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Building & Using a Tracking Mount for Astrophotography

RustierOne | Photo Techniques | Published Sep 5, 2012

August 26, 2012 

Introduction

Astrophotography may involve exposures of several minutes or more. Since the sky appears to move due to Earth's rotation, such exposures often require some way to track the sky's movement during that exposure. This article describes building and using a simple, relatively inexpensive "Barn-door" tracking mount based on an article by Gary Seronik in the June 2007 issue of Sky and Telescope magazine, page 80. Since reprints of that article may  be hard to obtain, you can view the plans online at

http://www.garyseronik.com/?q=node/52

I will not attempt to duplicate those plans here in my article. But I will add my comments and photos of my implementation of the plan. Be sure to consult the plans for construction details and important points not covered here.

The Barn-door mount gets its name from the two plates of plywood with a hinge connecting them, like a barn-door. We see it pictured above installed on an old Celestron equatorial wedge, which has adjustments for altitude and azimuth. The mount can also be placed on a heavy-duty photo tripod. A gear head on the tripod with altitude & azimuth adjustments can make polar alignment much easier.

 The view above shows the tracking mount and equatorial wedge on a very solid Celestron Tripod.

Building the Mount

For the most part I followed Gary Seronik's plans carefully. Since I intended to install the mount on an equatorial wedge, I modified the size and shape of the bottom plate to match the circular outline of the wedge as shown below. Also since my supply of scrap plywood did not include any 1/2-inch plywood as called for in the plans, I used some nice 3/4-inch plywood. This change could make the mount a bit heavy for installation on a photo tripod. So in that case it might be best to stick with the plan's use of 1/2-inch plywood.

One of the keys to this type of mount is the hinge between the two plywood halves of the mount. The upper plate with camera moves relative to the lower one via that hinge. In the Northern Hemisphere the hinge pin must be pointed at the North Celestial Pole (near the North Star) for the mount to track accurately. Of course for those located in the Southern Hemisphere, the pin must be directed to the South Celestial Pole.

 Gary's plan uses what looks like a pair of door hinges. I chose to use a single piano hinge, which allows the hinge to be the full width of the upper plate. I also relocated the ball-head camera mount from the center of the upper plate to the top edge, as shown in the above photo. This was necessary to avoid the problem of my articulating camera monitor not being accessible when the camera was pointed up.

Some Tips For Construction

The 10-32 threaded brass rod, which drives the upper mount plate and camera, is curved to a 7-inch radius. It is driven upward by a nut epoxied to the bottom of the large gear, which is turned at 1 RPM. Below the bottom plate is a 4 RPM, 3 VDC gear motor, which drives the rod through a 1:4 spur gear set (16-tooth and 64-tooth). Motor speed is adjusted with a variable DC supply (visible above) powered by a 9-volt battery.

I prefered not to install a stop nut on the drive rod below the bottom plate, as called for in the plans. Thus if the drive is inadvertantly left running, it will not stall at the end of the rod, possibly damaging the gears or the motor.

Further Tips For Construction

Another key feature of this mount is that the distance between the center of the hinge pin and the points where the curved drive rod penetrates the two plates must be exactly 7.14 inches. This is necessary so that when the nut (attached to the large gear) is turned at 1 revolution per minute on the 10-32 drive rod, it will rotate the upper plate and camera at exactly the correct rate to match the sky's rotation.

The image above shows the drive rod penetrating the upper & lower mount plates at a 7.14-inch distance from hinge pin.

The photographer can sight along the hinge pin to align it with the celestial pole. On my setup I installed a green laser to be parallel to the pin. The laser beam is then pointed at the celestial pole via adjustments on the equatorial wedge or photo tripod to ensure that the hinge pin is polar-aligned. The image above shows the camera pointed south of the celestial equator.

Aligning the Green Laser With the Hinge Pin

The green laser is installed in an adjustable mount, much like a telescope's finder scope. The method to align the laser beam to be parallel to the hinge pin is as follows:

When the camera is pointed north of the celestial equator as shown below, the laser and its mount can be  removed to avoid interference with the camera and its monitor.

 This image shows the green laser and its mount removed from the top plate to give clearance for the camera.

The Green Laser and Its Mount

It is important to use a green laser rather than the more common red laser. This is so because the human eye is much more  sensitive to green  light  than red. A red laser would simply not be visible in the sky during polar alignment. 

Shown here is the green laser in its mount made from white, 3/4-inch PVC pipe. Threaded into each end of the mount are 3 nylon screws for adjusting the aim of the laser. The middle screw on top activates the laser push-button switch. 

Finishing the Mount

After assembling the mechanical parts of the mount, the project can be completed by making the variable DC  power supply for the motor. I purchased most of the parts for this at my local electronics store. The 10-turn potentiometer for adjusting motor speed was purchased on-line. With a little planning, I was able to fit all of the electronics into a rather small plastic project box, which also houses the 9-volt battery. The box's metal cover serves as a heat sink for the adjustable voltage regulator integrated circuit. When the power supply is not in use, it can be disconnected from the motor by use of a set of quick disconnects on the wire leads.

My experience shows that with simple tools (jig-saw, drill press, screwdriver, hacksaw, soldering iron, etc.) it is possible for someone with limited skills to construct a workable tracking mount.

When describing this mount, I have used the term “inexpensive”, which  is of course relative. I estimate that my mount has cost around $185 (US).  If one is needed, a heavy-duty photographic tripod can easily add more than $100 to the total. Take note of the following link to “Mounts for Astrophotography” by Jerry Lodriguss:


 http://www.astropix.com/HTML/I_ASTROP/MOUNTS.HTM


In that reference Jerry states “… good, inexpensive German-equatorial starter mounts for astrophotography are difficult to find for less than about $750 - $1,000.” So in light of that observation, the roughly $200 cost of the Barn-door mount is very reasonable. And the results can be quite good.

