photo:telescope_mounts
telescope mounts
Alt-azimuth:
Dobsonian:
a type of alt-azimuth mount designed for cheap mounting of large aperture telescopes - usually Newtonians of at least 8“ aperture
allows large cheap Newtonians without expense of equatorial mounts (eg. 8” $700, 10“ $1200)
easiest to set up although bulky, so good for a quick look at an object to impress people
does not adequately lock the scope to allow sufficient stability whilst allowing tracking of a body at high magnifications & thus limits the useful magnification to ~200x, thus not as good for high magnification planetary observation
at 150x magnification, an object takes only 30secs to pass from one edge of view to the other before one needs to manually re-align the scope - this is a significant problem & worse at bigger magnifications - imagine having only 15secs at 300x to allow it to stop vibrating after you adjusted it and before it disappears again!
Equatorial:
for accurate tracking of stars:
NB. in southern hemisphere the polar axis is aimed at the south celestial pole not the north pole as shown in diagram
angle from ground must be changed when travelling to a different latitude as the angle = latitude
must be level with the ground
must be rotated so that it aims at south pole when setting up
uses sidereal coordinates:
stars remain fixed in this coordinate system, whilst sun, moon & planets move through this adjacent to the ecliptic (due to the obliquity of earth's axis)
stars remain fixed except for the small alteration due to the precession of the equinoxes where a star's sidereal position changes each year as a result of the intersection of the ecliptic with the equator making one revolution every 25,600 yrs & thus star positions are usually given according to a given year or epoch & corrections from this are given by:
annual change in RA = m + nsin(RA)tan(declination)
annual change in declination = ncos(RA)
where m = 3.074sec; n = 1.336sec; southern declinations are negative;
right ascension (measured eastwards - analogous to longitude - in 24 hours, 1 hr = 15deg) & declination (degrees north or south of celestial equator, ie. analogous to latitude)
sidereal time equals zero when First Point of Aries (zero RA) crosses the meridian
sidereal time = RA on the meridian
NB. Universal Time = time measured on the meridian of Greenwich
NB. EAST is 10 hours less than U.T.
can be easily converted to diurnal coordinates:
once calibrated to a location's latitude & aligned to south pole, can be used to find objects based on their Right Ascension & Declination, with the aid of a computerised motor drive, objects can be found by entering them into the computer.
one needs to correct for refraction of the atmosphere which causes the apparent position to be a little higher than its actual position, thereby enabling us to see below the horizon, examples for given altitudes are:
allows one to easily track an object to counter the effect of earth's rotation which is a signification problem
sub-types:
Fork mount:
commonly used on SCT's & Maksutov's
requires use of an optional equatorial wedge for astrophotographic quality tracking
undersized fork mounts may result in “tuning fork” vibration which takes 10sec or more to settle
additional weights needed if using additional accessories such as a camera
may have problems viewing zenith with cameras or other accessories attached as insufficient room at the base
eg. Celestron Nexstar series; Meade ETX series; Meade LX90; Meade LX200
German equatorial mount:
favored choice of astronomy buffs and astrophotographers because of its stability and portability.
More stable because the center of gravity is directly over the center of its base
sliding the counterweight for Right Ascension and moving the optical tube along its dovetail mounting for Declination accomplish balancing the weight of camera equipment and other visual accessories. This means that no additional weight needs to be added to balance the telescope when additional accessories are added.
more portable because it can be broken down into smaller component parts for easy storage and transportation.
For astrophotography, the German Equatorial mount offers easier balancing, unlimited space at the rear of the telescope tube to mount a camera, and whole sky access
Synta HEQ5:
$A1275; solid, basic mount with motor drive & ability to adjust in Dec & RA but no autoguide input
periodic error and tracking errors a problem for long exposures, but good basic mount for visual astronomy
can fit a 10” f/5 Newtonian but extra equipment such as cameras can stress the drive
Meade LXD-55:
Celestron Advanced Series (AS):
Vixen GP-DX 3813:
Meade LXD-75:
Losmandy G8:
Losmandy G11:
photo/telescope_mounts.txt · Last modified: 2013/02/08 01:11 by gary1