Subject: Re: [sdss3-boss 1607] Re: Quality of plate 3648 Mjd 55131
From: David Schlegel
Date: Tue, 3 Nov 2009 15:10:44 -0800
Michael, David, et al,
I believe I've tracked this problem down to the extraction
step of the 2D pipeline.  For some of these early plates,
the spectrographs were quite out of focus.  (The telescope
being out of focus or not guiding couldn't do this.)
So this gets down to the problems of optimal extraction
with terrible PSFs in essentially blended fields.
The bad news is that this isn't a quick fix, but rather a
longer-term piece of the code to rewrite.  The good
news is that if we are well-collimated, as we are on
other plates, then the problem is not nearly so severe.
BTW, this is also the cause of the spectral break sometimes
showing up at ~6000 Ang, more often for faint objects are
objects near brighter objects.
-David
On Nov 3, 2009, at 2:41 PM, Michael Strauss wrote:
> David Weinberg writes:
>>
>> That behavior (bigger fiber-spec for fainter objects) does
>> seem odd.  Can you think of any explanation?
>> I presume that even for the fainter objects the position errors
>> from the SDSS imaging are small enough that they don't
>> add to the error budget in getting light down the fibers.
>> Is that correct?
>>
>> David
>>
> Yes, I believe that's correct.  We saw such behavior in the first
> plates observed.  One could imagine the problem being due to
> systematic errors in sky or scattered light subtraction.
>
> 			 -Michael
>
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