Sky flats

You can take sky flats at zenith or at a `blank' field. Source names for blank fields can be found in Table 4. Once you found the name of the correct blank field, issue on the TCS User Interface

          USER> gocat blank2

When both telescope and dome have reached their destination, move the filter wheel and take the flats with

          SYS> filter Ha
          SYS> sky 2 "sky Halpha"

and check the count rates in IRAF (use DAS_GET and IMEXAMINE). Narrow band sky flats can be taken just before sunset, for broad band sky flats you have to wait a bit longer.

BEWARE that the CCD iris shutter takes about 0.1 seconds to fully deploy, so short flats aren't flat! Depending on the accuracy that you need, flats should have exposure times of >2 seconds.

 

Table 4: Blank fields from Christian et al., 1985, PASP 97, 363. These fields are all in the system catalogue.

name	RA (B1950)	DEC (B1950)
BLANK1	04 25 46.0	+54 09 03
BLANK2	13 04 33.0	+29 50 49
BLANK3	16 49 42.0	-15 21 00
BLANK4	19 19 09.0	+12 22 05
BLANK5	21 26 54.4	-08 51 41
BLANK6	23 54 08.9	+59 28 18