Choosing your camera according to your instrument

An astronomical camera is not bought in a vacuum. Its usefulness depends almost entirely on the telescope it is paired with. Pairing them poorly means leaving performance on the table.

Sampling: the starting point

The central concept is sampling, which expresses how many arcseconds of sky correspond to one pixel on the sensor:

Sampling (arcsec/pixel) = (pixel size in µm × 206) / focal length in mm

The factor 206 comes from the radian/arcsecond conversion (206,265 arcsec per radian). A 3.76 µm pixel on a 1,000 mm telescope gives approximately 0.78 arcsec/pixel.

Which sampling should you aim for? It depends on the local seeing, the atmospheric turbulence that blurs the stars. In Quebec, it generally hovers around 2 to 4 arcsec FWHM depending on the night. The practical rule: aim for 1 to 2 arcsec/pixel, which matches the sensor’s resolution to what the atmosphere actually allows. Below that, the signal is diluted over too many pixels with no gain in detail; this is oversampling. Benefits can generally be obtained from oversampling when it is up to two times smaller than the achievable resolution. Above it, fine details are crushed; this is undersampling, and resolution suffers directly. Some websites, like this one, let you calculate the optimal pixel size for the seeing.

For planetary lucky imaging, the logic is reversed. You deliberately aim for 0.1 to 0.3 arcsec/pixel, because exposures of a few milliseconds freeze the turbulence. You compensate with volume: thousands of frames, of which only the best are kept.

Focal ratio

Focal length alone isn’t enough to characterize an instrument. The focal ratio (f/D) has direct implications for the camera.

A fast instrument (f/4 or lower) concentrates light quickly, but the rays arrive at a steep angle. As mentioned in the article on filters, this causes band shift on dichroic filters, and aberrations at the edge of the field (coma, astigmatism) are harder to correct. A sensor with large pixels is more tolerant in this case, since it is less sensitive to sub-pixel aberrations.

A slow instrument (f/8 or higher) is more forgiving optically, but requires longer exposures. Very low read noise then becomes valuable, since short exposures can be stacked to compensate.

Sensor size and field of view

The field of view is calculated directly:

Field (arcminutes) = (sensor size in mm × 3438) / focal length in mm

An IMX571 sensor with a 28.3 mm diagonal on a 500 mm focal length covers approximately 3.3° × 2.2°, good for extended nebulae or open clusters. The same sensor on 2,000 mm drops to 0.8° × 0.5°, better suited to galaxies.

You also need to check that the telescope’s image circle fully covers the sensor. An instrument designed for an APS-C sensor will not cover a full-frame one without vignetting, which can sometimes be corrected with a focal reducer, and sometimes not.

Useful resolution

A point that is often misunderstood: more pixels does not mean more detail. If the seeing limits you to 2 arcsec and the sampling is already at 1 arcsec/pixel, switching to a sensor with twice the resolution and pixels half the size will give nothing more in the final image, just heavier files and a lower signal-to-noise ratio per pixel.

Useful resolution is the one that matches the seeing. Beyond that, stacking recovers nothing new.

Read speed

For planetary imaging, the number of frames per second is critical. 100 frames per second over 10 minutes makes 60,000 frames, enough to select the best 5 to 10% even with mediocre seeing.

For deep-sky, this criterion is almost irrelevant. A camera at 10 frames per second is more than enough when exposures last several minutes anyway.

A few concrete combinations

• A short f/5 refractor of 500 mm calls for pixels of 5 to 7 µm for reasonable sampling, and benefits from a large sensor to take advantage of the wide field that this type of instrument naturally offers.
• A 2,000 mm f/10 Schmidt-Cassegrain pairs well with small pixels (2 to 4 µm); the long focal length brings the sampling back to correct values even with fine photosites. This is the territory of galaxies and planetary nebulae.
• A 300 mm f/4.5 Dobsonian with a 1,350 mm focal length sits between the two. A large sensor only makes sense if the image circle covers it. And at f/4.5, dichroic filters at 45° see their band drift slightly at the edge of the field, not a major issue, but something to anticipate.

The right combination of camera and telescope above all allows greater satisfaction with regard to image quality. It is nevertheless worth noting that not meeting these criteria will not necessarily produce bad images ;)