Champlane Observation: CTIO 4m/MosaicII May 8-9, 2008

ChaMPlane Observation with CTIO 4m MosaicII

May 8 - 9, 2008

I. Target List and Priorities

Following is the target list. The first two fields are to be observed in the first 3 hours of each night, before the BLS fields are up. The observation of each of the two fields may need to be split into two nights. The six Mosaic BLS fields cover all the Bulge Wide Survey - North and South (36 Chandra pointings). Mosaic BLS-S were observed in 2007, but the images were not desirable (cloudy and bad seeing). So it's best to redo it. Priorities are given in the first column. Only 3 fields can be observed each night. So only the first six fields can be done even in perfect conditions.

     Telescope pointing Center of Mosaic II

Pri  Field   RA (J2000)    Dec (J2000)     l         b        Observed?
---  ------  -----------  -----------  ----------  ---------  ---------
7    J0737   07:37:52.80  -30:39:43.2  245.239062  -4.500286   y
8    J1701   17:00:58.46  -46:11:08.6  340.813293  -2.488318   y
1    BLS-N1  17:43:01.40  -28:25:48.0    0.139421   0.759262   y
2    BLS-N2  17:40:24.00  -28:31:48.0  359.743863   1.187018   y
3    BLS-N3  17:40:39.60  -28:11:24.0    0.064033   1.335264   y
4    BLS-S1  17:49:02.40  -29:14:24.0    0.189919  -0.758665   y
5    BLS-S2  17:48:34.80  -29:36:36.0  359.760529  -0.904929
6    BLS-S3  17:51:19.99  -29:41:24.0  359.996589  -1.455686
---  ------  -----------   ----------- -- -------  ---------  ---------
Click a field name to see the Mosaic and ACIS overlay on dss image.  North-up.

The following maps show the location of the six Mosaic BLS fields (big green squares), Mosaic fields previously taken ((big blue squares), Chandra ACIS pointings (red squares) in Galactic coordinates:

BLS    jpg  postscript 
BLS-N  jpg  postscript
BLS-S  jpg  postscript

The R - Ha - V - I exposure sequence is outlined in Sect. II. If time is running short, see the suggestions for skipping exposures in Sect. III. Back to Top

II. Exposure Sequence

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

           Exposure sequence for each field

N  Filter  Frame         Exp   Exp+readout Total Timeline
                        (sec)  (min/frame) (min)   (min)
---------------------------------------------------------
1  R       single image     2       1.7     1.7     1.7
2  Ha      single image    60       2.7     2.7     4.4
3  R       dither 1x5     240       5.7    28.5    32.9
4  Ha      dither 1x5    1500      26.7   133.5   166.4
5  V       single image     2       1.7     1.7   168.0
6  I       single image     2       1.7     1.7   169.8
7  V       dither 1x5     180       4.7    23.5   193.3
8  I       dither 1x5     180       4.7    23.5   216.8
---------------------------------------------------------
   Total time needed for each pointing            216.8 minutes = 3h36.8m

1. N is the exposure sequence number.
2. Exp is the exposure time for each image in seconds.
3. Exp+readout is the exposure plus 1.7min readout time for each image.
4. Total is the total time ((Exp+readout) x dither) for each filter.
5. Timeline is the accumulated time for each field.

Under Frame: 

A. single image: take one short exposure for each filter.  No dither.

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

It looks like this:
===========
# Tod Lauer'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. When short of time, dither 1x3 can be used for filters I and V: 
   Use command "mosdither" to do one set of dither pattern of 3.  Take
   one image at each position. Create and use a "ditherdb$dither3.dat"
   file like this:

===========
# 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.

III. Observation Plan

The astronomical twilight on May 9-11 ends at 19:24 and begin at 5:55 local time. There are 10h31m from evening to morning twilight. The first field of each night are time fillers before the BLS are up. So try to allocate only 180m to each of them at the beginning of the night (e.g. only do 4 Ha exps and 3 each for V & I). Then devote the rest of the night (7h31m or 451m) to the BLS fields. Two pointings will take 7h13.6m (2 x 216.8min = 433.6m). But this doesn't include slewing the telescope and acquiring the targets. So careful planning is helpful.

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

B. When time is running short, do the following to save time:

a. First reduce the number of I images to dither 1x3. (save 9.4 min)

a. Then reduce the number of V images to dither 1x3. (save 9.4 min)

b. Skip V and I short (2 sec) images. (save 1.7 min each)

c. Reduce one or two more I deep images, then V deep images. (save 4.7 min each) But keep at least one V and one I images.

When take only one deep V and one deep I images, the total time needed for a field is reduced to 175.8 min - a total saving of 41.0 min.

C. Don't need to take the standard (not enough time). We will apply for time on CTIO 1.3m telescope for the calibration.

Here is an observing elevation chart.

postscript CTIO 2008 may Ephemeris Back to Top

IV. Biases, Flats etc.

Biases and Domeflats: Take biases and domeflats in the afternoon before dinner.
Skyflats: Take twilight skyflats starting shortly after sunset at around 6:00pm.
Pointing and Focus: Do telescope pointing and focus after the skyflats at around 6:50pm.

1. Biases and Domeflats

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

2. Skyflats

Take some skyflats in the evening or morning twilight if possible. I filter skyflats are probably the most needed in order to correct the fringes. 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 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 at 6:05pm.
Turn the tracking on during the skyflat exposures and turn the tracking off during the readouts so there will be an offset between exposures.
Do the Ha filter skyflats first, then do the I filter skyflats.
Start at about 6:10pm, 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 1.7 min readout. To double the exposure time is just about to compensate the change in the sky.
Take at least 3 Ha skyflats.
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.

3. Pointing and Focus:

Stop the skyflats at around 6:50pm.
Do the pointing and focus at around 6:50pm.
Start to observe the first ChaMplane field at about 7:10pm.

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

THANKS!

Ping Zhao: zhao@cfa.harvard.edu