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Home > Astronomy > Telescope Operator Notes > TWFC1/2 QHY cameras at Prime Focus


TWFC1 and TWFC2 (TWFC stands for Tiny Wide Field Camera) were installed on the WHT in January 2021 to allow for testing of the optical characteristics of the focal plane for WEAVE. Since WEAVE was then delayed, the cameras have remained in place (until WEAVE is ready) and have been offered to the science community for astronomical observations.

There is quite a lot of detailed information on the Confluence web pages with information on the CCDs, filters, and response curves. On the public website too, there is information about the TWFC cameras, including field of view and linearity, etc.

Jure's seminar on the QHY cameras contains a guide to troubleshooting and using the camera's as well as the GUI, SkyTsar.

  • Download the presentation in ODP format.
  • (right click to) Download the video of the session [1hr40].


FTUs: At the start of each night, we perform an initialisation of the focus translation units (FTUs). From the PLC GUI in eng. mode, telescope at elevation 90 and mirror petals closed: FTU all home. Open petals when done.

TCS: In terms of the TCS, these cameras are at Prime Focus and are based on a configuration that is in preparation for WEAVE. So, unless instructed otherwise, we should configure the TCS in the same setup as will be used for WEAVE.

  • USER> FOCUS <Varies depending on the camera and filter. See the section on focus>.

On the TCS it will show that the optical configuration is "Prime (Nasmyth flat not stowed)" (NASMYTH DRIVE SIDE SET would be lit, GRACE). Should you have to move the Nasmyth flat (M3), please check that the telescope is at zenith and the petals are closed. Then press the NASMYTH DRIVE SIDE (GRACE) button.

Check camera settings: Overheating has been seen several times before, especially with TWFC2. OSAs should always check this at the start and end of night. Look in SkyTsar for the temperature and cooling power of the camera. If the power exceeds 85%, pause observing with that camera for 30 minutes and change the temperature set point to 0°C. Then ramp it up again and continue observing. Check that the camera gain is set to 30 (for all observations unless the PI asks for something different).

Using the camera

Exposures, darks and flats can be taken from the ICS command line using the 'indicam' command. There is a User Manual written by Jure.

The most frequently used commands are summarised here: TWFC indicam commands.

Telescope focus: The two cameras are both in fixed mounted positions in the focal plane. Their relative distance from the primary mirror is therefore fixed. This can only be adjusted by the Ops team during the day and involves opening the box at prime focus and manually adjusting the camera's position with a lot of trial and error. This means that whenever you switch cameras, the telescope focus will have to be adjusted. This is also true if you just change filters but continue to use the same camera. Many other instruments at the ING, such as ACAM or the WFC (on INT) will automatically change the focus of the telescope when filters are changed. This does not happen for TWFC1/2 so you will need to adjust the telescope focus every time you change the filter and/or the camera.

Since the cameras are being moved by the Ops team from time to time (check the WEAVE night log for any updates), the expected focus values will be changing. It is recommended to do a focus run each night You might not need to do all filters in both cameras, depending on the main work for the night. The nominal values for focus are being recorded in the QHY filters focus offsets - Records confluence document.

Typical focus values are shown below, but these should be checked against the most recent known values (check a recent WEAVE night report).

TWFC 1 (off-axis from April 6)
Filter position Filter Central wavelength
Tel focus
1 empty . 0.28
2 R 641 0.56
3 G 529 0.56
4 B 466 0.56
5 Sloan i 780 1.12

TWFC 2 (on-axis from April 6)
Filter position Filter Central wavelength
Tel focus
1 Sloan u 355 0.42
2 R 641 0.02
3 G 526 0.02
4 B 470 0.02
5 656 0.43
6 empty . -0.28
7 Sloan z 824 0.44

Autofocus: There is a wrapper script to help with measuring the focus for a particular camera and filter. The steps are as follows:

  1. First choose the filter you are going to use:
    eg. indicam filter TWFC1 -filter R
  2. Then point the telescope to somewhere in the sky :-), just make sure both the telescope and rotator are tracking.
  3. Take a 10s exposure using the SkyTsar GUI. Use the same binning factor that you will be using for the focus run (4x4 recommended).
  4. In SkyTsar, find a suitable star to use for a focus run. Peak counts for 10s below ~30k. Click on the star and note the X, Y positions.
  5. In the ICS command line, run the autofocus script:
    eg autofocusQHY TWFC2 -0.7 0.3 -n 21 --binning 4 --expTime 15 -x 884 -y 813 -sp
  6. The script should save the images ('-s' option), fit a parabola and show a plot of the fit ('-p' option).
  7. The script always prompts you to confirm the parameters you have entered. If you don't want this, specify the command line option '--noprompt'..

Autoguider focus: The autoguider focus cannot be adjusted independently from the cameras, so, depending on the filter you are using, the autoguider may be out of focus. This cannot be adjusted during the night. The plan is to have the autoguider parfocal with the most popular filters and camera for the science observations.


The Autoguider camera is AG7 and is mounted at a radius of 0.9 deg from the optical axis. This has implications for the accuracy of the guiding for long runs as the field rises or sets and the atmospheric dispersion becomes a factor. Nevertheless autoguiding is still better for long runs than relying purely on telescope tracking. Some simple tests of the autoguider drift have shown that this effect is quite small and on the order of ≤2 arcseconds per hour.

In order to autoguide, just go to the DAS machine hosting AG7 and start the autoguiding software:

  1. obssys
  2. startobssys
  3. start AG7
This will start the autoguider expose control panel and the DS9 window. Hopefully you will see a star that can be used for guiding. If not, then you will need to reposition the telescope (perhaps change the Sky PA?) in order to find a star. The autoguider can not be moved independently of the telescope, so no probe X Y commands, sorry.


Dome flats can be taken using the flat field lamps and pointing the telescope at the inside of the dome.

The suggested position for dome flats is:

  • Dome position: 270°
  • Tel elevation: 45°
  • Tel azimuth: 170°

The flat field lamps are turned on with the ics (taurus) command 'fflamp', for example 'fflamp all on'. There are 5 lamps in total and all of them can be turned on and off inidividually. The lamps' percentage illumination can be controlled between 0-100% using the command 'fflampspwm'. This command adjusts all lamps together (ie you can't control the lamps individually here). Also note the the pulse length for the PWM is about 1 second, so this won't be good for exposures below about 20s.


Camera not responding:

  • Check if miniwfc1 is offline (ping miniwfc1)
  • If it doesn't work, try power-cycling the miniwfc1 PC.
  • If miniwfc1 is responding to ping, Use shut account to restart miniwfc1.
  • Restarting miniwfc1 will automatically restart the server
  • After this, you will need to restore filter names using qhyreset command
  • Check cooling

Power cycle: If the cameras stop responding to commands (eg FR 23930), you might need to do a full power cycle of the cameras. This is done using the APC device at prime focus. Go to (the password can be found in the safe box in the control room) and switch the affected camera off and then back on. You might need to power-cycle both. It is also possible to power-cycle the computer but if it works (ping), please do not do it. You should rather reboot using the shut account.

After that you must follow-up with qhyreset and by setting the temperature to -10 degrees C from skytsar. Note that by default the temperature set point is at 0 degrees C. Please do report any further occurrences of such faults in the Fault database (not WEAVEFAULT).

After using the APC remote switch please do not forget to log out!

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Last modified: 10 May 2021