24-in User Instructions: Flat fields

Twilight flats

If you're taking evening twilight flats (which have to be done in twilight) they'll probably happen before anything else. The goal is to get 5 images of twilight sky with signal between 20,000 to 60,000 ADU in each filter needed. Note that because the sky brightness is continuously changing, doing twilight flats can be rather hectic, especially if you need to do several different filters.
  1. Telescope tracking stays off.
  2. Telescope to twilight flat position:
    • For evening twilight: Desired position is about HA 0h53m East and Dec +42d; this will put telescope about 10 deg east of zenith.
    • For morning twilight: Desired position is about HA 0h53m West and Dec +42d; this will put telescope about 10 deg west of zenith.
  3. Adjust dome so telescope view is clear through dome slot
  4. Start with "darkest" (for a CCD this is generally the bluest) filter you plan to use.
  5. test exposures at 1 s until image max less than 10,000 ADU (so that 6 s image max will be less than 60,000 ADU)
  6. When the signal is right - 5 flat images, 6 seconds each.
  7. Repeat for each filter you need, working from "darkest" (usually bluest) to "brightest" filters.
  8. With twilight flats especially there are ample opportunities for things to go wrong at exactly the wrong time. (e.g., if the sky darkens too much, say signal below 15,000 to 20,000 ADU, you'll need to increase exposure time. But then you can't get as many images in before the sky darkens too much yet again. Meanwhile the number of stars showing up in each image is increasing and the streaks are getting longer...) For morning twilight the sky goes from too dark to too bright which changes the strategy somewhat, but the goal is still to get 5 images of signal between 20,000 to 60,000 ADU in each filter needed.

Dome flats

  1. home the telescope (also turns off tracking)
  2. dome to "dome flat" position:
    • for telescope on east side: dome home position - blue arrow on dome at blue arrow on dome wall
    • for telescope on west side: blue arrow on dome at black tape arrow on dome wall just south of west
  3. Paddle telescope to point at flat screen: Put the ladder at the flat screen, grab the paddle, and climb the ladder to put your head at the middle of the flat screen. Now keep your head there and paddle the telescope until you're looking straight down the tube. (At the right spot you see a big reflection of your face in the primary mirror.)
  4. Leave the dome lights on; use the dimmer control to adjust brightness of dome lights so that 6 s test exposure of the flat screen gets about 35,000 to 45,000 ADU mean signal.
  5. 5 flat images in each filter you used