Twilight has never been my favorite time to photograph an international space station or shuttle pass but it has rewarded me with some favorites and a couple of unusual shots. The important thing is to embrace the challenge and use every opportunity to practice and pretty soon youll just know what to do.
Civil Twilight is when the Suns altitude is 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities.
Nautical Twilight is defined when the Suns altitude is 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.
Astronomical Twilight is when the Suns altitude is 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.
***above definitions excerpted from the USNO site
These periods of twilight occur before dawn and after sunset and are usually about 20 minutes apart. You can check http://www.heavens-above.com or any astronomical almanac type reference for exact times at your location.
By looking at the times of twilight, you can begin to make a more educated guess on how to prepare for an image that really stands out from the crowd!
Finding a Happy Medium
Dont let the exposure run too long since lingering or imminent sunlight will wash out the image in a bright sky.
Use a small aperture and low ISO. Be mindful that this combo has a two sided edge.
If your exposure is too short or youre using too small of an aperture, you also risk not collecting any of the light left behind by the ISS at all. This is not what you want happening so finding a happy medium is the challenge.
Bracket, Bracket, Bracket!
Practice using different exposure lengths, apertures and changing the ISO. It will take some trial and error to get the hang of it since light is always changing and is never the same from moment to moment.
Practice your bracketing technique using airplanes. I know this sounds corny, but you can learn a lot about your camera settings this way. Keep a small note book and use it to make detailed notes about the exposure length, aperture, ISO and time of day.
Compare the images to determine what worked best and use that as a starting place for making an image of the ISS flying by.
Check Heavens-above.com for twilight times and then use it as a guide for maximizing your exposure lengths.
As an example if you are planning to make an image of an ISS pass that occurs at 6:20 a.m. and twilight time tables indicate that civil twilight begins at 6:22 a.m. the light is not ideal for making a long exposure. You can practice a couple of mornings before pass day to determine just how long an exposure can run before it is washed out by the bright sky.
Ideally you want to make long exposures before astronomical twilight begins each morning or after it ends each evening.
Yeah, this is a drag because satellites dont heed your schedule, but once you get the hang of it, its bucket loads of fun!
Another thing to consider is the direction of the pass you are imaging. Try to plan your shots aimed away from the rising or setting Suns direction. The sky will be darker opposite the Sun and may give you just a few more precious seconds of exposure time.
Make a few quick shots and then stitch them, or make a short exposure when the ISS is by a pleasing target, like Venus.
Give your photo context and thoughtfully begin your exposure when the ISS is near a recognizable target in the sky.
Use a photo software adjustment program to enhance an image on the edge of being just about right.
You can adjust the levels of contrast, brightness, etc. to bring out faint traces of the ISS without going overboard and making your photo look garish or gritty.
Good luck and have fun!