Science Operations

Twilight Sky Flats

Blank fields

Blank fields can be found here: NOT Blank Fields

Sky Flats: About mkskyflat

The perl script mkskyflat creates a binocular OB that takes a series of 5 dithered exposures about a specific or “standard” position and a binocular “FlatTest” OB. The script can be run from any directoy. Corrections were made on 1 Jan 2014 to wrap values of RA that, initially, are equivalent to the LST at twilight, but, after some adjustment within the script, may fall outside the range 0-360 deg. These corrections affect the program output when the “e” or “m” option is used and for times of the year when LST at twilight is close to 0 hr or 24 hr, i.e. December/January and July/August.

The “standard” position for twilight flats is at an hour angle, HA ~ -00:30, and declination = 27 degrees, close to the zenith. However, if this coincides with the Galactic plane, or if there is a preferred position, the scripts has the option to generate the OB with these coordinates. When possible, twilight flats should be performed at blank fields.  Morning Twilight flats should be performed on field in the west to avoid pointing the telescope towards sunrise.  A table of coordinates of blank sky fields for sky flats is available from the NOT web site.

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.

The script will create a pair of OBs to take flat fields:

  • * the one starting with “SkyFlatTest…” will slew and obtain a strip to test counts.
  • * the one starting with “SkyFlat…” will slew and execute a 5-pt dither.

There are four options for input: The full syntax 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 “standard” flat field position and exposure time scaling are requested, the coordinates can be replaced with an “e” (evening “standard”) or “m” (morning “standard”) and no exposure times need be given. For example,

mkskyflat e 0 U Y

will create a pair of OBs at the evening “standard” position, 0 position angle, and using the U-BESSEL and Y-FAN filters with appropriate exposure time scaling to yield similar counts in both channels. The evening or morning positions can potentially point the user at a crowded field of field with a bright source so it strongly recommended to use a known blank field.  The minimum exposure time is 1sec (can be decreased by changing the scale factor on the User Interface) and when the needed exposure times differ by greater than a factor 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). The first letter of the desired filter is sufficient to specify the filter. for LBC-Blue:

S = SDT_Uspec
g = g-SLOAN
r = r-SLOAN

For LBC-Red:

r = r-SLOAN
i = i-SLOAN
z = z-SLOAN
F = F972N20
T = TiO_784
C = CN_817

The “standard” flat field position is: RA = LST at sunset/sunrise minus 15 minutes, and DEC = 27 degrees. mkskyflat1 uses the program ‘skycalc’ by John Thorstensen to determine the LST at sunset or sunrise on the current date. This syntax is to be used on the date (or within a few days) that the created flat field OBs will be run. The syntax is:

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

where a single letter indicates whether these OBs are to be used to take evening(“e”) or morning(“m”) flats.

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, 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.

Since 2017B, we have maintained a table of good sky flats (i.e. flats with counts within a suitable range) here. Times when the filters were cleaned are recorded here. And on any of the obsN machines, the observer can list flats by camera, filter and month using the command, lbcsky.

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

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*

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.