Astronomical Society of Coonabarabran

How To Polar align a fork mounted Schmidt-Cass.: the "Fast Method"

by Harry Roberts

In the days before "Go-To" it was necessary to polar align Schmidt-Cass (SC) 'scopes for long exposure photography. If you drive to remote sites with a fork mounted 'scope you need a "fast" method of polar alignment so that "clear sky" time is not wasted.

Older texts describe a "star-drift" method that gives good alignment but takes a long time. Many German equatorial mounts have a hollow RA axis with a small polar alignment scope - my Vixen mount has one, but given the faintness of the south polar stars it is almost useless. Fork mounts are not fitted with polar alignment 'scopes - so what to do?

Some years ago I had success with a fast method derived from basic surveying practice. While easy to do, it is hard to describe; but here goes!

POINT 1: While the southern sky has by far the brightest Milky Way objects and galaxies, it does not have a bright pole star at the south celestial pole (SCP). But It does have some distinctive faint stars that mark the pole, and the first requirement for finding the SCP is an accurate map of these stars. I made a map from parts of the Uranometria 2000, showing the present location of the (slowly moving) SCP relative to nearby stars to magnitude 10.

POINT 2: The fork mounted SC is itself a large polar alignment 'scope when its optical axis is precisely aligned with the RA axis of rotation.

POINT 3: Having got a map that shows where the SCP is, a method is needed to point the 'scope's RA axis precisely at the SCP.

ESSENTIAL EQUIPMENT 1: The 'scope must have manual slo-mo controls. Many new mounts lack these. Also a one-power (Telrad type) finder and an 8 X 50 finder are needed, both accurately aligned with the main scope when a high power eyepiece (ep) is used.

ESSENTIAL EQUIPMENT 2: the scope's polar wedge must have slow motion adjustment of both latitude angle and azimuth angle when the scope is attached (Warning: this is often not supplied as standard). I got a nice jack-screw from the US (Bob's Knobs) that allows fine adjustment of the latitude angle while looking through the 'scope's e.p. as well as azimuth. Note that in-built levelling bubbles and compasses are not needed, but can be handy.

ESSENTIAL PREPARATION: Before attempting a field set up, assemble tripod and wedge at home. Preset the angle of the wedge plate to the latitude angle of your intended location. And check that with the 'scope in place, the knobs that adjust latitude and azimuth can be worked smoothly without any fancy tools.

ON SITE - STEP 1. Align the 'scope's optical axis with the RA axis.

Firstly, assemble tripod, wedge and 'scope with the forks pointing roughly due south. Now insert a compact star diagonal and wide-angle ep, say 60 to 80 times. Then find a bright star in the ep, any bright star will do, and check that both telrad and finder are aligned with the main 'scope. Adjust the finders if needed.

Secondly, swing the 'scope to declination 90ºS and lock (Fig 1). With the 'scope set at 90ºS the scope's optical axis is almost aligned with the RA axis of rotation. For the SC to work as a polar alignment 'scope we must precisely align it with the RA axis.

We do this by looking in the ep at any stars in the field (do this after dark). With RA unlocked we rotate the scope by hand (in RA only) while at the same time watching the stars in the wide-angle ep. These stars will seem to rotate about a point in the field of view (Fig 2), or more likely, about a point outside the field of view (FOV). With only the Dec. slo-mo shift the FOV towards the centre of rotation. Repeat until the new stars that have come into view rotate around the centre of the FOV. It sounds complex - but It's easy to do, taking only minutes. You now have the optical axis of your 'scope aligned with the RA axis of rotation. Lock the declination axis and keep it locked until STEP 2 is complete.

This is a good time to check your declination circle and see if it reads exactly 90ºS. If not, adjust the dec. circle. Do not unlock the declination axis. It must remain fixed for STEP 2.

ON SITE - STEP 2: Pointing the RA axis exactly at the SCP.
Remember, you can move the RA axis only by shifting the wedge's latitude angle (with the jack-screw) and its azimuth angle. Do not change the declination setting - it must remain at 90ºS.

This is where the Telrad comes into play. I use binoculars to find the brighter pole stars in the sky. Then I look through the Telrad to see if the pole is in the field. If not, I adjust the wedge's latitude angle and azimuth - sometimes I have to pick up tripod, 'scope and all and rotate the whole assembly to bring the pole into the Telrad's field.

Now search for the polar stars in the 8X finder, using smaller adjustments of the jack-screw and azimuth controls. Finally in the main 'scope's 80X e.p. bring first the pole stars, and then the precise SCP location into the centre of the FOV. Unlock the RA clamp and rotate the forks by hand, and you will see the FOV rotate around the SCP. Lock the wedge's latitude angle and azimuth setting. Remember you must not shift the 'scope in declination during STEP 2.

You are done! You can now shift RA and Dec to acquire any target object. How long did it take? Once you learn where the pole lies in the sky relative to Crux, the Magellanic Clouds and other sky features the process takes much less time. Learning to recognise the fainter pole stars in the 80X eyepiece and the current position of the pole speeds things up greatly.

Of course this method assumes the pole is visible at your site (no trees or cloud). And all the above is irrelevant to fork mounts used in alt-az mode. They suffer field rotation however, and de-rotators are needed for long exposures. The above instructions apply mainly to portable 'scopes used in the field. For permanent mounts in observatories the method should be refined using the drift technique. Also the "fast" method only works with SC FORK MOUNTS. STEP 1 being almost impossible with German equatorial mounts.

How long will it take? With practice, five to ten minutes is enough, but if you miss a step you must start again. Make the most of clear skies!

polar_allign_fork_mount_files/polar_align_fork_mount_1

Figure 1

polar_allign_fork_mount_files/polar_align_fork_mount_2

Figure 2