Comatic Aberration and Chromatic Aberration

These two phrases are pretty much guaranteed to raise the blood pressure of optical aficionados: Comatic Aberration and Chromatic Aberration. There. Did your blood pressure go up? Then it is likely you have dealt with one or both of these issues before… and it is likely that you do not need to read further! For those looking around the internet for an example of these aberrations, seek no more!

Let us start with an image. This shot is of the December sky taken through a wide angle 20mm AFS Nikkor 1:1.8G ED lens on a Nikon D-810. The images were raw NEF files without any processing (except resize), either on board the camera or using software. Click on any image to see it in larger format.

Taurus and comet Wirtanen
Taurus and Comet Wirtanen

The image is a pretty typical night shot: 10 seconds focused at infinity and using 5000 ISO at f/2 (a little stopped down). The constellation Taurus is dominating the right side of the image. There is an airplane top-center moving to the lower left. If you follow the airplane’s future trail it leads to a faint greenish fuzzy object, Comet 46P/Wirtanen. This image is reduced in size…. but upon close, full-scale viewing, this image displays two of the common issues that astronomers and photographers aim to rid themselves of. Funny thing is that this lens gets fabulous reviews on sites like Amazon, and when I complained about these issues I was actually chastised! “Are you kidding? This is such a great lens!” Well, no. It’s not, and for the price, it really should perform a lot better. Add to this story the fact that the Nikkor 16mm fisheye actually is BETTER than this 20mm lens, and there you have an argument to not buy this 20mm lens. So, read on….

May I present to you comatic aberration:

comatic aberration in star images
Classic coma on the images of stars. These should be small round dots, not winged things….

This aberration is off to the sides of the image, off-the central axis. The further from the center, the worse this aberration gets. Some systems sprout seagull like wings from stars. This lens sprouts more than that. Ugly. The cause of this problem is in the optical design and is usually found in parabolic mirror systems like Newtonian reflectors. Alas, it also happens here in lens designs.

May I now present to you chromatic aberration:

Chromatic aberration in star image
An example of chromatic aberration: note the violet halo surrounding the bright star. The violet wavelengths focus at a different distance than all the rest. The fuzzy green object to the left is Comet Wirtanen.

Chromatic aberration has been the bane of the optical world for a long time, starting with those who first pointed telescopes up at the stars (i.e. those like Galileo, etc). A single lens acts very much like a prism in how it bends (refracts) light. The angle of refraction has to do with the light’s wavelength, so not all colors of light will come to focus at the same spot. This is usually handled with complex, multiple-lens systems like Petzval lens groupings using unique glass recipes than minimize chromatic aberration. Well, this lens? It suffers. When pointing at a bright white star, this lens gives an image very much like that of a simple two-lens refracting telescope, what is called an achromatic refractor. Well, they are notorious for having a violet to blue ring of light surrounding bright objects… and halos of blue around the moon and Jupiter. Not fun. Nope. This is why we have monstrously expensive systems like apochromats and Petzvals. We are talking expensive!

A Visit to SALT: The South African Large Telescope

During the last week of June 2018, I was in South Africa working with a school as they brought focus to their interests in students-centered, discussion-based learning. My host, Shaun Hudson-Bennett, a member of their Maths Department is a wonderful person and shared an interest in visiting the South African Large Telescope. We were based in the town of Somerset just east of Cape Town… about a 4 hour drive south of SALT. SALT is located in the town of Sutherland in a region that is largely rolling hills and flat open terrain reminiscent of what we would call “high chaparral” here in the American West. Sandy and silty soil is mixed in with scrubby shrubs and prickly plants that all struggle to get water to survive. The geology of the area is lovely to behold: layered metamorphic and sedimentary mountains, and one can even find a volcano remnant. Much of the area is a fossil lover’s dream-scape.

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Heading out of Cape Town, one must first get through some of the mountainous regions. It is very colorful, even in winter, with grape arbors and other fields planted.

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Headed into the high chaparral nearing SALT. The rods are very straight for a good length of time.

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Entering the town of Sutherland. Lots of eateries and dark sky observing sites are available here.

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Some of the smaller domes on the summit.

SALT itself is surrounded by what I call dark sky country: there are dark sky reserves in the area, and most businesses in the town of Sutherland are very much aware of the value of their dark sky commodity.  Small bed-and-breakfasts are spotted throughout the town, each advertising their private star-gazing pads and fields. Climbing the road out of Sutherland, one arrives at the summit region of the South African Astronomical Observatory, home to SALT and quite a few other facilities owned both by South Africa and a number of other foreign consortiums (Japan and Russia to name but two). At 1798 m (5899 ft), SALT is well placed for its work, primarily spectroscopy. The air is not too thin, and walking around was easy enough.

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The SALT dome and alignment tower.

SALT Facts:

  • Location: Sutherland, South Africa in the semi-desert region named the Karoo.
    • Latitude: 32°22′34″S
    • Longitude: 20°48′38″E
  • Altitude: 1798 meters (5899 feet).
  • Optical Design: Optical with a total of 91 hexagonal 1m diameter mirrors making a composite 11m diameter hexagonal primary.
  • First Light in September 2005.
  • Primary Science: Spectroscopy, polarimetry, imaging:
    • SALTICAM
    • Robert Stobie Spectrograph (RSS) (née Prime Focus Imaging Spectrograph).
    • The Berkeley Visible Image Tube camera (BVIT).
    • Fiber-fed High Resolution Spectrograph (HRS).
  • Website:  http://www.salt.ac.za/

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My host, Shaun (right), Frigid (the Penguin) and myself in front of the SALT primary mirror. The mirrors are very thin and lightweight. Combined they equate to an 11m diameter primary.

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The instrument payload package at prime focus (black painted objects). These move for tracking in both RA and Dec by sliding on rails.

The optics of the telescope are most interesting. The system has a fixed altitude for the primary, so pointing and tracking are not done by moving the primary and all attached equipment, as is done with most other systems. At SALT, tracking is accomplished by moving the equipment package at primary focus. So, to image, the camera system is slewed across the top of the observatory as the target moves through the field. The telescope can be rotated in azimuth to view other regions of the sky. Each segment of the primary is adjustable using remotely controllable actuators. This allows the system to be aligned rapidly. Alignment is accomplished using what I call a collimation tower, an interesting structure that looms high on the outside of the dome. Essentially the telescope is rotated until the tower-top is at the center of curvature for the primary. Each of the 91 mirrors of the primary are then tipped and/or tilted until they are precisely aligned to form a spherical primary system using lasers sent from the tower to each mirror on the primary.  To maintain alignment, the whole structure is air-conditioned to maintain conditions as close to ambient as possible. The interior was pretty chilly the day we arrived, as it was winter there (in June).

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The whole telescope can rotate in azimuth on air-bearings like this one. Once aimed, the bearings are deflated and the telescope remains static throughout integrations. Only the instrument payload set moves during data capture.

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Another view of the massive primary assembly.

Visiting?  They do allow visitors during daylight hours, twice daily (at this time), and reservations are very highly recommended. More information may be found on the SALT website:   https://www.salt.ac.za/