Twilight Sky Flats

Blank fields

Blank fields can be found here: Blank Fields from the Nordic Optical Telescope; and DSS images of these fields over a 40 sq arcmin field of view and with the LBC instrument overlay are provided at this link.

Sky Flats: About mkskyflat

The perl script mkskyflat creates two scripts:

(1) SkyFlatTest*   a binocular “FlatTest” OB which is used to slew the telescope to the desired position and take a single pair of LBCB/LBCR images, reading out only a small strip of rows of chip2. This OB is used to monitor count levels.

(2) SkyFlat*         a binocular OB that takes a series of 5 dithered exposures about a specific or “standard” position .

When possible, twilight flats should be performed at blank fields.  Morning(evening) twilight flats should be performed on field in the West(East) to avoid pointing the telescope towards sunrise(sunset). The “standard” position for twilight flats is at an hour angle, HA ~ -00:30, and declination = 27 degrees, close to the zenith. A list of Blank sky fields for sky flats is available from the Nordic Optical Telescope’s website; however these fields were selected to cover a much smaller field of view than the LBCs. DSS images of these fields over a 40 sq arcmin field of view and with the LBC instrument overlay are provided at this link.

The script attempts to scale the Blue and Red exposure times to yield similar counts in both channels, but this may need to be adjusted. The command line syntax allows flexibility in coordinates and exposure times.

There are four options for input: The full syntax where the user specifies, in addition to the required PA and filter set, the coordinates of blank sky field and the LBCB/LBCR exposure times is:

>> mkskyflat RA_hh RA_mm RA_ss DEC_dd DEC_mm DEC_ss PA Filter_Blue Filter_Red ExpTime_Blue ExpTime_Red

However, if the user wants just to point to the “standard” flat field position, the coordinates can be replaced with an “e” (evening “standard”) or “m” (morning “standard”):

>> mkskyflat [e|m] PA Filter_Blue Filter_Red ExpTime_Blue ExpTime_Red

If the user wants to use a specific field but accept the exposure time scaling for the pair of filters they wish to use, then they should use the following syntax:

>> mkskyflat RA_hh RA_mm RA_ss DEC_dd DEC_mm DEC_ss PA Filter_Blue Filter_Red

Filter_Blue and Filter_Red are indicated by a single letter as follows:

Filter_Blue: S = SDT_Uspec; U = U-BESSEL; B = B-BESSEL; V = V-BESSEL; g = g-SLOAN and r = r-SLOAN; and

Filter_Red: V = V-BESSEL; R = R-BESSEL; I = I-BESSEL; r = r-SLOAN; i = i-SLOAN; z = z-SLOAN; Y = Y-FAN; F = F972N20; T = TiO_784 and C = CN_81

The OB is created with a minimum exposure time of 1sec and when the exposure time scaling dictates that the lower throughput filter requires the blue/red channel exposure times differ by more than a factor of 3, a warning is issued asking whether the user would like to choose a different pair of filters (sometimes this may not be possible, e.g. for any combination which includes the narrow band F972N20 filter).

To create skyflat scripts for LBCB V-Bessel and LBCR r-SLOAN, using Blank Field Blank19+59 at PA=180 and the default exposure time time scaling, the command would be:

>> mkskyflat 19 15 00 59 33 00 180 V r

Sky Flats: Exposure Time Scalings

Because the LBC user interface only permits one scale factor for both blue and red channels, it is important that the relative exposure times in the script are correct. When ExpTime_Blue and ExpTime_Red are not explicitly given to mkskyflat, the following exposure time scales will be adopted. These were determined from the set of relative count rates, s: (tB/tR = 10(sB – sR)), for which current best measurements are listed in the table below. (Note s = -1 * log10(counts/sec)). These scaling factors still need to be fine-tuned, but are provided here to show the defaults which are assumed in the script, mkskyflat, and to serve as a rough guide for calculating exposure times to be entered on the command line, if needed.

Relative twilight count rates (8-April-2011, used in mkskyflat1.pl, but to be checked)
LBC-Blue		LBC-Red
filter	S = -log10(cnts/sec)	filter	S = -log10(cnts/sec)
(S)DT_Uspec	+0.55	(V)-BESSEL	0.0
(U)-BESSEL	+0.60	(R)-BESSEL	0.0
(B)-BESSEL	0.0	(r)-SLOAN	0.0
(V)-BESSEL	0.0	(I)-BESSEL	-0.1
(r)-SLOAN	0.0	(i)-SLOAN	-0.1
(g)-SLOAN	-0.35	(z)-SLOAN	0.0
		(Y)-FAN	+0.6
		(F)970N20	+0.9

As the sky becomes darker, it will be necessary to increase exposure times. Both the Red and Blue channel exposure times can be scaled by a factor which can be entered on the OB Execution page of the LBC User Interface. If you wish to change the scaling factor while the OB is playing, click the pause button and wait until the it is paused (the play button will change from grey to black), type the new scaling factor, and then click play again to resume.

Sky Flats: When were the last flats taken?

Sometimes it is not possible to get all of the necessary twilight sky flats during a partner block. Sky flats taken a few months from the run should be fine to use, so long as no changes were made to the instrument that might affect the flat field. Specifically, cleaning (or new debris) on the filters, which are close to focus, would change the flat field, but cleaning of L1 or L2, which are far from focus, should not affect the flat field.

We maintain a table of good sky flats (i.e. flats with counts within a suitable range) taken within the last year here. Times when the filters were cleaned are recorded here.  The table updates automatically each day, so it has made the lbcsky command obsolete. The table maintains lists going back only 1 year; if you need a list of good flats that are older than that, please send a request to sciops@lbto.org.

The script, lbcsky (/lbt/lbto/lbc/lbcsky on obs1, obs2 and robs) will list flats by camera, filter and month.

Some examples of acceptable syntax (this is seen by typing lbcsky ) are:

lbcsky camera filter -l list,
lbcsky camera filter -y YYYY for a year
lbcsky camera filter -y YYYYMM for a month
lbcsky camera filter -y YYYYMMD for any YYYYMMD*
lbcsky B -y YYYYMM any LBCB filter in YYYYMMD*
lbcsky V -y YYYYMM any LBCB or LBCR V-BESSEL flat in YYYYMMD*

where:

camera = [B] or [R], and
filter = [S]DT_Uspec, [U]-BESSEL, [B]-BESSEL, [V]-BESSEL, [g]-SLOAN or [r]-SLOAN for LBC-Blue, and
filter = [V]-BESSEL, [R]-BESSEL, [I]-BESSEL, [r]-SLOAN, [i]-SLOAN, [z]-SLOAN, [Y]-FAN or [F]972N20 [T]iO_784 [C]N_817 for LBC-Red

If both camera and filter are indicated, the camera must precede the filter designation. Specifying only the filter or the camera, or neither, is allowed. When not indicated, the search will be over both cameras and/or over all filters.