Manual Setting Circles - Basic Use

K. Fisher Rev. 10/31/2010 Orig. 10/8/2007

Table: 638 stars visible above -38° dec
Table: Cross-reference Bayer-Flamsteed-to-SAO for 1822 stars brighter than mag 5.5 visible above -40° dec csv Excel 2003 for GOTO star synch
Table: Messier and Caldwell list slews from bright stars above -38° dec by Messier Caldwell number sorted by constellation
Table: Messier and Caldwell list slews from bright stars above -38° dec by Messier Caldwell number
Table: Messier and Caldwell list slews from bright stars above -38° dec by bright star name
Table: Messier and Caldwell list slews from bright stars above -38° dec by right ascension
Spreadsheet (Excel): Messier and Caldwell list for alt-az slews from bright stars (2.5 megs)
Cartes du Ciel: External catalogue - bright stars and Messier-Caldwell list above -38° dec

Concepts Assumed skills Required resources Desireable resources Adding index marks Basic techniques Convert RA to Decimal Degrees Worked examples Messier and Caldwell list slewing aids


With GOTO computerized scopes being so prevalent, basic use of manual setting circles (also known as analogue setting circles) on a German equatorial mount (GEM) is not well covered in manufacturer instructions or basic astronomy books. This cheat sheet describes how to use manual setting circles. A reference table of bright stars with Bayer and Flamsteed numbers is provided here to aid in using manual setting circles. A table and spreadsheet of pre-computed slews for the Messier list and for the Caldwell list above -38° declination are provided.

In aid of computerized GOTO small scopes, some of the tables include for bright reference stars, the identification cross-reference numbers to the HD and SAO catalogues. HD (Meade) and SAO (Celestron) handcontrollers typically contain one or both of these stellar catalogues. For GOTO resynching of a hand-controller, the table titled "Messier and Caldwell list slews from bright stars above -38° dec by Messier Caldwell number" is helpful. With this list, the telescope user can slew to the nearest bright star to the object, resynchronize their GOTO controller to that bright star and then slew the few remaining degrees to the faint object. Resynchronization of the hand-controller allows for high-accuracy slewing, in particular, for urban and suburban light polluted skies. Some astronomical planning software, like AstroPlanner, build in intermediate bright star slewing utilities that automate the resynchronization process.

Basic concepts

Manual setting circles best work by making short right-angle slews from bright-star targets. The telescope is centered on a bright star with known coordinates. The setting circles are adjusted to show the (1) known right ascension and declination of the target bright star, or to (2) zero (0 °) right ascension and zero (0 °) declination.

Setting circles on high-end mount

Declination and right ascension setting circles on a high-end mount - GNU Wikipedia Image by Halfblue

A short slew is then performed to move the scope either to (1) the direct right ascension and declination coordinates of the target or (2) the indirect index (offset index) between the bright star and the target. Special cases for indirect index slews involve where the bright reference star and object are on the same right ascension or declination great circle. These special cases are called (3) "right ascension sweep" and (4) "declination sweep".

As any of these techniques will rarely put the target exactly on center with visual observing alignment tolerances, this technique is supplemented with traditional star hopping. The use of bright stars with manual setting circles is similar to alignment stars in 1, 2 and 3 star computerized GOTO alignment.

Assumed skills

  • You should be able to roughly polar align your scope.
  • You have minimized perpendicular error (aka cone error) in your scope. This is particularly important for Newtonian reflectors. If your scope is not aligned to remove perpendicular error, it is possible to properly align a mount using its in-built polar scope, to use your manual setting circles and still have your scope pointing 1 to 1 1/2° away from the desired target.
  • You should be able to perform a meridian swap with your telescope mounted on your German equatorial mount.
  • You should have the general observing skill to relate the current local sidereal time to your south meridian. You should be able to relate the LST (the current right ascension on the south meridian) to a star chart. You should understand the relationship between the LST at the south meridan and right ascension on either side of the meridian.
LST example

