Rahul gives unsolicited advice

If large cameras and telescope capable of probing the low surface brightness objects in the sky is one end of the state-of-the-art in astronomy, the following work would be the state-of-the-art at the other end of the spectrum. While surveys focusing on looking at all of the objects in the sky, choosing speed and number of astronomical objects found over spatial resolution, there are telescopes used specifically to study objects at the highest spatial resolution. The paper ( found here ) talks about a narrow, edge-on disk resolved around the star HD 106906 using the SPHERE instrument on the VLT telescope. Note that I used to word resolved and not detected. For an astronomer, those two words are vastly different. The same way dark matter is discovered but not yet observed, astronomers knew that the aforementioned star had a dusty disk around it. How you ask? Well, for starters, if you have dust in front of a light source, the light source looks dimmer. Secondly, whatever light that t...

I've been coming across a lot of work, in the optical and the radio domain to identify and understand dwarf/satellite galaxies, stellar tidal streams and neutral HI (HI is neutral Hydrogen and HII is singly ionized hydrogen i.e a proton) in the universe. This is one of many such efforts (a few of which I will write about later on) where the authors used the SKA pathfinder telescope BETA/ASKAP to observe the galaxy group IC 1459. The full paper can be found here . Let me first tell you why it's a challenge to observe neutral HI in the local universe and why these results are so interesting, IMO. All galaxies form from HI (again, neutral Hydrogen) but once the galaxy/stars start(s) forming, neutral HI close to the sources of radiation get ionized into HII. Only HI in the outer reaches of the galaxy is left unionized. And given that it's at the edge of the galaxy, it's not nearly dense enough to emit strongly. Don't get me wrong, there is still a good amount of HI i...

Well, I got to know a couple of days back that there was a Lunar-based Ultraviolet telescope. I finally got to read the paper today ( that can be found here ) and it's quite interesting. For those of you who remember, the Chinese landed a Lander on the moon and tried sending a rover on the lunar surface (which quickly failed). Apparently, that Lander had a UV telescope on it. As soon as I read the abstract, the first questions that came to my mind was when exactly they observe. If you think about it, we see the moon at night because it reflects the sun's light meaning that if it's night time for us, it is day time on the moon. The same way, if I were on the moon, I'd be able to see the earth because it will reflect the sun's light. Astronomers on earth prefer moonless nights because the night skies are darker. In the same way, the lunar based telescope had to take into account the fact that earth shines bright at night time when designing their telescope and plan...

If you follow science news regularly, you will come across news of the kind " Astronomers find brightest object of the early universe " or " Black hole-powered jets plow into galaxy ". I would just like to point out that when they mean "brightest" object, they don't mean that it's the brightest object in the visible part of the electromagnetic (EM) spectrum. They mean that it's the brightest object in terms of energy emitted across the EM spectrum i.e X-ray, UV, visible, IR and microwave radiation all put together. (And just to throw in a bit of jargon, an object's brightness is quantified in terms of it's luminosity and the luminosity of an object measured across the EM spectrum is referred to as the "Bolometric" luminosity.) Coming to the specific cases mentioned above, the quasars are in fact brightest in the X-ray, UV regime of the EM spectrum and astronomers use X-ray telescopes to look at them! At this point, I would ...

Radio and optical telescopes observe the radio and optical portions of the electromagnetic spectrum correspondingly. But radio telescopes are fundamentally different from their optical counterparts. Radio telescopes don't have  CCDs . They don't need finely polished mirrors and they are much, much bigger in size. So. Why the fundamental difference? Let's take a detour and remind ourselves of the wave-particle duality of light. The popular young's double slit experiment is explained using the wave theory of light where as photo-ionization is explained using the particle theory of light i.e using photons. A wave can be described using y = A cos( omega*t + phi ) where omega is two pi times the frequency, phi is the phase and A is the amplitude of the wave. A photon on the other hand can be described using h*nu where nu is the frequency of light. This fundamental difference is what, as you will see in a moment, lead to the differences between optical and radio tel...

