Nickel Direct Imaging Camera User's Manual: Observing Hints

Observing Hints

Filling Dewar with LN₂

It is the observer's responsibility to fill CCD-2'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 #2.
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 #2.

Focusing the direct imaging camera is accomplished by moving the telescope's secondary mirror. The focusing procedure consists of making a series of star images on a single CCD exposure frame by repeatedly pausing the exposure, offsetting the telescope slightly, moving the secondary mirror, and resuming the exposure, until the desired number of images has been made (typically 7-9).

The result is a line of images for each single star in the exposure. The images for a given star are then compared to one another and the secondary mirror position corresponding to the sharpest image is adopted as best focus. The procidure is described in detail below. (N.B. Nickel's broadband filters are par-focal, but narrowband filters may vary, and should be checked and, if need be, individually focused).

Acquire a focus star near zenith. A star of about 10th magnitude will work well. It is not usually necessary to find and set on a particular star, as most fields will have at least a few stars of suitable brightness, but it is a good idea to take a test exposure of about 10 seconds to confirm the presence an appropriately bright star, to estimate the best exposure interval for focusing, and to note its position with respect to other stars in the field and to the edges of the CCD.

Set the telescope secondary focus to a starting position (e.g. 340) using the Expanded POCO Focus GUI.
Set the exposure time to be equal to the the predetermined exposure interval (make this long enough to get good S/N in at least one star, but be careful to avoid saturation, remembering that the counts/perpixel will increase as best focus is approached), times the number of steps through which you will move the secondary mirror (typically 7 to 9 steps).
Start the exposure, keeping your eye on the elapsed time and the cursor poised over the "Pause" button. As soon as the predetermined exposure interval has passed, pause the exposure.
Using the Expanded POCO Focus GUI., move the telescope 15 or 20 arcsec (choose a direction that will prevent other stars in the image interfering with your chosen focus star, and that will keep it clear of the edges of the CCD). At the same time, change the telescope focus (secondary mirror) by 3 or 5 counts. Note that the POCO focus GUI allows telescope and focus motions to be combined in a single button click.
Resume the exposure and pause again after the same interval.
Move telescope and focus by the same step sizes again.
Repeat the previous two steps 5 to 7 more times. Moving the telescope double the distance before the final exposure interval will unambigously indicate the order in which the images was made -- an essential datum for analysing the resulting focus frame.
After the image is read out (if there was exposure time remaining when you finished your your series, you'll have to click the "Stop and Readout" button) examine it. Each star in the field should have produced a row of images, their number corresponding to the number of focus positions tried (if the series does not pretty clearly pass through and beyond best focus, instead getting monotonically better or worse, you will probably have to reapeat the focus exposure using a different starting point and/or larger step sizes). The final image of each series should be clearly identifiable by the additional separation from the rest. (If you forgot to lengthen the final move, you can recover the direction by reviewing your procedure, but remember that moving a telescope in a given direction will cause a star to move on the CCD in the opposite direction.)
Use the Itv tool in the XVideo image display software to measure the focus.
1. Make sure that the star you have selected to measure has good signal-to-noise, is not saturated even at its best focus, and is free from significan interference from overlapping stars.
2. Click on Erase Focus Data.
3. Click on Pick Stellar Stat Loc.
4. Click on first stellar image in focus sequence (skip outlying images if they are badly out of focus as the resulting measure will be unreliable, but remember that you'll need at least three measurements to get a fit to the data).
5. Click on Do Stats.
6. Click on Add to Focus Data and enter the telescope secondary focus value corresponding to that position.
7. Repeat steps 3 to 6 for each measurable star in the focus sequence.
8. The best focus value will be displayed in the focus fitting section of the statistics window.
Set the telescope focus to the best focus position using the Expanded POCO Focus GUI..

Flat Fields

Sky Flats: Sky-illuminated flats, taken at evening and/or morning twilight are the preferred method of flat fielding. (Alternatively, domeflats may be made using a continuum light source affixed to the top of the telescope, reflected off the inside of the dome. Dome flats provide an important fallback if weather or other constraints prevent skyflats from being made.)

Provided weather conditions permit, open the dome and then clear the telescope lightpath by lowering the windscreen, retracting the rainscreen, and opening the mirror cover. Dome, windscreen, and mirror cover are controlled from naux_fe; the rainscreen must be opened manually.
The shortness of appropriate twilight illumination, combined with the rapidly changing skybrightness at twilight, require concentration and efficiency for successful skyflats, especially if flats in a number of filters are required. Some control of illumination may be gained by pointing the telescope east or west, away from or towards the part of the sky illuminated by the setting or rising sun, but there is no substitute for being well prepared and undistracted.
It is advisable to begin early, taking unrecorded test exposures until the sky illumination is suitable (either dropping below saturation in the evening, or reaching adequate brightness as dawn). Once the illumination is acceptable, turn on recording and begin taking flats, checking the flux level in each new image and adjusting the next exposure accordingly. (Note that scripts for multiple exposures are not recommended for skyflats because of the constant need to change exposure time to follow the changing sky brightness.)

