Edge and Hot pixels

[andrew.bennett@ns.sympatico.ca (Andrew Bennett)]

I have been taking another look at edge pixels on CD5.
Results can be found in
http://www3.ns.sympatico.ca/andrew.bennett/tass/mkivedge.html

I have computed medians etc for edge pixels avoiding
the corners and classify them as follows (which differs
a little from Herb Johnson's classification)

                VDARK15	sd	IDARK15	sd     Type
Image area	-23237	24	-22977	41		
Columns
X1 =	   0	-23168	36	-22545	118	C?
	   1	-24900	1	-24733	1	B
	   2	-23250	5	-23006	9	D*
	   3	-24899	1	-24733	1	B
	   4	-23126	42	-22929	31	C
	   5	-24898	1	-24731	1	B
	2038	-23223	25	-22799	73	C
	2039	-18143	50	-17553	129	A
	2040	-23212	24	-22790	71	C*
	2041	-4791	3073	-22788	73	A, =2042 for I
	2042	-23224	24	-22788  73	C
Rows
X2 =	   0	-18237	485	-18374	497	A
	   1	-23164	50	-23007	57	C
	2034	-22734	105	-21837	527	A
	2035	-23268	9	-23026	15	D
	2036	-23269	7	-23022	12	D

A	No idea
B	Constant; I guess this is the -ve end-stop
C	"Dark". Increases with dark current
D	"Unused". Rather constant; try it as a reference value

* I arbitrarily selected X1 = 2040 for a "Dark" estimate
and X1 = 2 for "Unused" for use as reference.

I have extracted all the pixel values at locations
previously identified as "Hot" into a spreadsheet and
have compared the mean "Hot" pixel value with the estimate
of dark current obtained from median of col 2040 - col 2.

The results are reported in
http://www3.ns.sympatico.ca/andrew.bennett/tass/dark.html

Values found for these "Hot" pixels are remarkably
consistent and not visibly affected by noise sources such
as stars. The correlation coefficient between values from
two images ranges from 0.85 for two dark images up to
0.998 for two adjacent I images having rather high values
for their "Hot" pixels.

[for plots, see web page]

I conclude:
1) The edge pixels provide a good estimate of the "Hot" pixel
values.

2) "Hot" pixel activity varies by a large factor from image to
image. This is presumably a result of temperature variation. One
would expect a large improvement if lower temperatures could be
achieved without returning to the ice crystal problem. As it is,
correction should be possible using a scaled Dark image, as I 
hinted in an earlier post to the mailgroup.

3) The "Hot" pixels are consistent. I hypothesize that the
intermittent ones seen on the dark images when creating the
index are mostly the result of an odd 
phenomenon I read about in a book on CCD's. Yes - I do
sometimes read books. This is the phenomenon of hot pixels
turning on rather abruptly as the temperature is raised. And
conversely, turning off rather abruptly as the temperature is
lowered. If one assumes, as the data suggests, that the dark 
images were taken at a lower temperature than most of the star 
images, some "Hot" pixels would be inactive. This nicely explains
the lower correlation between "Hot" values on dark images compared 
to that between star images. This also suggests that the 
improvement on cooling could be rather great.

Andrew Bennett, Avondale Vineyard, Nova Scotia, Canada.