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2013_07_1819

Testing the “power” of LHiRes guider optics

Two nights ago I took spectra of each of the three WR stars (Wr134, 135, & 137) at different locations along the slit as seen in the guide camera.  Each star was consistently imaged at a particular location along the slit:

  • wr134 at x=120
  • wr135 at x=160
  • wr137 at x=200

I was trying to see if location along the slit made any difference to the “shape” of the profile of spectral traces.  That is still under review, but on thing I could determine without any ambiguity is the “power” of the guider optics.  The shifts, above, resulted in the spectral traces falling along different rows in the imaging guider as follows:

  • wr134 at y=184
  • wr135 at y=217
  • wr137 at y=250

Basically a shift of 40 pixels along the slit as imaged in the guider results in 33 pixels of shift in Y in the imaging camera.  The imaging camera’s pixels are binned 1×2 – so in y = are 18 microns.  The guide camera is being used in 2×2 binning mode, so each binned pixel is (7.4*2) = 14.8 microns.  The ratio of physical pixel size is 18/14.8 = 1.22 .  Also note that 40/33 = 1.22.

Said a different way; 40 pixels of shift along the slit as viewed in the guide camera = 40*14.8 = 592 microns, resulting in a shift of 33 * 18 = 594 microns in Y on the imaging chip.  So the relative power looks to be 1.0.

 Thoughts on the profile asymmetry

Last night one of the spectra of of Beta Aquarii was focused using the ghost image which appears below the actual stellar image in the guide camera.  Basically I focused to get that ghost as sharp as possible.  The resulting spectrum was a bit of a mess, as might be expected.  Here’s a trace in Y across that spectrum:

 

Vertical trace through an out-of-focus spectrum of Beta Aquarii

Vertical trace through an out-of-focus spectrum of Beta Aquarii

This is similar to what I’d expect of the telescope was badly out of collimation.  So one thing to try might be to rack the focus to either side of best focus to see if the shape inverts itself.  Since the slit is oriented essentially E-W, if the problem is collimation then I should be able to gently tweak the secondary collimation screw that is parallel to that axis (if there is one) to see if it improves.  Depending on how clear it is tonight I may just try both of these experiments.