Home of MagAO and MagAO-X.
Before we tell you our big results for the night… let’s look at some happenings up and down the mountain!
Here are Enrico and Alfio waiting for a ride up to the telescope:
Armando departed on Saturday, and now Derek and Marco have also gone down the mountain.
Derek saw a horse on his way down the mountain. We’ve seen them before on this blog!
Coming up … REAL on-sky results!!!
We passed a big milestone today with the ASM working in closed loop with 400 modes at 1 kHz (the most complex AO mode)! This 400 mode interaction matrix has been made possible by the excellent trouble shooting from our friends at Arcetri Observatory, Simone, Enrico, Alfio, Armando and Marco!
It was such an exciting event that Alan Uomoto made a movie:
How did this happen? Well, yesterday the AO loop was struggling to close on the bumps we were referring to as Viscachas:
When the loop tried to close on this, we would get a higher and higher unstable patch of actuators trying to correct it:
So Simone and Enrico figured out that we were actually getting cross-talk from the Pyramid, because the phase bump was so high. This is similar to a quad-cell Shack-Hartmann without a guard band, where a subpupil may wander into an adjacent subaperture. Here is Simone’s drawing where he works out the solution:
So the solution is kinda a hack, whereby we applied a negative sign to the interaction matrix for that patch — and the bump and the viscacha disappeared!
And so tonight we were able to close the loop with our new interaction matrix, and get a nice flat wavefront!
New arrivals today: T.J. Rodigas (Steward) and Runa Briguglio (Arcetri).
Here’s a riddle for all you AO fans out there:
What aberration can be sensed but not corrected?
Usually an AO system has the opposite problem: There are aberrations you can correct but not measure. And of course, there are all sorts of aberrations you can neither measure nor correct, like the very highest spatial frequencies. (And if you don’t have enough stroke, you can saturate your attempt to correct an aberration — but that wasn’t the case today.)
But today while we were making our interaction matrices, we found we had 3 high spatial frequency dots in our pupil images (a couple times the size of an actuator) and we spent some time trying to track down whether they were phase or amplitude, and whether it was a dirty optic, scattered light, or misalignment.
Jared had to power-cycle the CRO controller, and so he and Armando went up on the scissor lift to check if the CRO was in the right position and to inspect the optics.
When we closed the loop, these “dots” could not be corrected. We tried shifting some optics to see if we could move the spots, and we concluded they weren’t an amplitude error (too bright). Finally, we tried setting the ASM back to an older flat shape — and the dots disappeared! This means that these phase errors were somehow introduced to the system at the time we were calibrating the interaction matrices. So then when we closed the loop, we were driving toward that shape — not toward a true flat.
So the answer to the riddle is, the shape you can measure but not correct is an error in your null!
Now that we’ve solved the riddle, tomorrow we’ll re-do our interaction matrices with a flatter null.
Yesterday:
Simone: “Hey, I think we should try taking the pyramid out.”
Laird: “No.”
Armando: “No.”
Phil: “No.”
Jared: “No.”
Katie: “What? Is he serious???”
Today:
Laird: “Hey, maybe we should try taking the pyramid out.”
Simone: “No. The last thing we want to do is take the pyramid out!”
Today we are going to explore the MagAO pupils and their corresponding transforms in the image plane, courtesy of Fourier optics.
So let’s have a look at the pupil. Here is a photo of the ASM, taken with a digital camera. This was from before Clio was mounted, so that we just stood on the Nasmyth platform and put the digital camera where Clio is now. The light source is the sky, and the light path is primary + secondary + tertiary.
The main features of the pupil are the outer diameter of the mirror, the inner diameter of the secondary obscuration, the support spiders holding up the secondary, and the slot. Here, then, is the pupil mask:
Since we know what the pupil looks like, we can create simulated images of the focal plane by taking the Fourier transform modulus squared:
If we really stretch the color table, you can see the diffraction off the spiders, but it is not a big effect. Also, I couldn’t find the diffraction off the slot, so it is negligible:
Now, Clio is an infrared camera, going out to 5 um, and so it has its own pupil mask, a cold stop. So let’s look at the pupil through Clio, by taking a pupil image (which we did after Clio was mounted). Here is an image of the pupil plane through the whole system, taken with Clio by putting in a powered lens to the focal plane to make a pupil image:
Clio pupil image, 3.4 um
It’s pretty cool because you can see the 2 spiders holding up the secondary obscuration on the cold stop, but you can also see the 4 telescope spiders and the ASM slot! Here’s just the Clio cold stop pupil mask:
And here is its Fourier-transform-modulus-squared: The simulated PSF:
And here is the zoomed-out, saturated version so that you can better see the diffraction spikes:
Summary:
The two simulated PSFs look very similar, and diffraction off the spiders and slot has a very minimal effect compared to the Airy rings.
(And — despite how it looks from the outlets — we really are in Chile! The observatory is highly USA-compatible. I haven’t even used my plug adaptor yet!)
Update: And here is the MagAO-certified power protector I made to keep people from stepping on the plugs and cords above:
