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Sunday, August 8, 2010

DIY Kinematic Optical Mirror Mount and Schlieren Photography

This week I built a Kinematic Mount for a 8 inch F7.38 parabolic mirror that I picked up. To explain why I bought a mirror, in the last post about Schlieren photography (HERE) I mentioned that I was using a lens in the setup and was trying to get rid of the chromatic aberration that I had in the pictures. The obvious solution was to get rid of the lens and use a mirror as the focusing element. I found a guy nearby that makes telescope mirrors and he had this one for sale so I bought it. To explain why I built a kinematic mirror mount for it... well why not?!
Homemade Kinematic Mount

Click on the picture for a bigger view. UPDATE 11/27/10: I made a video explaining how to make this Kinematic Mount HERE. The mirror by itself is a cool and fun thing to have but being delicate and heavy (it's 1-3/4in thick) it's a bit awkward to hold in place accurately. The nice thing about a Kinematic mount is you can hold a something like a mirror or a lens and position it just where you want it with screw adjustments. The mount that I made has one adjustment screw for elevation and one for azimuth allowing for easy alignment.

After buying the mirror I didn't want to spend a lot of money on materials for the mount so I did it mostly with stuff I had lying around the garage. That means that the mount is made from 5/8" and 7/16" plywood. The only things that I had to buy where a 1/4" Rod End, a 11" diameter plywood round disc and a couple of compression springs. In the picture above you can see the mirror mounted to the 11" disc with the rod end bolted to the back, the springs and a couple of 1/4-20 screws with red knobs on them. The idea of how this works is pretty simple and I drew a picture (below) of the details.
Here is the short explanation of how it works:
The rod end has a spherical bearing that allows the wood disc and mirror to 'flop around' all over the place. To keep it from flopping around and provide adjustment there is a 1/4-20 adjustment bolt at the top and one on the side. Opposite the bolts are 2 compression springs that keep the wood disc and mirror pushed up against the bolts. The ends of the bolts are ground down to points and are not attached to the wood disc, just pressing against it by spring tension. If you screw the top bolt in/out the disc and mirror tilts up/down, pivoting on the other bolts pointed tip and the spring. If you adjust the side bolt in/out the disc and mirror move side to side pivoting on the top bolts pointer tip and spring. The disc is prevented from turning around because the pointed tips of the bolts are sitting in pockets drilled into the heads of carriage bolts. The mirror is held to the wood disc by three hexagonal standoffs and some silicone glue.
Here is a more detailed description of how it works:
I mounted the mirror to the plywood disc by drilling 3 holes 120 degrees apart  on a 8" (plus a little) diameter and bolted down 5/8" aluminum hex standoffs. I spaced the holes at a diameter of a little more than 8" so there would be room between the standoffs and the glass for some foam padding. That way the mirror isn't touching the aluminum standoffs. For extra holding power I also glued the mirror to the plywood using some silicone sealant with 3 spacers between the mirror and the wood. The spacers make the mirror more or less parallel to the wood and also make the silicone gap even and at a good controlled thickness.1/4-20 is a bit coarse of a thread for this adjustment and if I ever build another mount I'll use something finer - a lot finer.
As mentioned the bolts that I used to mount the standoffs are carriage bolts that have the rounded head with a square shoulder. The wood disc is oriented so that 1 of the hex standoffs is "up" and the other 2 are "down at the sides". The carriage bolts and the orientation are important as I'll explain below.
Kinematic Mirror Mount Design

Click on the picture above for a bigger view and look at the round sketch on the left. You can see the three hex standoffs at 120 degree spacing to support the mirror and a few additional things on the right and left of the wood disc and at the bottom. On the right side of the disc is an additional carriage bolt that is just bolted through the disc, right behind it is a 1/4-20 adjustment screw. On the bottom and left of the disc are compression spring mounts.

