Vixen Polarie Star Tracker Mount Review
1 CommentTuesday, 27 August 2013
A Review for Opticron by Steve Richards
As the Earth rotates around its axis, (an imaginary line drawn from the south pole through the centre of the Earth to the north pole) the stars above appear to move across the sky in what is known as diurnal motion. The Earth completes one full rotation in 23 hours 56 minutes and 4.09 seconds not quite the 24 hours that you might expect and this time period is known as a sidereal day. Stars that are visible from the northern hemisphere appear to move in an arc counter-clockwise around the north pole and those visible from the southern hemisphere appear to move in an arc clockwise around the south pole. The long exposure image above shows stars trailing around the north celestial pole (NCP) over a period of 1 hour and 12 minutes. The bright star just below the centre is the star Polaris, also known as the Pole star but note that it isn't actually located at the exact centre of rotation.
If you want to take photographs of the night sky, long exposures will be required as celestial objects are relatively dim but because of the apparent movement of the stars across the sky, you will have to `pan' your camera at the same speed to avoid the image smearing with the movement. This panning cannot be done accurately enough by hand even though the movement is very slow indeed so a special tracking mount is required for your camera. This special design of tracking mount is called an equatorial mount. Equatorial mounts have a motorised section that is tilted at an angle such that it runs parallel to the Earth's rotation allowing it to match the apparent movement of the stars above.
Vixen manufacture a wide range of equatorial mounts designed to carry telescopes and photographic equipment from smaller back garden units through to those designed for permanent installation in an observatory but these are not really designed for packing in a suitcase and taking away on holiday with you! The Vixen Polarie Star Tracker, however, is designed for just this purpose. Because it is small and light it can easily be taken as hand luggage aboard an aeroplane or simply taken to a nice dark location nearer home where it can be set up to capture beautiful wide field images of the night sky using just your DSLR camera and a lens.
A normal equatorial mount consists of two moving axes, the Right Ascension Axis (RA) and the Declination Axis (DEC). The RA axis defines the pointing position of the telescope or camera in an arc across the sky and this is the axis that is tilted at an angle to align with the NCP. The DEC axis defines the vertical pointing position. The RA and DEC positions of all known celestial objects are catalogued making it possible to locate them by pointing to the correct co-ordinates. Once the telescope or camera is pointing at the object, it is only necessary to track the object in RA and this is the beauty of the Polarie Star Tracker as it only has an RA axis. Because the Polarie is designed specifically for wide field astro- photography where exact pointing towards the chosen object is not critical, a full DEC axis is not required. However, accurately following the movement of the object is still critical to avoid egg-shaped stars and smeared images and this is the sole job of that single RA axis.
The Polarie doesn't look anything like a normal equatorial mount, in fact ironically, it looks more like a compact camera at first sight - a point made by my son who said "typical, the only piece of astro-photography equipment that you have that actually looks like a camera turns out not to be one!".
So, let's compare the Polarie with a typical equatorial mount so that we can see how it works. In the image above we can see an RA axis with the DEC axis mounted at right angles to it and how a counterbalance weight is required to balance the weight of the telescope or camera mounted on the DEC axis. The whole of the RA axis moves in an arc, taking the DEC axis with it and it can be seen that rotating the DEC axis will determine how high in the sky the telescope or camera will point.
The Polarie simplifies this enormously by using a ball and socket head to point the camera in the right direction and then the RA axis moves the camera in a smooth arc at sidereal rate following the apparent movement of the stars across the sky. This simple design concept results in an extremely compact and light mount.
In typical Vixen style, the Polarie is solidly constructed and presented in the company's standard livery of off-white with black trim. The attention to detail in the finish is very evident in the screw-less construction of the outer casing. Weighing in at 740 gm (without batteries installed), the mount feels robust and confidence-inspiring and its smooth lines simply ooze quality.
The case comprises aluminium front and rear plates with a rich, smooth powdercoat finish and black plastic side caps. The left hand side cap has an illuminated latitude meter built into it to help with polar alignment and this measures latitudes from 0° to 70°. The right hand cap has a small opening in its base for the connection of a USB mini-B 5 pin connector. This cap can be simply removed to expose a tiny toggle switch and a housing for two AA batteries. The two batteries will power the mount for several hours (depending on the equipment load and ambient temperature) making the mount completely self contained although it can also be powered from an external USB 4.4v to 5.25v source.
