Build a right-angled polarscope adaptor

Take the back strain out of the aligning process with our guide to building your own right-angled polarscope adaptor

Build polarscope adaptor

Do you enjoy getting damp knees, back-ache and a stiff neck as you struggle to use the eyepiece of your polarscope when you’re trying to get your mount decently aligned?

No? Neither do we.

There’s not much point in an equatorial mount unless it’s properly aligned but, marvellous as polar alignment scopes are, they have never been designed for comfort.

Being uncomfortable isn’t just unpleasant, it’s also a disincentive, so you have to welcome any way of making the polar alignment process less of a back strain.

Tools – Callipers for measuring the finder’s bayonet connector and scissors for cutting out the template. If you’re using a soft plastic tube like a film can, you will need a craft knife. If you’re using a hard plastic tube then a multi-tool with a small milling bit is ideal.

Materials – A right-angled finder for an SLR camera. A tube, closed at one end, that’s a loose fit on the polarscope eyepiece – a 35mm film can or 36mm pipe end-cap may fit. The ‘loop’ side of some self-adhesive hook-and-loop fastener tape.

Sundries – Card, electrician’s insulating tape, permanent marker, bench vice, safety goggles.

There are several software solutions, and you can even fit a cheap webcam to the polarscope so that you can view the alignment reticule on a screen.

But these all add complexity to what, in a perfect world, should be a quick and simple process.

The ideal solution for frequent polar alignment, especially when you’re using a portable tracking mount in the field, needs to be one that doesn’t require computers, electrical power or the skills of a contortionist.

The discomfort arises from two factors. Firstly, the polarscope, from UK latitudes, must be aimed at an elevation of 51° or higher.

Your neck has not evolved to the point where you can comfortably look up at that angle except for short periods. Secondly, the polarscope eyepiece may be at chest height or lower.

Forget about sitting: putting a seat in the required position results in a territorial dispute between the chair’s legs, the tripod’s legs and your legs.

Getting your eye in the right position requires either kneeling or bending your back at the sort of unnatural angle that contravenes every chiropractor’s idea of good posture.

Both bending and kneeling increase the temptation to use the tripod as a support when you get to your feet, affording you every opportunity to knock the system out of alignment again. Plus, kneeling leads to wet knees.

The simplest solution is the one that we all use at the eyepiece of a refracting telescope: a star diagonal.

Fortunately, the essential components are readily available at reasonable cost.

The heart of this project is a right-angled finder attachment designed for use with the viewfinder of a DSLR camera.

There are several different types available and your choice should be dictated by the ease with which it can be fixed to the tube that will go over the polarscope’s eyepiece.

The Pentacon-branded one is particularly suitable because it has a bayonet-type connector that merely requires you to cut a shaped aperture in the connecting tube and not have to bother about drilling screw-holes or making complicated mating pieces.

Finding a cheap finder

Many second-hand finders have lost the attachment that slips over the camera’s viewfinder surround, and so are available very cheaply.

A right-angled finder also gives a correct image, so you don’t have to allow for a reversed polarscope reticule.

If your polarscope has a 30mm diameter eyepiece, such as the common Sky-Watcher or Vixen ones, the tube can be fashioned from either a 35mm film can or a 36mm ABS waste-pipe stop.

Alternatively, it could be a useful 3D-printing project.

You need to use loop tape to make the tube fit snugly to the eyepiece.

Trial-and-improvement is the best method for determining the appropriate amount to use.

It needs to be enough to prevent the finder from falling off, rotating or tilting, but not so much as to make it difficult to get it on or off.

To use your new creation, first roughly align your mount.

A bright penlight torch shone up the polarscope eyepiece, producing a collimated beam that you can direct at Polaris, is useful for this (as long as doing so won’t inconvenience fellow observers).

If necessary, set the polarscope’s time and date, put the finder on the polarscope and place Polaris in the appropriate part of the reticule.

Then take it off and enjoy a comfortable observing or imaging session with dry knees and an ache-free back and neck.

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Step 1

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A card template is useful to test that you’ve measured correctly.

Draw around the tube to get a circle, then carefully measure the bayonet connector of the right-angled finder and draw its shape in the centre of the circle you’ve drawn.

Step 2

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Cut out the template and make sure it fits the finder.

If it does then – ensuring the template is properly positioned on the tube end – draw around the aperture with a permanent fine-point pen (non-permanent marks may not survive the next step).

Step 3

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Carefully cut out the aperture in the end of the tube, ensuring you follow any safety precautions recommended by the tool’s manufacturer.

Remember – because you drew inside the template aperture, you will need to cut outside of the marked lines.

Step 4

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Use a fine file to smooth any rough edges and ‘tweak’ the shape of the aperture.

Experiment to determine the appropriate amount of loop tape to give a snug fit to the eyepiece of the polarscope.

Film canisters usually need less tape than end-caps do.

Step 5

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If you used a film can, cut it to 25mm long and reinforce the cut edge with insulating tape.

Mate the tube to the finder and rotate it through 90°.

Tighten the locking ring on the finder until the tube will no longer rotate.

Step 6

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Fit the finder to the polarscope and ensure that the reticule is visible in the centre of the field of view.

If it isn’t, this is usually because it is tilting slightly, so you will need to adjust the amount of loop tape inside the tube.

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Stephen Tonkin is author of Binocular Astronomy and editor of binocularsky.com