Looking at object located at 10RA at time LST 8:00

Required resources

  1. A bright star chart
  2. Some choices include:

    Star chart example

    Star Chart - Wikipedia GNU Image by T. Bronger

  3. A bright star table with J2000 coordinates
  4. Easy use of manual setting circles is dependent on having a good Bayer name list of bright stars above magnitude 4.5 that also lists the J2000 or epoch-to-date coordinates of each star. The list should be in hardcopy and sorted by constellation for ease of reference in the field. Bright star lists in several popular publications either do not have lists of sufficient star density or magnitude depth to allow for easy lookup. Tirion's Cambridge Star Atlas has a list of the 95 brightest stars, ordered by magnitude, down to about magnitude 2.5. Peterson's Star Guide and the RASC Observer's Guides have a comprehensive bright star list based on Garrison's catalogue of absolute magnitudes. Garrison's catalogue typcially contains 315 stars in both hemispheres down to a magnitude of 3.5. This is too low a density for use with setting circles (approx. 150 per hemisphere). These lists omits stars in some faint constellations, e.g. Capricorn, that only have faint stars.

    A well-sorted comprehensive bright star list supplements the observer's preferred star chart which may identify the bright stars but not easy reference the J2000 coordinates for each bright star.

    Some online bright star lists available on the internet that have both a good breadth and good order (Bayer and Flamsteed name within constellation) are:

    • Richard Powells' Brightest Star List accessed Sept. 2007. Lists 300 brightest stars to magnitude 3.5. Can be imported to Excel and resorted by right ascension and declination.
    • UNSO Astronomical Almanac Bright Star List accessed Sept. 2007. Is available in pdf and ASCII tabular format for importing, sorting and filtering in Excel.
    • UNSO Astronomical Almanac Bright Star Search Server accessed Sept. 2007. Will generate a list of bright stars, filtered by right ascension, declination and magnitude. That the list can be filtered by declination is useful for preparing charts for the latitude of your observing point.
    • Harvard Bright Star Catalogue 5th Ed. accessed Sept. 2007. Can be used to generate filtered bright star lists by magnitude and declination in ASCII format for import into Excel.
    • HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index (Kostjuk, 2002) accessed Sept. 2007. Can be used to generate filtered bright star lists by magnitude and declination in ASCII format for import into Excel.

    Most of these lists can easily be imported into an Excel spreadsheet for easy sorting and filtering.

    To aid northern North American observers near 41° North latitude, a filtered list of 638 bright stars down to magnitude 4.5, above -38° declination and sorted by constellation name and Bayer identification is provided here. The list is based on Kostjuk 2002. The list is limited to stars that have either a Bayer or Flamsteed identifier. (Html; pdf).

  5. A low-power eyepiece with a precomputed true-field-of view
  6. Once the scope is positioned using direct or indirect indexing, the target is acquired using the lowest feasible power eyepiece. Low power eyepieces yield the widest true-field-of-view (TFOV). A favored low-power eyepiece, typically 32mm, is used. Right ascension and declination can be measured by counting off the number of TFOV eyepiece views that the eyepiece traverses. To translate eyepiece views to right-ascension or declination, the TFOV of your favored eyepiece will need to be found by the following procedure:

    • Lookup the apparent-field-of-view (AFOV) from your eyepiece manufacturer's website. Typically, the AFOV will be between 47° and 52°. If you cannot find a catalogue value, assume an AFOV 50°.
    • The TFOV of the eyepiece is equal to the AFOV divided by magnification. TFOV=AFOV/M. Magnification is equal to the focal length of the telescope divided by the focal length of the eyepiece. TFOV=AFOV/(D_fl/EP_fl)=AFOV/M.

    A worked example: I use a 32mm eyepiece with a 52° AFOV on a 1200mm fl telescope. The TFOV for that combination of eyepiece and telescope is:

    1.4° = 52° / ( 1200 mm / 32 mm ) = 52° / 37.25x

    With a field stop in the scope, the TFOV of this particular scope is closer to 1.2°. When pacing off eyepiece views, each view is overlapped to permit back-tracking across a star-field. The practical, working eyepiece TFOV of this telescope-eyepiece combination is about 1°.

