Effect on seeing of heat dissipation

The aim is to investigate the effect on seeing of the (probably) ~ 250 W to be dissipated by the WEAVE fibre-positioner motors, by using a test 500-W heat dissipator mounted behind M2. We also aim to measure the dependence of this effect on telescope elevation.

Key points:

• The test can be carried out an S/D night, in seeing <~ 1 arcsec.
• The test is carried out when any of ACAM (imaging), LIRIS (imaging) or AF2 (AG5) are available. If ISIS is available, but not ACAM, the test can also be made using ISIS in spectroscopy mode (i.e. a one dimensional measure of seeing, perpendicular to the dispersion direction).
• Avoid doing the test during LIRIS observations in H or K bands, since the dissipator might significantly warm the top end of the telescope (or if you want to do the test, monitor the background level on the science images, make sure that it doesn't change after switching on the disssipator).
• The aim is that this test lasts the whole night (while observing service proposals or doing other tests).
• The dissipators will be ON for periods of about 2 hours and OFF for about 1 hour and we will take seeing measurements during these periods. E.g. a summer night has about 7 hours, then: 1 hr OFF (sets of measurements) + 2 hrs ON (measurements) + 1 hr OFF (measurements) + 2 hrs ON (measurements) + 1 hr OFF (measurements)
• It would be good that each set of measurements includes at least 3 consecutive images of t~15-30 sec.
• Please provide the requested comments (in the requested format) in the night log, so it makes it easy to examine the data and you do not need to add extra information in the Task Report.
• Filling the Task Report: If in the night log is clearly indicated (as requested), in the Task Report just write e.g. 21:30-22:00 in the "time used" section, as 30 minutes is more or less what you will use to do this test (if for any reason it took much longer than 30 minutes then write the amount of time it took). Don't log the actual elapsed time of the test, since it will usually take place in parallel with other observations. Please write in the Task Report any other comment about observing conditions you consider relevant for interpreting the data.

[1] Ask the operator whether the 500-W dissipator is mounted behind the secondary (alternatively, ask Juerg or the ops team in advance). This can be checked in two ways:

1. By lowering the telescope to elevation 30 deg, and looking at the back of the secondary mirror. The 500-W heater has a triangular structure (the older 100-W heater comprised 4 elements).
2. Checking the current consumption on the APC page linked from the telescope-operator pages (suggestion from Juerg); when the 500-W dissipator is on, the current should be ~ 500 W / 230 V = 2.2 A.

(The 500-W dissipator is an upgrade of the former 100-W dissipator, built by Amado, mounted on an aluminium disk, and reaching a temperature of 35 degC above ambient.)

[2] Focus the telescope for the imager to be used (in r band, if ACAM is being used).

[3] Dissipators OFF (about 1 hour).

• We aim to have at least 3 sets of images (each set comprising 3 exposures t ~ 15 - 30 sec) with dissipators OFF.
• Comment in the night log for the first image of each set: "OFF, INGxx TNGxx IACxx", where xx is the seeing at the DIMMs (whichever are functioning that night).
• (With AG5 you can take the images while observing with the Acq&Guiding tool, changing the exptime to 15-30 sec)
• After ~1 hour switch ON the heat dissipator.

[4] Dissipators ON (about 2 hours)

• Follow the intranet instructions linked here to switch on the heat dissipator (the OSA will provide username and password for this).
• We aim to have at least 5 sets of images (3 exposures in each set, 15 - 30 sec, as above) with dissipators ON
• Comment in the night log for the first image of the set: "ON-hh:mm, INGxx TNGxx IACxx", where hh:mm is the time in UT (accuracy a few min) when you turn the dissipator ON"
• About every 30 minutes take another 3 images (same comments as above on the night log)
• After ~2 hours switch OFF the heat dissipator.
• We're expecting that the dissipator will not affect the seeing, but if it does significantly degrade any science observations requiring excellent seeing, repeat the observations with the dissipator off (and don't carry out further tests during that science programme).

[5] Dissipators OFF (about 1 hour)

• Repeat [3]
• Comment in the night log for the first image of the set: "OFF-hh:mm, INGxx TNGxx IACxx", where hh:mm is exactly the hour in UT when you turn the dissipator OFF"
• After ~1 hour switch ON the heat dissipator.

[6] Dissipators ON (about 2 hours)

• Repeat [4].
• Comment in the night log for the first image of the set: "ON-hh:mm, INGxx TNGxx IACxx", where hh:mm is exactly the hour in UT when you turn the dissipator ON"
• After ~2 hours switch OFF the heat dissipator.

[7] Dissipators OFF (until the end of the night)

• Repeat [3]
• Comment in the night log for the first image of the set: "OFF-hh:mm, INGxx TNGxx IACxx", where hh:mm is exactly the hour in UT when you turn the dissipator OFF" and xx are the seeing as measured by the 3 DIMMs.

Last updated: 8 September 2016

Please address any enquiries to:

Cecilia cf@ing.iac.es (or Chris crb@ing.iac.es)