For those so disposed, a Barn-door type mount can be purchased at

http://www.astrotrac.com

for $580 ( plus tax and shipping, subject to the current exchange rate for Euros). There are other commercially available tracking mounts such as inexpensive equatorial mounts for telescopes.

Using the Tracking Mount

Before using the mount, it is important that its drive rate be set so that the large gear (with its attached nut) rotates once per minute. This can be easily accomplished by putting a small permanent mark on one tooth of the large gear as well as a small index mark on the bottom plate next to the gear. One simply adjusts the supply voltage until it takes a minute for the gear to make one revolution. Additional accuracy can be achieved by timing for a longer period, say 5 minutes. Be aware that the power supply output voltage (and tracking rate) can vary as battery voltage drops as well as with temperature changes. So it might be best to check the drive rate before each photo session.

Also note that after about a cumulative hour of tracking, it is necessary to reset the mount  as follows:

If care has been taken in constructing the mount according to the dimensions in Gary Seronik's plan, with careful polar alignment, and careful adjustment of the motor speed, this mount can easily provide tracking for some nice astrophotos. I have found that ISO 800 or 1600 with a 2 minute exposure gives nice results. Some of these results are on the next page.

Some Astrophotos Made Utilizing The Tracking Mount

All the photos on this page were obtained using this mount with a Sony NEX-5N camera.  The lens used is an old 1971 Mamiya-Sekor 55mm f/1.4 stopped down to f/3.5. These images are composed of single JPEG shots with no stacking, flat frames or dark frame noise reduction (except where noted). Post processing was mostly just histogram stretching. So there is lots of room for improvement in these areas. The panoramas were assembled using Microsoft's ICE (Image Composite Editor, a free download).

The first three photos shown here were taken at a site in the Blue Mountains of northeast Oregon at  just over 3000 feet elevation. Exposures were around 2 minutes at ISO 1600. There was some light pollution from cars and trucks on a nearby highway as well as from a town (about 17,500 population) just over 10 miles distant and about 2000 feet below. Also the air was dimmed by a lot of smoke from forest fires. Still the results are quite nice.

The view here is in Sagittarius, showing a region in the direction of the center of our Milky Way galaxy.
This image shows a nice star field between the star Vega in the constellation Lyra on the right to Gamma Cygni on the left.
This panorama of multiple images covers a span of the Milky Way from Cygnus on the left to Sagittarius on the right. The change in color from left to right is due mostly to smoke pollution, which is thickest low in the sky to the right. Click to expand image.

 The following panorama consists of parts of 13 photos taken from an elevation of just over 1200 feet in the Coast Range of southern Oregon. Some clouds and  light pollution  affected  the images. Exposures were 2-1/2 minutes at ISO 1600. Long exposure noise reduction (dark frame subtraction) was activated on the camera. This resulted in considerably less noise as compared to the first panorama above.

 This view of the Milky Way extends from the Double Cluster in Perseus on the left through Cassiopaeia and Cepheus to Cygnus on the right. Click to expand image.

 Here's wishing you success in astrophotography with this mount.

Its easy to build and fun to use - give it a try!

Some Further Observations and Conclusions

After using the mount for a few months, I have made some changes in its construction and use.

As noted on page 2, I had moved the location of the ball-head from the center of the top plate to its top edge. The purpose was to avoid problems with my camera’s articulating monitor not being accessible when the camera was pointed up. What I discovered after using the mount for a while is that this move just changed the pointing directing where I encountered the interference. The best solution was to mount the ball head on a short wooden post centrally located on the top plate as shown in the image below. This prevents interference with the monitor for most directions the camera is pointed.

The only down side to this solution is that the camera and ball head are located further from the hinge pin, requiring a bit more effort on the part of the motor to lift the weight. But in this position, I seldom have to remove the laser and its mount for clearance.

On page 8 I noted: “Power supply output voltage (and tracking rate) can vary as battery voltage drops as well as with temperature changes”. With that in mind, I had recommended checking the drive rate before each imaging session. I discovered that this was time-consuming and subject to error as the drive rate continued to change during the course of the evening as battery voltage and temperature dropped.

I believe that the best solution to this variation in drive rate is to determine by experiment what voltage supplied to the motor results in the correct tracking rate. If the mount is made carefully according to plan dimensions, the correct rate for the drive nut is 1 rpm. Once the correct voltage is determined, one merely monitors the voltage during the course of an imaging session, making corrections as necessary. To that end I made a set of multimeter leads that can be connected to the power supply via an extra output cord and quick disconnects. This is shown in the following image.

 Ball head mounted on wooden stalk; power supply voltage monitored by digital multimeter

One important note on polar alignment: The north celestial pole is not exactly on the North Star (Polaris) – it is located at a point about a degree from Polaris. For best results align on that point rather than on Polaris. For short focal length lenses, Polaris may be close enough. For longer focal lengths, find the pole’s exact location from a good star chart and align there.

 Tracking mount with laser for polar alignment and Multimeter for checking supply voltage

Overall the mount has been a joy to use. It is quick to set up and polar align. And the results are satisfying. Here’s a nice image taken with a Samyang 8mm fisheye lens at f/2.8, 2-1/2 minute exposure.

 Milky Way From Sagittarius (Lt.) to Perseus (Rt.)