With external power supplied, the internal batteries are automatically disconnected. The toggle switch reverses the drive motor's direction so that the mount can be used in either the northern or southern hemisphere. Although adequate, this side plate seems a little flimsy in the company of the rest of the unit.
The placing of a `hot shoe' and a rotary selection switch on the top with a standard ¼" 20 tripod bush on the base only add to the mount's strong resemblance to a compact camera. The tripod bush is the industry standard and is used for attaching the unit to a suitable tripod, preferably one with a geared head to simplify polar alignment, although both a pan and tilt or ball and socket head can be used. The issue here is the fine adjustment required to achieve the most accurate polar alignment possible. Making fine adjustments with a ball and socket head is doable but rather awkward. A pan and tilt head helps enormously but unless it is a very expensive unit, there is usually some `sag' when finally tightening up after adjustment. The hot shoe fitting doesn't have any electrical connection, it is there simply to retain various accessories like spirit levels or the optional Quick Polaris Locator (see later).
Tracking and switch functions
The rotary selection switch has a good solid feel to it and from the `OFF' position, it will only rotate in a clockwise direction. Firm click-stops at each setting give an unambiguous feel in the dark although as soon as you move from the off position, the selected icon illuminates anyway. The switch has six positions; off, illumination, 1/2 sidereal rate, sidereal rate, solar rate and lunar rate.
All power to the mount, both internal and external is disconnected.
The bezel of the rotary switch is illuminated in either red for the northern hemisphere or green for the southern hemisphere as a warning of which position the toggle switch in the battery compartment has been set to. The latitude scale on the left hand side of the case will also be illuminated but in red only.
½ Sidereal rate
Including ground-based objects like trees or a building is an excellent way of adding character to your night-time images but as the mount tracks celestial objects, fixed objects in the foreground will become blurred over time. The ½ sidereal rate speed produces a reasonable compromise between blurred foregrounds and trailed stars for relatively short camera exposures. Images produced in this manner are called `star-scapes'.
This is the mode that you use to track the sky accurately for long exposure wide field images. The movement will blur foreground objects, of course but stars will be crisp and not elongated in your images. The longer the exposure, the more detail that will be captured in your images. The focal length of your lens will determine the usable maximum exposure length before star trailing becomes obvious. Shorter focal length lenses allow the longest exposures.
Provided you use special energy absorbing filters mounted on the FRONT of you camera lens, you can capture images of sunspots and other details on the surface of the Sun. As the Sun appears to move at a slightly different rate to the night sky as the Earth orbits around it, a different tracking speed is required.
The Moon orbits the Earth every 27.3 days and it slips backwards in relation to the movement of the stars each night so to track the Moon accurately this slower rate should be selected.
Front and rear components
On the front of the mount there is a removable brushed aluminium cap retained on the RA axis by two thumbscrews. This cap attaches to the base of the ball and socket head to hold the camera and lens in place. A neat spring loaded ¼" 20 tripod bolt can be pushed outwards from the rear of the cap to locate with the base of the ball and socket head. The camera/lens combination is then attached to the top platform of the ball and socket head and then the cap is re-attached to the mount with the two thumbscrews. By releasing the tension in the ball and socket head, the camera can be pointed in any direction required. On the rear of the mount there is a slim brushed aluminium disk with a serrated gripping edge that unscrews to reveal a liquid-filled magnetic compass that is used in conjunction with the last feature on the mount - the polar sighting hole - to roughly polar align the mount.
The Polar Sighting Hole is located in the top right hand corner of the mount when viewed from the rear.
I was keen to view the inside of the Polarie mount but would urge end users not to dismantle the unit themselves as the screw-less design means that parts of the casing are bonded and breaking this bond could not only damage the mount but will certainly invalidate the warranty. We've taken one apart so that you don't have to!
The inside of the Polarie is as finely crafted as the outer casing although the design is simple and functional just what is required for the job. The Polarie uses a small stepper motor to drive the mount's precision gear mechanism. The output from the stepper motor is geared down by a brass gear on the output shaft and another brass gear on the end of the worm shaft. The 9.0mm brass worm gear drives a 57.6mm diameter aluminium alloy axis wheel gear with 144 teeth around its perimeter. The RA axis itself is retained by two substantial bearings and I was unable to detect any play or backlash in the system by hand. I took a series of time lapse images to produce a video showing the gear system in motion and this can be viewed below:
The 6 position selection switch is fully enclosed to protect it from damp and dust ingress. Allen bolts are used throughout the assembly and the internal layout is uncluttered and well thought out with the gear assembly slightly offset to reduce the unit's footprint. In operation, the unit is all but silent with just the faintest ticking sound from the stepper motor if you listen very carefully.
