Frequently Asked Questions About Astronaut-Acquired Photographs

Questions about access to photographs:

What is the resolution of the photographs?
How does the resolution compare with data from robotic satellites like Landsat or SPOT?
What are the smallest objects that can be seen in the photographs?
How does pixel size or instantaneous field of view relate to the size of objects can actually be distinguished in a space photograph?
What is the modulation transfer function for astronaut-acquired images?
Since astronauts are travelling so fast in orbit, why don't the photographs look smeared?

What is the resolution of the photographs?

Resolution is a complex issue because it depends on aspects of the equipment (the altitude, camera, lens and film used), and on the environment in which the photo was taken (light, atmospheric interference, look angle, etc.)

A detailed summary of our technical information on resolutionof the systems used by astronauts to photograph the Earth is in preparation, and will be linked here. For general information purposes, a description of some aspects of resolution are given below.

How does the resolution compare with data from robotic satellites like Landsat or SPOT?

If the image is converted to digital form, the pixel size depends on the lens used, the look angle, and the resolution at which the image is scanned. One can look at the absolute best possible resolution (looking perfectly nadir of the orbit) based on the geometry of the altitude, film size and focal length).

Table: Maximum possible digital resolution (minimum pixel width in m) for hand-held photographs scanned at 2400 ppi.

		Shuttle Altitude
		Minimum	Median	Maximum
Camera	Lens	222 km	326 km	611 km
		(120 n. mi.)	(176 n. mi.)	(330 n. mi)
Linhof	90 mm	26.1	38.3	71.8
	250 mm	9.4	13.8	25.9
Hasselblad	100 mm	23.5	34.5	64.7
	250 mm	9.4	13.8	25.9
Nikon	300 mm	7.8	11.5	21.6
	400 mm	5.9	8.62	16.2

Because most views are not perfectly nadir, a more conservative rule of thumb for an average 176 n. mi. (296 km) mission is that the 100-mm lens offers spatial resolution similar to that of the Landsat Multispectral Scanner (approximately 80 meters) and the 250-mm lens has resolution similar to that of the Landsat Thematic Mapper (approximately 30 meters).

What are the smallest objects that can be seen in the photographs?

As far as ground resolved distance is concerned, our best case so far (with a 250 mm lens) is resolving 8 m objects on the ground. These have included building shadows, roads, and boats in harbors. Most photographs have much lower resolution.

How does pixel size or instantaneous field of view relate to the size of objects can actually be distinguished in a space photograph?

Even though people often say that TM "resolution" is 30 m, that is actually technically incorrect. For Landsat TM, pixel size or instantaneous field of view (IFOV) is what is actually 30 m x 30 m. This number does not mean that objects of 30 m in width can be identified in an image. The ability of the system to image objects of various widths depends on a number of factors i ncluding the contrast between the objects. A more inclusive and realistic (but also subjective) measure of resolution used primarily in aerial photography is ground resolved distance (GRD).

Instead of specifying a resolution for an entire system, you can also think about resolution from an image-specific point of view. In general every image has a ground resolved distance (GRD) that depends on spatial resolution, contrast and texture as well as on the characteristics of the system that was used to capture the image. As an example, image a winter scene and a summer scene of a nothern decisuous forest with clearcut areas. The winter scene will have higher contrast (snow vs. trees) than the summer scene (snow vs. green meadows). Thus, for a study trying to identify the size of clearcuts in the area, a TM image in winter will have better GRD than one taken in the summer, and smaller clearcuts would be able to be distinguished in the winter scene.

An image-specific GRD is something we can measure for space photos as well. From the Shuttle, the best GRD measured to date is about 8 m (using a 300 mm lens and a Nikon camera with 35 mm film). In other words, one could readily distinguish a road that was known to be 8 m wide. The worst case GRD for a space photo would approach infinity for some a photograph taken with a short lens, slightly out of focus, of a low contrast and hazy area.

What is the modulation transfer function for astronaut-acquired images?

By including a modulation transfer function (MTF) for a remote sensing system you get a more realistic approximation of how well that system can distinguish objects of various widths than you get from a simple calculation of instantaneous field of view (IFOV). For example, it is sometimes said that when the MTF is included Landsat TM resolution changes from 30 m to 60 m. However, the resulting larger "resolution" number you get is still an underestimate of ground resolved distance for many purposes.

One could calculate MTF for a photographic remote sensing system from the direction of lens distortion at given distances and other parameters such as ground smear, etc. For handheld photographs by astronauts, however, there are many uncontrolled variables that have a greater influence on resolution (like look angle and window distortion) so no one has ever developed a set of MTFs.

Since astronauts are travelling so fast in orbit, why don't the photographs look smeared?

If you do the calculation for the amount of ground smear expected based on the shutter speed of the camera and the velocity of the Shuttle relative to the ground, you would expect that the ground resolved distance could be no better than about 30 m. Because we have much better resolution (i.e. boats in harbors, narrow roads), we know that consciously or unconsciously, astronauts track the Earth when they take a photo, so the resolution is actually better than you would calculate.