Category: General

The SAO phasing prototype visits MagAO

“Without phasing, there’s no real reason to build the GMT.”
-Andrew Szentgyorgyi

The biggest optical/infrared telescope in world will be the Giant Magellan Telescope, which will be built on a nearby mountain peak within sight of the Clay and Baade telescopes at Las Campanas.  The telescope will have 7 primary mirror segments and 7 adaptive secondary mirrors, similar to the Magellan AO system.

The 25 meter diameter Giant Magellan Telescope
The 25.5 meter diameter Giant Magellan Telescope
Photograph of the GMT site from the Magellan footpath.
Photograph of the GMT site from the Magellan footpath.

If we could build any optic we wanted for the primary of the GMT, we would probably build a monolithic 30 meter diameter (or larger) mirror made of a single piece of glass, with a thin face sheet and a honeycomb lightweight structure on back.  However, at the moment, the largest mirrors in the world are built in the Steward Observatory Mirror lab under the bleachers of the football stadium at the University of Arizona and are limited to a diameter of 8.4 meters.  Depending on who you ask, this 8.4 meter limit comes from either the distance between the columns underneath the stadium bleachers, or the size of an underpass on the highway leading from Tucson.

An 8.4 meter mirror being polished in the Steward Observatory mirror lab underneath the football stadium bleachers. Making mirrors larger than this will require a larger football stadium.
An 8.4 meter mirror being polished in the Steward Observatory mirror lab underneath the football stadium bleachers. Making mirrors larger than this will require a larger football stadium.

Because of this limit, the GMT is designed to take 7 of the largest mirrors that can be made and combine them to form one giant 25.5 meter primary.  For this to be possible, the seven 8.4 meter segments must be “phased” to a fraction of a wavelength.  That is to say, they must be aligned to each other so that they act as if they are one large continuous mirror.

To achieve the phasing of the GMT segments using off-axis natural guide stars, SAO and our collaborators at GMTO and Flat Wavefronts have designed a sensor that creates dispersed interference fringes using subapertures spanning the 12 segment boundaries.  Phase shifts across the segment boundaries manifest themselves as tilts in the fringes.

Segment boundary subapertures for the dispersed fringe phasing sensor.
Segment boundary subapertures for the dispersed fringe phasing sensor.
Simulated fringes from one subaperture showing 0 piston phase difference (left) and 10 microns (right).
Simulated fringes from one subaperture showing 0 piston phase difference (left) and 10 microns (right).

To test this sensor technology, SAO has built a phasing prototype that simulates 6 of the GMT segment boundaries working in conjunction with the Magellan AO system.  Our three nights at the end of the MagAO run turned out to be a success.

Six sets of fringes as seen by the SAO phasing prototype working in conjunction with the MagAO system.
Six sets of fringes as seen by the SAO phasing prototype working in conjunction with the MagAO system.
The SAO phasing prototype team. Clockwise from top left: Derek Kopon, Alan Conder, Ken McCracken, Jared Males, Laird Close, Dan Catropa, Brian McLeod, Bill Podgorski.
The SAO phasing prototype team. Clockwise from top left: Derek Kopon, Alan Conder, Ken McCracken, Jared Males, Laird Close, Dan Catropa, Brian McLeod, Bill Podgorski.

We obtained phasing data both on-axis and off-axis, with AO on and off, and at two different wavelength bands (I and J).  This data, and data that we gather during another run possibly in February, will inform the design of the GMT phasing sensor, scheduled for first light in the next decade.

Lastly, a “song of the run:”  Phazing, by Dirty South:

https://www.youtube.com/watch?v=031hzipvnTY

GMT Groundbreaking

On November 11th, I had the great pleasure of attending the Groundbreaking Ceremony for the GMT. It was a very windy, cold, but happy occasion. I’ll be posting a more complete update in the next 24 hours, including hopefully some of the photos that I was able to take during the celebrations.  The event was well attended, including wise and often humorous remarks by the ambassadors to Chile from Australia, South Korea, and the United States. The President of the Republic of Chile, Michelle Bachelet, was the keynote speaker.  Everyone working to build GMTO (the GMTO staff, the many people from all of the partners involved, including the talented women and men of Steward Observatory and the Richard F. Caris Mirror Lab) should be very pleased to see the project reach this milestone.

