Timing Occultations with a DSLR (Digital SLR camera):
Most of the current generation of DSLR cameras have a video recording mode which can be used to time lunar occultations and asteroid occultations. The observing process is simple to describe: record a video of the star covering the predicting time for the occultation. The following paragraphs will provide some specific suggestions which should improve the quality of your observational data.
Measuring the time of an occultation is our primary goal. So it is important to keep in mind two basic elements of time measurements: Time resolution and Time synchronization. Time resolution refers to the time interval measured by each observational data point. If you record a video at 30 frames per second, your time resolution is 1/30 second. At 15fps your time resolution is 1/15 second. Time synchronization refers to accuracy of your times when referred to a time standard. Similar to most astronomical measures occultations are measured with respect to Universal Time (UT). Many IOTA observers using a GPS time inserting device to measure the time of each frame of a standard video camera to an accuracy of 1 millisecond (ms). For these observers, the time synchronization of their video observations is 1ms.
With the right equipment and technique DSLR recordings can yield time resolution and time synchronization of 1/30 second. This is a good level of accuracy for most occultation work. You can easily determine your time resolutions with a DSLR by choosing your frame rate (e.g. 30fps). With occultations we are normally aiming for a time resolution of 1/10 second or smaller. In some rare situations the expected duration of the occultation is unusually long and time resolutions of many seconds is acceptable. This is often true for occultations by distant Trans-Neptunian Objects (TNOs).
Time synchronization is the biggest challenge in DLSR recordings. With no time synchronization at all you can still measure useful data – you can determine that the event did not happen from your location (negative observation) or you can measure the duration of the event (a positive observation). However, good time synchronization makes your data more useful because it can be tied together with other observations to yield a more accurate profile of the asteroid. Read the page DSLR Time Synchronization for more information on techniques for synchronizing your DSLR with UT.
Image Size / Spatial Resolution
When timing occultations we do not need high resolution (large image sizes). 640×480 (or less) is good enough. We are not interested in fine grained details or nice colors. We are only interested in the measuring the light from a single star and we want good time resolution. For this reason it is better to choose a lower spatial resolution video mode with a higher frame rate rather than a higher spatial resolution mode with a lower frame rate. In other words, 640×480 at 30fps is better than 1024×768 at 15fps.
However, larger sensors are useful. With a larger sensor you will have a larger field of view in the camera and this can make it easier to find your target star. In addition, the larger field of view of a larger sensor may include additional stars in the field of view while recording. These additional stars can be using during data reduction (providing a photometric reference when measuring the light from the target star). If you have two camera which are equal in other respects, choose the camera with the larger sensor.
Because occultations measurements are photometric measurements, and not spatial measurements, you do not need superb optics. Very poor optics could be a problem because you may lose sensitivity due to light scattering. But most camera lenses and telescopes will have sufficient quality for occultation measurements.
Despite the loss in image quality, many occultation observers use simple focal reducers with their telescope setups. Focal reducers can be very useful for two reasons. The increase in field of view is useful for finding stars and increasing the chance of recording reference stars (as mentioned above). In addition, focal reducers yield a shorter focal length and this usually leads to better SNR (signal to noise ratio) and a dimmer limiting magnitude in your recording.
As is true in almost all areas of astronomy, more aperture is better. Occultations of bright stars are relatively rare. Most occultation target stars are dim. With more aperture you will be able to record dimmer stars and have better SNR in general. For this reason, many observers use telescopes as the primary optics for recording occultations. However, camera lenses work fine as long as the star is bright enough for the aperture of the lens. Obviously, unless the star is very bright, you should set the camera lens to the fastest f/ratio allowed.
