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2013_09_1920

Weather:

Very slight haze to begin the evening.  Otherwise clear, warm with lows around 60F,  fairly humid (60% at sunset – should get to 85% before sunrise).  Seeing is very good at around 1.7 arcsec, no wind.  This is likely the last clear night for a few days.

Update:  This night is truly great!  The bit of haze seen just after sunset has disappeared.  It is very clear, and the seeing is well above average, perhaps as good as 1.7 arcseconds.

Rig #1:  Nova Del 2013

Last night was basically a dry run.  I had all sorts of issues, some of the symptoms of which I’d see reported before.  One was that it appeared that the “slant” of absorption lines changed with wavelength.  The other was that when imaging the nova I got parallel spectra.  All turned out to be issues with focus.  How I managed to be that far out of focus (the collimator in the spectrometer) is a mystery.  But tonight I’m getting some good data.  I’m not sure what region of the spectrum is going to be of most interest.  I asked on the ARAS forum but as yet have no answer, so just to make things easy I’m shooting H-alpha tonight, using the 23-micron slit and 2400 l/mm grating.  I did see a message from Steve Shore suggesting 4500-5000 angstroms – I’ll go after that region next session.  Before that I need to identify a few weak Ne or Ar lines in that region for wavelength calibration.  A project for a cloudy night.  Also will get some lamp flats – hopefully tomorrow if the weather predictions hold.

Rig #2:  New CV in Cassiopeia: J004527.52+503213.8

This is pretty bright – shooting 25-second images.  The object is far enough north that I should be able to see a fair bit of image rotation throughout the night (due to polar axis mis-alignment).  It should allow a test of one of the theories I have about why there is occasionally such errors in the photometry – suspecting that the star matching algorithm and the offset and rotation matrix it outputs may be wrong, indicating a bug in the code.

Later – here’s the light curve from tonight’s data:

2013_09_1920Superhumps!  These large swings in brightness are thought to be due to the accretion disk being elliptical.  That can happen when the disk grows during a super outburst and the material in the outer parts of the disk reach a 3-to-1 resonance with the orbital period of the binary system.  The line of apsides for the elliptical disk (essentially the major axis of the ellipse) slowly precesses in a prograde direction – the same direction as the orbital motion of the binary itself.  The precessional period of the disk is usually some tens of days, whereas the binary’s orbital period might be a few hours.   But in the few hours from one binary orbit completion to the next the axis of the ellipse has precessed a bit and it takes a few minutes longer until the orbit of the two stars again lines up with the axis of the elliptical disk.  So what is observed is an increase and decrease in brightness whose period is a beat frequency of the orbital period and the precessional period.  For dynamical reasons well beyond the meager understanding of this author there is a surprisingly good relationship between the orbital period and the little beat cycle which leads directly to a measure of the mass ratio of the two stars in the system.  And that is why we measure it!

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