Notes on Polar Alignment Methods - Observatory Class Telescopes
The type & accuracy of polar alignment depends on the work to be carried out and the type of drive. If you intend to use the telescope primarily for astrophotography the polar axis should be elevated above the true pole by 60.4 cosec Ø at temperate latitudes. This will compensate for Dec drift due to refraction for ±2h either side of the meridian and between -10º & +30º Dec. If the RA drive is fixed speed, it can be set to the King rate (1436.46 mins per rev ), with minimal tracking correction required.
If you intend to measure PA's, the polar axis must be aligned on the true pole, using either Challis' method, Grubb's or Loomis' method, or Toshimi Taki's matrix method. The accuracy needed in within ± 1' arc.
If the telescope drive is computer controlled in both RA & DEC it is better to use true pole alignment and track using either an auto-guider or by compensating for differential refraction.
In the absence of sufficiently accurate setting circles or DSC's (digital setting circles), use a combination of Schaeberle's method and Dec drift elimination.
If the equatorial is equipped with a Polar 'Scope bear in mind that the resulting alignment in temperate latitudes will be approx. 50"arc above the true pole.
If most of your observations are carried out on objects at or near upper culmination, azimuth error is less critical than elevation error. If you observe objects near the 6h / 18h HA, azimuth error becomes critical.
Modern GOTO Stepper or DC Servo drive controllers can provide a read out of the instrumental pole based on either a 2 or 3 star initialization, and also compensate for polar misalignment and cone error. Do not rely on this feature, it results in field rotation about the optical axis or guide star. Use the read out to correct the polar alignment and reduce error to within ±1'arc.
There are several ASCOM standard drive controllers & / or embedded polar alignment softwares. See for instance PoleAlignMax (PC Winx) or AstroPlanner (Mac/Unix/PC).
METHODS FOR PORTABLE EQUATORIAL
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1) |
Polar’Scope - alphaUMi to etaUMa 58’arc offset
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2) |
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3) |
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|
4) |
| 5) | |
| 6) | |
| 7) | Grubb’s method |
| 8) | Challis’ method |
| 9) | Toshimi Taki’s method |
| 10) | Multi-star instrumental pole readout - goto IHS initialization |
The most effective methods depend on circle readouts (either engraved setting circles or digital setting circles - DSC’s), interpretation & adjustments, repeated until the desired alignment accuracy is achieved.
The least effective method is that most commonly employed, the adjustment of the equatorial head through the minimization of Dec drift. It is inaccurate, tedious and inconclusive.
If you have an equatorial telescope with an RA drive either fixed sidereal rate or variable over a narrow range (Solar / Lunar / Sidereal), choose either methods 1) or 3) if it is portable, or methods 6), 7), 8), or 9), if housed in an observatory.
If your equatorial telescope has either a stepper or servo drive in both RA & DEC, choose either method 1) followed by 2) if it is portable, or methods 9) followed by 10) if housed in an observatory.
If you are fortunate enough to own an equatorial with finely divided setting circles, RA readout to say 4s time by verniers & DEC readout to say 30”arc by verniers, use either methods 6), 7), or 9).
An observatory equatorial without circles can be polar aligned using method 8).
Setting an Equatorial Head:-
Abstract from Section 16 Amateur Astronomer's Handbook by J.B. Sidgwick -
Grubb's true pole alignment method for observatory equatorials -
[True Pole Alignment.html] True pole alignment
[pdf 76Kb]
[Mac .edc 104Kb] Apparent RA drive rates -
[pdf 4Kb formulae]
[pdf diagram] Challis' Method of determining the coordinates
of the Instrumental Pole _download
[derivation.pdf]
for description of method. Challis' method is very useful for measuring small misalignments of the hour axis off the true pole,
and for checking precisely where the hour axis is pointing. Polar Alignment in Double Star Astrometry:-
Use of Challis' Method
Challis' method of true pole alignment was developed for equatorials lacking setting circles, but having a filar micrometer.
Essentially it entails measuring the declination drift of a clock star as it culminates.
Select a star approximately 3 hours east of the central meridian, near the celestial equator. Centre the clock star and orientate the filar micrometer such that the driven wire if moving north-south (in plane of declination). Place clock star on intersection of zeroed wire.
Set telescope to track clock star and take drift reading reduced to arc when star culminates, and again when clock star lies approx. 3 hours west of central meridian.
Sidereal times for beginning, culmination and end are notes at Tau(0); Tau(1) & Tau(2), Dec drifts as Delta(1); Delta(2) & Delta(3).
Challis' reduction method is described in the accompanying
pdf download
For AppleWorks v6 or ClarisWorks v5 AppleMac spreadsheet download Challis_template_mod.xls.hqx
Toshimi Taki's matrix methods for determining polar axis misalignment and mount build errors.
For pdf matrix_method_rev_d.pdf
King Rate
For King rate drive formulae.