Dome Flats: There is a flat field screen in the dome or you can use the raised windscreen as a flat field screen. There is also a white patch on the inside of the dome shutter that can be used for flat fields. While one can use the telescope lamps and the windscreen or rainscreen it is advisable to use the flat field screen with the dome flat lamps. Benifits of the flat field screen include: if the dome is dark there are no shadows in the telescope plane vs using the telescope lamps project shadows, it reflects more blue light so is better for U and B flats as well as having less contamination from sunlight leaking in the dome. To do dome flats you will have to do the following:

Open the telescope light path, e.g. open the rainscreen and mirror cover (rainscreen is opened manually from the dome, the mirror cover is opened from the naux_fe GUI.

Windscreen flats

To use the windscreen for flat fields, simply put the Dome into auto mode, i.e., set DomeAuto in the main POCO GUI. This will align the dome slit and windscreen with the telescope. If you wish to use the white patch on the dome for flat fields, the windscreen will have to be in the down position and you may have to adjust the position of the telescope to properly align it. If one is doing flats around sunset then move the telescope 2 or 3 hours east to cut down on sunlight leak through the dome slit.

Turn on the telescope flat field lamps from the naux_fe GUI.

Select a filter and take a test exposure. With CCD-2 the best flatfields have count levels between 35K and 45K DN and exposure times of at least 3 seconds to minimize shutter timing effects. The brightness of the flatfield lamps can be adjusted as necessary from the naux_fe GUI.

Flat Field Screen

To use the flat field screen, you need to put the Dome into manual positioning mode, i.e., set DomeHold in the main POCO GUI. If you are doing flats at HA=0 then you will need to command the dome to move to Az=218. Then move the telescope to Dec -05:00:00. If you are doing them elsewhere then set the dome to +38 of the autodome's az.

Note: If the windscreen is down or removed, too much afternoon sunlight may leak into the dome. If that happens, you can use this alternate setting for the dome and telescope for dome flats with the flat field screen: HA=4:00:00, Dec=38:00:00, and Dome Az=328, which will block some of that stray light.

Turn on the dome flat field lamps from the naux_fe GUI.

Select a filter and take a test exposure. With CCD-2 the best flatfields have count levels between 35K and 45K DN and exposure times of at least 3 seconds to minimize shutter timing effects. With the reheostat on the lamps set at 30% the following exposure times yield these respective counts: U (60sec,1500), B (30sec, 3400), V (10sec, 9500), R (5sec, 25000), I (3sec, 40000).

The igetnick script is a convenient way of getting multiple identical exposures once you are setup for dome flats.

It is possible to use both the dome and telescope flat field lamps when using the flat field screen but the illimination will not be as uniform.

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 the current setup displayed in the data-taker GUI.

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

Observing Scripts

It is possible to write simple observing scripts. Script commands are:

igetnick - take exposure(s)
settime - set integration time
setfilter - change filter
setobject - set object name
telmove - move telescope

More script commands are likely to become available in the future. If you require something specific for scripting, contact a support astronomer to find out if it is feasible. Syntax for the script commands:

settime x
where x is the exposure time in seconds. x must be a floating point number. Invalid entries (e.g. negative numbers, or strings) are ignored.
Example: Set exposure time in current setup to 10 seconds
settime 10
setfilter y
where y is the filter slot position, 0, 1, 2, 3, or 4. Note: 0 is always the open position and no filter can be installed in that slot.
Example: Set filter to position 2
setfilter 2
setobject z
where z is the object name. If z has spaces or special characters you must put z in single quotes.
Example: Set object name to Alpha Gem.
setobject 'Alpha Gem'
telmove ra dec
where ra is the requested move in arcseconds in RA and dec is the requested move in arcseconds in Dec. +ra = East, +dec = North. Note, that these moves are relative to the current position of the telescope. Also, the guide camera does not know about the moves, so the guide reticle will not compensate for the motion of the telescope.
Example: Move telescope 5.5 arcsec East and 10 arsec South.
telmove 5.5 -10

If something is not set in the script, it will use the values displayed in the current setup in the data-taker GUI.

It is possible to abort scripts in progress. Scripts can be aborted by typing Cntrl-C in the xterm window in which the script was started. If an exposure is in progress at the time the script is aborted, the exposure will continue and finish normally at the end of the set exposure time.

If you abort an exposure during a script, the script will continue to the next command unless you have already aborted the script.

Sample Shell Script: (lines beginning with # are comments, except for first line which defines the shell)

#!/bin/sh

# Set object name 
setobject SN2006cx

# set exposure time to 30 seconds
settime 30

# move filter wheel to position 1
setfilter 1

# take 1 exposure
igetnick 1

# move filter wheel to position 2
setfilter 2

# set exposure time to 10 seconds
settime 10

# take 2 exposures
igetnick 2

Observers may write scripts to suit their observing needs. Please place your scripts in your own directory, e.g. /u/user/observers/yourname. Scripts placed anywhere else may be deleted without warning.

Bias Frames and Dark Frames

Bias frames (0 second darks) can be acquired by setting the exposure time to 0.0 seconds and the shutter state to closed. It is wise to have all lights off in the dome while taking bias frames as there is always the possibility of light leaks in the system.