Rough Polar Alignment
In the top right hand corner of the mount when viewed from the rear, there is a polar sighting hole that passes right through the mount. This simple alignment hole has a field of view approximately 8.9 ° and is used in conjunction with the magnetic compass to locate the Pole star, Polaris, in the Northern Hemisphere. Polaris is a medium brightness star clearly visible in the north and it is the closest naked eye star to the current position of the North Celestial Pole (NCP) which is an imaginary position in space through which the Earth's rotational axis passes. By ensuring that the mount is pointing at Polaris, as seen through the polar sighting hole, you can achieve an approximate polar alignment meaning that the mount will rotate parallel to the Earth's rotation. This level of polar alignment will be accurate enough for shorter length exposures.
This simple and quick alignment process for users in the northern hemisphere is carried out as follows:-
Instructions for aligning the mount in the southern hemisphere are included in the instruction manual that accompanies the mount.
More Accurate Polar Alignment
If polar alignment is not very accurate and you take long exposure images, the stars will appear to slowly drift in the field of view, resulting in what is known as field rotation. This drift will cause the stars to become elongated so it pays to achieve the most accurate polar alignment you can and Vixen supply two optional tools to help with this.
The first is a more accurate combined compass and latitude measuring tool called the Quick Polaris Locator which as well as being more accurate than the built in system, is designed for use in locations where there is an obstruction that hides the star Polaris from view or for solar imaging when Polaris will be invisible. The Quick Polaris Locator attaches to the hot shoe fitting on the top of the case. The locator is equipped with a better quality, finely engraved magnetic compass card that will allow you to compensate for the magnetic variation at your location. There is also a larger, more finely engraved latitude scale that works in conjunction with a built in bubble level so that you can dial in your latitude more accurately. You then simply adjust the orientation of the Polarie until the bubble is level to ensure that the mount is facing towards the NCP.
The second tool steals from the polar alignment method used on Vixen's larger equatorial mounts - a polarscope. A polarscope is a small telescope with a finely engraved reticule that shows the position of the NCP in relation to the bright star Polaris. Because Polaris rotates around the NCP just like any other celestial object, its orientation varies hour by hour and day by day so the polarscope has a simple circular slide rule built in to calculate the star's orientation at any date and time. Because of a process known as precession which has a 26,000 year cycle, the position of the NCP in relation to the Earth changes over time as the Earth `wobbles' slightly on its axis. The Vixen Polarie polarscope can even compensate for this movement making it a very accurate alignment tool.
Most polarscopes rely on the RA setting circles on an equatorial mount to ascertain a calibration point for calculating the position of Polaris at any one time. This calibration point results in the reticule being orientated such that the little engraved Polaris marking is positioned at the bottom of the view directly underneath the position of the NCP. As the view through the polarscope is inverted, this indicates the position of Polaris when the star is `in transit', i.e. at its highest point in the sky. Normally getting this orientation correct requires the careful centring of the reticule's crosshair within the polarscope, followed by the calibration of an `index mark'. However, in common with all of Vixen's higher end equatorial mounts, the Polarie polarscope arrives with the index mark factory calibrated and the calibration position of Polaris on the reticule aligned with a simple bubble level. This brilliant system of calibration has always been a great feature of Vixen mounts and it is a real bonus to see it available as an option for use with the Polarie mount.
To use the Polarie polarscope, it is necessary to remove both the front and rear caps from the mount and then the polarscope is inserted from the back of the case until it is a snug fit through the large exposed hole that is now revealed. There are three sprung steel ball bearings in the polarscope mounting hole and these align with a groove on the polarscope's tubular casing when pushed into the correct position forming a detent to hold the polarscope firmly and accurately in place.
The first task is to view a distant object through the polarscope during daytime and adjust the dioptre ring on the eyepiece until you can see the reticule and distant object properly focused. Next, adjust the orientation of the date scale until the difference between the standard time meridian of your time zone and your observing site aligns with the index mark on the rotating hub of the polarscope. This procedure can cause some confusion with first time users but it is actually simpler than it sounds, especially if you are located in the UK. For example, in the UK, the standard time meridian is 0° and it passes through London (Greenwich in fact) which has a longitude of 0°. If you are observing at the same longitude as London, for example in Boston, Lincolnshire, you should set the index mark to point at zero on the meridian offset scale. However, if you are imaging from Exeter in Devon with a longitude of 3.5° west, you should rotate the date scale until the index mark on the central hub points to 3.5° on the western side of the meridian offset scale. If you always image from the same location, you will only ever have to carry out this simple calibration procedure once.
