Priorities for SPARO observations/tests and analysis as of July 7, 2000. Updated July 12th to reflect: new suggestions for checks relating to bad reproduceability, revised plan for 20 km/s calibration, and third set of Galactic center fields. Updated July 19th and 20th to reflect: --> added a description of second Moon tests (1C) and new Sgr B2 tests (1F) that we've discussed. --> added a description of how much more we need on fields gc2 and gc3 (see item 2) --> added items for after gc3 is done (see 3A and 3C) ______________________________________________________________ Priority 1: Checks, tests, calibrations: ______________________________________________________________ (1A) grid tests - DONE (1B) Repeat polarimetry of 6334-I - Done, but... ...4 more hours at tau = 1 (or equivalent) is desireable just to be sure that the April 18th anomalous NGC 6334 result is really wrong. Pixel 5 pointed at the peak. Pointing checks periodically, as usual. For NGC 6334, each increase of 0.25 in tau doubles the required integration time. (1C) Polarimetry of the Moon - DONE (1D) Polarimetry of 20 km/s cloud - perhaps the best test for instrumental polarization of central pixel. OK, we can do this without changing the chop. Here's how it works: We'll chop in 'software' by differencing neighboring pixels. So you can leave the chop at 0.5 degrees for these observations. When we go to CSO we'll chop at a 22 degree angle from the horizontal (approx. perpendicular to the Galactic plane), and we'll chop by 3.5 arcminutes. Well process the SPARO data by differencing pixel 5 from the average of pixels 4 and 6. There will be additional errors involved, so I've increased the integration time from what we'd need if we were chopping only 0.1 degrees. Also, this measurement will be somewhat sensitive to the instrumental polarization of pixels 4 and 6, but still it will be mostly sensitive to pixel 5, as the flux from the 20 km/s cloud is quite a bit bigger than from the neighboring positions, I'm pretty sure. A factor of two I hope - Sharon can tell us as she knows how to do photometry on polarimetry files. All we really care about for this 20 km/s calibration is that the center pixel be pointed at the 20 km/s cloud. The best way to do this is to use Sgr B2 as a reference pointing position. So we just need a single polarimetry pointing for the 20 km/s cloud. For purposes of calibration, we will only use the results for pixels 4, 5, and 6. At CSO in mid-July, we will map about 200 pixels and average them together to compare with what SPARO gets in that central pixel. That will give us an independent check on instrumental polarization of that central pixel (independent of Moon). We will use a 3.5 arcmin chop at the CSO, and do the SPARO chop by differencing pixels. I think while we are at it we should do polarimery of Sgr B2 just as a check that nothing is changing. So the routine is similar to the Sgr A science observations - only just two fields, not four, this time. I.e. we do pointing on Sgr B2 about 4 times/day (3 by 3 scans are best, I'd guess), and the polarimetry of the 20 km/s position and of the peak of Sgr B2. Just two polarimetry fields. It would be good to spend about 4 times more time on the 20 km/s polarimetry as on the Sgr B2 polarimetry, as Sgr B2 is way brighter, and way less important. So, keep the same chop of 0.5 degrees throughout these 20 km/s & Sgr B2 observations. See "Plan for Winter 2000: science", under "Plans" for coordinates of 20 km/s cloud (and link to Sgr B2 coords also are found there too). This is the most time-consuming of the calibration observations. I think we need the equivalent of 18-36 hours of tau=1.0, which is like 1.5-3.0 days of tau=1.2, or 3.0-6.0 days of tau=1.4. The goal is a 0.1-0.2% calibration, but even a 0.3-0.5% measurement would have significant value, as a Moon-independent sanity check on instrumental polarization. So even if we can only get a fraction of the integration time we want, the test still has value. (1E) A check on whether we might have been pointed at the "negative reference spots" during our recent GC polarimetry and/or photometry. This is unlikely, and its OK if we did this error. All it means is we wasted half of our observing time. But if this is what we were doing we need to know about it so that we can do the instrumental polarization calibration properly. Here's how to test if we were mis-pointed in this way. Next time you set out to take a scripted set of observations on GC fields, increase the size of the photometry maps of Sgr B2 from 3x3 to a map that is wide enough to catch both of the refence beams. maybe 9 by 3, or 11 by 3. This will waste