User's Guide to IRCAL

Table of Contents

Summary Table
Quick start
Instrument Characteristics
Software Overview
Data File Info
Filters, Grisms, Polarimeter
Nodding and Dithering Scripts
Observing Procedures and Tips
Technical Details

Mt. Hamilton Homepage

Observing Procedures and Tips

Twilight Sky Flats
Dome Flats
Image Sharpening
General NGS Observing Procedure
General LGS Observing Procedure
Low Resolution Spectroscopy
High Resolution Spectroscopy
Creating Reference Biases
General Warnings and Information

Twilight Sky Flats

With IRCAL the pixel scale is so small that twilight flats can be done while the Sun is up. This is especially true for J-band flats. Typical exposure times for flats done while the Sun is up are (though can vary, so use these only as a general guideline):

Ks: 30 seconds
H: 10 to 30 seconds
J: 10 seconds

Dome Flats

Dome flats can be done with IRCAL, though sky flats are recommended. Turn on both the super blue and the blue CCD flat field lamps. Typical exposure times for flats are (these can vary, so use these only as a general guideline):

Ks: 1 second
H: 1 second
J: 1 second
BrG-2.16: 10 seconds
H2-2.122: 10 seconds
CaF-Kgrism: 30 to 60 seconds

As always with NIR dome flats, you should take images with the dome lamps on and another set with the lamps off so you can properly subtract of the thermal component from the dome.


IRCAL does not get focused on the sky, but instead gets moved (on a linear stage) to bring it to the same focus as the NGS WFS. To focus IRCAL you need to use the AO internal white light source and the imagesharpen script, selecting the focus mode. Note that focusing is typically done by the AO operator as part of the afternoon setup.

Focusing can also be done manually using the focfit function in the ircaldisplay software.

  1. Have the AO operator put in the white light source and close the tip/tilt and AO loops.
  2. Move IRCAL's filters to BrG-2.16 Open Open.
  3. Determine the proper exposure time so that the white light internal calibration source does not saturate IRCAL, but still gives high signal-to-noise.
  4. Check the auto fwhm checkbox in the image browser window.
  5. Take an exposure with IRCAL.
  6. Enter the current IRCAL focus position in the focval entrybox.
  7. Push the focfit init button.
  8. Change IRCAL focus (IRCamFocus window in AO software).
  9. Take an exposure with IRCAL.
  10. Enter the current IRCAL focus position in the focval entrybox.
  11. Push the focfit button.
  12. Repeat previous 4 steps until you get a good focus. After 3 focus positions a plot will be displayed in the IDL 31 window and the best focus estimate will be printed.
  13. Move to the best IRCAL focus (IRCamFocus window in AO software) and take an exposure to check it is indeed at the best focus.
  14. Open the tip/tilt and AO loops.

Image Sharpening

Image sharpening is a technique to fine tune the internal AO alignment to get the highest possible Strehl on IRCAL. Image sharpening is typically done by the AO operator as part of the afternoon setup and calibration of the AO system.

Image sharpening uses the same procedure as focusing except that instead of moving the ircal focus stage, you adjust zernike modes on the deformable mirror through the HartmannModes GUI in the AO software. As with focusing, you use the imagesharpen script or do it manually using the "focus" routine for each of the Hartmann modes 2 through 10. Do not do this for mode 1 because that is focus and you do not want to change the focus of the system (that has already been done in the IRCAL focusing procedure described above).

General NGS Observing Procedure

AO observing is not too disimilar from regular NIR obsering. The main differences are finding the natural guide star for each object, field steering the AO system with telescope nods, and PSF stars for calibration. None of these will be discussed in much detail, however the basic steps of an observation are listed below.

  1. Telescope technician finds guide star (GS) on 3m Guide TV.
  2. Telescope technician or AO operator points telescope so that the GS is on the WFS.
  3. AO operator closes AO and Tip/Tilt loops and tweeks parameters for best performance.
  4. Open loops and field steer (using WFS Steering/Nodding GUI) to science target.
  5. AO operator closes loops and does some WFS centering tweeks for best performance off axis.
  6. Observer takes data, dithering manually using the WFS Steering/Nodding window or using one of the Scripts.
  7. When done taking data on this object, field steer the WFS to zero offset (i.e. put the WFS back on-axis) and repeat this procedure for the PSF calibration star or the next science target.

Listed here are tips to make communication between the AO operator, Telescope Technician, and Observer most efficient.

  1. Have RA and Dec for the GS readily available for the telescope technician. The best option is for you to make a starlist file. With our system we always find the GS first and then move to the science target.
  2. For the AO operator's benefit, it is helpful to know the R magnitude of the GS. If an R magnitude is unavailable, a V magnitude is also useful for making an initial estimate of WFS parameters for best possible AO performance.
  3. Have offsets from the GS to the science target already calculated in arcseconds of RA and Dec (and cardinal directions E or W, N or S). This will make it easiest for the observer or AO operator to steer the telescope and AO WFS optics from the GS to the science target.
  4. Have finder charts for both the science target and PSF stars.

