Subject: Further word on echelle

From: Don York

Submitted: Thu, 20 May 1999 12:25:42 -0500 (CDT)

Message number: 7 (previous: 6, next: 8 up: Index)

We are reducing commissioning data on reddened stars. These are roughly
30 minute exposures. We want to add many together to get very high signal
to noise, beyond the values discussed in the commissioning report (S/N
>100) and we want to do it for the entire spectral region covered by the
echelle, to address the scientific problem of the diffuse interstellar band.
These were chosen for commissioning specifically to test the high S/N 
regime on all spatial scales (1 to 500 pixels).


After some early reductions, we placed a new slit in the focal plane,
1.6 arcsec x 3.0 arcsec, so the flats are wider than the stellar spectra.
This went in on May 6, 1999, compliments of Shu-i and Jeff Sundwall.
We will be using it for the above science program on May 29-June 1 (2nd
halves) and will report back on how it works, in comparison with the 
square slit.

In the process of this work, interference effects that I had not anticipated
and have not seen before are showing up. Julie Thorburn has evidently
been successful in removing these using the one-dimensional flats I mentioned
earlier. However, there is evidence of an extreme sensitivity to the angle
of the dipping mirror used for flat fields. Whatever the cause of these
fringes, there is likely to be an optimal procedure for removing them that
involves a specific order of flats and stellar spectra. We will experiement
with these possible procedures at the end of May and get back to this
group with results.
DON

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