Shutter Correction Images

Howell, February 2004

 

 

Using R band flat fields of 0.4 sec, 1 sec, 10 sec, the WTTM shutter effects were examined and a shutter correction image produced.  A few 10sec flatfields and combined to produce one flatfield exposure with no shutter effects due to it's exposure length.  Flatfield images are then obtained by taking Nexposures such that their sum equals the signal in the averaged, long exposure.  Note: The use of N short exposures is similar to the technique of using "focus" mode to direct N shutter openings and closings followed by a single readout. The resulting shutter correction images are identical.

The procedure for making a shutter correction image was as follows:

Using the flat field exposures, the shutter correction image (T) was constructed from
T = (L * s - S * l) / S-L
where L and S are the long and short exposure images and s and l are the exposure times for each (the exposure time requested by ARCON).
 
To apply the shutter correction image (T) to ones data, providing an exposure of equal exposure time across the image, use
Shutter_Corrected_Image = image *  D / (D+T)
where D = the exposure time (sec) (the exposure time requested by ARCON).

Conclusion:

Based on these tests, we do not recommend using integration times of less than 1 sec, and for integrations of 5 sec or less, shutter corrections are needed.
The results of the shutter correction exercise are as follows:
 
Integration Time (seconds)
Non-Uniformity
10
0%
5
1-2%
1
3-5%
0.4
5-10%
Integration Time  %effect of non-uniformity across WTTM image

The greater the radial distance from the image center, the larger the effect.  Observers using short exposures are likely to want to determine their own shutter correction image as flat field gradients in different filters may cause slight differences in the correction.