Changes between Version 1 and Version 2 of astrometry

Timestamp:

May 26, 2009, 3:38:09 PM (17 years ago)

Author:

beaumont

Comment:

--

astrometry

@@ -128 +128 @@
 }}}
 
-Running addstar with updates adds new entries to the end of the DVO database. At the end of this process, running addstar with -resort re-arranges the database so that measurements of common objects appear consecutively
+Running addstar with -update adds new entries to the end of the DVO database. This saves time, since the catalog need not be repeatedly resorted. At the end of this process, running addstar with -resort re-arranges the database so that measurements of common objects appear consecutively
 
 {{{
@@ -152 +152 @@
 }}}
 
-
 I found that repeating these commands 2-3 times converged on a stable solution.
 
-Further discussion on configuration file options, command line options, etc coming soon.
+
+Some options for relastro are summarized here. These can either be provided on the command line or in your camera's dvo.config file:
+ * RELASTRO_SIGMA_LIM
+   * If set and nonzero, than measurements with photometry errors larger than this value are not considered in the analysis
+ * RELASTRO_SRC_MEAS_TOOFEW 
+   * If an object has this many detections or fewer, do not consider the object during the analysis. This is particularly effective at filtering out spurious measurements.
+ * +photflags
+   * During reduction, the IPP sets a number of quality flags. Those flags which correspond to problematic detections (e.g. saturation, cosmic rays, etc) can be excluded during relastro analysis by using this option. The specific bitmask defining which flags are problematic is listed in the Photcodes.dat DVO table as ASTROM_BAD_MASK
+ * instmag
+   * Set this to a range of instrumental magnitudes to consider. This can be used to filter out faint or saturated objects
+
+=== Evaluating the Fit === 
+
+There are several ways to ascertain whether relastro did a satisfactory job during fitting.
+
+ * '''Textual output from relastro'''
+ Relastro outputs lines like this to the terminal during --update-chips
+{{{
+image 3 : Nstars: 591
+scatter limit: 0.826633 based on 403 detections; using 377 of 591 for fit
+scatter limit: 0.290518 based on 403 detections; using 376 of 591 for fit
+scatter limit: 0.288850 based on 403 detections; using 379 of 591 for fit
+}}}
+
+This says the following: For the third image (each chip is a separate "image"), there are 591 stars which pass all filtering criteria. By comparing these detections to each object's average position, the 3-sigma scatter is .826633 pixels. 377 detections have scatter less than this, and are being used to update the astrometry solution. This process repeats two more times, and the final 3-sigma scatter appears to be .289 pixels.
+
+ * ''' DVO table values '''
+The average (.cpt) dvo table has two fields -- RA_ERR and DEC_ERR -- which lists the positional uncertainty for each object (e.g. the error on the weighted mean of the measurements ~ rms/sqrtN). These values are updated each time relastro --update-objects is run. They are reported in arcseconds. These values can be extracted in DVO via the command
+
+{{{
+avextract ra dec dra ddec
+}}}
+
+The help information avextract --help fields seems to mislabel dra and ddec as being in degrees; I believe they are still arcseconds.