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Sky Flat Fields

Sky flat fields should be taken as soon as the sun has set. A set of coordinates of "blank" areas in the sky may be included in the catalogue on the TCS by typing:

USER>inc blank

Now type on the TCS:

USER>out term

This will display the objects currently in the catalogue including the following "blank" areas:

Name: R.A.(B1950): Dec(B1950):
BLANK104 25 46.0+54 09 03
BLANK213 04 33.0+29 50 49
BLANK316 49 42.0-15 21 00
BLANK419 19 09.0+12 22 05
BLANK521 26 54.4-08 51 41
BLANK623 54 08.9+59 28 18

Choose the "blank" field for which the R.A. is closest to the Local Sidereal Time (LST) on the clock in the JKT control room. Then, on the TCS (or ICS), type:

*USER>gocat blankn
Note that "blankn" refers to the "blank" field that you have chosen.

A minimum of three sky flat field frames should be taken in each filter required for the science observations. To change the filter that is currently in use, on the ICS type:

SYS>filter n
Note that "n" refers to the number of the slot in the filter wheel containing the filter or the name of the filter that was entered in the ICS.

The saturation level of the SITe2 chip is 65535 counts. Flat field frames should aim to have ~40000 counts in the middle of the chip. If you are unsure of the exposure time required for the flat field, then set a small 100 pixel square window in the centre of the chip and on the ICS type:

SYS>glance n

This command takes an n second exposure of the windowed chip and writes the resulting image to DAS1.fit in the current data directory. Each time this command is issued it overwrites the previous image stored in DAS1.fit and nothing is written to the log.

Display the DAS1.fit image and observe the count level in the middle of the chip. Scale the glance exposure time accordingly from n seconds to m seconds so that the m second exposure will have ~40000 counts. Remembering to set the window on the CCD chip back to whatever it was originally, on the ICS type:

SYS>sky t "description"

This command takes a t second exposure of the CCD chip, sets OBJECT equal to "description" in the header and defines the OBSTYPE in the header as "SKY". The length of the flat field exposure should not be less than 2 seconds because the CCD shutter takes a small time (~0.1 sec) to fully open/close.

When sky flats are taken at sunset and sunrise, the light levels are continually decreasing/increasing respectively. Hence the exposure time for each flat field will change from one to the next. You may either repeat the whole process of "glancing" above or you may prefer to take an educated guess as to how much the exposure time will change.

It is also useful to note that, for the same light level, narrow band filters (such as Halpha filters) require longer flat field exposures than the broad band filters (such as B or I) and that the broad band filters require decreasing flat field exposure times in the sequence U,B,V,R,I,Z. Hence, at sunset, start off with the narrow band filters and move onto the broad band filters in the order U to Z and vice versa at sunrise. Overall you have ~30 minutes of useful flat fielding time after sunset and again before sunrise. You should stop taking flat field exposures as soon as the first stars appear in the flat field frames.

Dome Flat Fields

In the event that it is not possible to take sky flat fields or that you simply want additional flat fields, you may take flat field frames on an illuminated part of the dome.

Firstly you need to close the dome if it is not already closed:


Now, on the TCS, with the telescope in computer mode, type:

*USER>park zen

This will stop the telescope tracking and put it in the upright position. Then type:

USER>dome 235

This will move the dome to a position angle of 235 degrees. Finally, put the telescope into engineering mode (typing "ack" at the USER> prompt on the TCS to silence the alarm) and drive the telescope to an airmass of 1.700 using the "DEC+" button on the JKT control panel. You may find it useful to vary the rate knob for the declination motor while doing this.

The telescope will now be pointing at a relatively "blank" part of the dome wall. Put the telescope back into computer mode, change the filter to the one that you require and make sure that the dome lights are off.

On the right on the JKT control panel there are four white buttons marked "LP1", "LP2", "LP3" and "LP4", and there is also a larger yellow button directly to the right of these marked "FLAT FLD LAMP ON". Press the yellow button which will then enable the individual dome lamps to be turned on via each of the four white buttons. Below is a table indicating which lamps should be switched on and at what level the dial control (marked "DESK" under "LIGHTS CONTROL") should be set to in order to achieve the counts indicated for the given exposure time. This table only applies to the SITe2 chip in "Quick" readout mode. Experimentation may be necessary if this is not the case or if the filter that you are using is not listed below, and as a result you may want to use the "glancing" technique outlined above for sky flat fields.

In order to take a dome flat exposure, on the ICS, type:

SYS>flat t "description"

This command takes a t second exposure of the CCD chip, sets OBJECT equal to "description" in the header and defines the OBSTYPE in the header as "FLAT". The length of the flat field exposure should not be less than 2 seconds because the CCD shutter takes a small time (~0.1 sec) to fully open/close.

In order to take multiple dome flat exposures, on the ICS, type:

SYS>multflat n t "description"

This command takes n exposures each of length t seconds, sets OBJECT equal to "description" in each of the headers and defines the OBSTYPE in each of the headers as "FLAT".

The table of suggested exposure times and light levels is as follows:

Filter: Lamps On: Lamp Level: Exposure Time: Counts (Inc. Bias):
24 Z 2 1 4 23000
27 B 1,2,3,4 8 10 23000
30 V 2,3 5 8 31000
37 R 2,3 2 6 38000
44 I 2 1 4 40000
62 Ha 1,2,3,4 10 6 36000
80 HeII 1,2,3,4 10 70 28000


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Last Updated: 19 Apr. 2001
Dan Bramich dmb@ing.iac.es