Calibration

### ***2.1. Arriving at the Telescope***

Ideally---to enable acquisition of good calibration frames---arrive at the telescope 30--45 minutes before sunset. It takes a while for the Apogee CCD to cool down (~20 minutes), and you want everything to be ready to start taking images by Civil Twilight. Follow the directions for Opening Procedures. I like to look on the *Astronomy*section of wunderground.com for sunset/twilight times.

For acquiring calibration images, you'll want to follow the general guidelines on the Observing page (under development, July 2015). There are a few special steps, though, detailed below.

In the directory where you'll be storing your images, makes separate subdirectories for bias frames (call it *biases*), dark frames (call it *darks*) and flat field frames (call it *flats*). If you have flats in multiple filters, save them in different directories.

You should always be able to observe biases and darks. If you are not able to take flats the night of your observations, copy the most recent flats into your directory (that match the filters you plan to use).

### ***2.2. Twilight Flats***

Flat fields are the calibration frames that are time sensitive. They need to be taken around Civil Twilight, when the sky is still bright enough to nicely illuminate the CCD, but not so bright it saturates. Flats aren't worth taking under cloudy conditions.

If you are going to take twilight flats, move the telescope to **Zenith Park** in *ACE*. Select a filter that you plan to use tonight in *ACE* ("Clear" is a fine option, if that's what you plan to use).

In *Maxim DL 6*, in the 'Camera Control Window', under the **Expose** tab, make sure the CCD is binned by just 1 pixel (**X binning** should be set to **1,** and **Y binning** set to **Same**). Click on the arrow next to **Options** (at the right-middle of the window), and make sure **No Calibration** is selected. Select **Light** from the **Frame Type** drop-down menu in this window, and start with an exposure time of 1 second.

Take an image, and look at the image statistics. If they are around 60,000 counts per pixel, the sky is too bright and the CCD is saturated. Wait a minute or two and try again. You want your flat frames to have counts of 20,000--40,000 counts per pixel.

Try to get at least five flat frames in this range of pixel counts (don't forget to save images with **Save As**!). It is ok to make the exposure time longer than 1 second (as the sky starts to fade), but you probably don't want to make it shorter than 1 sec.

Repeat the process in every filter you plan to use tonight. Typically, the sky stays bright longest at redder bands, so I like to take flats first for the *B* filter, then *V*, then *R* and *I*.

### ***2.3. Bias Frames***

Bias frames can be taken any time at all---these are essentially images where the CCD does not expose at all, but just reads out to tell you what the base level of counts in the CCD is.

To take bias frames, in Maxim DL, find the CCD Control Window and click on the **Expose** tab. Select **Bias** from the **Frame Type** drop-down menu and click the **Expose** button. Check that the image just looks like flat noise. You'll want to take lots of bias frames (15--20), so it probably makes sense to use the **Autosave** feature in *Maxim D*L to repeat the image acquisition.

### ***2.4. Dark Frames***

As with biases, darks can be taken any time at all. Here we let the CCD expose for some time (Below, 180 seconds), but the aperture never opens to let light hit the CCD. It is better to take darks when the dome is not very bright. So, if it's daytime, close the slit. If the lights are on in the dome, turn them off.

To take dark frames, in Maxim DL, find the CCD Control Window and click on the **Expose** tab. Select **Dark,** and set an exposure time of 180 seconds (to start). Click the **Expose** button. Check that the image looks like flat noise, simlar to the bias frames, but with slightly higher pixel counts.. You'll want to take several dark frames (5--10), so it might make sense to use the **Autosave** feature in *Maxim D*L to repeat the image acquisition. It is good form to take darks for all the various expsoure times you intend to use throughout the night.