Archive for January, 2010


Emacs is a class of feature-rich text editors, usually characterized by their extensibility. Emacs has, perhaps, more editing commands compared to other editors, numbering over 1,000 commands. It also allows the user to combine these commands into macros to automate work. Some people say: “Emacs is not an editor, it is an operating system”. If you are a fan of emacs (like me), you can use this short list of commands.


Emacs has options to show commands of a variety of programming languages, e.g., Fortran, C++, IDL, … along with common purpose stuff like LaTex.  You have also options to select parts of text based of columns, not just rows.

Sample of a C++ computer program in Emacs

If you prefer to use it as inline editor, i.e., without opening a new window, you can use no window option:

$emacs -nw your_file.f90

If you want to customize Emacs, put a  .emacs file in your home directory. It allows  you to  customize the behavior of Emacs. There are plenty of such sample .emacs files.  Here is a sample.

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Why do I love CATs?

A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as “catadioptric” such as surveillance catadioptric sensors.

Catadioptric telescopes

Catadioptric telescopes are optical telescopes that combine specifically shaped mirrors and lenses in designs that have all spherical surfaces that are easier to manufacture, have an overall greater degree of error correction than their all lens or mirror counterparts, have a wide field of view, take advantage of a folded optical path, or a combination of any or all of these attributes. Many types employ “correctors”, a lens or curved mirror in a combined image-forming optical system so that the reflective or refractive element can correct the aberrations produced by its counterpart.

Catadioptric dialytes

Catadioptric dialytes are the earliest type of catadioptric telescope. They consist of a single element refractor objective combined with a silver backed negative lens (similar to a Mangin mirror). The first of these was the Hamiltonian telescope patented by W. F. Hamilton in 1814. The Schupmann medial telescope designed by German optician Ludwig Schupmann near the end of the 19th century placed the catadioptric mirror beyond the focus of the refractor primary and added a 3rd correcting/focusing lens to the system.

Full aperture Correctors

There are several telescope designs that take advantage of placing full diameter lens (commonly called a “corrector plate“) in front of a spherical primary mirror. These designs take advantage of all the surfaces being “spherically symmetrical” and were originally invented to create optical systems with very fast focal ratios (wide fields of view) with little coma or astigmatism for use as astrographic cameras. They work by combining a spherical mirrors ability reflect light back to the same point with large lens at the front of the system (a corrector) that slightly bends the incoming light, allowing the spherical mirror to image objects at infinity.  Some of these designs have been adapted to create compact long focal length catadioptric cassegrains ( a cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes).

The Schmidt corrector plate

The Schmidt corrector, the first full diameter corrector plate, was used in Bernhard Schmidt‘s 1931 Schmidt camera. The Schmidt camera is a wide field photographic telescope, with the corrector plate at the center of curvature of the primary mirror, producing an image at a focus inside the tube assembly where a curved film plate or detector is mounted. The relatively thin light weight corrector allows Schmidt cameras to be constructed in diameters up to 1.3 m. The corrector’s complex shape takes several processes to make, starting with a flat piece of optical glass, placing a vacuum on one side of it to curve the whole piece, then grinding and polishing the other side flat to achieve the exact shape required to correct the spherical aberration caused by the primary mirror. The design has lent its self to many Schmidt variants.

Popular sub-types
  • Schmidt-Cassegrain telescopes are one of the most popular commercial designs on the amateur astronomical market, having been mass-produced since the 1960’s. The design replaces the Schmidt Camera film holder with a Cassegrain secondary mirror making a folded optical path with a long focal length and a narrow field of view.

Light path in a Schmidt-Cassegrain telescope.

The meniscus corrector shell

The idea of replacing the complicated Schmidt corrector plate with an easy to manufacture full aperture spherical meniscus lens (a meniscus corrector shell) to create a wide field telescope occurred to at least 4 optical designers in early 1940’s war torn Europe, including Albert Bouwers (1940), Dmitri Dmitrievich Maksutov(1941), K. Penning, and Dennis Gabor (1941). War time secrecy kept these inventors from knowing about each others designs leading to each being an independent invention. Albert Bouwers built a prototype meniscus telescope in August of 1940 and patented it in February of 1941. It used a spherically concentric meniscus and was only suitable as a monochromatic astronomical camera. In a later design he added a cemented doublet to correct chromatic aberration. Dmitri Maksutov built a prototype for a similar type of meniscus telescope, the Maksutov telescope, in October of 1941 and patented in November of that same year. His design corrected spherical and chromatic aberrations by placing a slightly positive shaped meniscus corrector closer to the primary mirror.

Popular sub-types
  • Maksutov Cassegrain telescopes are the most commonly seen design that uses a meniscus corrector, a variant of the Maksutov telescope. It has a silvered “spot” secondary on the corrector making a long focal length but compact (folded optical path) telescope with a narrow field of view. This design idea appeared in Dmitri Maksutov’s 1941 notes and was original developed in commercial designs by Lawrence Braymer (Questar, 1954), and John Gregory (1955 patent). The combination of the corrector with the silvered secondary spot makes Maksutov Cassegrains low maintenance and ruggedized since they can be air sealed and fixed in alignment (collimation).

Light path in a typical "Gregory" or "spot" Maksutov-Cassegrain telescope.

Schmidt-Cassegrain or Maksutov-Cassegrain?

The SC cools more quicker, and thus you get better images sooner than the very thick corrector of a Mak/Cass. The Maks are also heavier than SC, as my 8″ Mak needs a mount that would hold an 11″ SC.

