Changes between Initial Version and Version 1 of Background_Model_Stacks

Timestamp:

Dec 8, 2010, 3:14:23 PM (16 years ago)

Author:

eugene

Comment:

--

Background_Model_Stacks

@@ +1 @@
+== Background Model and Stacks ==
+
+
+The IPP performs the following sequence of operations to generate a stacked image:
+
+ * We start with a raw image: Raw(x,y)
+
+ * Burntool : We subtract fits to the persistence trails from bright
+  stars (from the immediate image and those left behind by previous
+  images).  Affected areas have a 'suspect' mask bit raised in the
+  mask image.
+
+ * Detrend : 
+  * We adjust pixel values to compensate for a bias sag
+
+  * We subtract a dark model of the form (C_0 + C_1 * exptime + C_2 *
+    exptime * dettemp + C_3 * exptime * dettemp^2). this is a function
+    of (x,y).
+
+  * We multiply by a flat-field response F(x,y).  This is measured
+    from a flat-field screen in the dome, then modified based on
+    photometric observations of stars (only for spatial frequencies <
+    1/1200 pixels or so).
+
+  * for y-band, we subtract a fringe frame fitted to the fringe
+    pattern.
+
+ * Each exposure has a 2D background model subtracted: this is
+  effectively a high-pass filter.  In fact, it is a high-pass /
+  low-pass filter: the individual chip images have the low-spatial
+  frequency model subtracted; the low-spatial frequency model itself
+  is saved for each exposure.
+
+ * Each exposure is warped to a standardized pixel grid in a
+  flux-conserving process.  The output image products are called
+  'skycells' and represent about 1/75 of the focal plane (~22 arcmin
+  on a side).
+
+ * sets of 'skycells' are combined in the stack with outlier
+  rejection.  This is not a median, but a weighted mean with
+  sigma-clipping.  The effect is similar to a median: the resulting
+  image consists of the temporally static signal.
+
+Considering the data as observed by the telescope, there are several
+important aspects:
+
+ * There is the (nearly static) instrumental response.  
+
+ * In addition to an instrumenal response, the signal landing on the
+  detector consists of a true astronomical signal, of which there are
+  dynamic and static components; and a terrestrial and/or
+  contamination signal, which has a signficicant dynamic component.
+
+ * the dynamic portion of the astronomical signal is nearly all
+  PSF-like 
+
+ * the dynamic portion of the terrestial signal has a wide range of
+  spatial frequencies:
+
+  * star glints can be roughly PSF in width
+  * ghosts can range from ~10 pixels to a couple hundred pixels
+  * moon glints tend to be hundreds to thousands of pixels in scale
+  * sky gradients are large-scale, but not completely spatially
+  * linear.
+
+I contend that we can recover the low-frequency component of the
+astronomical signal in the stacks from the collection of background
+models.  There is one model per exposure, but with ~25k pixels instead
+of 1.4G pixels.  The goal is to determine the temporally static
+component of the sky as seen in those models.  To do this, we would
+transform them to a binned version of the celestial coordiate system
+of the stack skycells.  We could then simply generate a median image
+of the that portion of the sky.  With enough inputs, this would filter
+out the spatially varying terrestrial / contamination signal, leaving
+behind just the astronomical portion (possibly with an overall
+gradient).  This model can then be added back to the stacks.