Polar Aligning an Equatorial Mount for Use of Manual Setting

Circles

Chris Waskeywww.geocities.com/waskeyc

waskeyc@yahoo.com

German Equatorial Mount

Useful Links• Tons of great information

– www.cloudynights.com

• Free Star Charts to Mag. 7 – http://www.cloudynights.com/item.php?item_id=1052

• Daily celestial events, updated weekly – http://www.skyandtelescope.com/observing/ataglance?1=1

• Source for most of this presentation – http://www.skyandtelescope.com/howto/visualobserving/3304206.html?

page=1&c=y

• Free TriAtlas for download and print, virtually all known DSOs– http://www.uv.es/jrtorres/index.html

Outline

• Background/Purpose

• Setting Up Initially– Checking and adjusting the mount in daylight

• Setting Up and Observing– Polar Alignment– Navigating– Troubleshooting

• Conclusions

Background

• Celestial Coordinates– Declination is measured from the celestial

equator• Degrees north or south• True at all times

– Right ascension (R.A.) is measured from the location of the sun at the Vernal Equinox

• Hours and minutes, increasing to the East• Local meridian R.A. changes as the earth rotates

Background: Celestial Coordinates

Background: Mount Axes

• Polar Axis / Right Ascension Axis – points to the celestial poles. When the telescope is rotated about this axis, the R.A. value changes.

• Declination Axis / Counterweight Axis – perpendicular to the polar axis, the axis that the counterweight bar screws into. When the telescope is moved about this axis, the Declination changes

Purpose

• Accurate polar alignment helps with:– Tracking objects at high magnification– Long-Exposure Astrophotography– Use of manual setting circles

Purpose: the Goal

Setting Up the Telescope

• Do once and forget

• Goal is to identify polar axis of mount, then adjust Declination axis to be perpendicular to it

Setup: Finding Polar Axis

• Setup Mount Polar Axis facing a horizontal line– May be horizon or horizontal roofline, etc

• Turn scope to 90o Declination and rotate in R.A. so counterweight bar is horizontal

• Adjust mount elevation so horizon is centered in the finder scope

• Rotate scope 180o in R.A. and determine whether scope is pointing high or low

• Adjust the Dec. to split the difference, then adjust the mount in elevation until the horizon is centered

• Verify setting, permanently move the Dec. setting circle to indicate the new 90o

Setup: Adjust R.A. Axis to be Perpendicular to Polar Axis

• After identifying 90o Declination, point the Polar Axis of the mount to a distant, identifiable object

• Rotate the scope in R.A. 180o about the distant object

• If it stays centered, you’re done

• If not, you may need a spacer between one tube ring and the mounting bar

Observing

• After preparing the telescope, it’s ready to setup and use

• First accurately position the scope to 90o Declination and adjust the mount to align the Polar Axis with the North Celestial Pole (NCP)

• Second, point the telescope to a known celestial object and set the R.A. dial

• Third, move R.A. to coordinates of desired object, followed by Dec. to coordinates of object

Observing: Polar Alignment

• Set up tripod roughly facing True North• Adjust Latitude to roughly local latitude

(~37o N for Virginia)• Turn scope to roughly 90o Declination• Rotate scope in R.A. and alternately adjust

Dec. and mount until center of rotation is on Polaris

• Use mount adjustment screws to move center of rotation to NCP

Observing: Polar AlignmentNorth Celestial Pole

Observing: Polar AlignmentFinder View Orientation

Observing: Polar Alignment

• To find the North Celestial Pole, look for the 6th magnitude star near Polaris

• In a straight through OR correct image finder, go from the 6th magnitude star to Polaris, then turn RIGHT (otherwise turn left)

• Move from Polaris almost halfway toward the first 6th magnitude star (λ) in the handle of the Little Dipper

Observing: Polar Alignment

X

Observing: NavigatingVernier Scale

• For approximate R.A., read the value on the dial closest to the zero on the vernier

• For exact value, find the closest match on the vernier to a line on the R.A. scale (doesn’t matter which R.A. line that happens to be)

• R.A. is # on vernier plus closest R.A. value below the zero

• See example

Observing: NavigatingVernier Scale

Observing: Navigating

• First, point the telescope to a celestial object with a known R.A.

• Adjust the R.A. dial on the mount to the R.A. of the object

• Move the scope to the R.A. of the desired object and lock the R.A. axis (if equipped, turn on drive motor)

• Move the scope in Declination to the Dec. of the desired object

Observing: Troubleshooting

• If all of the steps above were followed carefully and a reasonable magnification is used, the object should be in view

• If it’s not in view, check these things:– Is the object in the finder scope view?– Is the object visible in your size telescope and current

conditions?– Did too much time go by between setting R.A. dial

and moving to the object’s R.A.?• If you still can’t find it, but should be able to see

it, then try star hopping from a nearby known object

Conclusions

• After adjusting the mount to set accurate Declination and to make the axes perpendicular, polar alignment should only take a few minutes

• An accurate polar alignment can be used to find objects with manual setting circles, and can improve the accuracy of GOTO scopes

Questions?