Flamingos maskdesign
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Most of the things described here aren't that complicated, they're just things you don't necessarily need to do when designing a mask.

Rotation

Rotating images can increase the number of objects available per mask. Only the images have to be rotated, the original catalogue can be used if the "Rotation: " value is entered when maskdesign is started.
Note: Each telescope is only capable of observing at certain rotation angles, and the Kitt Peak 2m cannot be rotated at all. If you're not sure what rotations you can use, check with somebody first.

This is an image that would benefit from being rotated for MOS observation. The concentration of stars near the bottom of the image would result in many overlapping targets, increasing the number of masks needed to study this field. You can use the ruler (Region->Shape) in ds9 to determine a better rotation for this image.
This shows the ruler drawn on the image, the direction of the ruler has been chosen to maximize the number of stars the lie along it. If you double click on the ruler an information dialog box will open, with the angle of the ruler at the bottom. In this case the ruler is at approx. 37°. You can now use iraf to rotate the image by the amount required to make the ruler vertical, in this case 53°.
The rotated image, as the main axis of the cluster is now vertical you should be able to get more objects per mask.