A 200mm lens on a full-frame camera is better suited to wider deep-sky subjects than to small objects needing high magnification. Galaxies and the Moon will look fairly small at 200mm, so don’t expect large, detailed images of compact targets.

From a very dark site, your best bets are brighter, larger deep-sky objects and wide-field night-sky scenes. Dark skies especially help with faint objects that are hard or impossible to capture well from light-polluted locations. Large targets like Andromeda and the Pleiades are reasonable ideas; in general, focus on bigger deep-sky subjects rather than tiny ones.

A key point: test your setup before the trip. Since your barndoor tracker is homemade, real-world tracking accuracy, alignment, and lens performance matter more than theory. Do trial sessions at home to find:

• how long you can expose before stars trail
• whether 200mm is practical on your tracker
• how accurate your polar alignment needs to be
• which focal lengths in your 70-200 range work best

If 200mm feels too limited for small targets, a teleconverter could give you more reach, but the dark site itself is still best used for wide and bright deep-sky objects rather than highly magnified imaging.