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Sunday, June 07, 2015

Telescopes, Astronomy and (next post) Astrophotography

Once upon a time in San Diego, I had a roommate who bought a really expensive telescope.  It was a nice Celestron telescope with an 8 inch aperture, just like this one.



My roommate's obsession with astronomy got me hooked as well.  I ended up owning a dozen books about astronomy, and we had a great deal of fun packing the telescope away from the city lights and seeing first-hand planets and deep space objects.  I still have a powerful set of binoculars that I purchased at that time, just for sky-gazing.

Telescopes come in quite a variety of types, each with advantages and disadvantages.  The differences between telescopes are primarily due to the optics and how the telescope is mounted.

OPTICS:
The first telescope ever invented was the Refracting Telescope.  It was invented in the Netherlands in 1608, and improved and popularized by Galileo Galilei in 1609.  Refracting telescopes use the combination of a large Objective Lens and a smaller eyepiece to refract (or bend) light to a focal point, gathering more light than the human eye is capable of doing on its own.  This creates a virtual image that is larger, brighter, and clearer than the object perceived by the human eye.

These are excellent telescopes for viewing bright objects such as the moon and nearer planets.  They are also great for looking at earth-bound objects.  You will frequently find them at visitor stations to look at wildlife or mountain ranges, because they give very high magnification (although with a very narrow field of view), and they do not invert the image.  Think of a zoom lens, permanently zoomed in.  Refracting telescopes typically have long focal length, and therefore high magnification.  Perfect for well-lit objects.

Refracting telescopes have some disadvantages for astronomy though.  The larger the objective lens, the heavier it will be, and that lens will be at the top of the telescope.  The telescope tube must be very robust to hold a huge glass lens at the very top.  Flexing of the tube, even slightly, will throw the focus of the telescope off.  Also if the objective lens is large enough, its own weight will cause the glass to distort, and at that point it will not be able to focus incoming light.

Below, Yerkes Observatory.  The largest astronomical refracting telescope ever built.  It had a 40 inch (1 meter) objective lens.


There is another problem with refracting telescopes though, and this one is due to physics.  Glass bends light of different wavelengths at different angles.  If you want to focus blue light perfectly, red light will be out of focus.  This is called "chromatic abberation".


Visually, chromatic aberration looks like this:  It is most obvious at interfaces between dark and light objects.


Chromatic aberration can be mostly controlled by sandwiching higher density glass between low density glass in the objective lens.  These objective lenses are called "Apochromat" lenses.



Refracting telescopes also suffer small losses when light enters and exits each lens, and at each type of glass interface.  A little light is reflected off the glass, and minute air bubbles and flaws in the glass cause other distortions.  For these reasons, most amateur astronomers and professional observatories use reflecting telescopes.

Here is the amateur version of a refracting telescope.  This has only a 5 inch aperture, and the telescope alone costs about $3000.  Ouch!


The simplest and most practical telescope for viewing the heavens is the Reflecting Telescope.  The most basic telescope of this type is the Newtonian (Yes, Issac Newton!) Reflector.  In this design, light enters the telescope, bounces off a parabolic mirror, is taken outside of the telescope by a flat mirror, and sent through an eyepiece for focusing.  See diagram below.
Reflecting telescopes were not as useful as refracting telescopes for a couple of centuries, and the reason was technology.  The lenses used in Refracting Telescopes were well understood and could be made of fairly high optical quality over the last few centuries.  Mirrors, on the other hand, were still very primitive things.

Until the task of grinding a mirror into a parabolic surface and depositing a high-quality reflective coating onto the glass could be accomplished, reflecting telescopes were of marginal quality at best.  Now, however, the preferred telescopes (both professional and amateur) are reflectors.

Below is the amateur version of a Newtonian reflector.  The aperture is 8", and the cost (which includes the tripod, eyepiece, and digital readout) is about $1100.  The light-gathering area is about 2.5 times that of the amateur refracting telescope above, for about 1/3 the price.


The Newtonian design is the simplest and least expensive reflecting telescope, but there are a couple of other ways to obtain the image.   Below is a Gregorian reflector, whose design pre-dates the Newtonian, although one was not built until after the Newtonian telescope had been made.  This design requires a curved secondary mirror and a hole to be cut at the center of the primary mirror.



Lastly there is the Cassegrain Telescope.  This scope also requires a hole at the center of the primary mirror.  The difference between Gregorian and Cassegrain is this:  With the Cassegrain design, the primary mirror has a hyperbolic curve, and the secondary mirror is convex.  This configuration allows the the telescope to be very short in length, which offers several structural and optical engineering benefits.  A shorter telescope requires less material, a smaller enclosure, becomes thermally stable more quickly, and is less expensive.

The largest Cassegrain telescope, the 200" Hale telescope at Mt. Palomar.


A refinement of the Cassegrain telescope is is the Ritchey-Chretien design, which uses hyperbolic curves on both the primary and secondary mirrors to eliminate the coma and spherical aberration found in other reflecting telescope designs. All major new scientific professional telescopes are Ritchey-Chretien designs.

