Last updated on
4/11/
2006

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Cassegrain Mounts

When you magnify an area of the sky with a telescope you also magnify the effect of the earth’s rotation, causing objects in the eyepiece to drift in and then out of view.  Centuries ago astronomers figured out that mounting a scope on an axis parallel with the earth’s rotational axis - i.e. aimed at the celestial north pole (or south pole if you’re south of the equator) - and rotating the scope around that axis opposite to but at the same rate as the earth’s rotation, would keep objects stationary in an eyepiece.  Such a mount is called an equatorial mount, and originally the rotating drive mechanism was a clockwork so the drive was called a clock drive. (Even though clock mechanisms were replaced by electric motors many years ago the term clock drive is still often used.)  “Setting circles” were added to equatorial mounts; these are dials which indicate the position of the scope around the Right Ascension or polar (rotating) axis and the Declination axis (the axis perpendicular to the RA axis). In theory if you know the Right Ascension and Declination of an object in the sky you could aim your scope at it, by turning the scope to that RA and Dec as indicated on the scope’s setting circles.  Assuming the scope’s RA axis has been very accurately aligned with the celestial pole the object will appear in your eyepiece.  In practice this only worked well if those setting circles were of a very large diameter to provide a detailed scale; typically the setting circles on amateur scopes are too small in diameter to provide a fine-enough scale to accurately aim the scope. So locating deep sky objects with a scope took some practice and was much more difficult if you didn’t have someone around to help you learn how to do it.  Amateur astronomy remained a hobby for a fairly small group of dedicated souls.

The advent of inexpensive microprocessors changed all this by letting a computer help the observer locate an object.  There are three types of computer-assisted mounts, described here in increasing order of sophistication:

  • Digital Setting Circles (DSCs) - you can add optical encoders to the RA and Dec axes, which convert the rotational position of each axis to a digital signal.  Run both of those signals to a microprocessor that provides a readout of the position and if the optical encoders are sufficiently accurate you have digital setting circles more accurate than the mechanical setting circles on the scope. But don’t stop there!  Computer memory is cheap - why not add a database of thousands of beautiful deep sky objects? Then you can call up an object from the system’s memory, have it display the RA and Dec of the object as well as the current RA and Dec of the scope. All you need to do is move the scope until its position matches the database’s recorded position of the object, and the object will be in your scope’s eyepiece. Such a system is called a Digital Setting Circle system and Celestron and Meade sell them for many of their less-expensive scope mounts.
  • GoTo Systems take a DSC system one step farther. Add separate “slewing” motors to the RA and Dec axes of the mount, and have the computer command the motors to turn the scope to the object you’ve selected.  This system is called a GoTo system and it isn’t entirely necessary because all it actually does over a DSC system is move the scope for you, which any reasonably healthy human could easily do by themselves. But, that automated movement is totally cool and really appeals to citizens of a technological society, so GoTo systems have completely revitalized amateur astronomy over the last decade or so.  And GoTo systems actually do have one advantage over DSCs. Faint objects are more readily detected if they are moving, so are more readily detected when looking through the eyepiece of a GoTo scope as its motors bring the object into view.  You can’t as easily do that with a DSC system since you’re looking at the keypad, rather than into the eyepiece, while you’re moving the scope.

    Most GoTo mounts for SCTs (such as the Celestron NexStar or Meade 200LX scopes) have been fork mounts due to their compact size and ease of use. However, with larger OTAs (say, 11” and above) the combined weight of the OTA and fork mount becomes so large that a single normal human often has difficulty lifting that combined assembly onto a tripod, and even more difficulty lifting it at an angle to slide onto an equatorial wedge, without a second person’s help.  (The NexStar 11 GPS OTA + fork mount weighs 65 lb. and larger scopes, of course, are even heavier.) So for larger OTAs it is often more attractive to have the OTA mounted on a German Equatorial Mount (GEM) because with the GEM, the OTA and the mount are separate.  So even if their combined weight is greater (as a GEM assembly) than they would be as a fork mount, being separate pieces they are easier to transport and assemble. For example the Celestron 14” SCT’s OTA by itself weighs 45 lb. while the Meade 14” LX200 SCT’s OTA + fork mount weighs 125 lb.  So Celestron has introduced their CGE telescope line, which is a series of OTAs on a GEM that has GoTo capability, and GPS capability with the addition of a GPS accessory receiver (GPS mounts are explained next).  The Celestron CGE series is allowing amateur astronomers to move to up to a 14” SCT and still have a GoTo scope that can be transported and assembled by one person.
     
