Photometry |
Your goal is all-sky photometry at the 1% rms level. This requires clear and stable skies with reasonable seeing. Good transparency is nice but stability is more important. The CCD and telescope are stable over perids of days or weeks (until new dust on the mirrors can be noticed) so the most important measurement on any given night is atmospheric extinction. |
Extinction |
Extinction measurements are observations of red and blue stars, preferably the same ones, at low and high airmass, ideally at 1.0 and about 1.8 airmases. MOP does a good job of scheduling these so you needn't worry, but be aware that nothing is valid until you get an extinction solution. Observations above about 2.2 airmasses are probably not as useful as those slightly under 2.0. Check the computed k values in the MTPIPE follow mode plots (the lower set of five plots at http://galileo.apo.nmsu.edu/sky/weather/pt.html). Get to know the numbers (ku is 0.45-0.48, kg is 0.15 to 0.18, etc.) and use the fits to gauge the night quality. |
Flats |
You should get at least three and preferably five good twlight flats in u' every night you observe. It's easiest to do these at sunset, but if you miss, or if it's cloudy, rainy, etc., you can get them in the morning. You have time for at least one other set of twilight flats (g, r, i, or z) and you should get one of these as well, cycling through the different filters during the week. You should also get at least three sets of dome flats every night. |
Primaries |
You need at least ten different primaries for a photometric solution. A good strategy is to observe five primaries at the start of the night before going on to your program stars (secondary patches). This increases the chance of getting a solution if it clouds over later. After doing five primaries you can let MOP autopick a mix of program stars and primaries. The primaries should cover a wide range in color and airmass (MOP does a good job at this but it's worth checking occasionally). Continue observing primaries through the night since MTPIPE likes to break the night into pieces. Don't override MOP's autopicker by removing all the primaries or turning off primary selection. |
Seeing |
Photometric reductions degrade when images are larger than around 5 arc seconds (4.3 pixels) FWHM. If the seeing goes above 4.0 pixels, consider doing "primaries-only" until it gets better. Quit if it gets above 5 pixels FWHM. Note that image size at the 20-inch is roughly 1 arc second larger than at the 3.5 m. This is because the correction optics that give this telescope its wide field of view (it's an ordinary Cassegrain otherwise) creates intrinsic images around 1 arc second FWHM. If APO site seeing is 0.8 arc seconds, then the best we can expect to see is 1.3 arc seconds FWHM. This image would be undersampled (pixels are 1.15 arc seconds across) and create problems during reduction. Fortunately (?), our telescope has a closed tube (that traps warm air currents), Pyrex mirrors (that take a long time to cool), and an un-air-conditioned dome (that thoroughly warms things during the day) so the best image quality is usually a much bigger 2.2 pixels or so, at least until things settle late in the night. You should be happy when image size is 2.2 to 2.7 pixels FWHM. You needn't do anything about undersampled images, but it would be nice to put a note in the log about it. |
Moon |
If the moon is well above the horizon and more than one day bigger than quarter, go to "primaries-only" mode and do the bright time programs. If the moon is quarter or less, limit secondary patches to those more than 30 degrees from it. |
Closure Rules |
Closing is the 20-inch observer's decision. DUST: Your primary worry should be dust, since dust on the mirrors changes the photometric properties of the instrument. You should close if the other telescopes close for dust. While the dust meter definitely measures dust, it seems also to trigger warnings on other things, or at least on lesser dust problems, so gauge your worry on a flashlight test. Dust can move in quickly so it's probably too late to prevent a dusting but you can at least avoid a pileup. Dust can happen in the daytime; keep this in mind if you're working on the telescope with the louvers open or the telescope uncovered. PRECIPITATION: You should close if rain or snow threatens (or has arrived). If you're in a hurry, closing only the upper shutter is sufficient. A little water won't hurt the telescope or instruments and even a drenching is unlikely to cause permanent damage (the computer monitors should probably be your biggest concern). So don't do anything stupid like running on an icy walk or slipping on a wet dome floor. HUMIDITY: Humidity is not a big worry at the 20-inch since a closed tube telescope dews over much later than an open telescope. If it's only humidity, you should ignore the 20-inch while you take care of the 2.5 m. Check the 20-inch primary with a flashlight when you do close, though. WIND: Wind is unlikely to be a problem because dust and/or seeing will shut you down before the wind beomes dangerous. APO's tower mounted anemometer is not a good indicator of wind at the 20-inch and tells us nothing about our main concern, which is wind from below lifting and letting fall the dome shutter dropout. Go to the dome or listen for loud noises on the intercom to understand the real wind situation. If you're unsure or just waiting for things to get better, close when you see consistent gusts above 45 mph or single gusts over 55 mph. |
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