At the start of every night the Observing Assistant at the 4 meter telescope should send out a weather status report. This will show up in any xterm window on the VNC viewer window to Rust or VNC viewer window to Taupe as "Domes are open" or "Domes are closed due to...", etc. This statement is only a guideline - you are ultimately responsible for the decision to open or close due to weather. You can check the weather status at any time by typing "weather" in any xterm window on Taupe, Driftwood, Rust, or Emerald or can be found on the web at http://www-kpno.kpno.noao.edu/weather.shtml. Note that this is not always updated.
There are several criteria that have to be met before the dome may be opened. These criteria have been set with the safety of the equipment and the telescope in mind and are not flexible.
As an added protection measure, whenever the sky is too overcast to observe through, the mirror covers, and preferably the dome, shall be closed.
There are a couple ways to check the humidity and wind speed:
Weather Links
Data acquisition is done on the computer Emerald via the program VNC Viewer for both Mosaic and S2KB. VNC stands for Virtual Network Computing, a software program that allows one to view and manipulate windows on another computer. We use VNC to remotely view the Arcon control computer, Rust (used with Mosaic), and the ICE acquisition computer, Taupe (used with S2KB). For more information on VNC see http://www.realvnc.com.
Emerald has 3 monitors labeled emerald:0.0 (center monitor), emerald:0.1 (left monitor), and emerald:0.2 (right monitor).The right-hand monitor is for image display only, and is not used with S2KB. The computer Emerald is a fast Linux box with a 3.4 GHz (Pentium 4) CPU and 2 Gbytes of memory.
To begin your observing session with S2KB you should follow these steps.
To begin your observing
session with Mosaic you should follow these steps.
This process takes about 2 minutes and you must also answer one question. A small window labeled "ARCON Console" will appear near the left center of the screen in which various messages will scroll by. After a few seconds a larger window labeled "ARCON Acquisition" will open immediately below this; this is the window you will use for entering all data acquisition commands. A brief greeting message will appear in this window and, eventually, you should be asked
Do you want to synchronize parameters? (yes)
When you reply "yes" (or just hit [cr]), the detector parameters loaded into Arcon will match those stored in detpars and the positions of the motors recorded in instrpars will correspond to reality. We currently recommend saying yes at the start of an observing run, but "no" during subsequent restarts. This saves slightly on the start-up time. In either case the IRAF package menus will be printed and the cl> prompt will appear. Some further windows will also pop up at this point. The system is now ready for you to begin observing.
Occasionally things will get hung up during the process of downloading and initializing the Arcon software. If this happens you may see the message (but probably not)
**************************************** ** *FAILURE DURING ARCON STARTUP !!!* ** *Use re-start button to try again* ** ****************************************
In the majority of cases, simply performing the restart procedure will fix this problem, although it may be necessary to try this more than once. If after repeated attempts the system will not start, refer to the Frequently Encountered Problems section of the Mosaic Manual (www.noao.edu/kpno/mosaic/manual) for further advice.
The system is now ready for you to start observing.
It's best to check your pointing with a bright star at zenith. The first
thing to do is load the Bright Star Catalog into the TCS. To do this right
click anywhere within the TCS and select the first option "Open Existing Database".
This should take you to the file containing all of the coordinate catalogs.
The path is
ACE Control System (D:) => ACE => TelescopeControlSystem
The bright star catalog is labeled ACE_BSC5.cat. Select this and click "Open".
The catalog will open up into the center of the TCS.
Now find a star near zenith. You can sort the catalog by clicking on the title of any column. Clicking that same column title a second time will sort the catalog in descending order. Scroll down until you find the stars around the current Sidereal Time (this can be found on the right side of the TCS screen, labeled S.T. in purple). Select one that is close to the sidereal time and near a Declination of +32. A 5th magnitude star works best. Once you've selected a star double-click anywhere on that line in the catalog to send the coordinates to the appropriate RA boxes. Confirm the coordinates in the boxes are reasonable and click the "Go To" button.
