A couple of years ago, when I was trying to take wide field (nightscape) astro-images with a DSLR camera on a simple photo tripod, I was surprised to discover there were star trails in even a 30 second nightscape image. To fix this frustrating problem, I needed a way to get my camera to track the sky.
So I attempted to construct a barn door tracker. I started down this path because I wanted something inexpensive, small, lightweight and easy to set up. This project proved to be difficult to finish as I could not find a 1 RPM synchronous motor. I gave up on the Barn Door tracker project and started using my large telescope mount (CPC 1100 on the HD Pro equatorial wedge) to affix my DSLR camera for tracked nightscape images. It became obvious very quickly that this setup was anything but inexpensive, small, lightweight and easy to set up.
So fast forward two years. I started seeing Celestron GT/SLT goto mounts on sale on eBay and other places for $20 (minus the hand controller (HC) or <$60 with the HC included). Since I own a Celestron mount on an equatorial wedge, I realized that maybe this was the answer to get tracked nightscape images. So I did research on the GT and SLT mounts and found out it is fully capable of being put in equatorial mode. All that was left now was to get a wedge for this mount. I found several plans on the internet but none of them met my requirements for quick setup, ease-of-use and an accurate polar alignment. What follows are the plans, the construction instructions, operating instructions and photographic results of this project.
My version of the Equatorial Wedge
This wedge was designed and constructed to be lightweight but still carry at least a 5 pound payload. The GT/SLT Mount can only handle 8 pounds so my goal was to keep everything on the mount at 4 pounds or less. As this project progressed, I realized that it would be the perfect entry-level setup for beginning AP. So with that in mind I tried to keep this wedge construction as simple as possible and buildable in a single weekend. I wanted a setup that was easy to adjust to get an accurate polar alignment, and would allow decent tracking with a DSLR camera using an intervalometer (and no computer for image capture). I also tried to keep the expense down, thinking that I could put a $20 GT/SLT mount on a $30 wedge and it would produce some respectable results. As it turns out, I had several other necessary pieces in my "astro" accessories that the beginner would not necessarily have. The additional items that a beginner would need would be:
- A suitable tripod
- A Celestron hand controller (can be bought with the mount)
- A vixen style bar for attaching the camera to the mount
- A small telescope (50 – 80mm) for the goto alignment and then the All Star Polar Alignment (ASPA).
Wood: (see figure 1)
- 6” x 6”, 1” thick pine board for the Tripod Disk,
- 6” x 21” piece of 5/16” plywood for the Base and Wedge Plates
- 2) 3” x 1 ½” 1” thick piece of pine for the Dowel Blocks
- 5 ¼” long piece of ¾” diameter pine dowel
- 2) 1” x 1” pine board squares to secure the cup hooks (not shown)
Hardware: (see figure 2)
- 2) #10-32 1” long thumb screws
- 3) 5/16-18 clamping knobs (2 closed end and one open end)
- 5/16 – 18 Insert nut
- 5) Auto Trim panel plugs
- 6” continuous hinge (I cut a 12” hinge in half)
- 12” long, 5/16 – 18 threaded bar (7” for Altitude adjustment rod and 5” to connect the tripod, disk and base plate together allowing for the Azimuth adjustment)
- Acorn cap nut 5/16 - 18
- 7/16 x 2 ½ x .047 and 7/16 x 1 ½” x .047 -- 11 pound capacity spring pack.
- 2) cup hooks
- 2) 3/16 x 1” fender washers
- 6” disk of HDPE (I cut mine from a 1 gallon ice cream bucket lid, shown later)
- 2) 2 ½” wood screws (not shown), 6) ¼” beefy machine screws for the wedge plate hinge, 6) 1” wood screws (4 for the dowel blocks and 2 for cup hook blocks)
- Drill press (hand drill attached to a portable press)
- Jig saw
- Various drill bits (normal and spade bits)
- Drill sanding disk
- Wood glue
Cut, shape and drill the wood pieces according to the dimensions on the pictures below. The hole in the dowel blocks is ¾” wide using a spade drill bit at the block's center point. The hole in the 3/4" dowel’s center is 13/32 for the insert nut. Once this hole is drilled, screw in the insert nut all the way into the dowel. This is the stationary pivot point through which the Altitude adjustment rod passes. Next drill an 11/32 hole 1¼” from the hinge line on the centerline of the base plate. This is the hole that connects the tripod disk to the base plate and allows for the Azimuth adjustments. Now, using a ½” spade drill bit, drill a ½” wide channel 1½” long starting 3¼” from the end of the base plate. This channel is where the Altitude adjustment rod passes through the base plate.
