Canon Scanner Large Format Camera

As noted in a previous post, in 2015, I attempted to make a large format camera scanning back using old Canon scanners. I was inspired by this Make article. However, in the previous attempts, all I managed to do was destroy 2 otherwise perfectly good (though largely obsolete) scanners. Here are a scan:

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The black band is an unresponsive element, and the light green band is an oversensitive element. These two are probably related.

Getting Started

A good scanner camera should be portable, and not utilize external power. This limits the choice of scanners to portable document scanners and the LIDE series of scanners from Canon.

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The Canon  LIDE scanners are lightweight scanners that run on USB power. This allows one to operate them using only a laptop.

To get started, the scanner is opened up. The two sides of the scanner are plastic strips taped onto the scanner glass. After peeling them off, the scanner glass can be slid off the casing. This gives you access to the insides, which consists essentially of a guide rod and a scanner bar assembly, which in turn includes a motor, and the sensor itself (in the black housing). The first thing to do is to disable the light source, which is used for document scanning. To do this, remove the long, clear plastic light guide rod.

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Light guide removed.

Next, the LED can either be removed or covered up. Initially, I used a blob of black epoxy to cover it up.

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Epoxy-coverd LED

Initially, I had wanted to make minimal modification to the scanner. Later on, I decided to just cut the LED off the PCB entirely. I had thought that using a diffuser screen onto which the lens can project will improve things, since the light will be scattered in all directions, including perpendicular to the image plane. Indeed, it does, but the improvement was nowhere near sufficient. In fact, the image above was after the addition of the diffuser screen. A look at the image circle reveals why – in the absence of a fresnel lens, the image on the diffuser was clearly circular, with significant drop-off around the edges. This means that the light was in fact scattering only slightly into the correct direction.

 

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Shallow entry angle of the light prevents detection on the sensor on either side of the scene.

 

Project Revival

Recently, I’ve been thinking about this project again, after having been gifted another scanner by a friend who knows I like to tinker (Thanks Steven!). Having looked at the previous attempts, I narrowed my failure down to two things:

  1. Attempting to remove the sensor PCB from the plastic sensor housing, so that I can modify it to increase light intake, is extremely risky. The long, thin PCB has a tendency to bend, and the sensor elements are easily damaged. Each scanner bar has something like 8 linear sensor elements, which, when broken, renders the camera inoperable.
  2. Not thinking through the calibration process properly.

Making a scanner camera involves trying to project light from the camera lens directly into the sensor array, without any intervening optics. In principle this ought to work. However, the sensor array is recessed from the top of the sensor bar, and light is transmitted through a slit, via light pipes, to limit the direction from which light can enter the sensor. Under normal operation, where a light source is mounted on the sensor bar, this works well, since the direction of light between source, to the document being scanned, and back to sensor is always well-defined. On a scanner camera, however, the light from the camera enters at an angle. This angle is shallower (i.e. less ‘perpendicular’ to the scanning plane) the closer to the edges, and the shorter the focal length of the lens. At some point, the slit excludes the light from the camera. This results in a hotspot in the middle, and ugly vignetting on the left and right. This was also what necessitated the sensor bar modification.

This time round, I made sure to be extra careful when removing the sensor bar. Instead of pulling it out, I made sure to cut off all the retaining tabs, and test for free movement of the sensor elements. Then I gently pushed the bar out using a screw driver. Next, I used a Dremel and cutting wheel to cut out the plastic sensor bar.

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The black plastic sensor bar casing was cut up, to reveal the PCB and epoxy-encased sensing elements.

I pretty much destroyed the sensor casing, keeping only the tabs to mount back onto the rest of the assembly. You can see that the sensor elements are now wide open – this allows light to enter from any angle.

Fixing Calibration

I use the Scanner cam with my Macbook, controlled by the Vuescan software, which was purchased with this project in mind. In the settings, it is possible to perform a calibration procedure, which corrects for lighting evenness and sensor element responses. For the scanner cam, I expect that this will also be useful for setting the gain on the sensor. However, the actual process of the calibration is not completely clear. Once the calibration process is started, the software has a status bar that indicates the progress of the calibration, which happens in two steps. The black level reading is easy enough to achieve – simply put a large, black opaque sheet over the entire scanner. This is followed almost immediately by the white level reading, which I think sets the maximum level of light expected. The problem is that the white level is hard to determine. Should I be using room/environmental lighting? That will surely over-estimate the maximum amount of light the lens will let in.

