exposures, there are physical limits to how fast the iris shutter can open and close. For practical purposes, exposures of less than 1 second are discouraged because of shutter timing errors and uneven illumination of the field (though it should vary by less than 1% for a 1 second exposure). An exposure of 3 seconds or more brings the shutter timing errors down to insignificant levels.

Overscan Subtraction

An IDL overscan subtraction script is available, ccd2bias.pro (also used for CCD-2 Nickel Direct Imaging). This script will correctly identify the overscan region, regardless of binning or subregion of the chip read out. A third order legendre polynomial is fit to the overscan region and subtracted from the data region. The overscan subtracted data is written to a new file.

Syntax:
ccd2bias,FitsFile,NewFitsFile

FitsFile is the original FITS data file name.
NewFitsFile is the file to which the overscan subtracted data will be written.

Example:
ccd2bias,'d100.fits',d100_new.fits'

Bug reports, comments, and suggestions for ccd2bias.pro should be directed to Elinor Gates (egates@ucolick.org).

Target Lists

Considerable time can be saved and errors avoided if the observer comes prepared with a list of targets in a machine readable format. Target List formats are described elsewhere. Target lists should be placed on noir in /u/user/starlists/ or in your subdirectory under /u/user/observers/.

You can read your targetlist with the Coords program. This program can update frequently showing current hour angle, airmass, and zenith distance of each object in the list. You can also get finding charts for the objects and send the coordinates to POCO, the telescope pointing and control software.

Order Blocking Filters

Sometimes the use of order blocking filters is necessary with the Nickel Spectrograph, particularly when using the lower resolution gratings that have the broader wavelength coverage. Select an appropriate order blocking filter from the 40" or 120" filter libraries and install in the filter wheel (and updating the motor control software with the current filter name). Since the filters are below the aperture but before the collimator, you should focus the spectrograph (e.g., using nickelfocus) with the filter in place.

Direct Imaging

Limited direct imaging can be done with the Nickel Spectrograph. However, unless you are using an occulting spot, there is no way to guide, limiting the usefulness of this capability. Most observers only use direct imaging for target acquisition of faint objects.

If you want to do direct imaging you will need to

Select the open aperture (or one of the occulting spots)
Move the grating tray to the open position.
Set the collimator focus to the proper position (this is not necessary if just taking quick exposures to center your object rather than science data). NB: If taking direct science data, you will have to use the pinhole to determine collimator focus on a lamp source (either arc lamp or flat field lamp) - this is best done in advance during the afternoon. Contact a support astronomer for more information or instruction.
Move the diagonal mirror out of the way.
If using an occulting spot, you can start autoguiding.
Take exposure.

Support Astronomers (sa@ucolick.org)

Last modified: Tue Jul 17 19:07:24 PDT 2012

User's Guide to the Nickel Spectrograph

Table of Contents