With the polarscope now calibrated for your location you are ready to use it on the night sky. Start by rotating the time scale with a finger either side of the bubble level until the bubble is level and centred in the middle of the two etched lines. This sets the calibration point for calculating the position of Polaris at your location on a specific day and at a particular time.
The date circle is graduated in increments of 2 days with a larger graduation mark at every 10 days and the middle of the month indicated by the position of the month number from 1 to 12 (January to December). The hour circle is graduated in increments of 5 minutes with the hours shown in 24 hour format. Note the current time, subtracting 1 hour for British Summer Time (or Daylight Saving Time - DST - as it is also known) if it applies and rotate the date circle until the day of the month aligns with the current time. This relatively simple procedure calculates the position of Polaris with respect to the NCP. Now it is just a matter of finely adjusting the direction in which the Polarie mount is pointing until Polaris aligns correctly in the graduated scale portion of the Polarscope's reticule, noting carefully the position of the year engravings as these allow for precession.
Vixen Porta Mount II
The Vixen Porta Mount II is designed for mounting a small to medium telescope via a Vixen dovetail bar. Vixen produce a neat adaptor bracket specifically for the Polarie that has a dovetail bar at one end and a platform sculptured to fit the Polarie, fitted with a 1⁄4” –20 tripod bolt.
The Porta Mount II is ideal for use at or near home and a very smart shoulder bag is available for it as an option for easy transportation but for carrying on board an aircraft, a lighter set-up might be preferred. A standard lightweight camera tripod and a Manfrotto 410 Junior geared head also works well.
Quite fine adjustments are required to align the mount accurately and a geared altazimuth head installed on the tripod makes this task much easier. For this review I used Vixen 's excellent Porta Mount II which already has a geared altazimuth head making it ideal for the purpose of firm attachment and ease of polar alignment.
Whereas Vixen 's normal equatorial mounts illuminate the reticule in the polarscope, there is no provision for this with the Polarie version. The manual recommends shining a red torch at an oblique angle at the front of the polarscope to illuminate the reticule so that both Polaris and the reticule can be viewed at the same time. Although this is possible to do, it is very fiddly and uncomfortable to kneel, view, point the torch and adjust the geared head all at the same time. I made a simple reticule illuminator comprising a red LED mounted in a cardboard collar, powered by a CR2032 Lithium cell with a 510Ω resistor to reduce the LED 's brightness. The LED is turned on by the simple expedient of plugging the flying lead from the LED into a matching lead from the battery holder. The rolled cardboard collar that holds the LED in place simply slides onto the front of the polarscope with a negligible effect on the view as the LED is so out of focus but makes a huge difference to the ease of polar alignment. Perhaps Vixen should consider making a commercial version for use with the Polarie Polarscope?
Mounting and pointing the camera
With the mount polar aligned, the camera can now be attached via its ball and socket head. The easiest way of doing this is to remove the RA cap, attach the ball and socket head's base to the cap and then attach the camera to the top of the ball and socket head. The whole assembly can then be attached to the Polarie mount using the two thumbscrews.
Because of the widefield nature of the images that the Polarie was designed for, pointing the camera and lens in the correct general direction is not terribly critical. However, if you want to ensure that you capture specific objects, I found it useful to have a means of aligning the camera accurately with a specific area of the sky. To this end I used a red dot finder (RDF) attached to the camera's hot shoe. These are available commercially but I found it more cost effective to fabricate my own hot shoe foot and attach an RDF from an air rifle to the shoe. This allowed me to view the night sky with one eye and superimpose the red LED dot from the RDF directly over the section of sky I wanted to image. Aligning the camera with the sky was then a simple matter of moving the camera and tightening up the ball head.
Imaging the night sky
Keen to try the Polarie mount as soon as possible, I took a series of 120 second images of the constellation of Lyra using the bright start Vega to focus on and give me a reference point for the final alignment of the camera. I was very impressed with the star shapes I captured with the very first image using the Polarie straight out of the box with a Canon 450D DSLR camera and a 50mm standard lens! The image below is cropped to show the star shapes better with Vega just above and to the right of centre and the famous `Double Double' star, Epsilon Lyrae, just to the left of centre. The diffraction spikes on Vega that you can see in the following image are caused by the leaves of the aperture iris which was stopped down to f2.2.