The holes for the Tripod Disk are going to be unique to whatever tripod you choose to use. They need to be countersunk so the base plate sits flush on top of the tripod disk.
Don’t forget to precisely measure and drill the 11/32 center hole for the 5” x 5/16” threaded Azimuth adjustment rod (very important step). Lastly drill the holes for the auto trim panel plugs based on the ones you acquired. They act as a low friction point for the azimuth adjustments. So after this step you should have a 6” round, 1” thick tripod disk, a 6” x 14” base plate, a 6” x 7” wedge plate, 2 - 3” x 1 ½” dowel blocks and 5 ¼” long piece of ¾” dowel. All as pictured above. (see figure 3)
To drill the holes in the wedge plate, first remove the metal mounting plate from the bottom of the GT/SLT mount (this plate is only used for measuring and NOT for connecting the GT/SLT mount to the wedge plate). Find the center of the wedge plate (horizontal and vertical) and orient the GT/SLT metal plate along the vertical axis, mark all 3 holes. The two outer holes are for the 10-32 thumb screws and a ¼”center hole acts as the resting spot for the acorn cap nut. Countersink this hole to conform to the shape of the Acorn nut.
After you have drilled the above 3 holes, attach the mount, using the thumb screws and fender washers. Snug the thumb screws tight enough that the small plastic feet on the base of the GT/SLT mount makes 3 indentations on the wedge plate. Now drill 3 holes 3/16” deep using a ¼” drill bit. These holes allow the GT/SLT mount to sit flush against the wedge plate and act a guide making it easier to attach the GT/SLT mount to the wedge plate. (see figure 4)
At this point I spray painted all the wood pieces with ultra flat black enamel paint. Each piece got 3 coats.
Here is a close up of the Tripod Disk with all its holes and Auto trim panel plugs installed. 4 are shown, but a 5th one needs to be added in that big space. The HDPE disk will sit on top of the Tripod Disk. (see figure 5)Below is the HDPE (high density polyurethane) disk I cut from an ice cream bucket lid. This disk sits between the tripod disk and the base plate and acts as a low friction point to allow for easier and more precise Azimuth adjustment when doing the Polar alignment. (see figure 6)
The Tripod Disk attached to the tripod. (see figure 7)
Assembling the pieces parts:
After the paint has thoroughly dried it’s time to assemble. There are two glue & screw operations. The first is to glue & screw the dowel blocks to the base plate as shown (they sit flush to the edge of the base plate). Make sure to drill small pilot holes in the base plate and the blocks (2 screws per block). Don’t forget to insert the dowel between the two dowel block BEFORE you glue & screw. Also make sure you get no glue on the dowel as it needs to move freely. Then glue & screw the 1” cup hook blocks to the top of the wedge plate flush with the top edge on the centerline of the wedge plate. Then glue & screw the second block 1” from the back edge of the base plate on the centerline of the base plate (1 wood screw per cup hook block).
When attaching the 6” hinge to the wedge plate and the base plate pay careful attention to the “squareness” of the hinge. I drew two guide lines for the hinge edge so I could tighten it down making sure it didn’t “wander”.
Now cut your 12” 5/16-18 threaded bar into a 7” piece and a 5” piece. File the cut ends so that caps can be screwed onto it. Now thread the 7” bar (now called the Azimuth adjustment rod) into and through the dowel’s insert nut. Once you have gotten about 2” through, screw on the Acorn cap nut and secure tightly. On the other end secure the closed end clamping knob (I used a second nut to cinch up against the clamping knob to lock it in place).
