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Guide for Twilight Flats

Twilight flats have been forbidden effective 10 November 2012

Overview

A good goal for twilight flats is to obtain 10000 to 20000 electrons (and definitely less than 40000 electrons). This corresponds to an image with about 2000 to 5000 DN. A good time to start evening twilight flats in a given filter is when the sky brightness corresponds to about 5000 DN/s. Based on measurements from the first week of SV, this occurs:

Filter Minutes after sunset
u 15
Y 23
g 26
r 26
i 26
z 26

These measurements were obtained about 1-2hr east of zenith.
A good goal is to obtain ~5 exposures, dithered by ~10", in each filter. Note that tracking should be on for twilight flats to prevent star trails.

These are useful, approximate guidelines for when to take the first 1s exposure to measure the sky brightness (the g,r times are particularly uncertain). Some details about these measurements are provided below.

These data should also be useful to gauge when to plan to finish twilight standards in the morning with the telescope 1-2hr west of zenith.

Evening Twilight Sky Measurements

This figure shows the measured sky brightness (in DN/s) based on all twilight flat measurements obtained during the first week of SV:

These measurements are from chip N4 (Amp A) and were only measured in evening twilight. They represent a range in hour angle of 1-3hr east of the zenith (declination ~ -30), which is likely the origin of part of the scatter.