Most ground-based optical tracking still stops when the Sun comes up. For satellites in geostationary orbit, that's a manageable gap. For low Earth orbit (LEO), it's a serious one. A LEO satellite is above a given site for only a few minutes at a time, and a night-only telescope can see it lit by the Sun only in the short windows just after dusk and before dawn. As Zimmer et al. point out in their 2022 AMOS paper on daylight LEO measurements, that's not nearly enough to keep custody of a crowded orbital regime.
Daylight tracking closes that gap, and it changes what the camera has to do. This post covers those changes and the questions worth settling before you choose a camera for a 24-hour Space Domain Awareness (SDA) site. We use Finger Lakes Instrumentation (FLI) Kepler cameras as examples.
Why daytime tracking matters
A satellite passing over a sunlit site is lit directly by the Sun and indirectly by earthshine, the sunlight scattered off the Earth below it. By astronomical standards, LEO objects in daylight are bright. The problem is that the same scattering makes the sky bright too.
At night, the camera's job is to pull faint signal out of a dark background. In daylight, the job flips. The target is often easy to collect. The background is what you have to manage.
The sky is the noise floor
In daylight, the sky contributes far more signal per pixel than the camera's dark current or read noise. The photon noise from that background sets the noise floor, so the usual night-time priorities shift. Deep cooling still keeps dark current out of the picture, but it no longer decides whether you detect the target.
The background also changes fast around sunrise and sunset. A team reporting at ESA's 8th European Conference on Space Debris (2021) measured the sky background rising by about four orders of magnitude between a Sun altitude of -11.5° and about +10°, with the fastest change between about -5° and 0°. A camera and control loop that adjust exposure through that transition keep working; fixed night-time settings won't.
Short exposures and full well
A bright sky fills pixels quickly. Expose too long and the background saturates the sensor, burying the target. The fix is short exposures. The same ESA conference team reported that the exposure needed for a reference star fell from 0.01 s at nautical night to 0.002 s in daytime.
Pyanet et al. (IAC 2022) recommend keeping the image at roughly 50 to 75% of the sensor's range, enough signal for a good signal-to-noise ratio without saturation. That puts weight on full-well depth and dynamic range: the more charge each pixel holds, the more background it can take before it clips, and the more signal you collect per frame.
Kepler cameras address this with a high-dynamic-range readout. Their 16-bit image combines two 12-bit readouts at different gains, with less than 1% non-linearity. Measure the response of your own camera across the signal range you plan to use. Our post on acceptance testing for gain, noise and linearity shows how.
Choosing a band: redder isn't automatically better
The daytime sky is blue, so the instinct is to filter the blue out with a red or near-infrared long-pass filter. It isn't that simple. Zimmer et al. note that both sunlight and earthshine peak in the blue part of the visible spectrum, and many sensors are most sensitive there too. Their models put the best cutoff at typically 550 to 600 nm, with field tests underway.
Conditions matter as well. The ESA conference team found that the near-infrared sky brightens faster than other bands during sunrise, which hurts those filters during that transition. Test candidate filters at your own site, across the Sun altitudes you plan to cover. If you'll switch filters during the day, plan the filter wheel into the optical path. Our post on back focus, flanges and filter wheels covers spacing, and FLI's wheels are on our accessories page.
Frame rate, stacking and data volume
Short exposures mean many frames. When a single frame isn't deep enough, stacking short exposures builds up an equivalent longer one (Pyanet et al. 2022). A fast readout also loses less light between exposures.
All of that produces a lot of data. Plan the host computer, storage and processing pipeline for continuous high-rate capture, not just the camera. Our post on SDA sensor networks covers the data path at fleet scale.
Optics and pixel scale
Optics and sensor have to be balanced together. A longer focal length spreads the sky background over more pixels, so each pixel fills more slowly, but it narrows the field of view (Zimmer et al. 2022). Small daylight systems often run at a different focal ratio than the same site uses at night.
Work out your arcseconds per pixel before you choose. Our post on pixel size and sampling has the math.
Sun, stray light and heat
Daylight operation adds hazards a night site never sees. Set Sun-avoidance limits in the mount software, and use baffles and dew shields to keep stray light off the optics. Stray light raises the background in exactly the way you are trying to avoid.
Heat matters too. A dome or enclosure in full sun runs warmer than at night, so choose a cooling setpoint the camera can hold on your hottest afternoon, with margin. Our post on cameras for remote observatories covers cooling and unattended operation.
Choosing the camera
Kepler CMOS cameras are our SDA line. They're designed for fast, continuous, short-exposure imaging, with a wide field of view and high frame rates for continuous 24-hour use, including both day and night imaging. The KL6060FI uses a 6144 × 6144 sensor with an 86.9 mm format, and the KL4040FI a 4096 × 4096 sensor with a 52.1 mm format.
Kepler sensors use a rolling shutter with a global-reset mode, so ask how frame start times are defined across rows if your program depends on precise timing. A GPS timestamp option is listed for Kepler and Aurora cameras; contact FLI for current status on your model.
User-replaceable shutter, fans, and digital board make Kepler ideal for remote installations. The Kepler SDK runs on Windows and Linux, and FLI provides an ASCOM driver for Kepler cameras. Drivers and manuals are on our support page.
Some daytime programs also look beyond the visible band. FLI SWIR cameras are coming soon.
Questions to settle before you choose
- Which orbits and objects do you need to track in daylight, and how bright are they?
- What Sun altitudes and sky angles will you observe at?
- How deep must each frame be, and will you stack?
- Which filter band, if any, and will it change through the day?
- What focal length and pixel scale fit your field of view needs?
- What cooling setpoint can you hold on your hottest afternoon?
- Can your host and pipeline keep up with continuous high-rate capture?
Data-sheet values are typical. A production test summary exists for each camera and can be provided by serial number, which helps when you model daytime performance.
Talk to our sales team
FLI cameras are Made in USA, and our sensor chambers are assembled in our 1,000+ sq ft ISO Class 7 clean room. See how on our in-house manufacturing page.
Send our sales team your telescope, targets, observing hours and filters, and we'll help you choose a camera for round-the-clock tracking. Request a quote or email sales@flicamera.com.
