Sunday, May 1, 2016

First Light & on-sky observations with the LFC

After getting everything working in the HRS electronics room, it was time to route the fibre to the high stability bench & figure out how to inject the light into the instrument.  Thanks to Keith for getting the fibre routed safely, & then to Eben for making us a cool hinge for the bench cover!  The hinge makes it much safer for the fibres when we work on the bench :)
Éric trying out the new hinge on the high stability bench cover
Calculations to estimate the intensity of the comb light relative to typical astronomical sources were incredibly sobering, so we were sure to go completely overboard in terms of attenuating the beam emerging from the delivery fibre.  A series of 5 Fabry-Perot mirrors, each only transmitting 2% of the light incident on them, were stacked in front of an iris that we also stopped down as much as possible.  The comb light was visibly vastly fainter than the ThAr lamp, but we were taking no chances with the precious HRS CCDs!
Our ultra-conservative approach to injecting comb light into HRS
Clicking the expose button for a 1s frame was nerve-wracking none the less & we were delighted to see what looked like bias frames for the first several attempts.  We would step up the exposure times until 120s, then remove one of the F-P mirrors & start back at 1s.  After removing a couple of the mirrors, a 120s exposure eventually yielded a small patch of comb lines in one of the reddest orders!!  More perfect & beautiful than we could've anticipated :D  & hugely impressive to reach this milestone just 5 days after Derryck & Richard arrived in the country & dashed straight up to Sutherland from the airport!
First LFC lines on the HRS red detector!
With that, it was declared celebration time...  Nothing less than a stealthily imported traditional Scottish beverage: Irn-Bru - a carbonated soft drink that's also known as "Scotland's other national drink".  The colour & sugar content are both quite startling, but it's neat that all of us got to try it!
Celebrating first-light with a traditional Scottish beverage - but not the one you might expect!
Soon it was back to the business of steadily increasing the exposure time & removing the attenuating mirrors, one by one, to get well-exposed comb lines to fill the entire red channel.  This eventually required all of the mirrors to be removed, since we had deliberately been very inefficient in coupling the comb light into the fibre.  & also in conveying that light into the high stability mode's calibration fibre.  It was possible to reach an intensity level that would easily allow 10 minute exposures without any risk of saturating the comb lines.
After more tweaking & a longer exposure - we got comb lines Everywhere :)
By this point Rudi - who's a planet hunter by trade, & who's suffered mightily in pursuit of good ThAr wavelength solutions - was *absolutely* beside himself about the incredible possibilities that this calibration device presents.
An exceeeedingly happy Dr Kuhn!
An extraction of the échelle order that contains our navigational beacon & new favourite spectral feature, the 780.246 nm rubidium line (the tall, sharp spike), along with all the other comb lines in a small section of the red channel image.
A dense thicket of comb lines, + the reassuring rubidium spike at 780.246 nm
Rudi's code does a fine job of identifying all of the comb peaks in a given order.
The dashed lines show all the peaks that were successfully identified by Rudi's software
The next image provides a compelling demonstration of why the comb is SO much better than a ThAr lamp.  The individual bright lines from the ThAr lamp are irregularly spaced & often quite sparsely distributed, certain bright lines saturate badly even in the shortest exposures possible, some lines are faint, others are blended - none of which is conducive to good wavelength calibration.  The comb on the other hand provides about 50x as many lines, without any blends or saturation, their frequency spacing is fixed & traceable, & each one can be referenced to the 780.246 nm rubidium line simply by counting the number of comb teeth & multiplying that number by the fixed repetition frequency that sets the comb spacing.
Comparison between comb light down the calibration fibre (upper track) & calsys ThAr light down the star fibre (lower track)
Seeing this sort of data coming in was incredibly exciting & gave everyone plenty to think & talk about.  Soon it was clear that late submissions to major upcoming laser physics (CLEO) & astronomical instrumentation (SPIE) conferences would be in order! 
Much new information to process
Then it was time to prevail upon Encarni & Veronica - the SA/SO pair on duty - to point to a standard star so that we could obtain a stellar spectrum with the simultaneous injection of light from the laser comb!
Time to persuade Encarni & Veronica to let us observe a star with the comb
Here's the SALTICAM acquisition image - the spectrophotometric standard is the bright star near the centre of the frame, just to the left of the chip gap.  The pink outline represents the position of the HRS guide probe, which was used to ensure that the star remained well positioned on the HRS high stability mode's object fibre throughout the 10 minute integration.
SALTICAM acquisition image of our first star to be observed with the comb
While waiting for the first star + comb readout, our visiting scientists were each required to don The Astronomer's Hat to lend credibility to the proceedings...
Dr's McCracken (left) & Reid (right) - geared up for science!

A couple of minutes of engineering time very well spent!  Our first stellar spectrum + simultaneous astrocomb calibration :D  Note the prominent H-alpha absorption feature (dark vertical band) near the centre of the image.
Simultaneous star + comb observation of a specphot standard star LTT7379
That excitement got us through to well after 2am on Tuesday morning - a long but fantastic Monday!  Since Nic & Rudi would be heading back to Cape Town at 10am on the Wednesday, we took a break on Tuesday night & went down to Sutherland for a lovely dinner at Cluster d' Hote.
A well-deserved dinner out in Sutherland to celebrate a brilliant week!
Having trained the 4 of us (Éric, Rudi, Nic & Lisa) to run the comb, Richard & Derryck returned to Edinburgh on Saturday night.  The comb will remain at SALT until mid July, so during these next couple of months we'll aim to extract as much information about HRS & do as much science with the system as possible.  We have 20 hours of engineering time set aside for on-sky observations, but an essentially unlimited amount of off-sky time with which to monitor & characterise the HRS.  This is a truly amazing opportunity & hopefully the start of a long & productive collaboration with the Heriot-Watt team!


  1. Super story, great pictures and am I glad I can just skip lightly over the detail!

  2. Where is the comb going to after mid-July?
    And thanks for a really great 'view' of science / technolgy in action.
    Maciej Sotynski

    1. The comb will return home to Heriot-Watt University in Edinburgh at the end of its "field trial" at SALT. We can already anticipate a sad farewell when we have to relinquish our new favourite toy!