Observing HintsMinimum Exposure Times | Multiple Exposure Script | Focusing | Acquisition and Guiding | Overscan Subtraction | Target Lists
Minimum Exposure TimesFor dark exposures the minimum exposure time is 0.0 seconds, enabling observers to take zero second bias frames for calibration if desired. For normal exposures the software will allow any exposure time, however, the shutter will only properly work for exposures of 0.25 seconds or longer. Shorter exposures will probably cause the shutter to fail to close properly. The data taking software will alert you if the shutter is in an improper state at the beginning or end of an exposure. If you accidently take an exposure with too short an exposure time, the shutter can be reset by taking a normal exposure of 1.0 second or longer in duration (note that this exposure will not contain useful data because of the confused shutter state).
igetpfcam - Multiple Exposure Scriptigetpfcam is a script to take multiple exposures in a row with the current exposure configuration. The script runs from the user account on pfcampc. Syntax:
igetpfcam NumExposures PauseTime
NumExposures is the number of exposures you wish to take with the current exposure parameters.
PauseTime is an optional input and is the time (in seconds) between exposures. Examples: Take 10 exposures with no pause between exposures:
Take 3 exposures with a 10 second pause between exposures:
igetpfcam 3 10
Focusing PFCamThere are two mechanisms that need focusing on PFCam, the Guide Camera and the Science Camera. Both cameras look through the prime focus corrector and atmospheric dispersion compensator, contained in the Prime focus barrel, which can be moved up or down to adjust focus on the cameras from the PFCam Motor Control GUI. The science camera has an additional focus mechanism, also controlled from the PFCam Motor Control GUI. Because the guide camera has only one focus mechanism (the Prime focus), it should be focused first. You will probably be working in cooperation with the telescope operator for this step.
- Acquire a star on the guide camera.
- Adjust the Prime focus position from the motor control GUI. Small changes (e.g. from 3.2 to 3.3) are usually sufficient to see a moderate change of focus on the guider.
- Watch the guider as the Prime focus position changes. Continue to move the Prime focus position until you get the best looking focus.
- Set the Dewar Filter to the desired position from the motor control GUI.
- On PFCam, acquire a star that gives about 5000 DN or more above background in five seconds without saturating (stars between 10th and 12th magnitude are ideal).
- Set the Dewar focus at an initial position (e.g. 0.0).
- Set the exposure time for PFCam to 90s or 120s.
- Start the exposure. After 5 seconds, or whatever is required to get resonably good S/N, pause the exposure.
- Move the Dewar Focus by the desired step size (0.5 mm is typical).
- Move the telescope 15 or 20 arcsec in some direction. Choose the direction so that your focus-star will not trail out of the field after several such moves.
- Resume the exposure and pause after another 5 seconds.
- Move the Dewar Focus and telescope by the same step sizes again.
- Repeat previous two steps 5 to 8 more times, making sure you move the telescope a double distance on the final focus position so that you can tell which end of your focus star trail is which.
- Stop and read out the exposure.
- Use the Itv tool in the image display software to measure the focus.
- Choose a star that has high signal-to-noise and is not saturated in the image.
- Click on Erase Focus Data.
- Click on Pick Stellar Stat Loc.
- Click on first stellar image in focus sequence.
- Click on Do Stats.
- Click on Add to Focus Data and enter the proper Dewar Focus value corresponding to that position.
- Repeat this procedure for each star in the focus sequence.
- The best focus value will be displayed in the focus fitting section of the statistics window.
- Set the focus to the best focus position.
Field Acquisition and GuidingBecause the guide camera is 30 arcminutes off axis from the science camera, we do not use it for field acquisition. Typically, the telescope operator moves the telescope to the desired position. The observer then takes a short exposure (typically binned by 4 to speed up readout time) to see how centered the field is. The telescope operator then adjusts the telescope position to fine tune the pointing as needed.
Once the telescope is pointed to the observer's satisfaction, the operator looks for a guide star -- if none is already in the guide camera field -- by translating the guide camera E-W. The PFCam Motor Control GUI includes controls for translating the camera, but the operator generally does this from a GUI of his own. Once a guide star is found and guiding commenced, the operator will inform the observer that the exposure may begin.
Overscan SubtractionAn IDL overscan subtraction script is available, pfcambias.pro. This script will correctly identify each amplifier region and the corresponding overscan columns, regardless of the binning or subregion of the chip read out. A third order legendre polynomial is fit to each overscan region and subtracted from the data regions. The overscan subtracted data is written to a new file. Syntax:
pfcambias, FitsFile, NewFitsFile
FitsFile is the original FITS data file name.
NewFitsFile is the file to which the overscan subtracted data will be written.
pfcambias, 'pfc100.fits', 'pfc100_new.fits' Bug reports, comments, and suggestions for pfcambias.pro should be directed to Elinor Gates (firstname.lastname@example.org).
Target ListsConsiderable time is 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.
Support Astronomers (email@example.com) Last modified: Mon Apr 9 22:02:34 PDT 2012