[Aavso-photometry] Re: Dithering Techniques

[Wolfgang Renz]

Hello

There is an other application for dithering in photometry.

In photometry we do no image processing (except of bias, dark & flat
frames and sometimes image stacking) to not influence the measure-
ments. So in photometry single hot and cold pixels are not 'smeared'
over several pixel due to image processing.
To get high internal precession within a differential magnitude time
series, we usually try to keep the stars at the same pixel. So any
remaining anomalies stay constant and produce just a constant offset
in the data.

To get high accuracy magnitude values in the centi (and especially the
milli) mag range for all stars in a single image, the remaining anomalies
show up significantly.
One of the sources is flatfielding. Due to internal reflextions in the scope
etc., an evenly flatfielded background doesn't imply that the image is
also well corrected photometrically. To improve accuracy one can apply
an additional photometric flat field correction (photometric super flat)
after flatfielding. Bruce Gary is doing some research on this issue
currently.

An other major source of inaccuracies in photometry on UNDER-
SAMPLED images are the insensitive inter pixel structures (with full-
frame CCD chips like Kodak KAF chips and especially with inter-line
CCD chips like Kodak KAI and Sony chips) and the different blue
sensitivity of the two gates of the Kodak blue enhanced E and ME
chip pixel.
Microlensed chips try to focus most of the light onto the most sensitive
area of the pixel. At high f numbers that will for sure work in the center
of the image, but that might also fail or even make it worse at low f
numbers near the edges and in the corners.
The solution to overcome these inaccuracies would be to make several
undersampled images and dither these images on a sub pixel scale
(2x diagonal, 2x2, 3x3). As the aim of this dithering is not to improve
the resolution, the images might be added by a bi-linear sub-pixel shift
and add algorithm (so the count totals stay the same and the stars are
enlarged by just one pixel). As severely undersampled images just occure
at short FLs, the targets would be bright ones that would require multiple
short exposures to not saturate the CCD and to beat the scintillation
noise in such short exposures below 10 seconds.

An average camera lens has in the center a resolution of ~ "30 LP/mm
that equates to 16.7 microns per line". A good camera lens has in the
center a resolution of ~ "50 LP/m that is 10 microns per line" (Source:
email by Paul Howell). 
Many CCD chips have a pixel size of 20 microns. So it is no wonder
that the light of a star might fall just on 1, 2 or 4 adjacent pixel. Thats pretty
badly undersampled. But even with the 6.8 micron pixel of a Kodak KAF-
3200 chip and an average camera lense its still undersampled without
defocusing.


Is sub-pixel dithering already possible and supported by any existing
image aquisition software  ?


Clear skies
  Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO 



----- Original Message ----- 
From: "John Smith" <johnzonie at comcast.net>
To: <ccd at listserv.wwa.com>; <SBIG at yahoogroups.com>; <SoftBisqUser at yahoogroups.com>
Sent: Thursday, February 17, 2005 3:34 PM
Subject: [CCD] Dithering Techniques


> Hi,
> 
> I have posted a note on dithering techniques that discusses
> recommendations for setting the maximum dithering value.
> It also describes a newly developed dithering algorithm that
> improves CCD sensor pattern noise reduction. Pattern noise
> is any non-uniformity in a CCD sensor.  Hot and cold pixels
> are a typical example.  Here is the link:
> http://www.hiddenloft.darkhorizons.org/notes/dithering1.htm 
> 
> This algorithm is first implemented today in version 2.06 of
> CCDAutoPilot2.
> 
> John
> CCDAutoPilot2 - Innovation starts here.  www.CCDWare.com 
> www.hiddenloft.darkhorizons.org