    As a practical application, take M92, a globular cluster near iot Hercules. Using measuring calipers and a star chart, set the calipers equal to about one eyepiece TFOV, or 1° in the worked example. Use the declination degree scale on your chart to measure off one chart degree. Do not use the right ascension scale. Count off the number of eyepiece TFOV's in right ascension between iot Her and M92 - about 4° in right ascension and 3-4° in declination. Eyepiece views are overlapped to be able to backtrack the telescope. For the right-ascension leg of the iot Her to M92 slew, that translates with overlapping into 5 or 6 eyepiece views covering 4° decimal, as shown in the figure.

    M92 Right-Angle Slew

    Counting e.p. TFOVs from iot Her to M92

    Desireable resources

  7. Drafting or divider calipers
  8. Dividing calipers are used to step off the number of eyepiece views between the bright star and target, as shown on your star chart.

    Dividing calipers

    Dividing calipers - GNU
    Wikipedia Image by
    G. McKechnie

  9. Extra watch set to local sidereal time
  10. Before a night's observing, set a watch to local sideral time. This is the equal to your right ascension on the southern half of your meridian. The telescope is pointed at a star on the meridian and the right ascension manual setting circle can be adjusted accordingly. The USNO Local Sidereal Time Calculator can be used to set a secondary watch to local sidereal time on the observing night.

    Dividing calipers

    Watch - GNU Wikipedia Image by R. McLassus

  11. Hand-magnifying glass
  12. A hand-magnifier helps in read small text on charts by a dim red astronomy flashlight.

    Dividing calipers

    Hand-magnifer -
    GNU Wikipedia Image
    by Tomomarusan

Adding supplemental index marks to your telescope

Manual setting circles typically have two sets of reversed index scales. One reads from zero to 24 hours right ascension; a second reads reversed from 24 hours to zero hours right ascension. The upper scale usually is for use in the northern hemisphere and will match the progression of right ascension hours that you see when facing the south horizon.

As a general rule for mid-price range mounts, basic mounts designed only for non-GOTO use are the easiest to use and read. The more compatible a mount is for a GOTO upgrade, the less the manufacturer is concerned about making the mount's manual setting circles user friendly. See illustration in the following mounts.

Manual setting circles can come in two flavors - relative and direct indexing.

In the first type - direct indexing - illustrated by the Orion EQ2 basic mount or the Orion Altas, the right-ascension setting circle moves with the scope and the right-ascension indicator remains fixed on the bottom half of the right-ascension portion of the mount.

In the second type - relative indexing - illustrated by the SkyView Pro EQ5, the right-ascension setting circle remains fixed. The right-ascension indicator moves above the fixed position setting circle on the bottom half of the right-ascension portion of the mount.

For both types of scopes, the indicator arrow on the right ascension tube and the declination tube can end up in hard-to-read positions.

A useful modification is to add indicator point lines at 120° intervals. To make a useful temporary mark, simply use copier white-out. If you are dissatisfied with the marks, rubbing alcohol will quickly remove them. Supplemental indicator markings can be made on both the right-ascension and declination barrels. Since all positions are made relative to any indicator line, having multiple indicator lines does not change the overall process of using a manual setting circle; it changes only the ease of reading the setting circle.

For the SkyView Pro EQ5 type mount, add three additional indicator marks on the right-ascension tube above the setting circle at 90° intervals. Add three additional indicator marks below the declination setting circle at 90° intervals.

For the Orion Altas EQ6 mount, make three additional indicator marks on the right ascension tube below the setting circle at 90° intervals. The Atlas EQ6 declination circle moves with the declination tube and the indicator is fixed on the right-ascension tube housing. Make the three additional marks on the fixed non-moving right-ascension tube below the declination circle. See the Supplemental Notes for a further discussion of marking the Orion Atlas (Syntax) EQ6 Goto mount.

For a basic mount like the Orion EQ2, after polaring aligning, point your scope at any star on the meridian. Then move your setting circle to the current local sidereal time.

Basic Technique

  1. Pre-compute the direct index coordinates or indirect index offset between the reference bright star and the target
  2. Three methods can be used to compute the size of the offset between a reference star and a target object - graphic, direct coordinates and indirect offset. Pre-computation and data lookup vary by the technique used:

  3. Apply the direct or indirect index to the telescope
  4. There are two methods to use manual setting circles: direct indexing or indirect offset indexing. A "RA sweep" or "Dec sweep" are a specialized case of an indirect index offset slew. A spiral search pattern is another search alternative.

    For all methods, first lookup the coordinates of the nearby bright reference star and target and perform any needed pre-computation, as discussed above. Optionally, use a planetarium program to print a supplemental star hopping chart down to a deep magnitude, e.g - v10-13. Such a chart is useful to confirm faint planet and galaxy target acquistion and to aid in star hopping.

Slewing to Deep Sky Objects on the Messier and Caldwell Lists

The Messier and Caldwell lists are the core of beginner and beginner-intermediate observing programs. Those lists also cover the majority of objects target at public outreach star parties of local astronomy clubs. Pre-computed RA and Dec slews from nearby bright stars for the Messier list and the Caldwell list (above -38° declination) are provided as follows:

For both lists the right angle and declination slews are generally less than 7°s across 201 bright-star and object pairs. For the right-ascension portion of a right-angle slew, 85% of slews are less or equal to 7°. The median right-ascension slew is 2.9°. For the declination portion of a right-angle slew, 95% are less than 7°. The median is 2.1°.

Of the above slew lists, the one organized by bright star name is more useful for beginners and for star parties. The primary sort of the bright star ordered list is by constellation. Once a constellation at a favorable altitude is located, the bright star ordered table can be consulted for which bright stars to center on. Usually there are one or two per constellation. The table lists the corresponding slews to popular Messier and Caldwell list entries.

Right-angle slews

Right-angle slews from delta Cass
from slew tables

A altitude-azimuth mounted scopes, a single html list cannot be generated do to differences in local horizon coordindate systems. A dynamic spreadsheet in Excel 2003 is provided:

A user guide for the spreadsheet is provided below.

Converting a minute of right ascension to an arcminute of declination

A minute of right ascension (RA) is not equal to an arcminute of declination - that is an arcminute measured in decimal degrees. Right ascension can be converted to declination mathematically in order to find the number of decimal degrees and the equivalent eyepiece TFOVs in a right-angle slew.

When graphically measuring eyepiece views using a dividing caliper, the declination scale is used to measure one degree. Declination is measured in decimal degrees. Right ascension is a hybird measurement of time and angular distance in decimal degrees. The angular size of a minute of right ascension is not the same as angular size of an arcminute of declination. Furthermore, the angular size of a minute of right ascension shrinks as you travel from the celestial equator to the celestial north or south poles. This occurs because all the great circle lines of declination converge to points at the north and south poles.

The practical implication is if you want to determine the number of TFOV eyepiece views to slew in right ascension by computation (a.k.a. reduction) there is some trigonometry to apply. Hence, it is easier, although less accurate, to use measuring calipers to measure the number of degrees involved in a right-ascension slew.

The varying angular size of right ascension in terms of decimal degrees

The varying angular size of right ascension in terms of decimal degrees

Mathematically, there are 15 minutes of right ascension for each arcminute of declination at the celestial equator. For example, take the slew to our worked example of finding Neptune - a right ascension slew distance of 13 minutes right ascension. Ignoring that Neptune is about 15° below the celestial equator, 15 times 13 RA minutes equals 195 declination (decimal degree TFOV) arcminutes. Dividing by 60 arcminutes per degree gives a right ascension slew of about 3 1/4° decimal degrees - or about 3 eyepiece views in the worked example. 3 1/4° is about the right ascension distance you would pace off using dividing calipers on a finder chart for Neptune.

As you move further in declination away from the celestial equator, the more pronouced the shrinking size of a minute of right ascension becomes. Take our worked example of slewing the right-ascension leg between iot Her and M92 - a distance of 23 minutes of right ascension but at +47° declination.

Neptune Right-Angle Slew

Slewing from gam Cap to Neptune

Mathematically, the shrinking decimal degree angular size of right ascension is compensated by an additional multiplier of cos(declination) times 15 decimal arcminutes for one right ascension minute. (See Supplemental notes.) Rather than getting bogged down in trignometry, the following is a simple rough conversion table:

Table: Multiplier to convert right ascension to TFOV decimal degrees at various declinations
Declination Cos(Declination) 15 arcmin decimal
/ 1 arcmin ra
80 0.17 15 2.6
70 0.34 15 5.1
60 0.50 15 7.5
50 0.64 15 9.6
40 0.77 15 11.5
30 0.87 15 13.0
20 0.94 15 14.1
10 0.98 15 14.8
0 1.00 15 15.0
M92 Right-Angle Slew

Slewing from iot Her to M92

In the M92 example, the right ascension slew of 22-23 arcmins of right ascension is equal to about 3.8-3.9° decimal ( [23 * ~10 multiplier ] / 60 arcmins per decimal degree). If you measure the declination degrees on a star chart and the right acension slew leg between iot Her and M92 (without any overlap as shown in the figure above), it is about 4 times the size of a declination degree mark on the chart.

In summary, to mathematically convert the right ascension slew in arcmins of right ascension to declination, estimate the multiplier from table provided above, or use the general formula:

arcmins decimal = 15 x arcmins right ascension x cos(radians(declination))

decimal degrees = arcmins decimal / 60

Worked examples

Find M92

See M92 example, above.

Find Neptune

See Neptune example, above.

Find Planetary Nebula NGC6826

See NGC6826 example, above.

Supplemental Notes

  • Marking the Orion EQ6 mount. The Orion Atlas EQ6 mount has some special performance behaviors that may warrant adding another set of marks above the right ascension setting circle. The Orion Atlas EQ6 (a branded Synta mount) is really designed to be used either manually with the power off or as GOTO scope with the power on. The setting circles remain fixed in place when the mount is powered in GOTO mode. With the power off and the scope right ascension axis unlocked, the right ascension setting circle moves with scope. However, in power off mode, the unlock levels are difficult to reach. Short-slews are really best done using the power-on hand-controller slew. The Atlas EQ6 can be run in a hydrid mode - the scope not goto aligned, tracking on or off and the motors used only for slewing with the hand-controller. Add four white-out marks above the right ascension setting circle at 90° intervals. Point the scope at a bright star. Lock the right-ascension lever. Slip the right-ascension circle to an index above the setting circle. Now use the hand-paddle to perform the short slew to the target. Conversely, if you are working only in power-off mode by locking and unlocking the right ascension lever, only use the additional index marks below the setting circle.
  • Converting right ascension to decimal degrees
    Decimal degrees equals Right ascension
    360° = 24 hours right ascension
    15° = 1 hour right ascension
    15° = 60 minutes right ascension
    = 4 minutes right ascension
    1/4° = 1 minute right ascension
    15 arcmins = 1 minute right ascension
    15 arcmins = 60 seconds right ascension
    1/4 arcmin = 1 second right ascension
    15 arcsecs = 1 second right ascension

User guide for the Altitude-Azimuth spreadsheet

It is assumed that the user has a basic working knowledge of Microsoft Excel and Excel's drill-down capabilities via the "Data filter" menu option.

The spreadsheet uses Visual Basic for Applications (VBA) code to compute numerous values. You will need to respond "Enable macros" to the standard Excel security prompt to update the spreadsheet.

Only enter data in blue-background cells. Use notes by component spreadsheet and worksheet tab follow:

  • Mandatory worksheets in Spreadsheet MessierSlews.xls:

    The following worksheets need to have key settings entered for the lists to properly display the altitude and azimuth slews for a particular location.

    1. Worksheet SettingObservingPoint:

      Enter the degrees, minutes and seconds for the latitude and longitude of your observing point. The spreadsheet automatically computes the decimal equivalents that are used in later worksheets. A storage area for your common observing points is provided.
    2. Worksheet SettingObservingTime:

      This is the key worksheet that controls recalculation of all other worksheets, in particular the Main Catalogue. Enter your current UTC offset in Cell B5. Cells B30:C31 control computation of the rest of the worksheets. Enter a beginning and ending observing time range using UTC, not local time. Typically use a 2 to 4 hour range. Worksheet main catalogue indicates whether an object is visible within this time range. To force recalculation of all worksheets, change the time value in Cell B31. Time must be entered in the format "HH:MM:SS". A scratch pad for converting local to UTC time is provided by Cell F31. After recalculation, this worksheet provides for the current and planned beginning observing time: Julian Day, Local Sidereal Time, Rising LST, Setting LST, Solar position and Lunar position.