  • GPS GoTo Systems automate the setup of a GoTo system.  After you place a GoTo scope in position outdoors for an evening of observing the first thing you need to do is tell the GoTo’s computer where on earth you are (literally) so the computer can figure out where to aim the scope for a particular object. This “alignment” process involves aiming the scope at one or two particular stars and telling the scope’s computer which stars they are, so that the computer can align itself with the sky. To simplify even this simple step Celestron introduced scopes that added a Global Positioning System (GPS) receiver to the system, that automatically tells the scope where on earth it is and thus automates the alignment process itself.  Meade subsequently added a line of GPS scopes to their LX-200 series. 

So you can purchase a scope with good optical quality, mounted on any of the above types of computerized mounts. But there is another consideration that affects the price of the mount.  Telescope mounts can be classified by those intended for visual use, and those that can also be used for astrophotography.  The astrophotography mounts are much more rigid and are consequently more expensive (and heavier). For astrophotography that structural rigidity is important because vibration is deadly when you’re taking a photograph.  Unlike human vision, photographic films or digital cameras accumulate light and they will record vibration as a blurring of the image.  Also, the more rigid mounts can support more weight (such as a camera) while maintaining good tracking.  If you don’t plan to try astrophotography with your scope, you can spend less money.  You still want a quality instrument but your eyes will forgive a little vibration - you actually won’t notice it.

It should be noted that even the top-of-the-line mounts from the GoTo scope manufacturers have some tracking errors in them, a necessary consequence of keeping the overall telescope affordable for most people.  But those astrophotographers who commit to deep-sky object photography in a really serious way, end up replacing the original equatorial mount of their scope with a very expensive (>$5,000) specialized mount, upon which they attach the scope’s Optical Tube Assembly (OTA). (Or they move to a Schmidt Camera.)  GoTo capability can also be added to these top-of-the-line mounts, for another $2,000 or so.  But the Celestron Nexstar GPS or Meade LX-200 GPS scopes are a very good compromise between capability and affordability if you don’t want to make a $10,000 or more initial investment in your equipment.

The opposite direction from expensive astrophotography mounts is taken by those observers who aren’t interested in astrophotography but love observing deep-sky objects (galaxies, star clusters, and nebulae). In this case there is no substitute for aperture (light-gathering power) and you want to invest in the very largest mirror you can. Typically this is a mirror in a Dobsonian mount design, which is a very simple mount so that a majority of your financial investment is in the mirror and its supporting cell. Dobsonians can be purchased or you can assemble one, but large Dobsonians are heavy, must be disassembled and re-assembled for transport, and can’t easily be fitted with motorized tracking (much less GoTo capability), so they can’t be used for astrophotography. Also you must stand on a ladder to look through the eyepiece of larger Dobs.  Nevertheless, deep-sky visual observers use large Dobsonians almost exclusively, and the view of faint objects through them is spectacular - in the end there really is no substitute for aperture.

There is a wide variety of other (generally permanently-installed) types of mounts for telescopes, primarily developed to achieve a balanced weight distribution for larger scopes. A short but good and well-illustrated discussion of telescope mounts can be found on John Savard’s Telescope Mountings web page.

If you purchase a Celestron NexStar GPS scope (I really love my 11” NexStar GPS), Mike Swanson has just published an excellent book that is very helpful in learning how to use these scopes. It is The NexStar User’s Guide, Michael W. Swanson, Springer, 2004.

 

 


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