Hi all,

today at 14:30 we had a meeting about the ADC, here in La Silla, with WEC, JAL, and AGI. I'm preparing the minutes.

However, a couple of points of the Fiber Head design were touched as well, so I'll comment on these right away.

1. Alignment of the ferule inside the hole.
   If you take a look at WEC drawing with the dimensions (the one with
   the wrong 0.130 mm hole), there is a horizontal line that bears two
   numbers, 2.35 and 2.50 mm. The first number is the external diameter
   of the ferule, the second is the internal diameter of the hole. The
   point is, the difference between the two is so small that it turns
   out as a single line in the drawing. WEC could make the line thicker,
   but in any case, the 0.15 mm space should allow the horizontal
   alignment of the rod lens to the sky hole (see also attached fig)

2. The problem of vignetting inside the sky hole tunnel. AGI
   reconsidered it, and realized that, although the situation was the
   same at the E152, the f/ratio there was f/11, while here we are at
   f/8, so the problem is more serious. AGI will come up with some
   numbers, but the idea is that there are two effects: 1) for seeing
   ca. 1.5 and above, indeed we are going to lose light. 2) for better
   seeings, the light loss might be not dramatic, but the marginal rays
   are going to graze on the tunnel walls and be reflected back into the
   fiber. So after some discussion we came up with this idea: use a
   'fresa' (don't know the English word, sorry), to remove part of the
   material below the sky hole. The smallest tool has a diameter of
   0.5mm, and that's perfect to the job we want to do. The idea is to
   operate it manually and with a loupe, until ca. half of the tunnel
   lenght is worked out. That looks like a good approximation to a
   conical surface. See the second attached figure for having some idea.

--- Ivo

Dear John,

In my view is not that the time issue has become critical now, I think
that now we are becoming aware about the multiple details involved in
the practical realization.

On the mechanical side the first work estimation was 5 days in 2D and 8
days in 3D. After some design work and interactions with AGI an myself
WEC realized that the design time is at least 24 days and he said that
he will need to machine the parts together with the person doing the job
in the machine itself. It is hard to review a design were we cannot even
clearly specify the machining of the parts.

On the optical side the first work estimation stated that the optical
design was so simple that it was almost nothing, just order the prisms
and mount them. This was 2 days maximum including the assembly of the
unit. After some design work AGI realized that it is necessary to use
curved prisms due to the in/out function, otherwise we will need to
re-focus the instrument every time the ADC is moved in/out. It turned
out that the in/out function is necessary because there is no space for
the calibration mirror and the ADC simultaneously. On the electronic
side this requires some hardware interlock to prevent both functions go
in at the same time. After AGI found that it is necessary, for complying
with the requirements, to use the optics of the ADC for correcting the
spherical aberration of the telescope and in turn we need a test night
for characterizing the optics of the telescope and quantifying the
actual telescope's aberrations. Finally it also appears to be necessary
some means of adjusting the ADC unit position to bring it in focus.
According to WEC there is almost no means to provide the unit with an
adjusting mechanism due to the small space available. According to AGI
the adjustment range should be "around" one millimeter...

Due to the above mentioned we are still far from a final ADC design, and
together with the fact that WEC is leaving for holidays between August
27 and October 6 and most likely AGI will be very busy with the new
fibre head I don't see realistic that we could finish with the PDR and
FDR at least until the end of October.