Coma investigation, telescope nudging, and variable clouds
Instrument: ARIES + MMTAO · Conditions: Variable clouds, intermittent AO
Overview
Night 4 was a challenging one. Variable clouds prevented stable adaptive optics for most of the night, forcing us to work on engineering tasks in the gaps. We used the time productively: we revisited the coma issue first flagged on Night 3, inspected the hardware for a physical cause, and made meaningful progress on coordinated telescope nudging — a capability needed for precise slit acquisition and AGPM coronagraphy with MIRAC.
Coma investigation
Initial hypothesis: ASM–ARIES misalignment
Coming into the night, our working theory was that the coma seen in the wavefront sensor signal originated from a misalignment of the wavefront sensor. If one system was significantly more misaligned than the other, that would point to a clear culprit and a straightforward fix.
Result: both systems contributing equally
When we measured the coma contribution from each system independently, both the ASM and ARIES showed a similar magnitude of error. Neither was the obvious offender — the coma was distributed across both systems.
Hardware inspection
We physically inspected the ASM and the top box to look for anything obviously wrong — a shifted element, a loose mount, anything that could account for a coma error of this magnitude. Nothing was found. There was no visible mechanical cause that explained the level of aberration we are seeing.
Coordinated telescope nudging with AO closed
The problem
When the AO loop is closed, you cannot simply nudge the telescope to move a star — the AO system sees the resulting tip/tilt as a wavefront error and immediately corrects it, moving the mirror to put the star back where it was on the WFS. The star stays pinned to the WFS reference, not to the slit or coronagraph mask. This is fine for AO correction but a problem when you need to precisely position a target on a spectrograph slit or on the AGPM coronagraph with MIRAC.
The solution: coordinated WFS motor and telescope moves
We worked out the combination of moves needed to shift the star on sky while keeping the AO loop closed and the WFS in a valid sensing state. The sequence works as follows:
- A coordinated nudge is applied to the WFS motors, shifting the WFS reference position.
- The AO system now sees an apparent tip/tilt error — the star is no longer centered on the new WFS reference.
- The AO system offloads this tip/tilt to the telescope, commanding the telescope to move.
- The telescope slews to re-center the star on the WFS, effectively moving the star on sky to the new position.
The result is a controlled, AO-stable nudge: the star moves on sky, the WFS remains happy, and the loop stays closed throughout. This is the correct approach for keeping a target precisely on a spectrograph slit during long integrations and for AGPM coronagraphy with MIRAC, where sub-diffraction-limit centering on the mask is required.
Scaling factors for nodding
We also performed a large move with the AO system off to establish the scaling factor between the commanded move and the resulting on-sky displacement. This gives us the starting point for nodding variables — the numbers needed to define a nod amplitude that corresponds to a known angular offset on sky.
Weather and on-sky time
Clouds were thick enough for several hours that no light was reaching the WFS, and we were closed for that period. When the clouds thinned toward the end of the night, we recovered a few hours of on-sky time. We were able to test the nudging scheme on a real star — with the AO system both on and off — and confirm that the coordinated move sequence behaved as expected.
Summary and follow-up
Despite the weather, Night 4 produced two concrete results: a clearer picture of the coma problem and a working recipe for closed-loop telescope nudging. The nudging capability is now understood well enough to be used operationally for slit work and AGPM acquisition.
Follow-up items
- Schedule daytime realignment of ASM and ARIES together to address the distributed coma
- Document the scaling factor obtained from the large open-loop move for use as nodding starting values
- Formalize the WFS motor + telescope nudge sequence into a repeatable procedure for slit acquisition and AGPM work
- Monitor whether the coma level changes after the realignment to confirm the fix