Table of Contents
telescope mounts
- see also:
- on the web:
Alt-azimuth:
- similar to a camera tripod, not useful for astrophotography
- uses altazimuth coordinates:
- earth remains fixed, stars move with earth's rotation
- altitude (height above horizon, zenith = 90deg, nadir = -90deg)
- azimuth (angle from north)
- meridian is line from north to south crossing the zenith
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!
- if you must use one of these, strongly consider ultra-wide eyepieces to increase viewing time.
Equatorial:
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- 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:
- hour angle = sidereal time - RA
- ie. meridian = 0 hour angle & hour angles west of meridian are positive such that west horizon at celestial equator has hour angle of 6
- declination
- 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:
- 0deg = 34'.9; 1deg = 24'.4; 2deg = 18'.2; 3deg = 14'.3; 4deg = 11'.7; 5deg = 9'.8; 10deg = 5'.3; 20deg = 2'.6;
- allows one to easily track an object to counter the effect of earth's rotation which is a signification problem
- with the aid of a motor drive, tracking can be automated
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:
- 72mm worm gear; 2 journal bearings in each axis; 15 arc-min GOTO precision;
- aux.port:
- The AUX port will accept the Meade 1240 electric focuser and then give you 4 speed bi-directional control from the AutoStar. It will also supply power to the StarGPS - GPS system. It can also be used as a 12v - very low amperage power tap - say for a illuminated reticule.
- The AUX port is a 4-wire device. Two outer lines are power/ground, the other two are a clock and a bidirectional data line for the peripheral i2c Bus driven directly from the Autostar. Those data/clock lines are simply directly connected to two of the wires in the Autostar's 8-wire cable.
- CCD autoguider ports have at least 5 wires (on a 6-pin connector),
one for each direction, and a ground reference.
- autoguiding:
- the LPI is the most economical way of auto guiding the LXD series - unfortunately, it
requires a laptop. - The LXD55 can be autoguided through the Autostar's RS232 port. Current solutions like Meade's LPI or GuideDog control telescope guiding with a serial connection to the same port. Any autoguiding or planetarium application which correctly sends the LX200 subset commands should work.
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- Celestron Advanced Series (AS):
- CG-5: $A1270 + $A300 for motor drive;
- CG-5GT:
- ?$A7900 computerised version with NextStar GoTo;
- GPS optional with CN16 accessory;
- autoguider port; RS-232 port; aux port; max. 35lb load;
- Vixen GP-DX 3813:
- $A2215; worm gears twice the resolution of the GP series; max. 10kg load;
- Meade LXD-75:
- introduced mid-2004 to replace the LXD55
- new features:
- round steel legs instead of more flimsy square aluminium
- PEC, autoguiding; virtually backlash-free; four high-precision stainless steel ball bearings in both RA and Dec axes;
- Losmandy G8:
- $US1500 for small scopes to 8“ SCT or 6” refractor & max. load 30lb
- autoguide with SBIG via a relay box
- Losmandy G11:
- $US2100 for larger scopes & max. load 60lb (NB. tripod 35lb, head 36lb) 0.5arc-sec;