As mentioned earlier, depending on the science that we would like to extract from the telescope, we will have to decide on the instrument. The most basic science expected from a telescope is regarding the position of stars i.e astrometry. Precise and accurate mount design will help us track astronomical objects better and a CCD with large number of thin pixels will help us image the object's spatial distribution or position precisely. The Hipparcos and Tycho satellites were pioneering in this regard, resulting in some of the most extensive and accurate maps of the milky way galaxy. Data sets from the Hipparcos , Tycho and a combined data set  have paved the way for future work, inspiring Gaia . Equipped with precise positions of stars, one can begin a photometric study to quantify the temperature, luminosity, variability and other such properties of stars.  As mentioned yesterday, photometric studies can be used to construct the HR diagram, a primer to understanding...

Once we've decided on a place for the observatory and decided on whether we want it to be a wide-field telescope or a deep-field telescope, we will have to decide on the array of instruments to be setup. Again, depending on what our intentions are with the telescopes, the objects that we would like to observe, we will have to decide on a relevant telescope. For example, if we intend to observe transient events such as asteroids, comets and supernovae, we will need a wide-field telescope. Why, you ask? Well, a transient event, by definition, is an event that wasn't observed at the same place at an earlier time. As such, supernovae are stars that blow up and asteroids move faster than any of the stars do, from our point of view. Therefore, if we observe the same patch of sky for a very long period of time, we are bound to run into interesting transient events. And since there isn't any special direction or patch in the sky, wide-field telescopes are designed such that th...

Telescopes can be differentiated in two ways, firstly depending on the way in which they are mounted and secondly depending on the optical setup of the telescope. Altazimuth and equatorial mounts are the primary types of mounts used for telescopes. Equatorial mounts are (were) advantageous to altazimuth mounts as equatorial mounts, by construction, only require the motion of one of it's two axis to be able to track stars reliably, instead of two in the case of altazimuth. One is better than two simply because of intrinsic errors in tracking objects. Tracking stars or astronomical objects is the primary objective of most telescopes as several minutes of continuous observation are necessary to be able to study objects well enough. Now-a-days, altazimuth mounts are just as good as equatorial mounts given the complex feedback systems involved tracking. Coming to the optical setup of telescope, they are classified depending on the position of the eye piece with respect to the prima...

Following are posts on a practical introduction to astronomy, practical in the sense as to the steps taken before and after observations, practical in the sense that I shall try look at things from an engineering perspective instead of a science perspective. So, to start, before we look up at the skies, we need to look around on Earth to decide on a place to setup the observatory. How does it matter where we setup the observatory, you ask? Depending on the position of the observatory on Earth, the amount of sky covered by the telescope will differ. You might've come across `all sky' maps, for example the all sky map by the Planck telescope or the all sky map by Fermi telescope . Planck and Fermi are in-space observatories, orbiting the earth while observing the heavens. In contrast, here is the sky coverage of the SDSS telescope , located in New Mexico, USA. As you can clearly see, the SDSS telescope can only cover a portion of the sky whereas the in-orbit telescopes can co...

I was discussing with a friend of mine about why early pictures of moon, say from the apollo missions, were so blurry. Well, astronomers don't enjoy the kind of speeds you and I do every day on our mobile phones, laptops and desktops. Most of us get impatient about low internet speeds, curse our service if the internet connection changes from 3G/H/H+to 2G. On the other hand, the data transmission rate from space craft back to earth is in the range of 100s of kbps. Deep space network , the title of this post, is a collection of radio antenna distributed all over the earth which are used to communicate with space craft and  DSN NOW  is a site which shows which antenna is communicating with which satellite, what the transmission rate is, what frequency they are transmitting at and so on. There are a lot more satellites than there are antenna so i guess data is pulled from the satellites based on demand or maybe periodically. As of now, MAVEN, the NASA mars mission is operating a...

Photonic band gaps are created when materials (at least 2) of different refractive index are stacked on top of one another. If the thickness of these alternating layers is exactly right, these materials act like optical band passes i.e allowing only a certain portion of the optical/electromagnetic spectrum to pass through them. I was taught this last year as part of a course on Optics & Photonics. And now, almost an year from when i was taught this, i realize how they can be applied in astronomy. Astronomical filters, such as the Johnson UBVRI or the SDSS ugriz, are lenses which act like band passes. The band structures of UBVRI and the ugriz are shown. source : http://www.vikdhillon.staff.shef.ac.uk/ultracam/filters.gif Coming back, the idea is to look for a photonic band gap material which can be used as an astronomical filter. But before that, i needed to know what the actual composition of filters are! And this was harder than i thought. Google and wikipedia were of...