Drill a small pilot hole for the cup hooks and screw them in place. Only attach the springs when the mount is in use. These springs put a lot of pressure on the cup hooks. It is also easier to attach the GT/SLT mount with the springs off. (see figure 8)
Now insert the 5” Azimuth threaded bar (with a wide washer) up from the underside of the mount through the tripod disk, the HDPE disk, and then through the base plate. Put another wide washer over this threaded rod and then screw on the open end clamping knob. Snug this assembly down. Now you are ready to attach the GT/SLT mount onto the wedge plate. Have the thumb screws, fender washers and springs ready to go. Line up the GT/SLT mount “feet” with holes in the wedge plate and then screw in the thumb screws with fender washers in place. Once the thumb screws are tight attach the springs starting with the base plate then attach to the wedge plate. (see figure 8)
At this point you are done. All that is left is to attach your camera or small telescope to the vixen bar and clamp that in place on the GT/SLT mount. Next you will have to guess or measure the angle of the wedge plate so it matches your latitude. If you look closely you’ll see I’ve attached a clear plastic protractor marked with degrees to the base plate such that the center of the protractor is lined up with the apex of hinge. (see fig 9, 10)
When finished it should look something like this.
The 70mm f/5.7 scope used for aligning the mount prior to attaching the DSLR to start imaging. (see figure 11)
I believe it is easier to align the mount using a small telescope than trying to use the camera and its live view feature. Make sure all mount connections are snug.
First orient the tripod so that the Altitude (now Latitude) adjustment rod points to the North Star (Polaris). Next set the wedge plate to the proper angle to represent your latitude. As a general rule of thumb as you go higher in latitude the closer the mount arm point to the zenith. Or the lower you go in latitude the closer the mount arm points to the horizon.
After turning on the power to the mount you will need to give it all your local particulars. After that you will select EQ North alignment. I prefer to do a 2-star alignment. This is the "goto" part of the alignment.
After a successful 2-star "goto" alignment, slew to a bright star near the meridian and on or south of the equator for the following procedure. Now press the align button on the hand controller and select “Polar Align” then “Align Mount”. After you have synced up the star the mount will move to where it thinks the star should be if you were perfectly pointed at the north celestial pole (NCP). Here is where the adjustment knobs on the base plate come into play. Use the Altitude adjustment knob to center the star on that axis then loosen the base plate to tripod knob to move the base in Azimuth to center the star on that axis. Once you have centered the star in Azimuth, tighten the knob back down so the plate will not move.
After these steps are complete you will need to re-align the mount (because you physically moved the mount) for the gotos to be accurate. Once you get good at these procedures you can get this done in a matter of minutes. In fact I usually do it twice to make sure I’m as close to the NCP as I can reasonably get.
After the mount is "goto" aligned and polar aligned you can replace the telescope with your DSLR camera and start imaging away.
Here are some of the very first images I acquired using this setup at my dark site on July 6, 2016. These images were taken with just the Canon T1i, its 18-55mm kit lens or the 50-250mm kit telephoto lens, using an intervalometer for multiple images.
This is a single image of of the Sagittarius region of the Milky Way. Lens at 25mm, 64 seconds, ISO 6400.
This is the first image cropped with minimal levels and curves stretching in PhotoShop.
This image used the lens at 25mm and is a stacked 15 x 4 minute at ISO 1600 for a total of 1 hour of exposure time. It was stacked in Deep Sky Stacker 3.3.4 (DSS) and has been fully post processed in PhotoShop.
This image M31 was taken with great haste as I was running out of night. It uses my 50-250mm telephoto lens at 200mm. It is a DSS stacked image, each image was of a different exposure time (from 30 seconds to 2 minutes) but it adds up to 13 minutes of exposure at ISO 12800! It was fully post processed in PhotoShop.
Here are some follow up shots to further test out this setup.
M27 the Dumbbell. 15 30 second exposures for 7.5 minutes total, ISO 800 (from my red/orange backyard). 50-250mm lens ≈ 130mm
M3 Glob. 10 3 minute exposures for 30 minutes total, ISO 800. 50-250mm lens at ≈ 100mm
M104 the Sombrero Galaxy. 10 3 minute exposures for 30 minutes total. 50-250mm lens at ≈ 100mm
Well, there you have it. An inexpensive wedge for an inexpensive goto mount dedicated to your DSLR camera for wide-field imaging on a budget!