<p>I have begun reducing images I collected with a telescope and CCD detector. I read in the the "Handbook of CCD Astronomy" by Howell that I need to trim overscan regions (as well as possible amplifier turn on/off) regions before I can compute the gain of the detector. However, it is not thoroughly clear where this boundary is. </p>

<p>If we treat the CCD detector as a matrix of pixels, then I know that the overscan region is along the edge columns. To visualize this, I computed the average and standard deviation along each column of pixels to view the variation in intensity from column to column. Below I inserted the left side of two amplifier matrices on my CCD detector: </p>

<p><a href="https://i.sstatic.net/g5A4T.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/g5A4T.png" alt="Column-wise average and deviation"></a></p>

<p>The blue is the column-wise average and the green is the column-wise standard deviation. The y-axis is the (arbitrarily scaled) intensity and the x-axis is the pixel distance from the edge of the CCD array. </p>

<p>My question is essentially: <strong>how much of the blue and green curve should I cut-off?</strong></p>

<p>Edit: additionally, it should be noted that I am using the method outlined in "CCD Data..." by Massey &amp; Jacoby (1992) <a href="http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1992ASPC...23..240M&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf" rel="nofollow noreferrer">link</a> to compute the gain. </p>

CC BY-SA 3.0
2018-03-14T11:09:56.493Z
What kind of images are these? It's usually obvious where the overscan is if you have a flat field or regular observation, because the overscan will be much, much "fainter" than the actual data. (Also: are you sure the overscan is on the left side, rather than the right side?)
Peter Erwin
CC BY-SA 3.0
2018-03-22T21:57:42.977Z
@PeterErwin Thanks for pointing out that the overscan should be obvious for regular observations. This made me realize that I forgot these images are log corrected (these are regular observations with a log correction applied). I removed the log scaling and the boundary is now clear. Thanks!
Peter
<observational-astronomy><data-analysis>
How to trim CCD overscan and amplifier turn on/off regions
Peter
# How to trim CCD overscan and amplifier turn on/off regions

(Asked by: username_0 on 2018-03-09)

I have begun reducing images I collected with a telescope and CCD detector. I read in the the "Handbook of CCD Astronomy" by Howell that I need to trim overscan regions (as well as possible amplifier turn on/off) regions before I can compute the gain of the detector. However, it is not thoroughly clear where this boundary is.

If we treat the CCD detector as a matrix of pixels, then I know that the overscan region is along the edge columns. To visualize this, I computed the average and standard deviation along each column of pixels to view the variation in intensity from column to column. Below I inserted the left side of two amplifier matrices on my CCD detector:

[![Column-wise average and deviation](https://i.sstatic.net/g5A4T.png)](https://i.sstatic.net/g5A4T.png)

The blue is the column-wise average and the green is the column-wise standard deviation. The y-axis is the (arbitrarily scaled) intensity and the x-axis is the pixel distance from the edge of the CCD array.

My question is essentially: **how much of the blue and green curve should I cut-off?**

Edit: additionally, it should be noted that I am using the method outlined in "CCD Data..." by Massey & Jacoby (1992) [link](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1992ASPC...23..240M&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf) to compute the gain.

**Tags:** observational-astronomy, data-analysis

> **Comments:**
> **username_1**: What kind of images are these? It's usually obvious where the overscan is if you have a flat field or regular observation, because the overscan will be much, much "fainter" than the actual data. (Also: are you sure the overscan is on the left side, rather than the right side?)  
> **username_0**: @username_1 Thanks for pointing out that the overscan should be obvious for regular observations. This made me realize that I forgot these images are log corrected (these are regular observations with a log correction applied). I removed the log scaling and the boundary is now clear. Thanks!  

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## 0 answers

