Notes on using the imager at the 2.3m telescope at Siding Spring

What to bring with you:

• A set of DAT (DDS2 or DDS3) tapes or CD blanks (there are NO tapes available at SSO).
• Finding charts for the objects (FOV of imager is 6.7 arcmin diameter)
• Paper with standard star fields (UBVRI: Graham, 1982, PASP, 94, 244 or Landolt, 1992, AJ, 104, 340 , H_alpha: ).

Setting up on Arrival

• Check when the last liquid N-filling of the imager CCD happened.
• Login on the computer in the control room as user23 or in your own account.
• Start CICADA from a xterm window (cicada &), when CICADA comes up select CCD#11
• The CICADA interface should look similar like the window shown here:

  

• Change parameters of the CCD readout region 1 to the following values: X-bin=1, Y-bin=1, Overscan Columns=100, X-offset-1=206, Y-offset-1=140, Width-1=700, Height-1=700 (This shall be the setting for the entire observing run!!).
• Start ximtool within CICADA (see under Tools).
• Initialize the CCD (see under Action).
• Take some test images in the "show only" mode to check the performance of the CCD/electronics. If necessary, i.e. if CCD image shows strange features, reinitialize the CCD.
• If you don't take exposures for a longer period select continuous flushing on CICADA menu.
• Delete all data on the DATA disks which have been left by the previous observer. The disk space available to you is shown in the CICADA window e.g. Disk: 199MB). Choose a directory (Output Directory) where the images of your observing run shall be saved by going to Options: preferences in the CICADA menu.
• Open another xterm and start IRAF by typing "ncl" or "cl".
• Within IRAF type "!ximtool" to start another ximtool graphic display.
• Within IRAF change to the directory you have chosen as output directory.
• Prepare a ASCII file as log file where all the observing details shall be written. See example here .

Before dinner

• Take 10 bias frames every day (Exposure type: Bias, Number of Exposures: 10).
• Fill the CCD dewar with liquid nitrogen (wear protection gear!). One filling lasts for about 12 hours.
• Read off Sink and CCD temperature before and after filling and write the values into the CCD logbook.
• Select continuous CCD flushing on CICADA menu.

Back to the telescope after dinner

• Last control walk through the dome. Check that there is nothing in the vicinity of the Imager that could interfer with the instrument rotating.
• Go to the control room and switch off the dome lights.
• Switch on Power Control and Telescope System.
• Open the building shutter.
• Start fan.
• Open the mirror cover.
• press BREAK key on the keyboard below the TCS Monitor.
• TCS Monitor comes up with USERNAME. Type "telescope".
• PASSWORD: (cf. CCD Logbook).
• OBSERVER'S IDENTIFICATION: KCF
• -> startup
• -> configure instrument_ident imager
• -> configure focal_station nasmyth_b
• -> rotator/reference = position_angle
• -> configure focus_control automatic
• -> configure tracking_equinox J2000
• -> configure windscreen_control vertical_only_tracking
• -> enlist txa3 (This is to allow the auto guider to control the telescope)

Twilight skyflats:

• Start with the bluest passband in the evening and reddest passband in the morning.
• Select an empty field from the blank field list.
• Move the telescope to these coords by typing -> track/coord XX XX XX.X XX XX XX J1950
• REMEMBER: if the telescope moves accidently to a wrong place you type halt and the telescope will stop moving.
• Stop the continuous CCD flushing mode in CICADA.
• Define a small (50pix x 50pix) window in the center of the CCD as readout region 2 (see CICADA interface).
• Select readout only and test with short exposures (5sec) the number of counts that you get from the sky by moving. Check the counts with the cursor on the image display in ximtool.
• If counts are about 50'000 in 5 sec change back to readout region 1 and take and save the first skyflat with a 5 sec exposure. Write the details into your log file. This will help you the next day to start twilight exposures at the right time.
• While reading out the CCD, shift the telescope a few arcsecs by pressing the offset bottons at the telescope console.
• Double or triple the integration time after each exposure to get roughly the same number counts.
• Take and save 5 flats.
• Once you have finished with the first band you change filter and take 5 flats in the second band. Continue until you have 5 flats in each photometric band.
• Find the central pixel coords of the Field of View.

Measuring the pointing of the telescope:

• Type -> rot 0. (This will move the rotator of the imager to position angle 0.
• Choose a bright star from the Astronomical Almanac (The book should be in the control room) and move the telescope into this direction (Remember to choose the correct epoch by typing this number at the end of the coords string)
• Take a very short (100msec) exposure in the Display only mode. When the CCD image is displayed in the ximtool window the star should appear near the central pixel coords of the Field of View. If offset is more than a few pixels, detetermine the offset in units of arcsecs and correct the position of the telescope with the offset botton and take another exposure until the star is close to this position (Scale of CCD#11 is 0.6 arcsec pixel-1). If you are satisfied with the pointing, type -> calib point

Telescope Focus:

• Choose a standard star field in Graham, 1982, PASP, 94, 244 or Landolt, 1992, AJ, 104, 340 and drive the telescope to point at the centre of the standard star field.
• Take (Display only mode) an exposure (readout region 1) in the V-band with about 10 sec duration (have you changed the time unit back to sec?).
• In the ximtool window determine the pixel coordinates (X_s, Y_s) of a star in the field which is suitable for the focus procedure (~10'000 counts).
• Define a small CCD window centred on the star (In readout region 2 set X_start=X_s-60, Y-start=Y_s-60, Width=120, Height=120).
• Move the focus 10 units away from last optimum, e.g. if optimum was 10, type -> focus 20.
• Take an exposure of the same duration.
• IRAF window: "display "starimage1.fits", and "imexam".
• Analyse the stellar profile with the "r"-key to determine its FWHM.
• Write down the value. The last three numbers on the right of the graphic display give the FWHM (in pixels) from three different fitted analytical functions (Scale of CCD#11 is 0.6 arcsec pixel-1).
• Exit the imexam command with the "q"-key.
• Change focus in steps of 2. (Do this always from the same direction!)
• Repeat the last 5 steps until the best fit is obtained.
• Move the focus to the best position, e.g. -> focus 8

Flux calibration:

• Move the telescope back to the centre of the standard star field.
• Type object name in the CICADA window. As comment write the Filter.
• Take and save CCD images of the standard star region (30sec in B, 10sec in V, 10sec in R). Don't forget to go back to the full CCD window (readout region 1).
• Check the standard star field and the counts you get for the standard stars (e.g. with "display ccd0110; imexa" in IRAF). If the number counts for the brightest standard star are smallish do another exposure with a longer integration time. If the brightest star is saturated repeat the exposure with a shorter integration time.

Taking science data:

• Select one if the target fields and move the telescope to the source position.
• Take a short exposure of 20 seconds.
• Analyse the image to find out about the flux you get from the central surface brightness of the galaxy. For bright galaxies change the integration time so that the central counts will be about 50.000 counts. If the galaxy is faint, use t_exp=900 sec.
• if t_exp< 300 sec, take three exposures without auto guiding and move to the next object afterwards.
• Use the transparent overlay for the autoguider good area to get the (x,y) coordinates of a suitable bright field star for autoguiding. This should be within the cross-hatched area, although you should avoid the thin central bridge of this arch-shaped region when using the larger sized CCD#12.
• Add dx and dy to the coordinates of this star as read from the grid on the overlay and move the autoguider to the resulting position. Start the focus procedure in the autoguider monitor using about 5 sec integration time. With luck you will find the star within the autoguider CCD field. If not, you will have to hunt for it. When you find it, go to autoguider mode. i.e. "ESC", move to guiding in the menu, select go, wait for the image and centre the square box on the star, press return.
• Select the desired filter, type object name, etc. in CICADA. Remember to re-select readout region 1, otherwise you will only save the small focus box to file.
• Take the necessary number of long exposures, B: 6 x 600sec, R: 3 x 600sec. (Don't forget to write all details into the log file!!). You should move the telescope a little bit (few arcsec) between exposures so that defects in the CCD and cosmics can be median-filtered out. For this purpose stop autoguiding ("ESC"), move the telescope, restart autoguider.

Morning: Twilight skyflat

• When the sky is getting to bright for science exposures you move the telescope again to a blank field to do skyflats.
• Start in the R-band and expose the full frame for about 200sec to get about 20'000 counts from the sky.
• Shift the telescope with the handset a few arcsecs after each exposure.
• Half the integration time after each exposure to get roughly the same number counts.
• After 2-3 good exposures in R change filter to B and continue doing twilight flats until CCD is close to saturation (>55'000 counts) within 5 secs.

Closing down:

• halt telescope -> halt
• close mirror cover with botton
• park telescope -> park
• close building shutter with botton
• -> shutdown
• Switch on the dome lights.
• Refill CCD dewar
• Write temperatures in CCD loogbook.
• Select continuous flushing on CICADA menu.
• Go to bed!

Bias frames:

• Every two days a series of 5-10 biases shall be taken during the day.
• Object name: bias, Exposure type: Bias (this automatically sets exposure time to 0sec), Readout region 1.

Backup of CCD data:

• Every observer has to bring its own set of EXABYTE or DAT tapes. There are NO tapes available at the telescope!!!
• Allocate the tape drive ("allocate mt0" (DAT), "allocate ex0" (EXABYTE))
• Rewind the tape to the start: type "mt -f $MT0 ($EX0) rewind"
• Go to the directory where the CCD images are.
• Type "tar -cvf $MT0 ($EX0) *". This will copy all data in the directory to the tape.
• When finished type "mt -f $MT0 ($EX0) rewind" twice.
• Type "tar -tvf $MT0 ($EX0)". This will show you the content of the tape and confirms the successful copy process.
• When finished type "mt -f $MT0 ($EX0) rewind" twice.
• Get the tape out of the drive and make another copy on a second tape by following the last four steps.

General Remarks:

• If the CCD image looks very odd, e.g. shows prominent strips or a steep gradient you should initialise the CCD hardware!
• Use the ABORT button (CICADA window) only in emergency cases as this can confuse the CCD hardware.
• Moving the telescope during CCD readout can disturb the CCD readout process, i.e. the CCD image can be of poor quality. Thus move the telescope only after reading out. Small step movements with are OK.