If I were to use the camera lens on a camera body, exposed to a bright light, it might be better were it not a scanning back. However, since the sensor bar is parked all the way on one side of the camera, using a camera lens will surely result in vignetting in the corners, and this in turn will set the gain higher on the edges (since the corner received less light, it will have a higher gain setting). The resulting image will have bright edge bands running parallel to the direction of travel. The solution, currently, is to use a diffuser and ND filter together to bring the response to a reasonable level.

Another problem I was not expecting with the calibration was how sensitive it was to the environmental light. You can see in the image below that a ribbon cable (top right corner) is connecter to the PCB. It turns out that light reflection off the white ribbon cable was registered as more intense light by the sensor, and a consequently lower gain in those elements. This resulted in very uneven fields

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A light shield made out of black cardboard reduces stray light.

The solution was to tape a black cardboard strip along the sensor, to minimize errant reflection.

Results

The Scanner Cam is now finally ready for action. I have a 4×5 Wanderlust camera that I strapped directly onto the scanner.

First, a flat field was taken by putting a sheet of paper over the lens. This will provide information about light leaks and calibration quality.

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Flat field of camera. Lines are the result of either dust particles, or differences in sensitivities of the element. This is contrast enhanced.

The flat field looks pretty decent. Next up, and image was taken.

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First scan using the new scanner camera

I have to say, for a first scan this is not half bad. The camera was not focused, and there was light leaks all around. But all things considered this is a decent image. I tried to correct the image using the flat field, but was not able to.

Result
Flat field correction seems to have increased the number of artifacts.

After performing the correction, the image looked worse. However, I believe I have identified the problem – neither image was controlled for in terms of exposure, contrast, etc, so the correction was not valid. After the scan is performed, I will have to save the uncompressed raw image, rather than the aesthetically better, but modified image.

Conclusion

The scanner cam project is officially a success. I will be mounting a new lens on a cardboard box, yielding a 32cm x 22 cm camera. I will post an update once the build is complete!

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Explore Scientific Twilight I Mount Optimization – Tuning a Worm Drive Assembly

I have a manual Altitude-Azimuth mount that has slow-motion control. Typically the slow-motion is made possible using worm drives, which allows one to achieve large gear reductions easily. One issue I was having with the mount was that the amount of slop was pretty large. The gif below shows the amount of movement on the slow-motion rod needed before the gear engages. To figure out how to fix this, I decided to take the mount apart.

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Massive amounts of play in the gear

Firstly, the mount head was separated from the rest of the body. The first thing you’d notice in the image on the left below is that the slow-motion connector on the left side is bent. I am not really sure how this happened, since this was bought used. In any event, it was easy to straighten it using a socket wrench and firm pressure. At the bottom of the mount head, you’ll see three hex screws. Undoing them will allow the bottom mount adapter to be removed.

Next up, there’s a thread-locked nut on a center bolt, pressing on a large washer. I didn’t know what lay on the other side, so I slowly loosened this nut with the mount head sitting upside down on the work bench.

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As it turns out, there weren’t many moving parts. The nut compresses a plastic washer (possibly Delrin or PTFE/Teflon), which in turn presses on the large stainless steel washer. Below these two components is a so-called wave washer, which is basically a spring that permits adjustment of the stiffness of the movement in the Azimuth. Loosening the nut allows you to remove the azimuth collar. This exposes the worm drive mechanism.

As you can see, it’s a brass-on-steel configuration, which is fairly common. Initially, I noticed that the meshing seemed pretty good, with good response in either direction. This led me to look elsewhere for possible problems.

The locking mechanism for the Azimuth axis is a compression clutch system – basically, the screw presses on a piece of metal, that sits in a groove, and which locks the collar (mechanically contiguous with the tripod) to the stainless steel worm gear (note that the brass bit is simply called a worm). I realized that although there was some play between the compression plate and the groove in which it sits, it does not actually move once locked down.

Another possibility was that there might have been movement of the entire worm gear along the Z-axis. This is in fact possible, If the worm gear moves up and down, then the directional change of the worm will result, first, in a translation of the gear, followed by rotation of the mount head. To fix this, I simply tightened the center bolt nut down, fully compressing the wave washer. Unfortunately, all this did was to make the mount crazy stiff, with no appreciable effect on the amount of slop. Just as I was about to give up, I noticed a curious detail.

