Once the M1 model was fixed, we still had
a somewhat intermediary situation, where
elongation was present, but to a lesser extent. After a few simple tests, we determined
that the LGSWFS system was the problem,
since running GeMS with only the Deformable Mirror (DM) conjugated to 0 km (DM0),
used essentially in a ground layer adaptive
optics mode, showed no signs of the elongation seen while running in a full MCAO
mode with DM at an altitude of 9 km (DM9;
Figure 2).
This indicated that we had something odd
coming from the LGSWFS that was being
offloaded to the DM when conjugated to an
altitude of 9 km (DM9). During two further
runs in 2014 (May and June), we acquired
some science data using a software patch
that removed a semi-static shape on the
DM9. As several optical phenomena could
cause this issue, the AO team decided to
shutdown the GeMS system in concert
with the general telescope shutdown and
proceed with a more thorough investigation inside Canopus — the AO bench that
is the heart of GeMS.
To maximize the effectiveness of this shutdown, we approached previous GeMS AO
team members François Rigaut, Benoit
Neichel, and Marcos van Dam, all of whom
agreed to assist the current AO team in diagnosing and correcting the issue.
Figure 1:
Example of the
elongation issue present
on GeMS/GSAOI images.
Figure 2:
GeMS/GSAOI image
using a groundlayer adaptive optics
reconstructor.
12
In early 2014, during the first run of Semester 2014A, our team discovered that stars imaged with the Gemini South Adaptive Optics
Imager (GSAOI) were elongated especially at
the edges of the field, yielding poor performance (up to 250 milliarcseconds). One of
the issues uncovered was an incorrect procedure used for saving Zernike coefficients
(that control the figure of the Gemini South
8.1-meter primary mirror (M1)). Figure 1 illustrates the image elongation issue.
GeminiFocus
Finding the Source, and a
Solution, for the Elongation
During August 2014, we removed Canopus
from the telescope and installed it in the
instrument laboratory on Cerro Pachón. After several tests, we found that its five field
stops were not properly aligned with the
rest of the optical train, thus vignetting the
beam. The vignetting pattern was different
for each of the five WFS, which explained the
unusual semi-static pattern on DM9.
April 2015