If Using S2KB:
The best way to discern your pointing with S2KB is to take a very short image (~2 second exposure). Once the image has read out you can fine-tune the pointing if the star doesn't land exactly where you would like it. You can find how far you need to move the telescope from the image. (The pixel scale is 0.6 arcsec/pixel).
If Using Mosaic:
The easiest way to check pointing with Mosaic is to view a bright star on one of the guide cameras. There are two guide cameras at fixed positions (one to the North and one to the South) relative to the CCD field. They are both approximately 2400 arcseconds from the center of the CCD field. Sending the telescope to a specific object will send the object to the center of the CCD field. Using the Offsets tab on the TCS send the telescope (+ or -) 2400 arcseconds in Declination. A positive offset sends the star to the North camera and a negative offset sends it to the South camera. Turn on the appropriate ICCD camera with the ND filter On. Since the offset is not exactly 2400 arcseconds the star will not appear exactly in the center of your screen, but towards the upper right portion of the screen.
If you can not see your star in either of the guide camera fields (and you're sure Mosaic's shutter is not in the "Dark" position) try taking an image of the star on your CCD. Remember to take the offset out (ie reverse the offset) so that the star will land on the CCD field. Then take a short (~2 second) exposure. If you can see the star in the CCD field but couldn't see it in the guide camera field you may need to do some fine tuning to better center the star in your field. Determine how many pixels from the center the star is and use the (0.43arsec/pix) pixel scale to determine how much you need to move the telescope.
If telescope pointing needs to be rezeroed please contact the Site Manager.
There are two filter wheels in the Filter Shutter Assembly associated with S2KB. Each wheel contains 8 slots, giving the capability to house up to 14 filters at any one time. The filter wheels are stacked, meaning both filter wheels are in the light path at all times. For this reason it's important to make sure that at least one of the filter wheels is IN an "empty" position when taking data.
To change filters you need to bring up the Filter Wheel GUI (displayed below):
Currently, filters are listed in the headers with numbers as opposed to names. To convert these numbers to their corresponding names you can edit and run the kpfilt script on a specific image (or list of images). The script and instructions for it's use can be found here.
Send the telescope and dome to the flat field position by selecting the Dome Flat Park position from the Telescope pulldown menu on the TCS. A box will appear with HA, Dec and Dome azimuth. The correct positions should be HA 3:28, Dec 13:00, Dome az 74. Press "Park" to move the telescope and the dome to the correct positions. It's a good idea to go upstairs and sight along the telescope tube to check the telescope/dome alignment. You may need to bump the dome left or right via the control panel.
The Flat Field lamp control is in the computer room. It is located in the middle of the first electronics rack as you enter the computer room. Control for the High lamps is on the left, and control for the Low lamps is on the right. Each has a toggle switch to turn power on/off and a rheostat to change the voltage to the lamps. To turn on a bank of lights turn the voltage to 0 by turning the rheostat counter-clockwise. Turn the power on with the toggle switch and then turn the voltage to the desired setting. When turning off the lamps be sure to turn the voltage all the way down before turning the power off. Ramping of the lights like this will help preserve the lifetime of the lamps.
Suggested lamp settings and exposure times for each instrument are listed below:
S2KB |
|||
| Filter | ExpTime | LampSetting | Counts |
| U | 15s | High 100% | ~30,000 |
| B | 13s | Low 100% | ~30,000 |
| V | 5s | Low 100% | ~30,000 |
| R | 3s | Low 100% | ~25,000 |
| I | 3s | Low 100% | ~25,000 |
| Ha 6580 | 6s | High 50% | ~18,000 |
| Ha 6620 | 6s | High 50% | ~18,000 |
| Ha 6660 | 6s | High 50% | ~18,000 |
| Ha 6700 | 6s | High 50% | ~18,000 |
| Ha 6740 | 6s | High 50% | ~18,000 |
Suggested exposure times and lamp settings for MOSAIC are listed below. These exposures should produce images having 5,000-10,000 ADU per pixel to stay within the linear regime (note that each ADU represents ~3 electrons, so there is plenty of signal with these recommendations. To minimize thinking at the telescope, we tried to use the maximum voltage settings when possible (i.e. 100%).
