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Procedures for taking Twilight Sky Flats

  1. After the telescope operator opens the dome (just after sunset) and the telescope and instrument are ready, ask him to slew to a position near zenith. We have been using RA = LST + 00:30 and DEC = +27 degrees.
  2. The sets of twilight flat OBs are in /home/sdtobs/Calibs. There are two types:
    • SkyFlatTestX.ob will take a single 1-second image using a subregion ([1:2048,952:3000]) of chip 2. The "X" indicates the filter: U,B,V,R or G. This image should appear in /Repository within seconds. Display it and measure the sky counts. Note that sometimes it takes longer (30 sec) for the image to appear - this is a known problem which is not yet solved but being monitored (issue 452).
  3. Once the 1-second test image is reasonable - not saturated (65536 counts) and counts ~50,000 or less, load the OB to take a set of 5 full-field flats: SkyFlatX_5.ob. This takes 5 1-second flats with dithering. Aim for counts ~10-40,000. If exposure times less than (down to 0.5sec) or greater than 1 sec are desired, enter the scaling factor in the box in the upper left area of the OB Execution page in the LBC User Interface. Note that all 5 exposures in the OB sequence will use the same scaling factor.
  4. If you have time, repeat the flat field sequence at that filter with the instrument rotated 180 degrees. The OBs for PA=0 and PA=180 are SkyFlatX_5.ob and SkyFlatXrot180_5.ob, respectively.
  5. Once a good set of flats (5 at PA=0 or, if time, 5 also at PA=180) has been obtained at one filter, go on to another, running first the SkyFlatTestX.ob to check counts and then the SkyFlatX_5.ob sequences of 5, until it is too dark (or light) to continue. At evening twilight, unless aiming for a particular filter, try the filters in the following order: U,B,(g,r)?,V and in the morning: V, (g,r)?,B,U. (Not sure about Vgr relation, but generally use V in the darkest conditions, then B and finally U in the brightest sky).

-- OlgaKuhn - 12 Feb 2007
Topic revision: r3 - 21 Jul 2009, NormCushing
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