General LGS Observing Procedure

LGS observations are similar to NGS observations for the observer. The main differences for the IRCAL observer are the way in which the GS is acquired.

LGS observations occur from 11pm until 5am. The hours of laser use are limited because of heavy air traffic in the Bay Area (particularly San Jose Internation Airport). The switch from NGS to LGS mode should begin sometime around 10:45pm for greatest use of LGS time. Typically laser alignment and calibration will take anywhere from 30 to 60 minutes, depending on seeing conditions and laser performance (we are striving to reduce this time, but for the time being, the observer should be patient).

When moving to an LGS science target we use the following general procedure.

  1. Point at an 9th magnitude star within ~1 degree of the science target. There is significant flexure in the system between the tip/tilt sensor (LTT), the WFS, laser launch telescope, and acquisition camera. The 9th magnitude star is used to align these elements for science observing.
  2. Close LTT loop on the star and let the telescope "offload".
  3. Open LTT loop and send light back to the acquisition camera to mark LTT boresight position.
  4. Steer laser to the boresight position.
  5. Send LGS to the WFS and close LTT loop on the TT star.
  6. Close uplink TT and AO loops on laser. Adjust WFS focus to the sodium layer altitude using the Back WFS.
  7. [optional] Image sharpen on the sky. This is a useful option on the first target of a laser guide star run, but only if the seeing is good.
  8. Open loops and move to science target guide star.
  9. Propagate laser and close all loops.
  10. Field steer to science target.
  11. Take science data.
Information you need to prepare in advance:
  1. Target list for US Space Command. This needs to be submitted to Elinor Gates at least four days prior to the observing run if you have less than 40 targets. If you have more than 40 targets the list must be submitted a week in advance of your observing run. Targetlist format is detailed in the AO LGS Preparation section of the Lick Adaptive Optics Manual.
  2. GS positions, magnitudes, offsets to science targets
  3. PSF star positions, mags, offsets
  4. R~9 mag star within ~1 degree of each science target (for LGS alignment purposes). If you have many closely spaced targets the same alignment star will serve for the group of them.

Low Resolution Spectroscopy

The low resolution CaF Kgrism is mounted in the first filter wheel in IRCAL. The slit used with this grism is the 100um-H mounted in IRCAL's aperture wheel. The aperture wheel motor is not very accurate in positioning the slit, but the Jog function allows you to move the slit to approximately the desired position. The following is a cookbook of steps for doing low resolution spectroscopy with IRCAL and AO. There is significant flexure between the WFS and IRCAL, so the object will move off the slit in long exposures (i.e. more than 10 minutes). Flexure moves the object down on IRCAL's chip over time. This limits the amount of time one can spend exposing before having to reposition the object on the slit. A flexure compensation program has been written to account for this, allowing observers to take longer exposures.

  1. Move IRCAL's filter wheels to Ks Open Slit-100um-H.
  2. Take a short exposure (10s or so) to determine the position of the slit.
  3. Use the Jog function in the Motor control window to move the slit to the desired position (typical moves are 500 counts).
  4. Determine pixel position on slit where you wish to put the science object. Record the pixel position, because you should use it all night for easiest and most efficient observing.
  5. Take an image of the science target with AO loops closed and use the fwhm tool in the Image Browser window to determine its centroid position. Typically the filter wheels are in the following position for this step: Ks Open Open. Also make sure that the image isn't saturated so you get a good centroid measurement.
  6. Use the WFS Steering/Nodding window to move the object to the desired pixel position. The WFS steering is well calibrated and should put the object very near the desired position. Take exposures on IRCAL and fine tune the WFS steering to center object on the correct pixel. The AO loops should remain closed for the following steps.
  7. Move IRCAL's filter wheels to Ks Open Slit-100um-H.
  8. Take short exposures and Jog aperture wheel so slit is in the proper position.
  9. Move IRCAL's filter wheels to CaF-Kgrism K Slit-100um-H.
  10. Have the AO operator turn on flexure compensation.
  11. Take spectrum of the science target.
  12. Open AO loops and move telescope to get a sky spectrum.
  13. Take a dome flat (have telescope tech move dome in front of telescope and turn on the Super Blue and CCD Blue flat field lamps). A reasonable exposure is 5s x 10coadds, though the flat field lamps vary in brightness from run to run, so it is wise to check the exposure time.
  14. Turn off the flat field lamps and take another exposure (a dome dark, which will ultimately be subtracted from the dome flat during data reduction).
  15. Move filters to Ks Open Open and take an exposure of the science target.
  16. Determine approximately how many pixels the object moved during the spectrum exposure (this will help determine the flexure effects, even with flexure compensation, on your data).
  17. Turn off flexure compensation.
  18. Find a nearby SAO star for measuring the atmospheric absorption. Typically an A, G, or K star depending on the wavelength range and absorption lines of importance in your science data.
  19. Close AO loops and take short exposure of star (e.g. 0.1 seconds for an 8th magnitude K5 star)
  20. Use the WFS steering to move the star to the slit pixel position.
  21. Move filters to Ks Open Slit-100um-H. Take short exposures and Jog slit to proper position.
  22. Move filters to CaF-Kgrism K Slit-100um-H and take data (e.g. 1s x 20coadds for 8th magnitude K5 star).
  23. Open AO loops and move telescope to get a sky spectrum.
  24. Take another dome flat and dome dark.