Exposure / ISO
The final considerations are your shutter speed and ISO settings. DSLR video modes may have four settings: resolution, frame rate, ISO, shutter speed, and f/ratio. We have already discussed resolution and frame rate. If your DSLR is recording through a telescope, the f/ratio is fixed. If you are using a camera lens, set the f/ratio to the fastest value (lowest number – e.g. f/2.8). Shutter speed is the exposure duration of each frame of the video. The frame rate of your video implies a specific time interval between each frame. If the frame rate is X frames per second (fps), the time interval between each frame is 1/X seconds. Because we want to maximize the SNR of each frame we would like each exposure to use as much time as possible within the 1/X second interval between the frames. Thus, if your video rate is set to 30fps you would like to have your shutter speed set to 1/30 second. This sets the exposure of each field to the maximum possible value given the 30fps frame rate. If your video is set to 15fps, set the shutter speed to 1/15 second. This is the best approach but not all DSLRs will let you set the shutter speed independently of the frame rate in video mode.
The ISO setting is the final, but very important, consideration. In DSLR cameras the ISO setting corresponds to the ISO or ASA ratings for film used in film cameras. In film the ISO rating is a measure of the sensitivity of the film. In DSLR cameras, the ISO setting is essentially a gain setting for the sensor amplifier. But the film ISO vs DSLR ISO analogy is a good analogy. Higher speed (higher ISO) film is more sensitive but produces a grainy (noisy) image. Higher ISO settings make the DSLR more sensitive but also produces a noisy image. SNR is important for occultations. Better SNR generally leads to better accuracy in the data reduction. Therefore you should use the lower ISO rating necessary for your recording.
Choosing your ISO setting can require some trade-offs. Lower ISO settings lead to better SNR but may longer shutter speeds. And longer shutter speeds mean slower frame rates and therefore less time resolution. So you may have to choose between a noisy recording (high ISO) with good time resolution (higher frame rate) versus a less noisy recording (lower ISO) with less time resolution (lower frame rate). In general we want the highest time resolution that yields a reasonable image of the star. For your first attempts you should try different combinations of frame rate/shutter speed and ISO. Choose the combination with the highest frame rate in which you can still see the star on your cameras image monitor. But you don’t need to choose a frame rate faster than 30fps. Start with 30fps and the highest ISO your camera will support. If you can see the star, reduce the ISO until the star almost disappears or you think the noise level is low enough. If you cannot see the star with the initial 30fps and a highest ISO setting, reduce the frame rate until you can see the star. Then you can adjust the ISO setting downward until the star almost disappears or the noise level is low enough.
As you gain more experience you will learn more about the limits of your equipment and what magnitude range you can actually measure with different settings. But you will always find a few surprises. Every observing situation is different. Sky conditions will be different each time. And the posted visual magnitude for a star may not be a good indication of the brightness of the star when viewed by a DSLR. Whenever possible you should try to run some tests prior to the actual event (even some days in advance) to determine what settings will work best.
If you record one or more positive occultation events on video with your DSLR your next step is determining the time(s) of the event(s) in your recording. After downloading the video from your camera, the data reduction process is very similar to the data reduction for other occultation video recordings. The one difference will be UT time synchronization. Most current video observations use a GPS based video time stamp system to determine the UT Time synchronization. In contrast, DSLR video recordings will rely on the audio track recording of a UT time signal for UT synchronization. Many early video observations employed UT signals on the audio track. So this approach is not new. You will find information about extracting event times from video recordings in the IOTA user’s manual and in the information posted at occultations.org.
Finding the target star
Finding the target star for an asteroid occultation can be very challenging – particularly if the star is mag 11 or dimmer. It is easier when you are using a piggy mounted DSLR with a regular camera lens that has a wide field of view. And it is easier if you are using a permanently mounted telescope with accurate GoTo capability. But most asteroid occultation observers will not be this fortunate. If your DSLR is imaging through a telescope you have a choice of two basic strategies. You can mount the DSLR on the telescope, go to a bright star, establish focus, then move to the target star via GoTo or star hopping while using the DSLR as your “eyepiece”. Or, you can put an eyepiece on the telescope, establish focus with the eyepiece, move to the target star via GoTo or star hopping, take out the eyepiece and insert the DSLR (don’t bump the scope), and establish focus with the DSLR. If the star is dim, it will probably be easier to find the star with the eyepiece, but it will be harder to focus the star afterward. It helps if you runs tests beforehand to determine the number of turns in your focus knob between eyepiece focus and DSLR focus.