Changes
/* Sky flats */
If sky conditions are photometric during morning/evening twilight, you will be able to take sky flats. See Section~\ref{sec-domeflats} if you need dome flats. Sky flats can be obtained in the following way:
# Follow the '''[[#Start-up recipe|facility start-up''' procedure in Section~\ref{sec-facstartup}]].# Follow the appropriate '''[[#Instrument start-up procedure|instrument setup''' procedure (Section~\ref{sec-inststartup})]].
# '''Slew the telescope to the zenith''', with the open dome shutter facing in the opposite direction to the rising/setting sun. Leave the drives in FAST mode (i.e. not tracking).
# On the TCS, move the '''guide mirror out of beam'''.
# On the instrument PC, '''select the required filter'''. If you need flats in a variety of filters, be sure to cycle through them in the correct order (e.g. UBVRI during evening twilight; IRVBU in the morning). This also applies if you need a range of prebin settings (i.e. start with low binning in the evening; high to low binning in the morning).
# '''With STE3/STE4''', click "Expose", create a module, select "Flat" for the image type, and "Sky" from the pulldown menu beside it. Keep the count rates between 20000 and 50000. Remember to select "Yes" under "Save to Disk". Snapshots can be used to gauge integration times and can be saved to disk if appropriate. ''Section~\ref{sec-SAAOskyflats} gives a recipe for taking sky flats. '' See Section~4 of [https://www.saao.ac.za/wp-content/uploads/sites/5/SAAOCCDmanual.pdf "SAAO~CCD Camera Control Program, version 1.2", by J. W. Menzies, ] for more detailed instructions on setting up a module.
# Take a '''minimum of 15 flats in each filter/prebin setting''', keeping exposure times ≥1s with STE3/4 to allow the shutter time to open and close (SHOC has no shutter so does not have this limitation).
==== Sky flat recipe for the STE3/STE4: Modules ====
Since all exposures with the SAAO~CCD STE3/4 must be performed via a module (except for snapshots), it may seem challenging to set up and edit modules quickly enough to obtain sufficient sky flats during twilight. However, you can do this pretty quickly using the following example:
# '''Before twilight''', set up a module for your first filter and an exposure time of 1s (do not go below 1~s), with 1 repeat.
# For evening twilight, perform [[#Start-up recipe|steps 1-4 10 of Section~\ref{secthe start-sky} up recipe]] to setup the telescope and facility prior to sunset.
# Click the small grey "On" button below the SAAO CCD image display.
# Shortly before (morning) or after (evening) sunrise/set ~10 minutes after sunset for B) click "Snapshot" → Exposure time = 1s. Click "Ready" to expose.
