During the acquisition of this image, sunspot AR2673 (below and to right of centre) was growing to more than ten times its original size during the course of only 24 hours. It quickly became one of the largest sunspots of 2017.
The Sun's diameter is 109 x Earth's diameter. The largest sunspot on the image is three times Earth's diameter.
Date: 3 September 2017, 13:30 GMT
Telescope: Skywatcher Esprit ED 120
Mount: Astrosysteme Austria ASA DDM160
Solar filter: Thousand Oaks Type 2+, 90 mm diameter
Camera: Lumenera SKYnyx2-2M
Exposure: 11 ms
Number of images in stack = 35 of 200
Producing quality optics is a labour-intensive process. After coarse and fine grinding to the target focal length, the work consists of polishing and testing, polishing and testing and still more polishing and testing. This process is not completed until the difference between the mirror's surface and an optically perfect paraboloid is so minuscule that the breadth of a human hair is 300 million times larger.
One can make quality telescope optics by testing them against an optically flat mirror. One must use a mirror that is so flat that a hallway mirror is an alpine landscape in comparison. An optically flat mirror is easy to make by exploiting Earth's gravity with a pan of cooking oil. I prefer rapeseed oil.
The animation shows the set-up for testing the telescope's f/3.4 primary mirror. Light is shown as a wave-front produced by the light source below. The light first passes through a hole in the pan of cooking oil. The surface of the oil forms a very flat mirror with radius of curvature = Earth's radius = 6357 km. This is more than flat enough for a 62 cm diameter mirror. The spherical wave-front travels upward and meets the parabolic mirror, reflecting back down as a plane wave-front, and reflecting back up again after striking the flat surface of the oil. Paraboloids transform spherical wave-fronts to plane wave-fronts when the spherical wave-fronts have a radius of curvature which is equal to the paraboloid's focal length, - in other words, when the light source is located at the paraboloid's focus.
To and fro
The oil flat reflects the light back to the primary mirror, which reflects the plane wave-fronts as spherical wave-fronts where they meet at the focus where they originated.
This method of testing optics is called autocollimation. Its earliest documented application was in 1890, by Harold Dennis Taylor, an optical engineer employed by T. Cooke & Sons of York, England. Taylor used autocollimation for testing objectives of refracting telescopes1.
In optical jargon, autocollimation is called a null test. What one observes using this test is the sphericity of wave fronts converging at the focus.
All optimized, image-forming optics form spherical wave-fronts concentric to, and converging upon the focus. From the instant they leave the last element in the optical train until they converge at focus, the wave-fronts are spherical, and the focus is their center of curvature.
The auto-collimation set-up is made of three columns, 2216 mm long, made of Leca®-blocks. The are blocks made of fused clay spherules. Atop the columns, the 62 cm parabolic primary mirror rests on a steel triangle. The triangle is dimensioned such that cork pads placed in the middle of each side support the mirror 93 mm from the mirror's outer edge. This is the 70-percent zone, which produces the least amount of deformation of the mirror's surface.
Levelling screws are located in two of the triangle's corners.
A head high
The light-source, Ronchi grating2 and video camera is located below the oil pan in the central hole. To visually assess the surface quality of the mirror, there is just enough space below the mirror's focus to peer through the test apparatus. The focal length of the primary (2108 mm) gives sufficient space for the entire vertical structure in a room with a ceiling height of 2400 mm.
The ASA DDM 160 mount, controlled by Autoslew, was tracking on the comet for ten minutes. The monochrome Apogee U16M camera made two minute exposures five times consecutively through red, green, blue and luminance filters.
Twenty-five thousand light-years distant and 150 light-years in diameter, Messier 13 is one of the brightest globular star clusters in the northern sky. It contains around 300 000 stars. Stars in the cluster's core are spaced less than 0,3 light-years apart.
This photo was taken while the winter sky was flooded with the bright glow of a 98.5 % moon only 86 degrees away at an altitude of thirty degrees.
Date: 12 Feb 2017
Loc: Rennesøy, Norway
Telescope: Esprit 120 ED 840 mm f/7
Camera: Apogee Alta U16M
Mount: ASA DDM160
Exposure: LRGB 20/20/20/20 min
CCD-temperature -30 C
Processing: Maxim DL 6