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BarnDoor
BarnDoor
The Constellation Project
Revisiting a Barn Door Tracker
Back in the days of film based astrophotography a number of Web Pages were devoted to Scotch Mounts or Barn Door Trackers. The idea was to be able to cheaply build (Scotch) a camera platform that would allow 5-20 minute exposures of large areas of the sky with a 35mm camera and interchangeable lens. The actual motion of the tracker was provided by turning a bolt, usually at a rate of 1 rpm, by either hand in the original design or vibration motor. Wide angle lenses were tracked by hand at 1/2 turn per 30 seconds, while longer telephoto lenses may have to be adjusted 1/6 turn every 10 seconds. While the mounts tracked fairly well during an initial short time period of less tha
n 5 minutes, tracking errors (tangent error) started compounding after 10 minutes or so leading to out of round or streaked stars. Different types were developed to eliminate or at least minimize tracking errors at longer exposure times. Many of the WebPages are still around today, although they have not been updated to reflect changes brought on
by DSLRs vs. film. I doubt that very many people are using Barn Door Mounts today. I'd guess a number of people make them, then add automatic tracking, then move on to EQ mount piggyback astrophotography.
I propose to explore Barn Door Mounts in light of DSLRs and the ability to stack sub exposures with a free program like DeepSkyStacker (DSS). I plan to use a DSLR
mounted on a Barn Door Mount to take constellation pictures. This seems to be an easy entry route for beginning astrophotography. Traditional Barn Door tracking would use long exposures limited by tracking errors, unacceptable sky fog levels, user error, and improper polar alignment. I will explore shorter exposure times, from 30 seconds up to two minutes. The length of exposure for the subs will be short enough that tracking errors, sky fog, and errors from less than perfect polar alignment will be minimal. I w
ill be able to take as many sub exposures as necessary, but will probably start with a total exposure time of 15 minutes. While the total exposure time will be greater that usually used for the traditional film based exposure, the individual sub exposures should be short enough to prevent trailing. DSS has the ability to rotate images for alignment, so all the subs exposures should be able to be absolutely aligned. There should be minimal error in the individual subs and in the final image. All-in-all I expect good final images. As an added wrinkle I propose to use a somewhat uncommon scissors type of Barn Door Mount because it is both smaller and easier to make.
The hardware needed to capture images with a Barn Door Mount include the easily made mount, a camera, a ball head, and a tripod. While not absolutely necessary, a quick mount for your tripod and either a digital remote (cable release) or laptop computer with the camera's
software will make the process much easier. I will be using the intervalometer in MagicLantern on a Canon T2i to sequence the exposures and lengths. You'll need a few bolts, nuts, T-nuts, screws, and washers if you plan to make your own mount. Do you really need to make a Barn Door Tracker to take astrophotos of large regions of the sky? Truth be told, if you choose a wide enough focal length and if your camera's longest exposure time is less than ten seconds or so, you can probably just put your camera on a tripod, bang away and forget the Barn Door Tracker entirely. DeepSkyStacker will do the rest.
I propose to use a scissors type Barn Door Tracker (seen at the left) with a 32 tpi pitch to minimize the size. The difference between a traditional Barn Door and Scissors type is a lack of a hinge. The Scissors type swings on a pivot point made with a bolt to hold the two boards together. The pivot bolt is hidden between the black laser pointer and wing nut in the pictures to the right and left. The directions below are for a 20 tip pitch because it is the norm. The 32 tpi pitch tracker fits in a suitcase much easer. You can see the difference in sizes in the picture to the right. Polar alignment of a Barn Door is always a bit of an adventure. The hinge or pivot point needs to point to the North Celestial Pole (NCP). Sighting along
a hinge in the dark without any optical advantage is difficult at best. Marv's Tracker above shows a sighting tube that helps. I've made Barn Doors in the past with a polar scope as the pivot point and a rifle sight or telescope finder mounted in line with the hinge, but getting them aligned with the hinge isn't easy. In this version I've add a provision to align with a laser pointer that can be mounted above and parallel to the axis of the pivot point. You can see the laser pointer in the pictures. Assuming I can correctly identify the position of the NCP, alignment is easily, with no bending, stooping or crawling. I have noticed that the Barn Door mount and tripod can flex a bit as the camera is positioned at different areas of the sky with new objects, so I plan to realign to the NCP before every new sequence.
Making a Barn Door Mount using a 1/4"x20 bolt:
A scissors type Barn Door Mount can be made from a 1"x2" piece of lumber with three cuts and five drilled holes. The drawing in the other column below shows the layout of a small hand powered mount. The three cuts give the three boards, two longer of equal length and one shorter. Two of the holes hold the longer boards together at the pivot point, one of the holes attaches the ball head, one of the holes provides the contact point, and the last hole provides a way of attaching the mount to a tripod. The only critical dimension for any Barn Door Mount is the distance between the pivot point (hinge) and contact point. Assuming a tracking rate of 1 rpm, this distance depends only on the threads/inch of the tracking bolt and is given by the expression: d = 228.56 / tpi . See Marv's page for the math behind it. The following table shows the pivot point to contact point distance in for the most common thread pitches used in the United States:
Materials needed:
30" of 1x2 lumber (2 @ 14", 1 @ 2") for Tracker sand one side smooth for bearing surface
6" of 1x2 lumber (3 @ 2") for laser pointer holder
1 4-6" 1/4"x20 round head bolt and matching nut for drive bolt
2 1/4"x20 T-nuts for drive bolt and tripod mount
1 3" 1/4"x20 bolt, matching wing nut, and 2 washers for pivot point
2 1-1 1/2" wood screws to attach small wood block
wood glue
Tools needed:
saw
drill and 1/4" bit
bit to match the diameter of your laser pointer
screw driver
Guiding Times:
Back in the days of film a general rule was that you could let the camera sit for 650/focal length before the stars started to show enough trails to matter. You needed to make sure you made the corrections faster than the guiding time. The good news is that you can always view the picture in the field, hit delete if trailing is a problem, and turn the guiding bolt more frequently the next time.
