How big are photos taken in space?

Question

How big are photos taken in space? (Megapixels)

Furthermore, are there any standards in regards to photo size, quality, etc or do most entities building satellites for exploration or scientific data gathering purposes just try outfit the probe or craft with whatever their budget will allow? (Do they use techniques like stitching, onboard or after transmission? etc)

Would a company be perhaps required to put in a more expensive camera unit than the budget allows for because of an existing standard?

Answer 1

How big are photos taken in space?

It depends. The two extremes are:

• the Voyager IRIS (Infrared Interferometer Spectrometer and Radiometer) instrument, an IR detector with 1 pixel. The Pioneer 10/11 Imaging Photopolarimeter (IPP) also used 1 pixel.
• Gaia, with a 938 megapixel CCD. These images are not downloaded in full. From an image, only the pixels that contain stars are downloaded.

These missions are 50 years apart, so of course there are going to be big differences.

There are no set standards. Every mission uses the cameras it needs. For scientific missions, older instrument designs are sometimes reused to reduce cost. One recent example is the L'LORRI instrument on Lucy, which is derived from the New Horizons LORRI.

Stitching is done extensively. The Pioneer PPI images were stitched together from strips 1 pixel high. As far as I know stitching is mostly done on the ground.

Would a company be perhaps required to put in a more expensive camera unit than the budget allows for because of an existing standard?

No.

Image sizes

I'm not going to use the above examples to discuss image sizes, as they're both not well-suited for that. The Pioneer IPP used 1 pixel, they could build images of arbitrary size by moving that pixel in X and Y directions.

Gaia raw images are not sent to Earth, but are processed on board.

So, different examples.

1. Voyager Imaging Science Subsystem. Built in the early 1970s. Used an 800 x 800 pixel x 8 bpp camera which took 48 seconds of readout time per frame. so 640 kbyte per uncompressed image. I get the impression they did not use image compression.

2. New Horizons LORRI: 1024 x 1024 x 12 bpp CCD with 13 ms readout time. Images can be sent uncompressed or JPEG compressed. Uncompressed images are 1.5 Mbyte.

3. Hubble WFC3: 4096 x 4096 CCD. No details found yet on compression, bit depth.

Answer 2

From this paper about the LORRI camera:

LORRI is a narrow angle (field of view=0.29°), high resolution (4.95 μrad pixels), Ritchey-Chrétien telescope with a 20.8 cm diameter primary mirror, a focal length of 263 cm, and a three lens field-flattening assembly. A 1024 × 1024 pixel (optically active region), thinned, backside-illuminated charge-coupled device detector is used in the focal plane unit and is operated in frame transfer mode.

If we take the field of view, 0.29 degrees or 5.061 mrad (milliradian) and calculate the angular resolution of the telescope for a medium wavelength of 600 nm and a diameter of 0.208 m, we get 2.91 µrad using the formula

$R = λ/D$

$R$ is the angular resolution, $λ$ the used wavelength and $D$ the diameter of the telescope.

See wikipedia for the formula of the angular resolution of a telescope.

When we divide the field of view (5.061 mrad) by the calculated angular resolution (2.91 µrad), we get the maximum number of pixels 1737. Using a 1024 × 1024 pixel sensor is therefore a good choice.

A 2048 * 2048 pixel sensor could deliver more pixel data, but not more image information. The maximum angular resolution of the telescope is defined by the diameter of the telescope and the used wavelength. To get a better resolution, we need a telescope with a larger diameter.