Confusion on the border: photography versus astronomy

One of the biggest confusions you might notice when you venture into astrophotography is that photographers and astronomers measure their equipment differently. Photographers tend to refer to their lenses in terms of focal length, while astronomers refer to their telescopes by the diameter of their aperture. For example, a 75mm f / 6 telescope has a focal length of 450mm. Meanwhile, a 75mm f / 6 lens has an aperture of 12.5mm. If a photographer learns that someone is shooting with a 155mm lens hand-held, it won’t raise an eyebrow, but an astronomer tells them the same would be incredulous! I can barely lift my 155mm telescope on its mount!

As another reference example, check out the gear in the main photo above. A 107mm refracting telescope sits alongside a 105mm camera lens – big difference! See below for photos of the two configurations. The 107mm telescope sight is circled in the 105mm lens shot.

Another point of confusion is that although photographers’ lenses are made almost exclusively of refractive elements, astronomers use a wide variety of optical designs, many of which also include internal mirrors. As telescopes get bigger, mirrors are preferred for the primary because they can fold the light path back on itself, and producing large precision mirrors is not as expensive as producing large lenses. . Large lenses get heavy and require optically perfect properties throughout glass, while only surface area is important in a mirror.

Main lens vs. main focus

Another source of confusion is the use of the term “first”. In photography, a primary lens has a fixed focal length, as opposed to a zoom lens which can vary the focal length. In astronomy, all telescopes have a fixed focal length (with the exception of some visual telescopes that have a zoom eyepiece), but you may hear references to “primary focus.” Technically, the main focus is the position at which the main element (lens or mirror) comes to focus. In the past, for reflective telescopes, only specialized telescopes (e.g. the Schmidt camera) and larger telescopes, such as the 200-inch Hale (Palomar) telescope, allowed the sensor (film) to be placed in the primary focus.

In general, some additional optics (lenses) are needed at the main focus (near the imaging plane) to correct the distortions introduced by the main optic. In astronomy, you may hear references to coma correctors and flatteners, which are necessary to get good images across the imaging plane. Focus reducers or Barlow lenses, which shorten or lengthen the effective focal length of the telescope, are also sometimes used. Barlow objectives are the equivalent of photographic teleconverters, but are available for more extreme magnifications of 2x, 3x and above.

The other general difference is that camera lenses (even primary lenses) have complex optical designs with many elements (sometimes more than a dozen). On long focal lengths, the additional optics help to shorten the overall (physical) length of the lens. On very short focal length lenses, additional optics are required to move the focal point far enough away from the lens to reach the sensor inside the camera body.

In astronomy, telescopes are designed with as few optical elements as possible. Each optical element introduces places where interfering internal reflections can be generated or unwanted absorption of light can occur.

Magnitudes and F-stops

Even the way we think about light can be confusing in photography compared to astronomy. In general, photographers are more concerned with the total amount of light reaching their camera in terms of diaphragms (a factor of two). In the “exposure triangle” (digital), photographers can choose between aperture, exposure time or sensitivity (ISO).

In astronomy, one of the corners of the exposure triangle (aperture) does not exist for astrophotography, other than switching to a larger or smaller telescope. With a few exceptions, astronomers use wide-open telescopes. Additionally, since most objects of interest emit light, astronomers refer to the “greatness” of targets. These are measured in steps of 2.512 for historical reasons, but the easiest way to think about it is that a magnitude difference of 5 is a brightness difference factor of 100 (about 6.6 stops).

Another source of confusion is that a difference in magnitude of +5, means 100x dimmer, and a difference of magnitude -5 is 100x brighter. In addition to this, a visual magnitude reference point has been established so that the brightest stars in the sky visible to the naked eye are around visual magnitude -1, again for historical reasons. With the naked eye, the limit of stars that we can see is usually around a visual magnitude of +6.5.