It is quite surprising just how narrow the field of view is when using a 50mm focal length camera lens to image the night sky so with the Milky Way high in the early August sky, I used a 28mm lens for my next imaging session. I aligned the camera with the Cygnus region of the Milky Way and having focused on the magnitude +2.2 star Sadr, I started capturing 150 second exposures at ISO 800 with the lens stopped down from its native f2.8 to f4. You might wonder why I didn't take full advantage of the wide open f2.8 aperture to collect as much light as possible but there are several good reasons for this. Firstly, with the iris fully open, the risk of colour fringing (chromatic aberration) on the brighter stars is greatly increased. Secondly, stopping down the aperture reduces the effects of astigmatism and spherical aberration. Finally, stopping down increases the depth of field which has the effect of making the focus sweet spot a little wider.
This session produced further excellent results with a lovely swath of the Milky Way from The Elephant's Trunk Nebula at the far northeast of Cygnus to Albireo at the other end of the constellation. Along the way, the unmodified camera captured such memorable objects as the North America Nebula, Pelican Nebula, Butterfly Nebula with Vega and the rest of Lyra to the far west and hints of the Veil Nebula to the southeast. Star shapes were excellent proving yet again that the Polarie mount has a great tracking capability despite its diminutive size.
Achieving the best focus
Achieving focus is not quite as simple as you might imagine as your camera 's autofocus is unlikely to work on the night sky with anything other than the brightest objects like the Moon. A ‘Liveview ' feature helps here if you choose a bright star near to the part of the sky you want to photograph and focus on that – if the star is in focus other celestial objects will be too as they are all effectively at infinity (oo). Remember that you can 't just trust the ‘oo ' setting on the lens as this is only a rough guide.
Either side of the focus ‘sweet spot ' there is an area that is also in focus with a slightly larger region on the object side and a smaller region on the camera side. This extended region of focus varies with f stop setting – a smaller aperture (larger f number) gives a greater depth of field thus a larger region of focus.
Images of the stars quickly show up a major deficiency in all but the very best and most expensive camera lenses – field curvature. The camera lens focuses light in a slightly curved plane but the sensor is flat so stars at the centre of the field of view appear circular but those towards the edges of the frame become elongated. Peter Smith from the Stargazers Lounge (SGL) forum has come up with a clever way of helping this problem by taking advantage of the depth of field discussed above. If you focus on a star at a ‘thirds ' position in the field of view you will have a wider range of in-focus stars. The intersections of the lines in the image below are all thirds positions and the top right hand intersection shows a star in the correct position on the intersection for focusing.
Alternative uses for the Polarie mount
Time lapse photography is a very popular method of producing fascinating videos that show changes in the subject matter that happen so slowly that the eye doesn't detect them. Images of the stars wheeling across the night sky or clouds forming and boiling in the daytime sky are typical subjects, especially when there is an interesting foreground in the frame as well. These images can be captured by simply mounting a camera on a fixed tripod and taking still images every few seconds but the Polarie adds a new dimension to this art - panning. By setting the Polarie up so that it is facing straight up in the air, it becomes a horizontal driven platform on which to mount your camera. During the capture sequence, the foreground will be slowly panned across from left to right with the hemisphere switch set to `N' and from right to left with the hemisphere switch set to `S'. A quick test example of this technique can be viewed at below - watch out for the visiting horses!
The Polarie mount has been designed with portability and widefield imaging in mind and it achieves its goals admirably. You could easily carry the mount, a lightweight tripod, your DSLR camera and a couple of lenses on board an aircraft and have a formidable imaging tool at your disposal when you reach your destination.
Polar alignment using the built in polar sighting hole and compass will allow you to take acceptable casual images of the night sky and the Quick Polaris Locator will certainly help in the northern hemisphere if the pole star, Polaris is obscured. However, the much higher accuracy gained by the Polarie polarscope is a must for the more serious imager who wishes to take longer and deeper images without star trails. The Polarie polarscope is beautifully made and is a great match for the quality of the Polarie mount itself. However, Vixen do need to address the illumination of this otherwise superb polarscope.
I was sufficiently impressed with the Polarie to make this one a ‘keeper ' and it will be accompanying my wife and I on our biennial pilgrimage to the dark sky island of La Palma next year and more importantly to the Roque de los Muchachos Observatory at the summit of the volcano.
Steve Richards is the author of ‘Making Every Photon Count’ and writes for The Sunday Times, BBC Sky at Night Magazine and BBC Focus Magazine.