MOSAIC |
||
| Filter | ExpTime | LampSetting |
| U | 10s | High 100% |
| B | 18s | Low 100% |
| V | 8s | Low 100% |
| R | 5s | Low 100% |
| I | 6s | Low 100% |
| [OIII] #2 |
3s |
High 100% |
| [OIII] +29 #2 |
28s |
Low 100% |
| Halpha |
60s |
Low 100% |
| Halpha +4 |
60s |
Low 100% |
| Halpha +8 |
60s |
Low 100% |
| Halpha +16/[SII] |
60s |
Low 100% |
| SDSS g' |
4s |
Low 100% |
| SDSS r' |
5s |
Low 100% |
| SDSS i' |
6s |
Low 100% |
| SDSS z' |
11s |
Low 100% |
| Wash M |
3s |
Low 100% |
| DDO 51 |
18s |
Low 100% |
| White |
5s |
Low 50% |
Due to the mechanics of the secondary system there is some inherent focus drift (~50 units). To try to eliminate this drift there is a routine that keeps the telescope at the last commanded focus position. If you would like to use this routine, simply check the box beside "Servo" in the Focus GUI. Each time you change the focus the system will automatically keep the focus at this new position. It is highly recommended to keep this box checked.
WARNING: Only click buttons in Focus GUI once and wait until action has taken place!
WARNING: Do not perform other tasks in TCS while changing focus!
The best way to focus the telescope is to run a focus sequence "manually".
You will be running a focus sequence from the Data Acquisition window but
setting the focus on the TCS manually at each step.
Start a focus sequence and you will be prompted to set the focus each time. Type "observe" and then "focus" in the Data Acquisition window. If you are trying to find focus at the beginning of the night it's best to set the number of exposures taken in the focus sequence to 9. If temperature from night to night has remained constant you can use the previous night's ending focus as your middle focus value. A typical step sizes to use are 50 or 100 units. The smallest focus increment detectable is ~25 units. Once you have entered in all of the information for the focus sequence you will then be prompted to manually set the focus for each exposure in the sequence. To do this you need to bring up the Focus Inspector Gui on the TCS. You can bring this us from the Telescope menu on the main toolbar: Select Telescope => Focus...
You can enter a desired focus into the box beside "Go To Focus" and then hit the "Go To Focus" button. Or you can use the JOG+ or JOG- buttons to jog the focus a specified amount. The current focus is displayed on the toolbar of the Focus GUI, on the main TCS screen near the middle left and at the bottom of the ACE SoftPad.
The range of focus is from 0 to 60000. One step is approximately 0.5 microns. You always want to increment the focus to larger numbers to remove backlash. Nominal focus for both Mosaic and S2KB is around 31000.
Once your focus exposure has read out you can use the mscfocus routine to determine the best focus.
When using S2KB it's best to run a focus sequence manually where ICE prompts you to enter the desired focus in the TCS. In the Data Acquisition window type observe and then focus. You will then be prompted for exposure time, starting focus number, etc. If you have a good idea of where focus should be, a good step size to use is 50 units. Otherwise use a step size of 100 units. The smallest step size you will be able to detect is 25 units. You should always move the focus in the positive direction, (ie your focus sequence steps should be +50 units).
Below is a typical focus sequence as seen in the Data Acquisition window.
When the image has read out you can use the kpnofocus routine to determine the best focus. See the IRAF help page on kpnofocus for more information. In your Data Reduction window type kpnofocus. Once you have determined the best focus type it into the Focus Inspector GUI (see above).
Mosaic has two fixed guide cameras - one North of the science field and one South of the science field. Guiding is accomplished by selecting a star from one of these fields. Both guide cameras are at fixed positions relative to the science field (~2400 arcseconds North/South). Suitable guide stars are almost always available without moving the telescope from the desired position. The field of view of each camera is approximately 5 arcminutes on a side. It is possible to guide on stars as faint as V~17 near full moon.
