What spectral resolution can I get?
The spectral resolution is about 80,000 for the spectroscopic mode 'star
only', and 68,000 for the spectroscopic 'star + sky' mode and the
polarimetric mode.
What is the relationship between the 2.6
km/s bin mentioned in the Exposure Time Calculator and a CCD pixel?
The 2.6 km/s bin corresponds to one CCD pixel. The spectra are actually
recovered with spectral bins of 1.8 km/s (in other words, the spectral
sampling is 1.8 km/s, or 0.6923 CCD pixel).
What is the wavelength
coverage? Where are the gaps?
Each exposure covers the
40 orders, from 370nm
to 1050nm, with 3 very small gaps: 922.4-923.4
nm, 960.8-963.6 nm, 1002.6-1007.4 nm.
What is the size of the aperture hole?
The aperture hole through which the light is gathered is 1.6 arcsec
across.
What is the Stokes parameter that you call
'W'?
In this circular polarization mode, the top rhomb rotates at about
0.1Hz, eliminating the cross-talk from linear to circular polarization
by averaging the linear polarization. This is useful if your target is
highly polarized in linear polarization.
What is the wavelength at the center of each order?
| Order | Wave- length (nm) |
Order | Wave- length (nm) |
Order | Wave- length (nm) |
||
|---|---|---|---|---|---|---|---|
| 61 | 372 | 47 | 482 | 34 | 666 | ||
| 60 | 377 | 46 | 492 | 33 | 686 | ||
| 59 | 384 | 45 | 503 | 32 | 708 | ||
| 58 | 390 | 44 | 515 | 31 | 730 | ||
| 57 | 397 | 43 | 527 | 30 | 755 | ||
| 56 | 404 | 42 | 539 | 29 | 781 | ||
| 55 | 412 | 41 | 552 | 28 | 809 | ||
| 54 | 419 | 40 | 566 | 27 | 839 | ||
| 53 | 427 | 39 | 581 | 26 | 871 | ||
| 52 | 435 | 38 | 596 | 25 | 906 | ||
| 51 | 444 | 37 | 612 | 24 | 944 | ||
| 50 | 453 | 36 | 629 | 23 | 985 | ||
| 49 | 462 | 35 | 647 | 22 | 1029 | ||
| 48 | 472 |
What is field of view of the guiding
camera?
The total field of view is about 100 arcsec. A subraster of 5-15 arcsec
on a side is usually used to reduce the readout time and speed up the
guiding corrections.
Is it possible to guide on my target?
Yes, and this what is done by default. The guiding algorithm takes into
account that it can only use the ring of light outside the star hole to guide.
Is it possible to guide on a nearby
star?
Yes. It is not possible to do this with ESPaDOnS right now (this option
does not work at the moment) but it is possible to use the Cassegrain
guiding instead.
What if it is not possible to guide on my
target and there are no nearby stars to guide on?
It is possible to use the Cassegrain guiding camera instead.
What is the magnitude of the faintest
target observed so far?
About magnitude 14.
How can I get an estimate of the
seeing?
Put your star close to the aperture hole, but not in it. Make the
guiding zone big enough to contain the star. Start the mode "Calculate
guiding corrections" (without sending the corrections to the telescope):
seeing and magnitude estimates will be given.
When should I use the Atmospheric
Dispersion Corrector?
An Atmospheric Dispersion Corrector is an optical device usually
comprising two thin prisms which can rotate to compensate for the
elongation of a star image caused by the wavelength dependence of the
refractive index of air. At high airmasses, it is very easily seen on
the guider that the star image is not round and small. Using the ADC
will make the star look round and small again. The ADC can be left in
the beam at all times since it does not absorb a lot of light and does
not introduce polarization. It should be noted that the telescope focus
will change dramatically when the ADC is moved in or out of the beam;
the star image will look like a big ugly donut. The telescope has to be
refocussed if the ADC is moved in or out of the beam, by about 100 'old'
focus units, or about 1.0mm (new units).
What is a Hartmann mask?
A Hartmann mask blocks one half of the beam at a time. In ESPaDOnS, the
mask can block the UP part or the BOTTOM part of the beam of light. This
is used to focus the spectrograph. If the spectrograph is not focussed,
each position of the Hartmann mask will produce spectral lines at
different locations on the detector. If the spectrograph is focussed,
the spectral lines are superimposed. This is a
figure to explain how
the Hartmann mask is used.
ESPaDOnS has a fiber agitator; what's
that?
Like Gecko, ESPaDOnS has a fiber agitator, which agitates (shakes) the
fiber just before it enters the spectrograph. This device is used to
remove modal noise present in optical fibers, and ensures a S/N ratio
compatible with photon noise.
How many exposures are taken to measure
each Stokes parameters?
Four.
Is it possible to take only 2 exposures to
measure a Stokes parameter?
No. Four exposures are required.
How much time does it take to move the
rhombs from one position to the next?
The rhombs are moved while the CCD is read out, and this is what takes
the longest time (40 sec in Normal mode).
How much time does it take to reduce data
in polarimetric mode?
After the geometric and wavelength calibrations, which take about 5
minutes total, 4 exposures taken in the polarimetric mode can be
completely reduced in about 3 minutes.
What are the telescope limits?
The
CFHT
Observatory Manual presents the telescope's range of movement,
although from experience, the following limits are more practical
(especially for the hour angle):
Can I do CONTINUUM polarization with
ESPaDOnS?
Generally speaking, ESPaDOnS is not a good instrument for continuum polarization. There are mainly 2 reasons:
This problem will not affect extended objects that overfill the aperture hole (because the whole circular edge will produce a polarization that will cancel itself by symmetry).
One might also imagine that long exposures with random guiding errors will result in an averaged out continuum polarization, but this has not been tested.
The exact extent of that effect has not been measured, but it might be minimal for extended objects.
What is the stability of the spectrograph
throughout a night?
As the night goes on, the spectra move with respect to the CCD (or the CCD
moves with respect to the spectra, or both). When looking at the
corrections provided by Libre-Esprit (position of telluric lines), the
corrections are increasing towards negative values (for an initial value
around 0.0). At the end of the night, the correction ends up
around -0.3 km/s with respect to the beginning of the night. This is
only a small fraction of a pixel, about 0.1 pixel (one CCD pixel = 2.6
km/s), but it is a clear and systematic effect.
Note that the latest version of Libre-Esprit incorporates analysis of telluric lines to provide a better wavelength calibration. Therefore, it should not be necessary to take calibration exposures during the night to track that effect.
Can I take daytime flats?
Observers usually take about 25 flats in the evening, and 25 more in the
morning. If your project requires an exquisite flat field correction,
you may take additional flats during the day. You may start those
additional flats in the morning before you leave the summit, or in the
afternoon from Hale Pohaku. It has been verified that if lights are
turned ON in the dome, the flats are not affected (the polarimeter is
light-tight and the dome lights are not very bright compared to the
internal flat field lamps).
What is the maximum number of flats that
Libre-Esprit can process?
Libre-Esprit can process up to 100 flats. If you absolutely need more
than that, there is a (somewhat involved) work-around:
What are the units in the reduced spectra?
The units of the polarization column of the reduced spectra depend on
the use or not of the option -c:
Guiding (with either the ESPaDOnS guider or
the Cass guider) does not keep the star centered in the hole, I'm losing
flux, what can I do?
At some telescope positions (especially at high airmass or high Hour
Angle), the telescope can start to "oscillate", usually in
Declination. If too much flux is lost or the star really moves around
too much, it is always possible to stop all guiding, and just use the
Guider Control button "Center star" to adjust manually the position of
the star, every 30-60 seconds.