For Excel v5 Win9x spreadsheet download
Challis_method.xls.zip
For further information on coordinate transformations refer to: Toshimi Taki's website-
For information on professional telescope pointing analysis software refer to: TPoint -
For information on professional telescope control software refer to: COMSOFT Telescope Control Software
For further information on exceptional telescope control software refer to: Sky Chart 2000-
Sky Chart 2000 Win3.11 demo version - download SC2000.exe
[860Kb]
Rate of Dome Rotation - [pdf 176Kb]
Dome Slit Synchronisation - [pdf 72Kb]
Appleworks spreadsheet - dome_slit [260Kb]
Xcel2000 spreadsheet - dome_slit.xls [60Kb]
Appleworks spreadsheet - Synchro_dome_azimuth [56Kb]
Xcel2000 spreadsheet - Synchro-dome-azimuth.xls [24Kb]
DomeSync Software that calculates the necessary dome azimuth and shutter altitude to allow a telescope to "see' through the center of the dome opening. This software has been developed by John Oliver based upon algorithms by Chris Lord and Toshimi Taki.
Calculating the minor axis and offset of a Newtonian secondary mirror - [pdf 20Kb diagram] [pdf 12Kb formulae]
Diffraction Spike Analysis - [pdf 8Kb]
USEFUL DOS6.2_WIN3.11 programmes:-)
Newtonian Secondary Minor Axis DOS6.2 - sec.exe
[116Kb]
Primary Mirror Fan Assisted Cooling Times DOS6.2 - cool.exe
[112Kb]
Sx Image Processing & Capture - sx.zip
[224Kb]
Orbit determination - coaaorb.zip
[92Kb]
Basic programmes - basprogs.zip
[116Kb]
Eyepiece articles
Topical articles on eyepiece properties, current observing trends and why following the
heard is not always best practice. Articles on various eyepieces in my collection acquired
over the past 40 years.
[ATT WideEyed article pdf 5.2Mb]
[30mm UWA eyepieces - GSO comparisons pdf 54.9Mb]
[New Angles on an old eyepiece pdf 60kb]
[Unusual eyepieces pdf 2.8Mb]
Barlows and Telecentric Amplifiers
Barlow lenses and how they operate to increase the effective focal length of a telescope
are described, together with the equations needed to calculate their amplification factor,
or determine their focal length. Telecentric amplifiers act in a similar way to Barlows,
but their operation is significantly different. There is a current gap in the literature
describing fully how telecentrics work, and why they behave somewhat differently to Barlows.
This short article describes the action of Barlows and Telecentrics and provides mcuh
needed background information.
Calibrating The Effective Focal Length Of Catadioptric Cassegrains With Moving Primary Focusing
Catadioptric Casegrain telescopes with moving
primary focusing have a variable focal length. Unlike conventional
fixed focal length telescopes, focus is effected by adjusting
the separation of the primary and secondary mirrors. This short
paper deals with the correct method of calibrating the true effective
focal length, and provides an EXCEL chart of EFL for specific
backfocus for most commercial SCT's and Mak's.
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
The EVOLUTION of the ASTRONOMICAL EYEPIECE
a description of the evolution of astronomical
eyepiece designs from 1610 to the current day. Includes ray trace
diagrams of all principal eyepiece
types, and comments on their performance
with various types of telescope.
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
Telescopic Resolution of Unequal Binaries
considers the historical aspects of the various
aperture dependent resolution limits applied to telescope objectives
figured to provide diffraction limited images, and how such limits
can be modified to predict the diffraction limited aperture required
to resolve pairs of unequal brightness.
[pdf 228Kb]
[Mac .edc 188Kb]
For Sissy Haas January 2002 Sky & Telescope article "Enjoying Unequal Double Stars" goto [Sky Publishing]
To download a hi-res.pdf of my nomogram [click here - 2.6Mb] & for instructions sheet. [here]
Contrast and Definition
dissertation written whilst resident in Santa Monica in 1979;
a preliminary study into the effect of contrast on the resolution of detail in extended
objects and the resolution of unequal double stars.
[pdf 220Kb]
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
For an AppleWorks spreadsheet and charts which accompany the above paper download [contrastindex.sit]
History & Development of the Divided-Lens
Micrometer & Calcite Double-Image Micrometer - >
detailed historical description of the double
image micrometer, separated into the divided-lens and crystal
micrometer. Includes description of the Muller & Lyot micrometer.
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
Calibrating a Bifilar Micrometer by Measuring
Declination of HIC Stars in the Pleiades -
specialist monograph on micrometer screw
constant calibration.
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
Measuring Close Double Stars with a Lyot Micrometer -
description of the use of the Meca-Precis
Lyot double image micrometer, designed by Jean-Louis Agassi &
Rene-Georges Huret, on behalf of the double star commission of
the Societe Astronomique du France.
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
Hints on Reflecting & Refracting Telescopes and their accessories
by W. Thornthwaite, F.R.A.S. 6th. edition 1895.
Trade publication of Horne and Thornthwaite,
416, Strand, London. One of a series of like booklets printed for their customers,
the earliest users of commercial silver-on-glass reflecting telescopes.
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
Designing & Constructing a Domed Astronomical Observatory -
[domeobs.html]
[pdf 238Kb]
detailed description of the design and construction methods
of an aluminium alloy domed observatory intended for a
large amateur equatorial telescope.
A full set of CAD generated engineering drawings created in Cadintosh v4.1 -
[download .hqx 2.5Mb]
(nb: unless you have either an ISDN or ADSL broad bandwidth internet
connection please do not single click on the link _ instead click and hold
and use the pop-up menu to select download link to disc).