      Again, remember Cell B31 in this worksheet. It forces recalculation of all the remaining worksheets.

    3. Worksheet MainCatalogue:

      This is a key worksheet of approx. 201 objects on the Messier and Caldwell lists. The list can be filtered by the "Alt_obj_selector" column (column S) to limit display to those objects currently above the horizon.
  • Optional worksheets in Spreadsheet MessierSlews.xls:

    The following worksheets contain either reference materials or do not materially alter the basic functioning of the worksheet to properly display the altitude and azimuth slews for a particular location.

    1. Worksheet SettingRefernce-TimeZones:

      Lookup your UTC offset value. Links to rules for applying Daylight Savings Time are provided. Tables for North America are provided.
    2. Worksheet ReferenceGreekLtrsConst:

      Reference table of Greek letters and constellation name abbreviations.
    3. Worksheet SettingsTelescope:

      In Section 1, enter the characteristics of the current telescope, barlow and eyepiece. This data is used in numerous worksheets to compute visibility, projection magnification and limiting magnitude. In Section 2, enter the characteristics of an observed star for the purpose of determining the telescopic limiting magnitude. Section 5 shows various resolution characteristics for the current telescope-eyepiece combination, e.g. the FWHM angular and linear sizes, the Airy disk radius, Dawes limit and Rayleigh's limit. A storage area is provided for data on your telescopes and binoculars.
    4. Worksheet SettingsSkySeeing:

      Record your estimate of the current seeing condition in terms of the FWHM seeing disk. This data is used in later astrophotography estimation worksheets. Includes reference tables for Pickering's scale and Antonaidi's scale.
    5. Worksheet SettingsObserver:

      Enter your age and experience scalar. This is used in limiting magnitude computations in later worksheets.
    6. Worksheet SettingSessionObservingRange:

      Enter your usual degree altitude range for telescope and binocular observing. This is used in later spreadsheets for computing the visibility of objects.
    7. Worksheet SettingSkyBrightness:

      Estimate your sky brightness in the magnitude and MPSAS systems. External content links to International Meteor Organization visual limiting magnitude areas are provided in Section 1. Sections 2 and 3 provide means to convert your Naked-Eye Limiting Magnitude Estimate into Magnitudes Per Square Arcsec (MPSAS). Enter these final values in Section 4. In later worksheets, this information is used to estimate whether an extended Deep Sky Object (DSO) will be visible. A storage area is provided for your common observing points.


Garrison, R.F. & Beattie, B. 1996. The Brightest Stars. Url: accessed Sept. 2007.

Kostjuk N.D. 2002. HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index. CDS Cat. IV27. Url: accessed Sept. 2007.

MacRobert, A.M. 2007. The Setting Circles on Your Telescope. Sky & Telescope Online Article. Url: accessed Oct. 2007.


Centre de Données astronomiques de Strasbourg - Catalogue Service: This project has made use of numerous catalogues redistributed through the CDS Astronomer's Catalogue Service, operated at CDS, Strasbourg, France. The use of those sources by this reference is acknowledged and appreciated.

Chevalley, Patrick: Cartes du Ciel Planetarium Software. This project makes use of celestial charts prepared using Cartes du Ciel v. 2.76. The use of this source by this reference is acknowledged and appreciated.

No copyright asserted

No copyright is asserted to any original content materials developed and included by this author in this document and the same are released to the public domain. No copyright is asserted as to any scientific fact recited herein or to any external content materials incorporated in this document.

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  • 10-10-2007: Corrected "arcminute of right ascension" to "minute of right ascension.
  • 03-09-2008: Merged and expanded with Messier and Caldwell list pre-computed slewing tables.
  • 03-10-2008: Added Cartes du Ciel external catalogue file
  • 03-14-2008: Added simple slew calculator
  • 03-26-2010: Revised Messier Caldwell slew tables for SAO reference star numbers
  • 10-31-2010: Added cross-reference table for 1822 stars brighter than mag 5.5 - Bayer-Flamsteed-to-SAO