Mosaic's guide cameras are intensified fiber-optically coupled CCD cameras (ICCD's), and can be damaged if exposed to bright light, so it is important to protect these cameras from bright stars. The camera controls (shown below) reside on the shelf above the TCS computer, Olive, and are labeled "North" and "South". The video switcher above the controls allows you to switch between cameras. The buttons needed to select the North and South cameras are labeled.
Video signal is routed from the cameras, through the controls and to the guider computer, Moss. Select the desired camera from the video switcher and adjust the camera from the ICCD Control Panel:
When switching between the two TVs, be sure to turn the high voltage potentiometer fully counterclockwise and turn off high voltage on the TV no longer in use.
The computer Moss is the guider control computer. Moss is a dual-boot machine with Windows2000 and Lunix RedHat. When the 2K camera is in use Moss is booted into Windows and the MaximDL software is used to control the guider. When Mosaic is in use Moss is booted into Linux.
By default Moss should boot into Linux. However, if it stops booting and you see lilo: in the upper left hand corner of the screen, type linux. This will boot Moss into Linux and you will eventually get the login prompt. The login and password are on the white board.
You should be at the kpno36> prompt. At this point you'll need to restart the guider software from Emerald. To do this, right-click anywhere on the background on Emerald to bring up the background menu. Select "Restart Guider". This should bring up an xterm window. Eventually this should bring up the video screen on Moss and then the Guider GUI on Emerald. You may see the message "waiting...." repeated many times in this xterm window on Emerald. If so, be patient as it may take a minute or so for the video screen to appear. The Guider Gui on Emerald should appear immediately after the video screen on Moss appears. If not you'll need to bring up an xterm window from the background menu on Emerald and type "xhost moss" (no quotes) in that window. Then select "Restart Guider GUI" from the background menu on Emerald. This should bring up the Guider GUI.
When the guider is running, Moss will show a video picture with two white boxes. The smaller box is the cursor, and the larger box is the guide box. There will also be one or more status lines at the top of the screen containing useful information.
For more on the Guider Control GUI, see the section "More on the Linux Guider" in the appendix.
You will need to make sure the video screen is running on Moss and the Guider Gui is running on Emerald. If not, follow the instructions above under "Guider Software" to start them.
Video signal is routed through the ICCD controller (the grey box above Olive). Make sure the gain on the controller is turned all the way down. Turn on the controller power (left-most toggle switch). Push the red INTEN button on. Make sure the ND filter is on (middle toggle switch). Slowly turn up the gain to ensure there are no bright stars in the field. If you see no stars, turn the gain back down (counter- clockwise), remove the ND filter (toggle switch down) and slowly turn the gain up again, watching for guide stars on the video screen.
There are 2 fixed cameras to choose from - North and South. You can switch between them with the Monitor Input Selector that's just above the ICCD controller.
There are 2 ways to select guide stars.
|
Now select Guider => Guider On from the Guider GUI on Emerald. You're not guiding yet. You must also turn on the guider on the TCS. On the main screen at the bottom left under "INSTRUMENTS" you'll see "Autoguider". The box to the right of the "Autoguider" displays the status of the guider (On or Off). Click this box to bring up the guider GUI on the TCS. In the Control Status box on this GUI you'll see OFF, ON, N, S. Disregard the N, S buttons as these are only used with S2KB and not with Mosaic. Click the On button. Near the bottom of the TCS autoguider GUI you should see numbers changing in the boxes below "Corrections". If you do not see these numbers changing, you are not guiding. See the troubleshooting section for help. You will need to turn guiding off in both places (on Olive and Emerald) before moving the telescope. On Olive, click the radio button beside "Off". On Emerald, select Guider => Guider Off from the Guider GUI. Order counts, be sure to turn the guider off via Olive first. |
|
The orientation on the guider video window: North is down and East is to the left.
In October 2006 new SBIG guide cameras were installed while the original guide cameras were sent out for repair. These SBIG cameras are also controlled through MaximDL and operate the same as the original cameras. The only differences are in connecting to the cameras and switching between the North and South cameras.
