ChaMPlane Observation: KPNO 4m/Mosaic October 25, 2001

ChaMPlane Observation with KPNO 4m Mosaic

October 25, 2001

Time Line of Observations

All images to be taken with unbinned 8k X 8k Mosaic. Here is the 2001 KPNO Observer's Almanac.

Biases and Domeflats: Take biases and domeflats in the afternoon before dinner.
Skyflats: Take twilight skyflats starting shortly after sunset at around 6pm.
Pointing and Focus: Do telescope pointing and focus after the skyflats at around 6:40pm.
3EGJ2227+6122: Start to observe the first field (3EG J2227+6122) 10 min before the astronomical twilight at above 6:55pm.
Landolt SA 92: Observe Landolt Standard SA 92 after the first field.
3C 129: Observe the second field (3C 129) after Landolt SA 92.
3C 123: Observe the third field (3C 123) after second field.
Landolt SA 97: If there is still time, observe Landolt Standard SA 97.

I. Biases and Domeflats

Take 15 biases and 7 domeflats each (V R I & Ha) in the afternoon. Dark is not necessary.

In order to correct pupils in the images, skyflats on I and Ha filters are needed. Ideally, dark skyflats made from object fields (or blank skys) are the best. But there are too many stars in the Galactic plane which makes this impossible (and we don't have time to spend on taking blank skys). So we need to take some (evening) twilight skyflats. See CHAMP KPNO 4meter Observing Cookbook for more complete instruction for doing twilight skyflats. Timing is very crucial for this. Here are some simple steps for doing evening twilight skyflats:

Point the telescope to zenith at sunset - 5:48pm.
Turn the tracking on during the skyflat exposures and turn the tracking off during the readouts so there will be a offset between exposures.
Do the Ha filter skyflats first, then do the I filter skyflats.
Start at about 6pm, take a 2 seconds Ha skyflats. Examine the image and make the adjustment of the exposure time so the flat has about 10000 counts.
Take the first Ha skyflats with adjusted exposure time (remember to turn the tracking on during exposure and off during readout).
Take the second Ha skyflats with exposure time twice as long as the first exposure (e.g.: first is 5sec, second one should be 10sec). The sky gets darker during the 2.5 min readout. To double the exposure time is just about to compensate the change in the sky.
Take at least 3 Ha skyflats.
If the exposure time is still under 30sec, continue to take the Ha skyflats until the the exposure time is over 30sec.
Take the first I filter skyflat with exposure time equal to 1/10 of the last Ha skyflat exposure. Examine the image to make sure it gets about 10000 counts. Adjust exposure time if needed.
Take the second I filter skyflat with exposure twice as long as the first.
Take at least 3 I skyflats.
Don't let the skyflats interfare with the observation. Stop at about 6:30-6:40pm and do the pointing and focus.
try to start to observe the first ChaMplane field at around 6:55pm.

III. Target List

Three Chandra fields are selected for the ChaMPlane optical imaging observation at KPNO 4m/Mosaic on October 25, 2001. All fields have Exp >= 20ks and with clean optical and X-ray backgrounds. Two of the fields were observed with Chandra ACIS-S. One of the fields were observed with Chandra ACIS-I. All were operated under ACIS TE mode and with 6 CCDs chips turned on.

Here is the target list in the order of observing during the night:

                      Champlane KPNO 4m/Mosaic Target list, October 25, 2001

                     Telescope pointing
                     Center of Mosaic                            Chandra  Chandra           Chandra
Field               RA        Dec (J2000)   glong       glat     Detect    Exp      nH      Status
----------------------------------------------------------------------------------------------------------
3EG J2227+6122   22 29 17.00  +61 19 00.9   106.71110   3.00561  ACIS-I   20.00  9.95E+21   observed
3C 129           04 49 19.20  +45 02 30.0   160.42455   0.18386  ACIS-S   30.00  7.33E+21   observed
3C 123           04 36 28.00  +29 37 49.0   170.52626 -11.78734  ACIS-S   50.00  1.90E+21   archived    
----------------------------------------------------------------------------------------------------------

The RAs and Decs are for the 4-m telescope pointings (not the coordinates of the targets or the Chandra pointings), based on the overlay of Mosaic on ACIS or HRC. NO MORE OFFSET IS NEEDED.

