Changes between Version 1 and Version 2 of GPC_BrightStarAstrometry_Issue

Timestamp:

Mar 31, 2011, 8:30:58 PM (15 years ago)

Author:

watersc1

Comment:

--

GPC_BrightStarAstrometry_Issue

@@ -5 +5 @@
 The STS astrometry showed offsets in the positions of stars in about half of the chips, with mean offsets of about 0.8 pixels.  Gene first characterized this, and showed that changing the stars used for the astrometry from the brightest non-saturated stars to fainter stars improved the agreement for the majority of stars, although it created a trend of bright stars having larger position offsets, as shown in the following figures (x is the reference catalog magnitude, y is the shift in RA in arcseconds).
 
-|| [Image(o5368g0473o-o5368g0474o.v0.png)] ||
-|| [Image(o5368g0473o-o5368g0474o.v1.png)] ||
-|| [Image(o5368g0473o-o5368g0474o.v2.png)] ||
+|| [[Image(o5368g0473o-o5368g0474o.v0.png)]] ||
+|| [[Image(o5368g0473o-o5368g0474o.v1.png)]] ||
+|| [[Image(o5368g0473o-o5368g0474o.v2.png)]] ||
 
 The original thought was that these offsets were due to burntool oversubtraction damaging the star and skewing the centers, or that the STS observing method was promoting persistence effects.  However, on closer examination, no obvious persistence effects were seen, and the small offsets in the pointing prevented stars from directly falling on the same pixels on subsequent visits to that pointing.  In addition, burntool only modifies the very brightest stars, with no stars fainter than m_instrumental = -14 having any correction applied.  The trend continues fainter than this.  Running the same exposures through processing with burntool disabled shows the same effect, so the shifts cannot be caused by anything that burntool is doing to the image.  A final issue pointing to a different source is the fact that RA aligns with the x-direction of the detector in these exposures.  Burntool operates along columns, making it unlikely to create a horizontal shift in a star position.
@@ -16 +16 @@
 
 || OTA34                             || OTA42 ||
-|| [Image(o5407g0385o.ota34.dR.png)] || [Image(o5407g0385o.ota42.dR.png)] ||
-|| [Image(o5407g0385o.ota34.dD.png)] || [Image(o5407g0385o.ota42.dD.png)] ||
-|| [Image(o5407g0385o.ota34.dx.png)] || [Image(o5407g0385o.ota42.dx.png)] ||
-|| [Image(o5407g0385o.ota34.dy.png)] || [Image(o5407g0385o.ota42.dy.png)] ||
+|| [[Image(o5407g0385o.ota34.dR.png)]] || [[Image(o5407g0385o.ota42.dR.png)]] ||
+|| [[Image(o5407g0385o.ota34.dD.png)]] || [[Image(o5407g0385o.ota42.dD.png)]] ||
+|| [[Image(o5407g0385o.ota34.dx.png)]] || [[Image(o5407g0385o.ota42.dx.png)]] ||
+|| [[Image(o5407g0385o.ota34.dy.png)]] || [[Image(o5407g0385o.ota42.dy.png)]] ||
 
 The lack of any trend in dy illustrates that this shift occurs along the rows of the detector.  
@@ -37 +37 @@
 
 || OTA24/dx || OTA36/dx ||
-|| [Image(o5407g0385o.ota24.dx.png)] || [Image(o5407g0385o.ota36.dx.png)]
+|| [[Image(o5407g0385o.ota24.dx.png)]] || [[Image(o5407g0385o.ota36.dx.png)]] ||
 
 
 Checking the differences in positions shows that in the raw cell coordinates, a bright source in GPC1 is at a smaller x position than the 2mass position. This means that an example object in cell00 has been shifted in the direction of cell01. There is some evidence of asymmetry in the raw images as well that supports this shift:
 
-[Image(asymmetry.png)]
+[[Image(asymmetry.png)]]
 
 This effect starts around instrumental magnitude of -13.  Using the SMF file to convert puts the peak flux for these stars around 5000 counts.  The size of the shift is dependent on the brightness of the star, with the brightest objects having the largest shifts.