The computer Moss is the guider control computer. Moss is a dual-boot machine with Windows2000 and Linux RedHat. When the 2K camera is in use Moss is booted into Windows and the MaximDL software is used to control the guider. Linux is used with Mosaic.
As of January 2003 both North and South guide cameras are fully functional, increasing the likelihood of finding suitable guide stars. See the section below on switching between cameras.
Field of view = 3.3 arcmin x 2.2 arcmin
Offsets:
To launch the MaximDL software click on the icon on the desktop.
When the program is launched you will get a window like this.
After launching the program select View => CCD Control Window. Click on the Guide tab and then the Settings button. Check to make sure the ControlVia box reads COM1.
Click on the Setup tab in the CCD control window.
Make sure the box below Main Camera reads "Fingerlakes" and the box below Autoguider reads "Same as main camera". You can change what's displayed in these areas by clicking the Setup buttons beside each area. On the same Setup menu now select Connect and wait for the mouse to return to an arrow. Then select Cooler On. |
|
A star field should soon appear. The program will pick a suitable guide star and place its coordinates into the Guide Star X and Y boxes. You can select a star manually by entering different coordinates into these boxes, or by double-clicking on the star of your choice.
If you get a message that there are no guide stars available, you will
need to try taking a longer exposure or shifting the field slightly. You
can change the length of the exposure by using the up and down arrows below
Exposure on the Guide Menu. Recommended exposure times are between 3 and
5 seconds. To shift the field you will need to jog the telescope slightly
via the ACE SoftPad on the TCS.
If you are having trouble finding suitable guide stars you can use the Guidestar Search program to search for
stars nearby.
After the guide star has been chosen, click on Track and then click Start.You are not guiding yet. You need to then turn guiding on via the TCS Autoguider GUI.
Bring up the Autoguider GUI from the main toolbar: Select Instruments => Autoguider. Click On. When the guider is on you should see numbers changing in the boxes below Corrections. Make sure these numbers are changing and that your star is not wandering before starting your exposure.
The guide camera focus is controlled via the TCS Autoguider GUI displayed above. The focus range is between 0 and 28000, with typical focus values around 12000 to 14000. The current focus is indicated in the yellow box beside Encoder. To change the focus, enter a desired focus value into the box beneath the current focus setting and click on GO TO.
You can setup the MaximDL software to take continuous exposures
of a single star while you change the guider focus on the TCS.
|
|
You can do a rough analysis of your guide star a few different ways:
|
|
| Filter | Guider Focus |
| U | 26,000 |
| B | 18,000 |
| V | 18,000 |
| R | 18,000 |
| I | 15,000 |
When changing between guide cameras you need to disconnect the communications to the camera currently in use and reconnect to the other.
If you are having trouble locating guidestars (when using S2KB) you can run a search for guidestars near your field.
To search for guidestars:
Note:The guide cameras are sensitive to V magnitude of 14.5.
There are two different autologing systems available at the 0.9m. The first is a GUI interface log sheet and recommended for S2KB. The second is an IRAF based log sheet and is recommended for Mosaic.
Both autologs runs on Emerald, and pull information from your image headers to create the log. In order for autolog to run, you must have the TCS/Instrument communications up and running.To start the S2KB autolog program, on Emerald:
|
|
The images (.fits images only) are written to the log after the exposure has readout. An example log is show below. Every time an image is logged, the log page is written to a postscript file and saved. These files are stored in the same directory as your images and are called autolog_page1.ps, etc. Each page is then printed when it is full. One nice feature of this log is that you can type virtually anywhere within the log - you can edit any field or add comments wherever you like.
Note:Image root names that contain a period will not show up correctly in the log (e.g. n001.0002.fits). To avoid this, make sure your image root names do not contain periods.
Note:If you use ccdlog2 test images and focus sequences will not show up on the log.
To start the Mosaic autolog program, on Emerald:
Instructions for writing data to DVD can be found at http://www-kpno.kpno.noao.edu/observing/dvd-writing-instructions.html.