You can use the information in the following table as a starting guide:
Most traditional Barn Door Mounts are made with 1/4"x20 bolts, 4-6" long. At 1 rpm an exposed 3" long 20 pitch thread will give 3 inches * 20 revolutions/inch divided by 1 revolution/.minute = 60 minute tracking
A finer pitch thread will give you a smaller mount, but bolts are harder to find in longer lengths, meaning you'll need to "rewind" more often. Only rewind between different objects!
From the table above, a 1/4" x 20 pitch bolt needs a distance of 11.42800 inches (29.02 cm might be easier to measure) between the pivot point and contact point. Adding a bit to each end means the two boards for the scissors would be about 14" each. The third short length of wood that holds the drive bolt should be about 1 1/2" to 2" long. So you would need abut 30" of 1x2 for a mount using a 20 pitch thread. You need about 6" more if you plan to add the laser pointer holder.
A bolt, nut and two washers are used to hold the two boards together at the pivot point. A T-nut runs through the short board to hold the drive bolt. You can see the T-nut to the left of the rubber band in the picture below. The mount can either be used to open (pivot point on west side) or close (pivot point on east side) the scissors. I personally believe it is better with the pivot on the west. In either case, mount the T-nut on the top side of the smaller board so it bears the weight of the camera without coming out of the wood. Insert the drive bolt and position it the correct distance from the pivot point and glue/screw the smaller board on the bottom scissor board.
The ball head mounting hole can really be anywhere along the top scissors board. Check the size/pitch and length of the bolt needed to attach the ball head before drilling. The head of the bolt will need to be recessed so it doesn't block or bind on the lower board.
The tripod mounting hole should probably be centered on the lower scissor board. Since most tripods have a 1/4"x20 screw at the top, a 1/4"x20 T-nut should probably be recessed in the top of the lower scissors board.
The holder for the laser pointer is made by gluing three thicknesses of short 1x2" together. Drill a 1/4" inch hole through all three and then enlarge the hole to just fit your laser pointer. Make sure that the head of your pivot bolt fits down into the hole and place the wing nut on the bottom. Slide the laser pointer into the hole and it should be perfectly centered above the pivot bolt. You can just see the head of the bolt in the picture below.
To get really short guiding times you'll need a 1 rpm motor.
Using a Barn Door Tracker
Before using the Barn Door Tracker to take pictures you have to mount it and the camera on a tripod. You'll then need to align the Tracker with the north pole. The pivot point bolt needs to be pointed to the NCP close to the North Star. The better the alignment, the less possibility of star trailing, but short exposures and DSS do help minimize the problem. The camera can be pointed anywhere in the sky, but the Tracker has to be aligned N/S and tipped up to the correct altitude.
I would assume most people today have a digital camera. It would seem to me that any digital camera could technically be used with a Barn Door Mount and stacking assuming that the FLASH feature can be turned off and that the camera has a self timer to prevent camera shake. It would be nice if the shutter speed could be controlled and set at the slowest possible speed, although the camera's auto exposure should probably set exposure to maximum under the dark sky conditions. If the longest exposure is still fairly short (as with a cell phone) results probably won't be too good. A quick trip to a dark closet should tell you what the maximum exposure time length. If it is less that 15 seconds or so, you won't get many stars in your pictures. Out in the dark with the camera on the tracker and aligned to the NCP, pointed to an object of interest in the sky, put the self timer on the longest delay possible to minimize camera shake, and take the exposure, spinning the Tracker as needed to follow the sky and discussed above. If you see trailing in the resulting pictures you'll need to twist the drive bolt more frequently during the exposure: first try 1/4 turn every 15seconds, then 1/8 turn every 7-8 seconds. The other option is to zoom the lens out so you can see more sky. Remember if you choose a wide enough zoom setting and if your camera's longest exposure time is less than ten seconds or so, you can probably just put your camera on a tripod and forget the Barn Door Tracker and polar alignment entirely. DeepSkyStacker will do the rest of the work for you.
If you have a DSLR with all the bells and whistles (30 second exposure max, bulb, computer control, digital cable release possibilities, etc.) try a 60 second exposure first. You'll probably need to do a 1/4 turn every 15 seconds to prevent trailing. Remember the wider the lens, the less intermediate adjustment it will need, the longer the telephoto effect the more you'll need. No matter what the lens, you'll still need 1 turn per minute. If digital noise in the picture isn't too bad, you may want to capture a bit more depth in the pictures by going to a two minute exposure. Ideally you should expose based on a time needed to get the "hill" in your image's histogram from 1/3 to 1/2 way across the display.
So lets see how I did: (to be added during the next few months)
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