2021 February 6

Using a polarscope

David Arditti

Introduction

Many of us possess German equatorial mounts supplied with a polarscope, a small refracting telescope that passes through the polar axis. It is provided with a reticle, an etched piece of glass or plastic at the focal plane, sometimes illuminated, containing a diagram that is supposed to aid in aiming the polar axis at the true pole. While an observatory mounting is likely to be carefully aligned on the pole over many nights by the drift method, the polarscope is intended to be useful with a portable setup, allowing far more rapid, though not so precise, alignment in the field. How, exactly, to use the polarscope, however, is often unclear to owners, unsurprisingly, because of the many different designs, and frequent lack of clear (or any) instructions from the manufacturers. Fortunately, the smartphone has now come to our aid, providing an easier and better way of using the polarscope than was ever possible before.

Collimation

Good polar alignment will be achieved only if the polarscope’s optical axis is exactly aligned (collimated) with the polar axis. This alignment is adjusted using three screws of some kind around the polarscope mounting, and is easiest if performed in the day. For this purpose, the telescope and counterweights need not be on the mount. Loosen the altitude bolt to lay the polar axis close to horizontal, so you can view some distant, precise target such as an aerial through the polarscope. Note that for some mounts, you cannot see through the polarscope unless the declination shaft is in an orientation that allows you to. Note, also, that the polarscope is focused by twisting the eye end. You should be able to get the target and the reticle simultaneously in focus. Place the target on the centre of the reticle (which in many models is marked with a cross), and rotate the declination section of the mount (that would carry the telescope) about the polar axis; any shift in the target against the cross will be due to mis-collimation. Adjust the screws to try to counteract the error, repeating until there is no image shift. If you must do this collimation at night, Polaris may be used as a target (in the northern hemisphere), as it moves only slowly.

Rough alignment for the northern hemisphere

Polaris currently lies 44 minutes of arc from the North Celestial Pole, but this figure is gradually decreasing year by year. When the mount is precisely aligned, the centre of the polarscope reticle will coincide with the pole, not with Polaris. There are various reticle designs, but many of them feature a circle around the centre, or several circles for different years, that correspond to the path of Polaris over the night. The picture shows one such design commonly found, used on many mounts from Sky-Watcher and Orion, including the very portable Star Adventurer. It contains three closely-spaced concentric graduated circles marked like a clock face, plus a diagram of some stars in Octans, and some other hieroglyphics to the right. Though Octans is a southern constellation, this reticle is actually intended for use in both hemispheres. When the polarscope is pointed precisely at the north pole, in 2020, Polaris will lie somewhere on the middle circle of the ‘clock’. But where?

A rough method of determining where involves noting that Polaris is displaced from the true pole in a direction approximately opposite the direction of the other bright star in Ursa Minor, Kochab. So if you can see Kochab with the naked eye, and can imagine the line between it and the centre of the reticle, you can estimate where Polaris should go on the circle, taking account of the fact the polarscope inverts, so, looking through it, Polaris will be seen displaced from the centre towards the naked-eye position of Kochab. You adjust using the altitude and azimuth adjustments of the polar axis to place it there, not worrying about the orientation of the clock-face. This method will likely place you within a few minutes of the true pole, accurate enough for, say, 60 seconds exposure with a telephoto lens or very short focus telescope, and it is applicable to all reticles with the basic circle design. It does not require levelling of the mount.

Using a smartphone

Most mountings intended for use with polarscopes feature a puzzling series of graduated rotating metal circles that are supposed to somehow enable you to determine the correct location of Polaris on the reticle circle using the date and time. I doubt many people really use these. Far easier is to use an excellent app available for both Android and iOS called Polar Scope Align. There is a free version of this, but you have to put up with advertisements. The app is programmed with many reticle designs, and contains clear instructions for each of them. To use the Sky-Watcher and similar cross and circle designs, the declination section of the mount needs to be rotated about the polar axis so one of the cross axes on the reticle is vertical. Provided the mount is first levelled, centring Polaris on the reticle and then moving the polar axis up and down in altitude will allow you to get this vertical exactly, so Polaris stays on the line as you move. Then you can simply read from the app where, on the circle, Polaris should go.

As mentioned, actual distance of Polaris from the pole varies slightly over the years, and this is the meaning of the ‘hieroglyphics’ in the right of the Sky-Watcher reticle. Ignoring this won’t result in significant error for many years, however. The diagram of the stars in Octans is, of course, for use in the southern hemisphere. It shows stars that should actually be seen through the Polarscope. When the pattern is placed over those stars, alignment will be accomplished.

David Arditti, Director.
Equipment & Techniques Section