Pre-IRTF Optics Recap

In planning/executing tests to characterize our optical performance at the IRTF, I'm summarizing the steps taken in the runup to the observation using visible light with a bare mux as the imager...hopefully this will be useful for diagnostics in comparison with the IRTF data.

By November 5 we had the bare mux running in the instrument, and we did a focus run in lo-med mode (Vol. III, p. 5).  We selected 25T as the best focus for the echelle paraboloid.  Here is a comparison of image quality in med-lo mode, one using the SB375 bare mux and the other using the SB226:

 SB375 bare mux image obtained on November 5, 2010 with

our laser fiber @ cold stop and the echelle mirror set at 25 turns absolute.

Similar image obtained September 22 using the SB226

bare mux at an 

absolute echelle position of 23.5 turns.

The SB226 image looks sharper to my eye, but there are plenty of reasons these images should not be directly compared.  Probably most significant is the fact that we had done no bias/clock optimization on the S375, and we apparently used a much stronger laser in the SSB375 image, creating an image about four times as bright as that in the SB226 image. I think it's a little odd that the odd/even effect in the SB375 is really only evident in the brightest part of the inset column cut...not sure this is really significant given the guessed-at operating setup.

At some point (in November I think) we received the foreoptics from Corning.  We installed them on the instrument and performed an alignment.  Amazingly, there is no record of this process in our lab book that I can find. According to my e-mail log, we received the mirrors about 15 November 2010, and Matt came down to Ames on the 16th to test them.  The EXES computer has only one file that looks like it was taken with the foreoptics installed, and it's dated 18 November:

SB375 Bare mux image through foreoptics Nov. 18 (25T)

This image does not appear to be in great focus....the signal level is very high so keep blooming in mind as you compare the images.  The odd/even is pronounced in the coulumn-cut, moreso than in the first image above.  Note the diagonal rays that are not apparent in the first SB375 image above.


I'm frankly surprised that we don't have more in the can from the lab test of the foreoptics, but we were trying to get ready to ship and maybe trying to do too many things at once.


Meanwhile, here is an image taken in the IRTF prep room December 9 with the laser phisterous mounted at the cold stop (similar conditions to the first image in this post but the signal is bombing bright, with no apparent diagonals even at this flux:

SB375 bare mux image obtained on December 9, 2010 @IRTF with

our laser fiber @ cold stop and the echelle mirror set at 25 turns absolute.

Andhere is an image at LN2 in the IRTF prep room with laser light coming through the foreoptics:

SB375 bare mux image at 77K at the IRTF, lo-med mode laser fiber imaged through foreoptics

There are again diagonal (plus quasi-vertical) rays coming off the different mode-images of the laser fiber that were not evident in the pre-foreoptics images.  I don't remember ever seeing these rays without the foreoptics, but it's possible they have always been there at a low level and we happened to accentuate them with the settings we used for these particular tests.  More likely these rays are related to the foreoptics and/or their alignment??


We did some foreoptics tweaking of the foreoptics, with an example image shown here (much lower laser power so it's easier to see the spots formed by the laser modes:

SB375 bare mux image at 77K at the IRTF, lo-med mode laser

fiber at 0.05mW imaged through foreoptics

The spots are elongated in this image, though the diagonal rays don't show up (at least at this power level)