Click a object name to see the Mosaic/ACIS or Mosaic/HRC overlay on DSS image. This should help to identify the field. Note that the DDS images are E-left & N-up. But the Mosaic images at KPNO 4-m are E-down & N-left.

IV. Exposure Sequence

Take the images with unbinned Mosaic (8k x 8k), which covers 36'x36'. The readout time is 2.5 min each frame.

Doing dithered observations for the long exposures are very important. Not only it fills the gaps between the Mosaic chips and eliminate bad columns to make a cosmetically clean image, it also greatly reduce the "bleeding" effect caused by saturated bright stars.

For each field, do the following Exposure Sequence to achieve the project goal - to reach 24th mag.

           Exposure sequence for each field

Filter  Frame         Exp   Exp+readout Total Timeline
                     (sec)  (min/frame) (min)   (min)
------------------------------------------------------
R       single image     3       2.5     2.5     2.5
Ha      single image    30       3.0     3.0     5.5
R       dither 1x5     240       6.5    32.5    38.0
Ha      dither 1x5    1380      25.5   127.5   165.5
V       dither 1x3     180       5.5    16.5   182.0
I       dither 1x3     180       5.5    16.5   198.5
------------------------------------------------------
1. Exp is the exposure time for each image in seconds.
2. Exp+readout is the exposure plus 2.5min readout time for each image.
3. Total is the total time ((Exp+readout) x dither) for each filter.
4. Timeline is the accumulated time for each field.

Under the Frame:
A. single image: take one short exposure each for R and Ha filter.  No dither.   

B. dither 1x5: Use command "mosdither" to do one set of standard dither pattern 
   of 5 positions for R and Ha filters.  Take one image at each position. 
   The standard dither pattern is at "ditherdb$todd.dat".

It looks like this:
==================================================================
# Todd Boroson's canned dither scheme for the NOAO mosaic

        # Offset relative to current telescope position

        # RA (pixels)   Dec (pixels)

           0               0

         200            -200

        -200             200

         100             100

        -100            -100
==================================================================

C. dither 1x3: Use command "mosdither" to do one set of dither pattern of 3
   positions for V and I filters.  Take one image at each position.  
   Create a file at "ditherdb$dither3.dat" that looks like the
   following.  Then set the "mosdither" parameter: 

           offsets = ditherdb$dither3.dat

==================================================================
# Dither 3 positions.

# Offset relative to current telescope position

        # RA (pixels)   Dec (pixels)

           0               0

         200            -200

        -200             200
==================================================================

OR, simply use the standard dither pattern and stop it after 3 exposures.

V. Landolt Standards

Take following Landolt standards with 5 seconds exposure each for V, R, I filters.

                      Landolt standards

                     Telescope pointing
                     Center of Mosaic              
Field               RA        Dec (J2000)	   Comment
------------------------------------------------------------------------------------------
SA 92 field 1    00 54 15.84  00 44 52.8   Take this between the first (3EG J2227+6122)
                                           and the second (3C 129) fields.

SA 97            05 57 19.20  00 03 00.0   If possible, take this in the morning twilight 
                                           after finishing all three fields.
------------------------------------------------------------------------------------------

VI. Observation

The astronomical twilight on October 25 starts at 19:04:45 and ends at 5:16:20 with a duration of 10:11:35 hours (611.6 min). Above exposure sequence requires 198.5 min (including readout time) for each field. Three fields need 595.5 min. This leaves a small amount of time for everything else, including slew the telescope, locate the new field, and do the standard. Therefore careful planing is needed.

A. First of all, the about exposure sequence is the most time can be spent on each field. When time is running short, first reduce the number of I images, then V images. But keep at least one V and one I images. When only take one V and one I images, the total time needed for one field is reduced to 176.5 min.

B. Try to start 10-15 min before and end 10-15 min after the astronomical twilight zone to get a little more observing time.

Here is an observing chart showing the locus of the objects during the night. Horizontal axis is the time in hours. Space between two gray vertical bars are the astronomical twilight zone. Vertical axis is the distance of the object above horizon in 10-degrees. Loci from left to right are for 3EG J2227+6122, SA 92, 3C 123, 3C 129, and SA 97.

postscript and txt

Here is an observing almanac made for this run. Download it here and print it out.

Call me anytime if there are questions: 617-496-7582 (office), 781-646-0611(home)

THANK YOU VERY MUCH, ROB!

Ping

Ping Zhao: zhao@cfa.harvard.edu