Making a lightcurve

I don't actually care that much about the lightcurve of GX5-1 -- it has no real bearing on this particular data analysis.  However, a lightcurve that shows the number of counts per second coming from the whole field and from my source is almost always the first data product I extract.  The reason is that the lightcurve can show all sorts of bad things that you'd hate to discover late in the game.  There are perfectly good instructions in the Chandra thread, so I shan't repeat them here. 
The lightcurve shows one major, and entirely expected, problem.  The count rate in the full field is practically at the telemetry limit - 184 cts/s for the HRC-I.  This is the fastest rate that Chandra can accumulate data with the HRC-I.  Data that comes in at higher rates are simply dropped on the floor and lost.  An observed rate as high as 170 cts/s doesn't mean we've just skated under the limit -- for a number of reasons, hitting the exact limiting rate is unlikely.  Much more likely is that we've lost a bunch of data and there is no way to get it back.  This isn't a disaster -- at 130 cts/s from the source, and almost 5000 seconds of data, I'm swimming in photons (1 count = 1 X-ray photon), but it does mean I need to keep careful track of just how many fewer photons I've got than I would otherwise expect.  This is called the detector 'deadtime', and in the case of the HRC-I is stored in a file called hrc****_dtf1.fits.  Right now it looks like the average deadtime is about 30.5%, based on the data in this file.  But we'll check that using WebPIMMS, a handy tool that can calculate the expected count rate in almost any X-ray satellite & detector, given an input source model.  I can use the values I got in a previous post, or values from my earlier paper, and get a predicted source count rate.  In this case, I used NH = 2.8e22 cm^-2, a bremsstrahlung model with kT = 10.5 keV and FX(0.3-10) = 2.8e-8 ergs/cm^2/s.  The WebPIMMS result is a prediction of 220 cts/s from GX5-1 alone, not including the rest of the HRC-I.  The lightcurve above, however, shows about 130 cts/s from a 2' radius around GX5-1, and about 40 cts/s from the rest of the HRC-I.  The Chandra Proposer's Guide Document for the HRC lists the HRC-I quiescent background as 1.7e-5 cts/s/arcsec^2.  For a 30x30 arcmin detector, this amounts to 55 cts/s when everything is just 'ticking over'.  So pretty clearly there are some 'issues' here.  On the plus side, adding 220 + 55 = 275 cts/s, and then comparing to the telemetry limit of 184 cts/s suggests that the deadtime should be (approximately) 1 - 184/ 275 = 33%, which compares quite well with the 30.5% number in the dtf file.  Now I just need to keep this in mind as I progress - basically, the source flux and the background have been suppressed by 30-33% due to photons that were 'lost in space'.