# Choosing the Scan Width

The scan width deterimines the number of resolution elements to observe on either side of the central resolution element in LWS02 or LWS04 AOT mode. When Fast Photometry is desired from LWS02, the width can be set to 0. Zero should be selected if only one continuum setting per detector is desired in the continuum.

If the Photometry mode is not the selected mode, then the minimum number of spectral resolution elements on either side of the central element is 2 of 3, depending on whether LWS02 or LWS04 is being requested. This number usually forms a secure baseline, unless a significant deviation from a horizontal baseline is anticipated. In such cases, the scan width may be requested to be larger.

Note that it is not determined where the satellite will be, nor where the Earth will be when the measurement is conducted. The Earth motion relative to the Solar motion is about 30 km/s. This is why the minimum number of Scan Widths for LWS03 is three. In this way the Vearth and Viso with respect to Vhelio will not offset the line more than a resolution element.

This example will let you figure out how your samples will be arranged vs. wavelength and how many will be done for LWS02 and LWS04. Suppose the scan width is 3. There will be 3 resolution elements on either side of the central one:

```        X   X   X   X   X   X   X   X

wavelength -->

If there are 4 samples per resolution element,
these are filled in like this:

X...X...X...X...X...X...X...X

wavelength -->

Finally, the LWS02 and LWS04 algorithm leaves
the endpoints off:

...X...X...X...X...X...X...

wavelength -->
```