All told, they both hold alignment fairly well, and the optics are kept cleaner due to sealed OTA.

The Mak/Cass should give better images but I find it difficult to tell the difference between 8″ scopes of these two types. I hear people say when they look through mine, that subtle planetary details are a bit more apparent.

The Mak is certainly more expensive inch for inch compared to an SC. They are also fairly uncommon.

Since all the optics in an MCT are spherical (the primary, the secondary, and both sides of the corrector), and since making and testing spherical optics is easier, it takes less work to shape and polish MCT optics to a higher precision. In comparison, SCT corrector plates, with their weird curve, are difficult to shape correctly and next to impossible to polish to maximum smoothness. So it’s not really that the MCT design gives superior images, it’s just that MCT optics tend to be of higher overall quality than an SCT of the same size.

Most MCT’s have a proportionally longer tube than a comparable SCT owing to the longer focal ratio of the primary mirror. Thus in the larger sizes, an MCT will end up being bulkier, and it will be more expensive (due to the cost of the corrector, mentioned previously). Also, due largely to the thickness of the meniscus corrector, the time required for an MCT to adjust itself  to the outdoor temperature will be greater than for an SCT. In the plus column is the fact that the image sharpness in a well-made MCT is generally better than in any SCT.

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Sometimes I face this simple question from students: how can it be that the total radiation of a black body does not depend on any of its detailed atomic properties?! The key is to understand the concept of thermodynamic equilibrium.

Assume I(ν), the intensity at frequency ν, differed for two enclosures at the same temperature, T, over some frequency interval, . Connect the enclosures with a filter that passes only radiation of that frequency interval. Radiant energy would now flow from one enclosure to another, making it possible to extract useful work from two reservoirs at the same temperature, T, in violation of the second law of thermodynamics. Thus our assumptions must be in error, and the monochromatic specific intensity of a radiation field in thermodynamic equilibrium with matter must be a universal function of T; in modern notation;

Kirchhoff    law :    I(ν) = B(ν, T)

Kirchhoff set a goal for future generations of finding the correct functional form of B(ν, T). Max Planck found the approximate formula.

Reference: The Physics of Astrophysics: (I) Rdiation, by Frank Shu

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Sunspots are intersection of the solar surface with a large magnetic flux tube. They appear in the activity belt, latitudes up to ±40° at the beginning of a solar cycle and tend toward equator at the end of a cycle. Formation time-scale of large sunspots is from a few hours to a few days.
The central part of a spot is the umbra. It is the darkest part of the spot and show the strongest magnetic field. A radial structure surrounded the umbra which shows an outflow at the photospheric layer. This layer is called penumbra because has an intermediate intensity between the umbra and quiet sun.

A simple sunspot. The central dark area is the umbra and the gray filamentary structure is the penumbra.

The radial outflow in the penumbra is known as the Evershed flow , which was discovered by Evershed at the Kodaikanal observatory, India, more than a century ago. The outflow velocities are typically 3-5 km/s. In chromosphere and transition region, it reverses into a rapid inflow inverse Evershed effect.  Nevertheless, the mass flux carried by thee inverse Evershed flow is over an order of magnitude smaller than in the photospheric Evershed flow.

A complex sunspot. The bright area inside the umbra is called light bridge.

Umbra is 1,000-1,900 K cooler than the quiet sun while this temperature difference in the penumbra is about 250-400 K. Relative brightness of the umbra to the quiet sun is 20-30%, while for the penumbra, it is 75-85%. The ratio of total to umbral area, r_A=A_t/A_u ~ 5 ±1. It seems that r_A is smaller at the solar cycle maximum.  Another interesting feature is that integrated intensity over wavelength of sunspot umbrae are approximately 17% darker at the beginning of the solar cycle than the end.

A complete active region.

Dimension of sunspots spans a wide range, 3,500 km < D<60,000 km. Smallest sunspots are smaller than large pores. The size distribution of spots is  log-normal. Typically, the product of a fragmentation process exhibits a log-normal distribution. The log-normal distribution of sunspot areas thus implies that the associated magnetic flux tubes  are the end products of fragmentation of a large flux tube (at the bottom of the convection zone).

The formation of sunspots is intimately related to formation of active regions as a whole. Lifetime of sunspots vary between ~ hours to  ~ months.

Sunspots start to decay immediately after formation. The decay rate of the sunspot area, dA/dt, is supposed to be linear (α A) or quadratic function (α \sqrt{A}) if  the erosion of the magnetic field happens  for the whole body or only at the boundary of the spot.

Credits of photos: http://dotdb.phys.uu.nl/DOT/

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Unison: backup package

Unison is my favorite backup tool. You can run it either on the command line or GUI. It synchronize two directories. You will have the option to check each update before pushing the “go” bottom.

After installation from the repository, go to command line and type:

$ unison-gtk

a small window will open and ask you either to select a profile or create one. To make a new profile, click the bottom. A panel like this  image appears:

Creating a new profile in Unison.

Type a name. Then, it will ask for the address of the folder you want to backup.  Call it the root folder. You will face a window like this:

address of the root folder

Similarly, you will be asked for the address of destination folder.  After that, there is a warning that it is the first time you are synchronizing these two folders. So everything will be renewed in the destination folder (I always use a new (empty) folder for destination).

Just one click remains: Go !

Next time when you open unison and select a profile, you just check the difference between the two folders and apply update as you need.

Note 1)  if you modify a file in destination folder and do an update, it will update the file in the root folder.

Note 2) if you modify a file both in the root folder and  destination folder, it get confused ! You have to explicitly tell it which one do you prefer.

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