Nordic Optical Telescope

Atmospheric Dispersion Corrector (ADC)

As of Dec 2010 we have the ADC on offer for regular use. The ADC can be used to minimise the wavelength-dependent fiber-aperture losses that are due to the atmospheric dispersion at large zenith distances.

The ADC swings into the telescope beam before the FIES/STANCAM pickoff mirrors, and provides an unvignetted field of view of about 3 arcminutes. The ADC consists of a set of two rotating prisms, that together compensate for the atmospheric effect. The ADC was primarily designed for use with FIES, but can be also be used with ALFOSC or NOTCAM.

For use with FIES the ADC is expected to be advantageous in terms of throughput and radial-velocity precision for sources at airmass>1.5 (or so). Detailed information about throughput etc. can be found in the commissioning document.

ADC transmission as observed by ALFOSC and NOTCAM

U B V R I J H K

88% 88% 90% 91% 8 9% 84% 85% 67%

Note: in the K-band significant emission from the ADC prisms should be expected, leading to about 0.5 mag higher background level.

Commands to operate the ADC

The ADC can be controlled from the TCS user-interface, and more commonly through sequencer commands on the instrument control computer (marissa, selena). The below table lists the most useful sequencer commands.
A complete list of ADC sequencer commands can be found here.
The full explanation of the commands is given in the main TCS command reference .

Some useful ADC Sequencer commands

adc-arm-in

adc-arm-out

adc-mode-automatic-when-presetting

adc-mode-automatic-off

adc-actual

adc-explicit

wait-adc

Note that the continuously updating ADC mode (adc-mode-automatic-continous) should be used with caution, as this mode will displace the star(s) during an exposure. The continuous ADC mode can only be used in combination with the FIES fiber guider.
In general, we advise to use the automatic mode adc-mode-automatic-when-presetting which updates the ADC prisms every time the telescope is pointed to a new target.

Telescope focus offset

When the ADC is in the beam the telescope focus should be offset by about 560 units. This offset has to be applied by the user (with focus-position, or focus-delta).

When using the ADC with ALFOSC, the offset will correctly be taken into account when using the command alfosc.focus-offset .

Use with FIES

See the FIES CookBook

Use with ALFOSC

Note that the Field-Of-View of the ADC is only about 3 arcminutes!

To use the ADC, issue the following commands before pointing to a new target:

[obs@selena][ALFOSC]$ tcs.adc-arm-in
[obs@selena][ALFOSC]$ tcs.adc-mode-automatic-when-presetting
[obs@selena][ALFOSC]$ alfosc.focus-offset

and then the ADC will then adjust itself while presetting to new targets.

To have the ADC adjust itself to the current telescope altitude, e.g. when following the same target for an extended period:

[obs@selena][ALFOSC]$ tcs.adc-actual

but never do that during an exposure, as the target will move somewhat with respect to the fiber.

To stop using the ADC issue the following commands:

[obs@selena][ALFOSC]$ tcs.adc-arm-out
[obs@selena][ALFOSC]$ alfosc.focus-offset

Last modified: April 19 2023