Tuesday, January 20, 2009

Observing strategy

The main goal for these observations is to obtain accurate relative photometry with sufficient cadence to resolve the events (ingress and egress each last ~15 minutes, but there might be interesting structure within) and sufficient accuracy to detect the event (Namaka's disappearance during an eclipse will make the overall brightness of Haumea decrease by only about 1.3%).

The optimal exposure time for your observations depends on your telescope and on your camera. The main things to worry about are:
-readout time for your camera
-time to saturation
-time until you are sky background limited

I would shoot for having exposure times be at least 2-3 times the readout time of your camera, as long as you are not coming close to saturating Haumea. Even with a very fast readout camera, exposure times under a minute or two are probably not worthwhile. The best cadence, if it fits your camera characteristics, is probably around 2 minutes per exposure. At Palomar, we will be doing 2 minute exposures with 45 second readouts in between.

Previously I said this:
In general, the optimal filter is the one that gives you the highest signal-to-noise. Haumea is almost precisely solar colored, so observing in something like a V filter is a good bet, though it is possible that you might want to go totally unfiltered (at Palomar we will be using a special filter that cuts out some of the worst of the sky lines from the bright Palomar sky).

However, after the 1/31 event I now believe that it is best if everyone observes in as common a filter as possible. Because some of the events will have heavy moonlight, R seems the best choice. At Palomar we used a slight variant of R that cuts out the strong sodium lines from San Diego, but as long as everyone is R-ish I think we will be able to compare all of the data well.

At some point in the night you will want to insert a single image in a different filter. This single image will be used to get the colors of all of the stars relative to Haumea. The relative photometry can then be done using only stars that are close to Haumea's solar color. This selection will be important given the very wide range of airmasses Haumea is likely to go through (at Palomar we will start at an airmass of 4!).

Before Haumea rises (or on an earlier night, at twilight, or with dome flats):
  • take images of the night sky/twilight sky or just dome flats
  • look for a particularly well behaved region on the chip where the flat field is as flat as possible, there are no bad pixels or columns, nothing funny is going on for a moderately good sized region.
  • Note that location for later; that is where you will put Haumea

The observing strategy is quite simple:
  • point to Haumea as early as your telescope possibly can.
  • place Haumea in the very nice region of the chip that you identified earlier.
  • track at Haumea's rate; do not dither! Dithering is great if you want to average out CCD variations over time, but we instead want to keep them constant with time.
  • observe all night until you can't anymore
  • pause for nothing.
  • if you fear focus changes, focus on an image-by-image basis rather than stopping to focus.
  • Be sure to insert one or two images in a second filter.
We believe that the best strategy will be to track on the rate of Haumea but perform no dithering of the telescope. If your telescope cannot track at a moving target rate, track siderally, but, again, perform no dithering.

If you believe your night to be photometric, good solar colored standard stars at the begining and end would be helpful, but not essential.

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