some observing time but will provide this good sanity check if you do this for one remote observing session. OK, so that should work for photometry, but what about polarimetry? How can we be sure that the polarimetry is done at the same pointing as the photometry? Maybe a bug in your script or some weird difference between Bob's polarimetry and photometry code could mess this up? Well, its easy to check...start your usual scripted observations, with the extra-wide photometry map, but hang around long enough to watch the photometry execute and, when the script tells you that you are observing the central position of the map, write down the left and right beam offsets as they appear on the comsoft screen. Also write down the Mac file-number during that file - the central file of the map. Then, when the polarimetry of Sgr B2 starts, do the same: write down the left-beam and right-beam postions as they They should match. Record everything in the logbook. (1F) Cross calibration of i.p. using Sgr B2 - measures i.p. of outer pixels relative to center pixel The idea is to successively put each pixel on the peak of Sgr B2 and do polarimetry. some points to keep in mind when writing the script... ...cycle through all the pixles rather than staying on one pixel for a long time. That way the "i.p.-difference" will be less vulnerable to any potential changes in i.p. due to weather/snow/etc. ...spend about 3 times as much time on pixel 5 as on any other pixel. (that's best way to reduce overall errors in the "i.p.-difference" between an outer pixel and pixel 5.) ...we don't need to do pixel 3. it is not used for polarimetry. We don't need to do pixels 2 and 7, as the i.p. for these pixels has been pretty well determined by Moon tests. That leaves 1, 4, 6, 8, 9 and of course 5. ...We need the equivalent of about 1 hour at tau=1 for each pixel. (3 hours for pixel 5). Thats a total of 8 hours at tau = 1. For GC, each increase of 0.2 in tau increases the required integration time by a factor of two. ...The occasional 3 x 3 pointing check (as usual) is always a good thing, as needed, to keep us reaonably well pointed at peak. ...Pointing is more impt. for this test than for science observations. But, even if we are off by an arcminute, the tests will still be quite valuable and will give us most of what we need. ______________________________________________________________ Priority 2: Getting more vectors on gc2 and gc3: ______________________________________________________________ We have finished gc1. As of July 19th we still want the equilvalent of 24 additional hours at tau=1 on each of gc2 and gc3. Field designations refer to Greg's tiling that is posted at "Plan for Winter 2000: science", under "Plans". gc2: (-1/2, 0) (-1/2, 1) (0,2) (0,4) gc3: (1/2, 0) (1/2, 1) (0,3) (0,4) (-1/2, 2) (0,5) Note that each increase of 0.2 in the tau makes the needed integration time double, for declination of GC. ______________________________________________________________ Priority 3: Beyond gc3... ______________________________________________________________ (3A) Here is gc4 gc4: (0,-3), (0,-2), (0,6), (0,4) 48 hours at tau=1 should be enough for gc4 (3B) If the tau is above 1.6 or so, it is best to extend the polarization map of 6334, rather than integrate on the GC. The penalty for higher-tau is less for NGC 6334 than it is for GC. I aoplogize that we have not worked out pointing postions for 6334, but if you look at the IRAS map you can see there is a whole ridge of emission trailing off from the peak we have been mapping down towards the SouthWest. Just follow the ridge. Ten hours at tau = 1.6 should secure some detections at any of these new positions. Another valuable thing to do is just to make a photometric map of the whole 6334 ridge. Look at the IRAS map on the web page and make a judgement as far as the size of map needed. It may be better to make the map first as a way to pick pointing positions. So, bottom line: if 1.6 < tau < 2.0, do 6334. Of course use 0.5 degree chop for photometry/polarimetry of 6334. It will also be good to repeat the polarimetry of that main field. It does not take long to get a good result as that main field is very bright. We don't need much more than the equivalent of 12 hours of data at tau of 1.3 for this "main" NGC 6334 position, called NGC 6334-I. (3C) After gc4 (if we get that far) we want to try to increase the chop and see how bad the microphonics are. This test will tell us what we need to do next summer, and if we manage to increase the chop successfully this winter, we will take a peek at some off-plane positions ______________________________________________________________