The devices in the 0.9m are physically located in the computer room - each labeled with their IRAF and Unix names. Typing devices in an IRAF (Data Reduction) window will produce a list of all the available tape drives with corresponding IRAF and Unix names. Note that most devices are associated with Emerald and are not listed in the devices file. Correct names/id's are located below.
cl> devices
| Dome | Computer | IRAF Name | Unix Name | Description |
| 36inch | emerald | mta | /dev/nst1 | Exabyte Eliant 820 |
| mtb | /dev/nst2 | DLT | ||
| mtc | /dev/nst0 | DDS-4 Dat | ||
| 36inch | Taupe | mta | /dev/nrst8 | Exabyte 8705 |
| mta8500c | /dev/nrst16 | 8705 - compressed | ||
| mtb | /dev/nrsx2 | Exabyte 8200 | ||
| mtc | /dev/nrst3 | Seagate Dat |
You can also write your Mosaic data to tape with the tar command in Unix,
but the preferred method is using mscwfits. Whichever method you use, you
should read part of your data from tape to make sure it is there.
If you get errors when trying to tape your data, set the blocksize to zero
and try again.
To set the blocksize type:
mt -f /dev/nst* setblk 0
To check the blocksize type:
mt -f /dev/nst* status
**Remember to bring plenty of tapes for the duration of your observing run.
You can record your data by either using the IRAF wfits task or the Unix tar task
IRAF
In the IRAF Data Reduction window:
Allocate the appropriate drive
UNIX
In a Unix xterm or xgterm window:
Write your data to tape with the tar task
See the ICE manual for more help on the tar task.
Technical Specifications
Date: 4/10/97
CCD NAME : S2KB
CCD SN : 1383BR03-01
PIXEL SIZE : 21x21 microns
PIXEL SCALE : 0.60 arcsec/pixel
CHIP SIZE : 2048 x 2048
DIGITAL LIMITATION : 65,534
BIAS LEVEL : ~750 (gain=#3)
MICROCODE : "Harcon1009"
PREFLASH, e- : none
NOISE, e-/RMS : 14 9 8 7
GAIN, e-/ADU : 7.5 5.0 3.0 2.0 1.0
GAIN, DETPARS#: 1 2 3 4 5
LINEARITY,0.1%,e-: ~210,000
LINEARITY,1.0%,e-: ~230,000
COLUMN SPILLOVER, e- : ~240,000
INTERNAL RADIATION EVENT RATE, EVENTS/HR : ~2700
CONTROL TEMP. deg C (thermocouple) : ~-107
COMPUTER TEMP. deg C (ccdinfo) : ~-109
DARK CURRENT, e-/hr/pix : ~5-10
DEAD COLUMNS : HOT COLUMNS : [1571:1571,1134:2048] bias
LOW COLUMNS : [1103:1103,744:2048] flat field
The quantum efficiency of S2KB is given in this figure. The abscissa is wavelength in nanometers and the ordinate is dQE (%).
Here are examples of the original calibration frames for S2KB. Click on the link to display the (BIG) image.
More recently, S2KB has developed an enhanced bias level in one corner amounting to a peak of ~1500 ADU above an overall level of ~965 ADU. Here is a bias exposure which shows this effect. The enhancement is limited to the region X < ~240 pix and Y> ~1800 pix.
You can estimate exposure times for S2KB using the CCDTIME package in IRAF v2.11.3 which should still have the necessary information in its ccdtime$kpno.dat database file. If it does not, here is the kpno.dat file.
A. While the overscan (or DC offset) region records the absolute value of the bias level as each observation is taken, zero exposure images (i.e. bias frames) are used to correct for two-dimensional structure in the bias level. These are typically taken in the afternoon before the dome flats. Be sure the inside of the dome is dark. You will probably want to obtain 9 bias frames, which will later be combined with the zerocombine task in IRAF. It is advisable to intersperse a few bias frames throughout the night as you obtain your program observations in order to track any temporal variations in the bias structure.