High Resolution Spectroscopy

High resolution spectroscopy is accomplished with IRCAL using the K-Silicon grism and cross dispersing the orders using the CaF-Kgrism. Instead of using a slit, a pinhole (usually pinhole-100um) is used as the aperture. Otherwise, the observing procedure is similar to doing low resolution spectroscopy.


A polarimeter is now installed for use with IRCAL. The waveplate is mounted just in front of IRCAL, while the Wollaston prism is mounted in a filter wheel. This is not yet a facility observing mode. Observers wishing to use the polarimeter should contact James Graham and Marshall Perrin at UC-Berkeley.

  1. Point telescope to object of interest.
  2. Close AO loops.
  3. Move IRCAL's filter wheels to Ks Wollaston Half-field. Jog the aperture wheel to properly position the Half-field aperture mask.
  4. Set the appropriate exposure time, number of reads, coadds, etc..
  5. Use the polarim script to automatically rotate the Waveplate and take exposures at each waveplate position. Waveplate positions in the script are: 0, 45, 22.5, 67.5, 90, 135, 112.5, and 157.5 degrees.
  6. Dither telescope using the WFS steering and repeat script.
  7. Repeat on PSF calibration star, polarimetry standards, etc.

The waveplate rotation positions are not absolute, thus from one run to the next, the 0 degree rotation may shift. Calibration of the waveplate rotation is done through twilight flat field images because the sky is polarized.

The aperture wheel motion is inconsistent, thus when positioning the Half-field plate it is advised to position it properly using Ap Jog (typical steps 500 or 1000 counts), then not move it for the remainder of the polarimetry observing.

For more details on the polarimeter, refer to Perrin and Graham "The IRCAL Polarimeter: Design, Calibration, and Data Reduction for an Adaptive Optics Imaging Polarimeter," PASP 120:555-570, May 2008.


Darks are typically done at the end of the night using the Dark Widget. All exposure times you used during the night are recorded in the Exposure Palette. When you start the Dark Widget it will assume you want 10 exposures of 10 coadds each for each of the exposure times used during the night. This will be overkill, especially for the long exposures. You will need to reduce either (or both) the number of coadds and exposures until you get a total time for darks down to 2 hours or so.

After you adjust the number of coadds and exposures, you should then move the filters to their Dark position by using the Config - Dark menu command in the Motor window. This command immediately moves the filters to Br-Gamma, Blank, and Pinhole-100um.

At this point, you are ready to click the Go button in the Dark Widget.

Creating Reference Biases

Note: This should only be done by a Lick Observatory support scientist or IRCAL support person from UC-Berkeley or UC-Santa Cruz. If there appears to be a problem with the IRCAL reference biases, please contact help instead of creating new reference biases.

  1. Click on Exposure Palette - File - Update Reference Biases
  2. Check createbias and coadd
  3. Press Go. This will take a while, about three hours. Wait until this step is done before proceeding to the next step.
  4. Uncheck createbias and coadd
  5. Press Go. This is to check that the refbiases are actually ok and that there wasn't a bad read in one of them. This will take about 30 minutes.

Reference biases are located on ircalbox2 in /data/bias/YYYY.MM.DD/, where YYYY.MM.DD is the creation date code for the biases. The file /data/bias/biaslist.txt is a link to the biaslist file in the proper (usually most recent) reference bias directory.

General Warnings and Information

  1. There is a ghost image approximately 20 arcsec east of the guide star caused by the 1 um dichroic beamsplitter in the AO system. It will be easily visible in short exposures if you have a bright (< 8 mag) guide star. Ghosts may be present for fainter guide stars in long exposures. If your science target is in the same field as the ghost, it could complicate data reduction
  2. See Other Information in the Lick Adaptive Optics Manual for more issues when using the AO system.

Elinor Gates
Last modified: Wed May 14 15:29:41 PDT 2008