If you work with 135mm film cameras, you probably know an important function named "multi-exposure", which allows a photographer to do multiple open-close actions for shutter on one single frame of film. With a film camera, you have no choice but to use the "multi-exposure" function to shoot a star-trail photo with blue sky. For this, firstly, you need to do an under-exposure for the sky during the twilight hour to obtain the highly saturated blue sky. Without moving your tripod nor film, you need to wait until total dark to do the second exposure on the same frame to obtain the star trails and ground elements.
Using a digital camera without the multi-exposure function, you can still play "multi-exposure" by using a black cloth or lens cap, but there is a better approach for most of us - long exposure HDR. (However, a manual HDR blending is needed.) I have used both methods for tulips shots so I have both versions. Since HDR is already a well known technique, I am going to talk about the other approach which may be more interesting. The camera was a Canon EOS 7D with Sigma lens in 10mm wide angle, ISO 200 in bulb mode and manual focus. I arrived after sunset and set my tripod in front of the tulips and windmill as soon as I determined the best composition. I started at the magical twilight hour. I began the first exposure of about thirty seconds, mainly for the sky. With star trails in plan, I knew the blue sky needed to be an underexposure.
To end the first exposure, I put the lens cap on but left the shutter open. I took a short nap until I felt the sky was dark enough and, after removing the lens cap for the second exposure, I used a super bright flashlight to do a 60-second light painting on all land elements including the tulips and windmill. Then I took a long nap and left my camera exposing in total dark until I felt the star trails would be long enough. The darkness of sky is highly dependent on weather and location and a thin layer of fog or cloud reflecting remote city light may make a significant difference, as you see in this case. A circular polarizer filter may also make a big difference. I don't have a specific scale for that, I just bet for luck. This star trails exposure was about one to two hours.
In addition to photography techniques, the weather conditions were essential to shooting star trails with still flowers in the scene like this. Not only did the sky need to be clear, but also the air needed to be calm. I observed the weather daily before I took this photo and the wind speed was as calm as zero mph the night I took it. Noise control was also important. I used both in-camera noise reduction and software noise reduction afterwards to deal with it. The unsharp mask, curves and hue/saturation in Photoshop CS3 helped me sharpen the star trails and adjust the overall brightness and colors. - William Lee
Equipment and settings: Canon EOS 7D, Sigma 10-20mm f/4-5.6 EX DC HSM lens set wide at 10mm, manual focus - ISO 200 in bulb mode.