Back to amateur telescopes though - the ones hobbyists can afford... How about making an amateur telescope that uses simple and inexpensive to manufacture spherical-curved mirrors?  You can use spherical curved mirrors if you add a corrective lens at the opening of the telescope.  This design is called the Schmidt-Cassegrain, and it is hugely popular within the amateur astronomy community.  The resulting telescope has a large aperture for gathering light, and is very short and compact.  An additional advantage of this design is that the corrector plate seals the telescope optics from accumulating dust!  This sealed design makes it possibly the lowest-maintenance telescope of any design.


Below is an 8" Schmidt-Cassegrain telescope.  It costs about $1100 with tripod, eyepiece and digital readout.  It is more powerful than the refracting telescope, and is far easier to transport to a dark site than an equivalent aperture Newtonian.

Time to talk about telescope mounts - designs, advantages and disadvantages.

First up, the Dobsonian mount.  Advantages:  It's inexpensive, sits low to the ground, and it will hold a lot of weight (like a really big primary mirror!!!).  This is the simplest and cheapest Altitude-Azimuth mount there is. Disadvantage:  It won't allow you to keep a star in view using a motor drive.  For this reason, telescopes on a Dobsonian mount usually have a short focal length (for a wider field of view), so that stars and planets don't drift out of the field of view too rapidly.

Below is a Dobsonian mounted Newtonian Reflector telescope.  It has a 12" aperture and costs about $1000.  It has 2-1/4 times the light-gathering power of the 8" scopes above, and 5-3/4 times the light-gathering power of the $3000 refracting telescope.
The scope below has a 16" aperture and costs about $2000.  It has 4 times the light-gathering power of the 8" scope, and about 10 times that of the $3000 refracting telescope!  

Dobsonian-mounted scopes are a favorite among amateur telescope builders.  Below is a 30" scope, behind the late John Dobson.  Yes, THAT Dobson.

Dobsonian mount telescopes offer great value for your money, because very little of the expense goes into the mount.  The majority of your money goes to the optics!  But...  You can't track objects, so you cannot take photographs.  Nor can you tackle amateur science projects, because you cannot track the desired object in view very well.  Those seem like interesting things to do with a telescope...

Let's discuss the rotation of the earth while tracking an object in the night sky.  First off, telescopes must be able to point to any point in the sky.  They also need constant adjustment to counter the rotation of the earth.  If a telescope is fixed, whatever you are looking at in the eyepiece will drift out of view as the earth rotates on its axis.  At high magnification, with a fixed telescope, the object will drift out of view before you can even show it to a friend!

Below, a long-exposure star picture with a fixed camera.  The stars *appear* to drift, because the earth (and the camera) are rotating.  The only star that didn't appear to move is Polaris (The North Star, Alpha Ursae Minoris), because by circumstance, it lines up very closely with the earth's axis of rotation.

The next mount we will look at is the computerized Altitude-Azimuth mount.  This mount provides simple up-down (altitude) and horizontal swivel (Azimuth) motions to a telescope, just like the telescopes you drop a quarter into to look at the mountain goats (or a gun turret).  We live on a spinning ball though, so following one of the stars as it travels in an arc (as in the above image) requires some continuous clever adjustments with an Alt-Az mount.

Thanks to modern computing, the Alt-Az mount can now automatically keep an object in view, without adjustment by the observer.  This type of telescope is called a "GoTo telescope", and it is very user-friendly.  To set this type of telescope up, take it to the observing site, point it at a known star and tell the scope's computer what star it is (using the keypad).  It is desirable to enter a second star, far from the first, in case the scope is not perfectly level, so that the computer can compensate for that error as well.  The built-in GPS will provide the tracking computer with the correct time, longitude and latitude, and then it will be able to make the telescope "GoTo" thousands of objects in the computer's database.  Yay for technology!

There is one problem with the standard alt-az mount:  Neither of the telescope's axis of movement are aligned with the earth's axis of rotation  If you had hoped to take a long-exposure picture of a dim object, it will rotate during the exposure.  Your photo of saturn might show the rings as a blur.  There is a simple fix though - The wedge mount and the German Equatorial mount.

The wedge mount and the german equatorial mount work by tilting the plane of the azimuth until it aligns with the earth's axis.  The angle of the tilt will vary with the observer's latitude of course, so the angle is adjustable.  Simply install this wedge between your tripod and your fork-mounted telescope, and now you have an equatorial mount!
You have also switched from an earth-bound measuring system to a celestial measuring system.  

All star and deep space objects have mapped coordinates in Declination and Right Ascension.
The German Equatorial mount works in a similar manner, by placing one axis of rotation of the telesscope in alignment with the earth's axis of rotation.  Below, the German Equatorial Mount.

With this arrangement, a simple 24-hour clock drive in the base of the telescope will now keep any object in view for a very, very long time, assuming you leveled the base and properly aligned the forks of the telescope with Polaris.  If you've set the telescope up well, the clock drive will precisely counteract the spin of the earth, regardless of where the telescope is pointed.  Furthermore, the field of view will no longer appear to rotate, and you will be able to take long-exposure pictures without blurring due to that rotation.



If I were to get back into amateur astronomy and astro-photography, my dream setup might look something like this:  Big scope, CCD camera, computer-driven (not keypad) GoTo feature, and a little shack to cover the thing up when not in use, so it wouldn't need to be set up and adjusted every time it's used.  Nice rig!!!!

The post after this one is about Astrophotography

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