To take sky flats:
1. Take sky flats if the sky is clear.
Sky flats when it is cloudy are no good.
The telescope is open and pointed towards zenith.
2. Wait for the sky to get dark enough:
To monitor the sky brightness,
Camera Wheel N 3.75
Grating Unit: Mirror
FW2 = Clear
Mask to use: NO MASK
Mask to position: NO MASK IN USE
Remember to hit COMMIT INSTRUMENT SET UP after changing filters, etc.
in readout manager gui, change SAVE IMAGE
to MANUAL <--- not saving
DIT = 2
(2 second exposures)
Set No. exposures = 100
hit SET NEW VALUES
hit START READ
This will start a stream of exposures.The images are displayed in the small Lucifer display window.
Put the cursor in the window and look at the counts.
When the counts in the Br-gam filter are about 1500, then the counts in
Ks will be about 15000 and it's ok to start.
to stop the exposures.
3. Take Ks flats
FW1 to clear
FW2 to Ks
************* MAKE SURE YOU NEVER HAVE
FW1=clear and FW2=clear at the same time ***********
Start 100 exposures with the Ks filter to verify that the counts are OK;
(like DIT=3, counts are 16,000)
set SAVE IMAGE
FRAME TYPE = FLAT
# Exposures = 10
SET NEW VALUES
This takes 10 sky frames.
4. Take H-band flats
Monitor the sky brightness with the FeII
filter, instead of the Br gamma,
Repeat as in Steps 2 and 3.
H sky counts will be about 10x the FeII
For H and J, take a set of short flats and a set of long flats
5. J-band flats can be taken as soon as the H band flats are done.
No need to do monitoring.
J. Bechtold, F. Bian Jan 2010, May 2011