Debris finder, PolCor-2
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The mechanical box's surfaces:
Here is the instrument cabinet opened. First Göran shows the instrument's overview and outer parts that are included, in the image below we can discern the internal parts.
Here Göran had just taken the lid off. Look to the right and see the revolver for coronagraph stick up and the camera (grey) to left underneath the instrument.
The entire chassis and the internal parts are coated in a black color / anodizing to dampen reflections. Reflections brings unwanted stray light into the detector and must be reduced to a minimum. The camera has its range of sensitivity from the visible and well into the infrared range (infrared, above 650 nm). The color that appears to be black to the eye does not necessarily mean that the camera see it as black. IR can behave so that the could be reflected in black surfaces and one must carefully select the right type of surface treatment to obtain adequate suppression of reflections over the entire wavelength range to be used, not easy when they directly to the eye not appears.
In the image below you see the coronagraph and here you did have part of the explanation of the name of the instrument, Cor of PolCor-2.
Here appears the coronagraph's revolver with the various masks (beam blockers) and mirrors.
Its function is to block the star's direct light, just as when the sun's corona to be studied have the blinding light from the central part blocked. The turret contains a full set of masks to suit different object sizes. As you may recall, I have in a previous tutorial said that all the stars are point objects in normal telescopes. But optics, atmospheric and other things means that the observed angular diameter gets larger. Depending on circumstances, this diameter can be unequal, that's why several sizes of mask are needed.
The mask consists of a very thin layer of metal evaporated on a glass substrate, it is like a small dot in the middle of the filter. The sizes that can be selected on the masks is 1.5", 3" and 6" (when instrument is mounted on NOT). 3" means 3 arc second and one arc second is 1/3600 part of a degree. The mask does not block the light completely, a very small part of the light penetrates to give the opportunity to set the camera's and telescope's, focus and tracking. Three different magnitudes of damping for each size can be selected: 5, 8.75 and 12.5. The blocking dot also has "softed" edges to reduce edge effects. Notice the "pits" in the outer edge of the revolving wheel, they have the function to fix the various filters in the correct position. A motor controlled by a computer, select the correct mask.
Mirror set one:
The mirrors that are included in the system has a very high reflectivity, better than 99.5% Göran enlighten me. They are of two types, the flat one is used to redirect the light beam, and the parabolic divergent and focuses the light beam. The mirror together serves as a relay lens with 1: 1 ratio.
Filter holder to the right.
Image above shows the filter holder in the system. This is activated when one wants to study the narrow wavelength ranges, 0.3 nm to 0.5 nm. Some interesting resonance lines FeI 386 nm, CAII 393 nm, 589 nm Nal and KI 767 nm. Even the more normal filters U, B, V, R and I are available. The filter holder can be loaded with two filters at a time.
We follow the light path ahead and change thus chamber in the instrument box.
At the top shows the unit that compensates away diffraction from the secondary mirror holder.
In image above, at the top is the optical device that reduces the diffractions spikes. The diffraction spikes we've talked about earlier causing trouble here, just like in the amateur astronomer Newton telescope. The spikes gives disturbing light on the vague light from the dust. It lowers the contrast and something must be done about it. Using a technique called Lyot Stop based on Fourier transform, these spikes is compensated away or at least reduced. One thing that complicates the matter is that NOT (the telescope normally used) does not have an equatorial mounting without an azimuth. A good technique for larger telescopes because it simplifies the mechanics but with the disadvantage that the image field rotates. Field of rotation compensates for the telescope itself but spiders (secondary mirror holders) are not. It must be dealt with within this instrument, it is the computer's task to control the servo motor and the rotating filter (Lyot Stop) so it is in the correct position relative the spiders and the spikes it caus. A disadvantage of the Lyot mask is that it screens of some light.
Apparently this instrument contain a lot of motors. To design and build new motor system from scratch is time consuming and expensive. Göran, who is a very clever person, based on the designs in other areas which can be advantageously used to this. Believe it or not, but part of the servo motors are of the same type used in radio controlled airplanes!
The system includes a polarizer, here did you also had the other part of the instrument's name, Pol of PolCor-2. Its task is to filter out the polarization angle that is most beneficial. Often the angle that give as high a contrast as possible. By turning this filter and analyze the data (image) obtained as a quality measure can be used to obtain the optimum angle.
Polarizer is visible in the image above as the device that follows the Lyot Stop filter. Also this is provided with a servo motor that transfers torque through a timing belt. The servomotor sets quickly filter into the angles of 0, 45, 90 and 135 degrees. It also has a position where it blocks the light path. The final image may consist of several combinations of angles. The polarizer is designed to let through the wavelength range from 410 nm to 750 nm.
Parabolic and flat mirror set two:
After passing the polarizer remains to focus the light beam back and align it with the camera that is mounted on the outside of the cabinet.
The parabolic mirror to the right and the plane mirror (tilted) to the left.
