photflat versions |
We have used two variations on the flat-field photometric
correction image (see discussion in Scattered
Light ). The first version used an ad hoc image created by
covering the bottom of the mirror covers and scaling the resulting
(vignetting) pattern to minimize the standard star scatter. The
improved correction was determined using stellar photometry from
multiple images at offset positions and construction a correction
frame to minimize the scatter. We call the first version
'scatter-A.0' and the second 'scatter-B.0'. The flat-field images
corrected with these scattered light frames are noted as
'photflat-A.0' and 'photflat-B.0'. All data distributed after 2002
July 15 uses scatter-A.0.
There are two ways to check which scattered-light correction was
applied to the flat used to process your data: from the flat-field
images distributed with your QSO distribution packet, or from the CFHT
Elixir Web pages. In either case, the first step is to identify the
name of the flat-field used on your image. To do this, look at your
image header. There are a variety of image processing HISTORY
statements written by the FLIPS processing system. Look for the line
that identifies the flat-field image:
HISTORY imred: Flat-field: * 1.0 / 02Bk01.flat.I.00.01
This says the image was normalized by the flat-field image
02Bk01.flat.I.00.01. The first part of the line shows that no
re-normalization was applied; this is generally the case for
Elixir-processed data since the registered flats are defined to have
normalization set to 1.0. The name of the flat-field image defines 5
quantities separated by dots: the camera run ID (02Bk01), the detrend
type (flat), the detrend filter (I), the CCD number (00), and the
version number (01). See top
for more details on these definitions. Having identified the
flat-field image, it is nessecary to determine how that image was
processed.
All QSO-distributed data comes with the associated detrend frames. If
these are conveniently available, examine the header of the
appropriate flat-field frame, in this case 02Bk01.flat.I.00.01.fits.
If the image has been corrected for scattered light, it will include a
comment in this form:
COMMENT Scattering Correction B.0 applied
The alternative method is to find your flat-field in the CFHT detrend
database tables on the Web. Start from the table of camera run IDs. Select the appropriate
run ID (in this case 02Bk01) and you will see a list of all possible
detrend types available on the web. Select the 'all' entry for your
flat & filter, and you will see a table of all versions of flat-field
images for this run ID, type and filter, along with a comment. The
comment will either be 'elixir' for the uncorrected images, or it will
indicate the version of the scattered light correction (photflat-A.0,
photflat-B.0).
To correct images already flattened with either uncorrected flats or
the old scattered light version (scatter-A.0), follow the discussion
below:
We have an uncorrected flat F
We have a scattered light image version A: SA
We have a scattered light image version B: SB
The corrected flats FA, FB are created with:
FA = F - SA
FB = F * SB
Given a raw image Io, it is flattened by doing: Ix = Io / Fx where Fx
may be one of F, FA, FB, and the result is called I, IA, or IB:
I = Io / F
IA = Io / (F - SA)
IB = Io / (F * SB)
So, if you have I and want IB, that is straightforeware:
IB = I / SB
A bit more tricky is going from IA to IB:
IB = IA * (F - SA) / (F * SB) = IA * (1 - SA/F) / SB
This would be fine if you have the uncorrected flats, but it can be
simplified with a minimal loss of accuracy, as follow. The difficulty
in applying the above is that each camera run has a different set of
flats for each filter, so there will be alot of work identifying those
flats, downloading them, etc. To avoid this, we make use of the fact
that all flats of a given filter are stable across many months at a
level of about 2%. Since the error introduced by using the wrong flat
for F is of the order SA*F1/F2, the amplitude is quite small
(~5%*2% -> ~0.1%). Thus, we can construct a correction frame Rab
using a single uncorrected flat from a single camera run and apply
this to all camera runs with a reasonable accuracy. This would be
generated as:
Rab = (1 - SA / F) / SB
and would be applied as:
IB = IA * Rab
You can download SA and SB from our website. go to:
http://www.cfht.hawaii.edu/cgi-bin/elixir/detrend.pl?good=1999/09/01,00:00:00.scatter.B
and you will get a list of both scatter-A.0 and scatter-B.0 images for
the B filter (change B to VRI in the above query to get the other
filters).
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