The reason for using the carriage bolts is I had to rotationally constrain the disc in a way that still allows it to pan and tilt. I did that by drilling a hole part way into the head of the top carriage bolt that holds the top hex standoff and into the carriage bolt on the right side that is just bolted through the wood disc. As mentioned above the 1/4-20 adjustment bolts are ground down to a point and sit in these drilled holes (with a bit of grease to smooth things out). The size of the holes in the carriage bolts is a little bigger than the 1/4 inch bolts and because it's a drilled hole the bottom is conical from the point of the drill bit. The conical shape lets the point of the ground down 1/4-20 bolt center in the hole.
The adjustment screws are screwed into threaded t-nuts that are pressed into the wood base. Those nuts lock into the wood so they can't turn. The compression springs are held in place by small faucet washers screwed down with wood screws. I used faucet washers because I have them left over from fixing a faucet and they fir the springs nicely. In the sketch picture above on the right is a cut away view of some more details. I won't get into the specifics because it's all pretty self explanatory. If you are interested email me at or leave a comment. I think the only thing left to do on this project is to build a box that will for over the entire mirror base like a lid to protect the mirror.
On to the details about the mirror and some exciting pictures and cool videos!
Schlieren image of candle burning with colored filter
I didn't have a lot of time to mess around with taking cool pictures this weekend because most of my time was spent building the kinematic mirror mount. The above picture and below video are just the first attempts at using the mirror. I'll improve things over the next week.

Above is a picture of a candle burning with the 8" mirror. You can compare that to the same candle pictures that my son and I did with a lens HERE. In the above picture you can see the really small flame right at the bottom and all around it the hot air rising. There is a lot more detail and much better color in this image using a mirror instead of a lens. Instead of using a knife edge for the 'Schlieren stop' I used a piece of clear plastic with colored lines on it as a filter. I made the filter by cutting a small piece of plastic and drew a bunch of different colored vertical lines on it with fine point felt tip pens. The refracted light from the different air densities passes through different parts of the filter and are given different color tints making the image.

The detail using the mirror is a lot higher than using a lens because the light is passing through the air once on it's way to the mirror then reflects back to the camera passing again through the same air. Passing through the same air twice doubles the refraction and gives better detail. HERE is a link to the same setup I am using now with a mirror and HERE is a link to the lens setup I was using before if you want to compare the differences.
The parabolic mirror that I have is an 8 inch F 7.38 which gives a focal length of 59.04 inches. To get the Schlieren setup correctly I have to have the light source and the camera twice that distance at 118 inches. (As a side note I'm not sure why the setup isn't right at the focal point but that is a question for another day) In order to get the camera to zoom onto an 8" object from 118 inches away and fill the frame I have to use the "digital zoom" function of the camera. The optical zoom doesn't make the mirror look big enough and the digital zoom+optical zoom reduces the quality of the image. I am going to work on fixing that so I can have a good quality image at that distance. Maybe I'll use the homemade zoom lens system I made HERE that I was using to try and correct the chromatic aberration problem last week.
One of the first things that became really obvious with this mirror was getting the setup right at 118 inches. At that distance even small adjustments in mirror / camera / light cause a big change. Just walking across the room can mess up the alignment so I am going to have to find a good stable way to hold everything in place and reduce vibrations.
The other thing I noticed is how sensitive the mirror setup is. I could see cool air blowing around the room and even the heat from my hand rising up off my skin! Check out the below video to see what I mean. In the beginning you can see the heat left over from where the candle was sitting. Then you can see my hand at just body temperature then the lighter and candle.

Heat from my hand, candle and air
I think I am going to get much better results with the mirror than the lens. Judging by the long focal point and the sensitive setup I think the Kinematic mount was definitely the right way to go. Even though the adjustment screws are a course 20 turns per inch it was good enough to get everything set up pretty well. Now all I need to do is work on getting the camera zoomed in without using the digital zoom and minimizing the vibrations and sensitivity of the setup. A better colored filter would be a good idea too!