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To improve the meshing, the worm must be moved closer to the worm gear. This is often done by placing the worm shaft on a slotted bracket that can be moved around for best meshing. One problem is that the worm gear is close to – but not perfectly – centered. So, the optimum position is often a compromise. Many manufacturers send mounts out with the gears meshing loosely, perhaps because this will at least ensure the gears won’t end up binding (that is, getting jammed together) and burning out a motor. In any event, it often falls to the user to optimize the meshing.

The strange thing about this mount was that there wasn’t any slotted bracket. In fact, the insides of the mount were pretty barebones. From a manufacturing point of view, it is almost impossible to make something without allowing for adjustment. It would have required extreme confidence in the manufacturing tolerance, something I was sure Explore Scientific did not have in its entry level mount.

The worm shaft sits in a pair of bushings or collars, which can be turned using a spanner wrench (that’s what those two little holes are for).

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A spanner wrench

Careful inspection of the mount revealed that the bushings were in fact eccentric – that is, not round. Turning them will thus allow the worm shaft to be moved back and forth. To tighten the meshing, one just has to turn it such that the fatter side of the eccentric bushing points away from the worm gear. Two grub screws (see first 2 images for the two holes on the mount) hold the bushings in the proper orientation. Just a small turn made all the difference in the slop, which is now comparable in both Altitude and Azimuth, and pretty much negligible!

All that remained was to reassemble everything. 2018-06-24 01.45.02

What a beauty. 🙂

 

Building Tip: Edge Drilling Jig

Suppose you have to drill holes into the edge of a large piece of wood. Unless you’re particularly skillful with a drill, you’ll want to avoid doing this freehand. Even if you want to use a press, there is no good way to secure the workpiece, since most clamps will only be able to secure one end to the work bench, and there is bound to be significant movement on the free end. Furthermore, it can be tricky getting the drill press high enough to drill the hole.

One work around is to make your own hole drilling jig. If you are not drilling too many holes, it’s possible to make what is essentially a disposable drilling guide. To make one, first find a piece of scrap wood that is exactly the same thickness as your workpiece. This is important, since any play in the fit will result in a mangled hole. Next, cut a convenient size, with a long axis for leveling the jig, and a short axis for the guide hole. It is important that this long axis is very straight, since it will determine how true the resulting hole sill be. The piece should be small enough to mount securely on a press, with the long edge facing down. Use the same drill bit that you will be using on the final work piece to drill into the edge of this small work piece. The precise size does not matter too much.

Drilling

Next, get two pieces of scrap wood, and glue the three parts together. To ensure that the two side pieces do not taper inwards, use another scrap piece (same thickness as the workpiece again) to allow the glue to cure at the correct spacing.

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That’s all, really. I didn’t even bother to clean up the glue. Ideally I would have used a longer short axis for this M6 hole, but I was short on scrap wood then.

You can cut an additional hole on one of the side pieces to help with positioning. Since this is a one-use jig, I didn’t bother making it pretty. I just drilled a few holes close to each other so I can see the marking.

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To use the jig, simply place it on the edge of the workpiece, and place the drill bit into the guide hole. You don’t need a press to do this – simply push the bit in, guided by the hole. Most drill bits are not designed to cut on the sides (those are routing bits), so the damage to the guide hole will be minimum if you don’t drill too many holes. And when you’re done, just toss it! It only takes 5 minutes to assemble, and 45 minutes for the glue to cure.

Upcoming Posts

Over the years, I’ve had some pretty ambitious – and frequently abortive – projects, that were a lot of fun nonetheless. Amongst these were

  1. Debayering Canon 1000D sensors for monochrome conversion;
  2. DIY cooled camera in custom box;
  3. Celestron CGEM mount conversion to stepper motor;
  4. Large format scanning camera.

A few of these I intend to continue. Notably, #4 is on my list of projects to revive. For the rest, I will probably write up a couple of posts each to describe the project. It’s gonna be a doozy.

Fixing Your MacBook Air Charger

My MacBook air charger suffered a catastrophic failure today, when my attempts to re-tape the fraying bits went awry.

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Fraying ground cable snapped clean off.

As a result, the charger stopped working. As a self-respecting engineer, I couldn’t let this challenge go unmet. So, I went round the house to scrounge up some supplies.

The most important part is this: The heat shrink. I cannot remember why I bought this, but it is a 3M HDT-A 12/3 heatshrink. Basically, that means that its unshrunk diameter is 12mm, and after shrinking it goes down to 3mm. This was just about perfect for what I want to do.