A. Dome flats are used to correct for pixel-to-pixel sensitivity variations in the CCD. They are usually taken in the late afternoon 1 to 2 hours before you open the dome slit to begin your evening of observing. If it is not possible to obtain the dome flats in the late afternoon, they can also be procured in the morning after your night of observing is done. Making sure the lights in the dome are off, position the dome at an azimuth of 73 +/- 1 degrees and point the telescope at HA = +3:28 and Dec = +13d (this is usually done using the 'flat-field park position' selection in the TCS GUI). Go to the computer room and turn on the flat-field illumination lamps (high intensity for the U filter or narrow band filters, low intensity for BVRI and other broad band filters); then, ramp up the rheostats to achieve the proper level of intensity (usually 100%). You will need to expose the CCD to achieve a mean level equal to about half the saturation of the A-D converter - ~30,000 ADU - making sure that the CCD itself has not become nonlinear, which occurs at ~210,000 electrons (the limit of the A-D converter for a gain of 3e/ADU). Recommended exposure times for the UBVRI filters are given in the manual. You will probably want to obtain ~5 dome flats in each filter, which will later be combined with the flatcombine task in IRAF.
A. The dark current on the S2KB CCD is low (5-10 e/hour/pixel), which means that there is no need to obtain dark current exposures.
A. It is unfortunate but true that the upper left corner of the CCD is plagued by higher than average background counts - even in a bias frame. This results from a faulty LED that is illuminating one corner of the chip. The good news is that the standard technique of bias (image) subtraction does an excellent job of removing this gradient - to better than 1%.
A.
A. Twilight sky flats are taken just after sunset before the sky is completely dark. The idea in taking these is to correct for any illumination differences between the dome flat white spot and the actual sky. Since time is limited, one usually tries to acquire at least 3 twilight sky flats in each filter to be used for the program. You should aim to expose each flat to a mean level equal to about half the saturation of the A-D converter - ~30,000 ADU - making sure that the CCD itself has not become nonlinear. If enough twilight sky flats are obtained in each filter (i.e. > 4) on each night with the appropriate mean intensity levels and no stars, then they can be used to flatten the program images in place of the dome flats. However, if less than ~3 twilight sky flats have been obtained on a given night, it is better to use them as an illumination correction (if one is needed) after the dome flats have been applied to correct for pixel-to-pixel sensitivity variations.
A. Generally speaking, the answer to this question is 'no.' However, if you feel particularly uncertain about the quality of the dome and twilight flats you have obtained, you are encouraged to also take dark sky flats. The direct imaging manual lists the locations of 'blank' sky fields on page 16 to be used for this purpose.
A. The S2KB shutter moves linearly across the field of view. As a result, any shutter correction will be an additive offset that is applied to the requested exposure time. On two separate occasions, this correction was measured to be +0.016 +/- 0.002 sec. Thus, a requested exposure time of 1 second will actually be 1.016 seconds. This means that requested exposure times of 2 seconds and longer will incur errors of less than 1%.
A. The CCD has a full well capacity of ~210,000 electrons which means that at a gain of 3 e/ADU (index = 3), the A to D converter uses the full dynamic range of the CCD up to 65,536 DN. Gain values as high as 7.5 e/ADU are available. The read-out noise of S2KB is 10 electrons so that there is no gain value at which the read noise is quantized.
A. The recommended trim ([1:2048,1:2048]) and overscan sections ([2049:2080,1:2048]) are given in the image headers. Feel free to check these by plotting intensity vs column number to be sure that the trim and overscan regions are appropriate.
A. The native image file format for IRAF is the imh/pixel format. However, all of the tasks will also run on FITS files. When you run a task, be sure to have only one version of each image in the working directory (i.e. either FITS or imh/pixel) and do not provide the image file extension to the task. To convert between these formats, use the rfits and wfits tasks. Converting images from imh/pixel to FITS makes them more portable allowing you to put them on tape with TAR and/or FTP them to your home institution.
A. The problem you have encountered is not covered in the manual, the troubleshooting guide, or this FAQ. Telephone numbers are on the whiteboard.
Last updated September 26, 2007
