# Question about start of usable twilight time

Hi,

For twilight proposals the Cadence Optimization White Paper Call has said there’s about 20-30 minutes per twilight available.

Doing some math for Cerro Pachón for current sunset/sunrise times: The time between nautical twilight is 11.58 hours. The time been 18 degree astronomical twilight is 10.6 hours. That leaves 0.98 hours between the 12 and 18 degree mark, divide that in two it’s about 20-30 minutes in the morning and evening twilight between 12 and 18 degree twilight. This matches the rough estimate in the document.

Should we assume that a twilight survey would start no earlier than 12 degree twilight and end at 18 degree twilight? In the current operations simulator when do observations start (does it vary by filter?)? Is there expected to be time taken by twilight flats or early observing in Y filter?

Does Project recommend that we focus on what can be done between 12 and 18 degree twilight and make a brief comment if we could use more twilight time later for the proposed twilight observing strategy if it is deemed safe?

Thanks,
~Meg

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In the current opsim runs, observing is configured to start as early as 12 degrees twilight; observations will be acquired after 12 deg twilight as soon as the skybrightness falls below the limits set by the configuration parameters.

For these runs, we’ve set those skybrightness limits as follows for the WFD:

``````configId    Session_sessionId  paramName                                                         paramValue
----------  -----------------  ----------------------------------------------------------------  ----------
248         2026               science/general_props/values/WideFastDeep/filters/u/bright_limit  21.3
256         2026               science/general_props/values/WideFastDeep/filters/g/bright_limit  21.0
264         2026               science/general_props/values/WideFastDeep/filters/r/bright_limit  20.25
272         2026               science/general_props/values/WideFastDeep/filters/i/bright_limit  19.5
280         2026               science/general_props/values/WideFastDeep/filters/z/bright_limit  17.0
288         2026               science/general_props/values/WideFastDeep/filters/y/bright_limit  16.5
``````

So typically y or z band will kick in during the early twilight times, although you will note that sometimes we’re observing in other bands beyond 18 deg twilight – because the sky can be darker than these limits, especially near zenith.

I don’t believe there is any limit regarding observing earlier than 12 degree twilight, if it’s suitable for observations, but I will check. There is not any plan to reserve this time for calibration (twilight flats) or early y band observations.

Thanks Lynne. Can you explain a bit more where the ~ 20-30 minute estimate quoted in the cadence call for twilight comes from?

it’s a good question! @ivezic? Maybe the point is that time beyond even 12 degree twilight could be used for something?

Does Project recommend that we focus on what can be done between 12 and 18 degree
twilight and make a brief comment if we could use more twilight time later for the proposed
twilight observing strategy if it is deemed safe?

I love it when questions are actually excellent answers!