The dark current on the CCD is very low, so dark frames are usually not necessary. However, if you wish, dark frames may be acquired by setting the shutter state to closed and the desired exposure time. Again, it is wise to have all lights off in the dome while taking darks in case there is a light leak in the system.

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.

Target Acquisition

Traditionally, target acquisition has been done by first identifying the field on the Guide Camera, but the pointing accuracy that has come with the present pointing and control system (POCO) has prompted many observers to bypass this step and acquire directly on the science camera. Both methods are described below. (Note that instructions for autoguiding can be found in the Lick Autoguider manual.

Acquiring with the Guide Camera To use the guide camera for target acquisition you will want to set to Guider on Axis in the POCO GUI. The orientation of the Guider image is indicated by the compass rose, imprinted on the guide TV image display. Telescope pointing is generally good, so your target will most likely be in the field of view of the guide camera at your initial setting. Once you have identified your target, move the telescope so that it the is at the position on the guide camera corresponding to the center of the center of the science camera (this will lilely have been marked by an electronic marker on the guide camera display, or you may have to determine the position as described below). In the POCO GUI, now change to CCD on Axis. This will cause the telescope to move the necessary distance to place your target at the center of the science CCD.

If there is no marker and/or it is the first night of your run, do the following:

Center the object in the Guider field of view and then move the telescope to the CCD on Axis position and take a test exposure.
Move the telescope using the Offset Telescope function of POCO to place the target where you want it on the science CDD. Take another test exposure. Iterate telescope moves and test exposures until you are happy with the target's position.
Move the telescope back to Guider on Axis and put a marker on the object to designate the centered position (markers are controlled from the "Guide Control" tab of the "Guider Control Window. See Guider Control and Monitoring. You should also click on the Mark Centered button in the POCO GUI to make sure that telescope pointing is updated accordingly.

Acquiring Directly on the Science Camera To use the science camera for target acquisition you will want to set to CCD on Axis in the POCO GUI. Point the telescope to your target and take an unrecorded test exposure on the Direct Imaging Camera. Adjust telescope pointing as needed to place your target on the CCD as desired.

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 Direct Imaging camera for 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 2 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. This script will correctly identify the overscan region, regardless of binning or subregion of the chip read out. Each row of the overscan is averaged, and optionally a third order legendre polynomial is fit to the overscan region. The overscan (or Legendre fit) is subtracted from the data region. The overscan subtracted data is written to a new file.

Syntax:
ccd2bias,FitsFile,NewFitsFile,[/legendre]

FitsFile is the original FITS data file name.
NewFitsFile is the file to which the overscan subtracted data will be written.
/legendre is an optional input which indicates a Legendre polynomial fit to the overscan and then the fit is subtracted from the data.

Examples:
ccd2bias,'d100.fits','d100_new.fits'
ccd2bias,'d1000.fits','d1000new.fits',/legendre

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

A python script to do overscan subtraction from a list of FITS files is also available: overscanLickObs.py (requires python 2.7) or overscanLickObsP3.py (for python 3). This script will correctly identify the overscan and data regions regardless of binning or subregion of the Lick Observatory detector read out or whether one or two amplifiers are used. Each row of the overscan is median combined and optionally a third order legendre polynomial is fit to the overscan. This is then subtracted from the data region and written to a file.

Syntax:
overscanLickObs.py -f -i inputfilelist -o outputfilelist
or
overscanLickObsP3.py -f -i inputfilelist -o outputfilelist

-f is an optional argument to indicate that the Legendre polynomial fit is desired (this is recommended for most data).
inputfilelist is a text file containing the names (one per line) of the FITS files for which you wish to do overscan subtraction.
outputfilelist is a text file containing the names (one per line) of the output FITS file names. This file should have the name number of lines as inputfilelist. outputfilelist may be the same as inputfilelist, which will overwrite the original files with the new overscan subtracted files.

Examples:
overscanLickObs.py -f -i rawdataList.txt -o overscanSubtractedList.txt
overscanLickObs.py -i inFitsList.txt -o outFitsList.txt

overscanLickObs.py requires python packages numpy and astropy. Also, this script assumes that the path to python is /usr/bin/python. If that is not the case on your machine, you may have to edit the first line of the program with the proper path. Bug reports, comments, and suggestions for overscanLickObs.py 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.

Saving scratch images to data file

Non-recorded data images are saved to the file /scratch/nickel/scr.fits. This file remains until the next non-recorded image is started, at which point it will be overwritten. In the case that you want to save the data to a regular data file, copy the file to /data/ on noir, giving it a unique name so that you don't overwrite any existing data in the /data directory.

Example (for the user account on noir):
cp /scratch/nickel/scr.fits /data/saved001.fits
will copy scr.fits to saved001.fits in noir's /data directory. This location will put the file in the same location as the recorded data and allow the archiving software to properly archive the file.

Support Astronomers (sa@ucolick.org)

Last modified: Wed Apr 5 16:59:52 PDT 2023

User's Guide to the Nickel Direct Imaging Camera

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