Keeping in mind the eye’s visual magnitude limit of +6.5, if we take the maximum pupil opening to around 5mm and increase it to 50mm using a small telescope because the amount of light captured depends on the area of ​​the aperture, we can receive 100x more light, or a boost of magnitude 5 to +11.5. So even a small 50mm telescope (the typical size of a finder or binoculars) greatly increases the magnitude limit of the stars we can see or photograph. Of course, this is theoretical. In practice, the brightness of the sky due to light pollution or auroras decreases our sensitivity, and looking at a low angle through more atmosphere also decreases our ability to detect a faint star.

For sky objects with an extended size, the visual magnitude estimate is the total light coming from the object, so compared to a star of the same magnitude, it will be more difficult to see. For example, the visual magnitude of the Andromeda galaxy is around +3.4, which is equivalent to a moderately bright star, but in practice it is much harder to see than its visual magnitude suggests since ‘it stretches over a large area of ​​the sky (several times larger than the moon).


Another area of ​​confusion is the notation of filters. Photographers often use filters to dim the light enough to take long exposures so that bodies of water or cloud movement are smoothed out. Astrophotographers don’t do this, but a filter is needed to photograph the sun, where the problem is too much light. The sun has a visual magnitude of -26, which is beyond even what the fastest shutter speed on consumer cameras can handle, and will likely burn a hole in a device’s shutter. photo in seconds.

For solar photography, extremely dense filters are used, which are rated at optical densities of 4 to 5, which are powers of 10 in light reduction (10,000 to 100,000, respectively). One of the areas of confusion is that optical density ratings differ from neutral density (ND) ratings of camera filters, which offer much less light reduction (up to 4 or 5 stops) and are not. consistent in their density rating scales. See the Wikipedia table for a convenient way to see the correspondence between the different optical and neutral density scales.

Alpha-hydrogen filters

Another source of confusion is that astrophotographers often use very narrow band filters to improve the visibility of glowing gases commonly found in deep sky nebulae. Using these filters will increase the contrast between the nebulae of interest and the sky background. Typically, these filters will pass 5 to 10 nanometers of light around the wavelength of interest outside of the visual band which extends from about 400 nanometers (purple) to 700 nanometers (red). While these filters may look like ordinary screw-in camera filters, they are usually made from precision multi-layered coatings instead of colored glass or gel and therefore are more expensive.

These deep sky astronomical filters also differ from solar astronomical filters, which isolate even narrower bands. These specialized (and expensive) filters are typically less than 0.1 nanometers wide, so details can be seen on the solar surface and the atmosphere has enhanced contrast. These special sun filters can be chunky, multi-part arrangements for the front and back ends of the telescope instead of the thin pieces of glass or gel that photographers use in front of camera lenses.

Sky coordinates

With many modern astrophotographers using computerized mounts, familiarity with the sky coordinate system is less common these days, but it is still helpful to understand the sky coordinate system. Basically, the angular distance used in the NS direction corresponds to the latitude coordinates on Earth. The coordinates, called Declination, range from zero degrees directly above the Earth’s equator to +90 degrees above our North Pole, and likewise down to -90 degrees above our South Pole.

In the direction corresponding to longitude, however, astronomers call this coordinate “Right Ascension” (RA). Just as Earth’s longitude has 0 at Greenwich, with degrees East (+) and degrees West (-) measured from it, Right Ascension has a point of reference to the Sun’s location at l spring equinox. But the most important thing to note is that instead of using degrees / minutes / seconds, astronomers use hours / minutes / seconds, increasing eastward from 0 a.m. to 11 p.m. and returning to 0, like a clock. 24 hours. This means that 1 minute of angle in RA is not equal to 1 minute of angle in declination. It is 15 times larger in AR than in declination. And to add to the confusion, when the separation between objects in the sky is discussed, a “normal” angular measurement is used – 360 degrees = 21,600 arc minutes = 1,296,000 arc seconds.

Still confused?

If you have been confused by any of the above points, don’t worry. I suspect that many astrophotographers are not even aware of what could be a subtle and confusing overlap of terms in photography and astronomy. I may be in the same situation. If you have encountered any points of confusion that I did not raise, comment below!

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