Subject: APO echelle (aka CARCES Chicago ARC Echelle Spectrograph, but probably

From: Don York

Submitted: Mon, 11 Jan 1999 17:31:10 -0600 (CST)

Message number: 331 (previous: 330, next: 332 up: Index)

       not for long.)

We have an echelle spectrograph on the telescope that works.

We have observed 3.5 nights with it (Dec. 14, 1998; Jan. 6,7,8(second 1/2).
The instrument is operationally stable. It has now been cold on the telescope
for about 20 nights with no problems. APO staff routinely fill the dewars
and have had no problems other than a sticky valve, which all know how to un
stick now. 

We have obtained data on drift in cross dispersion and in wavelength (echelle
dispersion) over long sequences with systematic moves of the telescope;
on repeatabiltiy night to night (same stars, different nights);on all aspects
of spectrograph thruput (except of course the critical three mirrors of the
telescope) including slit function (amount of light actually through the slit
for given seeing; on instrument operability in the coldest conditions; on dark
current; on stray light signals; and on stars used for various types of
science (interstellar lines, halo star abundances, stellar line profiles,
nebular emission lines) that is already done and published (this, to look
for subtle errors). We have not yet done a stable series for radial velocity
stability (end to end, with reductions) or on broad line/continuum separation
at high signal to noise. These data were obtained in essentially three 
instrument setups, in the subtle sense that various foci were changed,
widgets twiddled etc. so the detailed state was changed three times, the
longest stable run being 48 hours.

These data are in various stages of reduction and I shall not here quote
values for the various parameters derivable from above. Everything looks
very good. Drifts of 1/2 pixel have been observed over many hours. The design
goal is 0.1 pixel in four hours. Whether the difference is due to unmonitored
observing conditions or represents an instrument limitation will only be
known after another month of observing.

To the eye, the spectra are beautiful. The full format is well included with
in the big CCD. The echelle orders are clearly and cleanly separated from
each other (2 pix, FWHM). The resolution (emission lines) is 2.2 to 2.5 pixels,
and we expect the order FWHM to resolution (Nyquist) to be 3/2. We may have
an aberration that is forcing this ratio and Shu-i is working on this. We
know the optics delivers the proper ratios (measured with the 800x800 15 micron
chip in 1989), but that was over a small field. Clearly, much more work has
to be done on this.

The spectra can be reduced, to first order (all we have checked) with standard
IRAF packages without evident failure. We have not carried the reductions
through to completion to learn of subtle effects in photometry. All of that
is underway.

The echelle software works without failure (or at least, only with understood
failures). The remote software works as well. The system has been completely
controlled from Chicago (slew, acquisition on the slit, guiding for up to 30
minutes) all without site intervention. We have various things on the to do list
(such as remote focus within the remark echelle window, etc.) that should be
done by the end of February.

The main uncertainties at this point are throughput and use of the guider
on faint objects. The former is low (1% overall) including top of the atmosphere
down to the computer. We are documenting all of the surfaces in between and will
publish the error budget and any out of spec performance issues once we are
done with the reductions. The low number does not include precise gain,
precise slit/seeing/tracking corrections, degraded telescope optics (73%
overall assumed), etc. We have also not verified that we are photon noise
limited at over 100 counts per pixel (i.e., that the flat fielding is perfect).
However, there is no reason to suspect a problem. The chip has no defects of 
note. As noted above, we have all the data for the throughput calculation,
which I will publish when all the reductions are done and I am satisfied
we have thought of everything.

At the moment we are using only one slit (1.6secx1.6sec, square). We have
other slits, and will make a few more (especially if the slit function
is ambiguous and we need a straight large slilt measurement for calibration.

We have a number of nights in February to finish the undone work (radial
velocities, wide lines, tests on QSOs), to verify any realignments necessary
and to verify the comparisons between our data and existing data for the
stars and gas noted above.

The teritary is not automated, though the work is underway. I hope we can
test it completely in March 1999.

If I try to list all the folks who made this possible, I would certainly
miss someone without lengthy research into years of notes. All who contributed
know who you are. My personal thanks. This message is being sent to the
echelle commissioning team, the group currently defining tests and working
daily for the next nights work. Their hard work is evident.

Questions may be sent to me directly. Everyone on the echelle commissioning
team mailing list has a copy of our current working task list if you wish to
see it.

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