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3M HDT-A 12/3 Heatshrink

Next, I stripped what remained of the cable on the mains side, and got a short length of cable.

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Stripped cable – a gap too far.

There was a gap that needed bridging. So I did that by soldering a short length of copper cable.

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It looks awful, because I didn’t want to risk cutting any more wires by trimming and cleaning it up. I made sure there was plenty of contact between the original cable and the graft.

Finally, the heatshrink needed to be threaded over the very large power connector on the laptop end. As it turns out, it was barely large enough, and the friction prevented it from being slid on. So, I applied some lithium grease to the inside of the heatshrink, and was able to slide it on with some effort.

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Repaired charger.

Next, I used a propane torch to shrink the material, a voilà! Good as new.

Building an Equatorial Platform from Scrapwood – Part 3 – Rebuild

As I discussed in the last post, there were some issues with the radius of the bearings because of a miscalculation (or rather, a total lack of calculation!) on my part. As a result, it became necessary to re-do both the North and South bearings. Thankfully, I have access to engineering software, that allows me to draw precise circular segments, that I then printed out to true scale. I pasted this onto the old South bearing and through extensive use of a wood rasp, managed to trim the surface pretty close to the design curvature.

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A trace of the desired curve was designed on a CAD software and printed.

However, for the North bearing, the holes that I had drilled into the segment precludes any trimming, since the remaining bearing surface would end up being too thin to support the scope. Instead, the North segment was newly cut from a piece of plywood using a jigsaw.

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New North segment screwed and glued onto the platform top board.

To ensure sufficient strength, the segment was glued and screwed onto the top board. A quick test confirmed the stability of the structure with the telescope mounted on top. Next up, I painted the whole thing black, glued on the bearings, screwed on PTFE sliders, attached a spirit level in front, and it was done.

Here it is in action:

The scope tracks very well. Using a 30mm eyepiece, a centered object remains within the field of view even after about 20 mins. This makes observation much easier.

Building an Ultraportable 10″ F/4 Dobsonian – Part 4 – Finishing the Scope

From the previous update, the scope was already mostly done. In this part, I’ll just discuss the little details of the scope.

Ventilation Holes

The scope has a pretty open configuration for cooling. The edges of the mirror are exposed on either side, and there are four holes drilled into the back for added ventilation. I also have a small 12V fan, but have not installed it yet.

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Ventilation is important for mirrors!

Mirror Cover

The primary is not secured in any way to the cell. It slides freely up and down, adjusted with three M6 knobs, that are epoxy-tipped (to prevent metal-on-glass, which is not good. Not good at all.) To prevent any mishaps during transport, the scope features a snug-fitting, felt-lined cover, that is held in place with two swinging plastic tabs.

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The mirror cover is held in place with two swinging plastic tabs, and a roller bearing serves as the handle. The mirror-facing side is covered in felt.

Getting the Components to Fit when Closed

It turned out to be non-trivial getting everything to fit properly in without too much stress. The base of the scope has a recessed region in the middle, that allows the secondary hub and collimation screws to sit in. Four small holes are drilled in for the truss clamping screws. When the Upper Truss Ring is placed upside down in the box, the ring itself sits flush on the dob base.

The primary collimation knobs are backed all the way out, so that the mirror sits on the mirror cell, and not on the collimation screws. Although epoxy-tipped, there is just too much stress concentration for my liking. Instead, six 3mm felt pads are distributed in the mirror cell for the mirror to sit on. Because the backed-out screws are so long, when the mirror cell sits on the Upper Truss Ring, the knobs extend past the ring, into the base. Three larger holes are thus drilled into the base for the collimation screws to pass through.

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Primary collimation screws sit in the three holes right in the base.

The side bearings, when stored in the box, are held in place by the same knobs that attach them to the side of the mirror cell. However, the knobs were a little too tall. You can see that the phenolic plastic parts have a 5-lobed portion that is larger than the ‘stem’ part right next to the screw. To save me some space, I used a Forstner bit to drill a counterbored hole for the stem portion.

Here’re all the components laid out.

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All components laid out.

Conclusion

 

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Completed Ultraportable Dobsonian

The truss poles are covered with heat-shrink. I attached a red dot finder on the top of the scope, and also cut a little shade out of a thin plastic folder. And with that, the scope, all 17 pounds of it, is complete. Refer to the different parts of this build below:

  1. Conception
  2. Rough Cut
  3. Assembly