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Posts Tagged ‘three-star-alignment’

German Equatorial Mount – Part 2

The first part of this article gave an introduction of German Equatorial Mounts (GEMs) and discussed polar aligment and how it is been done.  This second part of the article talks about Declination, Right Ascension, setting circles, balancing the mount and load capacity.

Declination

German Equatorial Mounts are polar aligned so their declination is always the same, this independently of their location.The celestial equator is a projection of the earth’s equator in the sky. Declination of an object describes the angle to the celestial equator. Similar to the latitude scale of the earth, Declination is measured in degrees. Values are positive (0 to 90 deg) for objects north of the celestial equator, and negative south of it (0 to -90 deg). The advantages of an equatorial type mount are clear: because we have aligned the mount exactly with the polar axis, we have calibrated the mount’s celestial equator as well. The celestial equator is always orthogonal (right angle) to the polar axis. With that, the Declination of an object is completely independent of location or observing time.

It may be a bit confusing for beginners to imagine the celestial equator and the related Declination. A really simple way to picture this is to replace the sphere of the earth with a flat, circular platform on which the telescope stands. The platform has the size of the earth’s equator and the axis is the polar axis. Even when moving the telescope to any place on this disc, the

German Equatorial Mounts offer an Declination axis and Right Ascension movement to compensate for the Earth's rotation

Declination of the object will be always the same. The famous Andromeda Galaxy (M31) can for example always be found at DEC 41° 16’, this is independent if we observe it from New York, Los Angeles or Munich.

 

Right Ascension

The last degree of movement at a GEM is called Right Ascension (R.A.). After proper alignment, the R.A. axis points exactly at the NCP / SCP and with that, any R.A. rotation describes a circle as do the stars in the sky. Motors (or hand controls) can follow the apparent movement of a celestial object perfectly. Right Ascension is commonly used in units of time (hours, minutes and seconds).

The star Omega Pisces in the constellation Pisces is very close to the zero point of Right Ascension and serves astronomers as easy to find reference.
The star Omega Pisces in the constellation Pisces is very close to the zero point of Right Ascension and serves astronomers as easy to find reference.

Greenwich has been arbitrarily selected as start point for the earth’s longitude scale (0°), similarly the celestial zero point for Right Ascension (00h 00min 00sec) has been chosen arbitrarily to be the vernal equinox. A useful reference for astronomers is located in the constellation Pisces. With R.A. of 23h 59m 19s, the star Omega Pisces is currently very close to the zero point. Right Ascension of all other objects are designated by how long they lag behind this coordinate after it passes overhead moving toward the west.

Setting Circles

German Equatorial Mounts are equipped with setting circles for R.A. and DEC and slow motion controls (or motors) that move the telescope in these directions. Setting circles are scaled discs attached to the R.A. and DEC axes.

Even faint celestial objects, like nebulae and galaxies, can be found with setting circles.
Even faint celestial objects, like nebulae and galaxies, can be found with setting circles.

They are marked with a 90 – 0 – 90 degree scale for DEC and a 0h to 24h scale for R.A. Setting circles are ideal to point the telescope at a particular object just by setting its coordinates. Most GEMs have movable setting circles. Their use is quite easy; astronomers point the telescope to a star with known R.A. and DEC coordinates close to the wanted object. They adjust the setting circles to the exact coordinates of the known star and move the telescope until the scales show exactly the wanted coordinates. It is surprising how easy it is finding faint objects with this method.

Balancing the Mount

All German Equatorial Mounts need one or more counterweights to balance the telescope. The position of counterweights is variable for balancing any load. Balancing is of utmost importance because it minimizes the burden on the gear and bearings. Clutches that hold the telescope in place are also much less stressed with a balanced setup. Conversely, a highly unbalanced set-up will most certainly damage the gearbox (and probably the motor). Furthermore, it is much more convenient to work with a balanced telescope that stays put when released instead of shifting and slipping at its own.

It is often forgotten that the focuser is extracted when the telescope is in use. Due to the weight of diagonal, eyepiece and sometimes even a camera, this extraction shifts the center of gravity significantly. This has to be considered during the balancing procedure.

Load Capacity

Most manufacturers are quite “generous” when describing load capacity of their GEMs. Specifications should be taken with care, particularly for entry level mounts. Load capacity includes, with the exception of the balance weights, everything that is put on the mount: telescope, finder scope, diagonal, eyepiece, accessories, additional mounting hardware, and when shooting astrophotography, camera and addition accessories.

Do not overload the mount, it will make it unstable and results in unwanted mount wiggling and mount shake. This effect is multiplied by the magnification power of the telescope. Weak, wiggly mounts can make observing indeed quite unpleasant. Stable solid mounts are absolutely paramount for any kind of astrophotography. Astrophotographers tend to load their mounts only with 50% of the actual load capacity for best stability. There is much truth in the astrophotographers proverb:

 “A good mount with a mediocre telescope will provide much better Images than a good telescope on a weak mount.”

Motorized Mounts

Motorized GEMs come with one or two motors to control either only R.A. or both, R.A. and DEC. Most manual mounts can be retrofitted with small motors. For stargazing it is often more convenient to have only a R.A. motor drive and correct possible DEC deviations manually.

Completely different types are computerized GoTo mounts. These mounts have not only coordinates of thousands of celestial objects stored in their memory; they have also electronic versions of setting circles built in; they are called rotary sensors. A computer calculates the position, counts the necessary sensor impulses and drives the motors to the right point. Before the computer can find any object, it needs calibration on reference points, which are usually three known stars (three-star-alignment). Some stargazers conclude falsely that this alignment substitutes for proper polar alignment of the mount – this is not the case. After a successful three-star-alignment, the computer can point the telescope automatically at any wanted celestial object.

Conclusion

German Equatorial Mounts offer many advantages that are desired for observing, but are definitely required for astrophotography. Portable GEMs are heavier and more difficult to set up than Alt-Az mounts and they always need to be propperly aligned before they can be used in the intended way. The biggest advantage of GEMs is that they are suited to eliminate apparent star movement and with that, they are clearly the mount of choice for astrophotographers.

Further Reading

German Equatorial Mount – Part 1 @ Astronomy Source

The March Equinox @ timeanddate.com

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