Also here, an off-axis parabolic mirror. See it as a mirror in a Newtonian telescope where only one circular outer part of the mirror is used. The great advantage of this is the output light beam from the mirror is not moving in the same direction as the incoming. In other words, do not descendants equipment sit in the way of the input light beam. A major drawback is that the optics imaging becomes more deformed than the normal parabolic mirror which already severe coma. But you can not get everything, but choose the one that is most optimal for what you want to achieve. Using mirrors instead of lenses also means that the instrument can handle a wider range of wavelengths.
If you want to study dust accumulations around other stars you study faint objects with very small angular spread. See the introductory image, the entire image field is only 1' (one arc minute) and details of only fractions of the field. Total system resolution thus becomes a very important parameter. A major limiting factor is the atmosphere. NOT telescope located at a carefully selected spot, at 2400 meters hight at La Palma in the Atlantic Ocean, where there is very good properties in this regard. The atmosphere is changing constantly and cause bad seeing. If you can keep the exposure time very short, fractions of seconds you can be lucky and get a picture just when the atmosphere gives small contributory distortion. If you take a lot of pictures with short exposure you can get several sharp images. The technology is called the Lucky Imaging.
Now, this technology is excellent on our own planets around the Sun, which is very bright and one can therefore take short exposures, typical 1/100 second, and many images, shifting to lie exactly against each other and add them up (stacking). The dust emission around other stars, however, is very weak and noise arises whether to keep exposure short. It can be compensated by adding together more than (100 to 1000's) exposures. However, it requires that the signal is significantly higher than the camera read-out noise, so unfortunately it is not for the normal camera under these circumstances.
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How do you solve this? You have probably seen all of these night vision googles, the Russian with the image intensifier. They have an internal valve which electronically amplifies the signal. That's one way to go, at the binoculars at least. Other they have to much drawbacks to be useful.
Now there is a more refined method where the image light reinforcement is built into the CCD circuit. The technique is called EMCCD, Electron Multiplying Charged Coupled Device, in which an internal electric field detaches a burst of electrons for each detected incoming photon. That in itself gives no less noise but the signal comes up at a higher level so that the camera read-out noise impact is reduced. The camera dynamics however reduced accordingly as the gain is increased. One of the manufacturers who have such cameras is Andor Technology and is also the camera Göran has chosen for this instrument.
Camera shown here mounted in the bottom of the instrument unit. Above the vents visible are the water connection for cooling.
This camera can be seen as a photon counter, it detects the individual photons. It may, however limit the range of wavelengths to achieve this. An impossible dream before, normal amateur CCD cameras feature a noise of 8 e- to 25 e-. The system's pixel scale mounted on NOT is 0.12" / pixel, with the option to install a Barlow lens with x2 or x3 focal extender.
Something that perhaps many people reacts to are the few pixels the camera have, 0.5 million only. But for professional astronomers, it is not the number of pixel, it's the quality of the measurement it performs that are important. If the camera cost anything? Well, 30,000 Euro you have to pay for it.
Here the camera is disconnected from its mount on the coronagraph.
The computer that control the system.
This complex system is controlled by a computer. Here, a standard industrial PC, which provides a flexible and economical solution. It is for including this as H.G. comes in as software developer, many different components to be controlled, measurement data to be collected and processed.
Test of the instrument that has been done:
"One thing that must be mastered is the backlight (light pollution and the noise from this). One method that is common in IR is to move the image field (chopping and / or node) to the side where no object are and take a picture. This image is then subtracted from the object image. Proven technology, but more than half of the exposure time is lost in the overhead. The optical chopper carrying out this is not on the pictures we've seen here. Tests show that the instrument PSF (Point Spread Function) behaves well and the instrument is well suited for high contrast imaging."
The test has been performed including the red halo surrounding a compact blue galaxies and verified, object Mrk900. Simply put, the focused image is well composed. In this test the camera's gain is set to 100, and 12,000 images were collected respectively. Of these 85% were used. The frame rate was set at 10 Hz. To read the full report there is a link at the end where it can be downloaded as a PDF file, written in English.
Mrk900 in Visual filter (left) and Infrared filter (right).
The instrument has also been used successfully to detect AGB envelops (star set to become a planetary nebula), and more normal high-resolution images without the pole filter and the coronagraph mask that normally sits in the path. The instrument, however, is in an early phase and still has no measurements made on young stars and the dust ring that was written about in the beginning of this article. Watch NOT's website if something comes up there in future.
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Should we conclude PolCor-2 instrument we can say that it is an instrument cluster consisting of Lucky Imager, Polarization meter and Coronagraph with the following advantages over a more general instrument as ALFOSC (you can read about ALFOSC at NOT's website):
After this day at Albanova one can not be other than very impressed by the instrument that has been designed and built here at Albanova. And you shall know that Göran has done the most amazing things besides this!
End of demonstration:
After this tumultuous day in the technology front area concludes Göran to reassemble the lid.
Göran mounts back the protective lid again.
Many thanks to Professor Göran Olofsson of all the material and the time he dedicated to making this article possible.
Useful links here:
When I google at PolCor-2 today (2017) I can't find any recent observation with it, maybe it has been replaced with a new version.