Fortunately for the MAGAO team, the more than 150 people that were visiting the mountain yesterday have now left, leaving hopefully enough  room for you to work and sleep during your upcoming long observing block!

New Results In The Chamaeleon

Now that the MagAO team has (mostly) recovered from our epic 6 week stay at LCO, we are turning our attention to processing all of the great data we’ve been taking. We’re also happy to announce two new publications based on MagAO data which have been accepted to the Astrophysical Journal, both of which looked at stars in the constellation Chamaeleon.

Ya-Lin Wu has been studying the young star CT Cha with VisAO. You can read about his results here.

Steph Sallum used Clio2, in combination with some other instruments, and used a resolution-boosting technique called non-redundant masking to take a look at T Cha. You can find out about her results here.

We have a bunch of other papers in the works, and we’re already starting to plan for our next run, which starts May 3rd. Stay tuned!

Comm2 Day 17: Calibrating Clio while clouded out

We were clouded out tonight:

Tonight
Tonight
A few nights ago, for comparison
A few nights ago, for comparison

Also, a truck on the highway had an accident, which closed the highway, so the new turno couldn’t get here so the day crew had to also be the TO’s at night. We’ve been away from home for ~3 weeks and everyone is tired, but we stayed up all night in the control room working.

So, instead, I’ve been working on reducing my data, which includes determining various necessary calibrations for Clio. One important correction needed for high-contrast photometry is calibrating the linearity of the detector. To do this calibration, I took a set of measurements of increasing integration time, and determined the counts per itime:

Raw data that are not linear.
Raw data that are not linear.

I then fit various functional forms to the data until I found the best calibration of the linearity is a third-order polynomial that must be applied to pixels with counts above ~27,000 DN in the raw images, giving the result here:

The result of applying the linearity correction to the raw data.
The result of applying the linearity correction to the raw data. The linearity calibration must be applied to pixels with values above ~27,000 DN, and is not valid for pixels with values above ~45,000 DN for high-contrast photometry (there is margin up to ~52,000 DN for low-dynamic-range photometry).

Note that the data cannot be well-corrected above ~45,000–52,000 DN (depending on your tolerance for photometric error), and these values should be considered saturated in the raw images. I apply the calibration through an IDL function I wrote called “linearize_clio2.pro”. This is going onto the Clio observer’s manual.

Jared and I are working on astrometry, comparing Clio and VisAO measurements and exploiting our capabilities for boot-strapping: Clio has a wider field (up to ~30”) and can get a longer lever-arm on astrometric measurements, but VisAO has a finer pixel scale (~8mas) and a tighter PSF and can get precision astrometry on close companions. We are starting by verifying that we both get the same measurements for the locations of stars in the Trapezium cluster. To do this, I look at our Trapezium images and identify which star is which, then I compare the positions I measure to the positions measured by an earlier author. Here’s an illustration:

Measuring astrometry using Trapezium stars. (Left) Trapezium as imaged at the LBT/PISCES in the IR a year or so ago by Laird. (Right) Trapezium as imaged with MagAO/Clio here at LCO by me.
Measuring astrometry using Trapezium stars. (Left) Trapezium as imaged at the LBT/PISCES in the IR a year or so ago. (Right) Trapezium as imaged with MagAO/Clio here at LCO. Can you identify the stars in the picture on the right? (Ignore the black splotches which are negative star images, from the sky subtraction.)

I’ve measured the plate scale in both cameras and various filters, as well as the rotation offset to orient the images with north-up, which I’ve written in the IDL function “derot_clio.pro”. We’re working on a code repository for these data-reduction utilities that we’re calibrating on these commissioning runs.

In fact, Clio has two cameras, a wide and a narrow camera. Here is a comparison of the fields of view, including an illustration of the overlap:

The narrow field is shown within the wide field.
The narrow field (16” x 8”) is shown within the wide field (28” x 14”).

VisAO’s field of view is similar to Clio’s narrow camera along the short direction (8” by 8”).

Finally, I’ve been having a bit of fun experimenting with the APP coronagraphs that I want to use for following up GPI planets:

Fun with APP
Fun with APP

Well, it’s been a long night, good morning!