Spin Alignment

By installing a pinhole light source of a known diameter pinhole size in the focuser at the focal plane, and then suspending a video camera over the scope, focused at infinity, looking down into the scope parallel to the optical axis (offset to the side from the secondary mirror), you can spin the scope around the azimuth axis and be able to adjust the vertical stop in the inside of the front panel of the rocker box to have the scope be perfectly aligned with the azimuth axis in that plane when the scope is against that adjustable stop. By adjusting the "fore and aft" position of the altitude trunions along the Optical Tube Assembly (OTA), the side to side tilt of the scope can be made to be parallel to the azimuth axis in that plane. The alignment in each plane can be accurately and easily measured in terms of arc seconds.

With the scope aimed vertically (not necessarily vertical as with using a bubble level, but instead make it parallel to the Azimuth axis) with the OTA against the hard stop and secured there with a bungee cord, suspend a video camera above the scope, looking down into the OTA. Position it just off set from the secondary mirror to look past it down towards the Primary. Set the focus at infinity (and turn off autofocus). Take the camera video feed and send it to a monitor/TV convenient to the scope. At the eyepiece, you need to have a pinhole light source and have it located at the focal plane. An easy way to do this is to cut the end off of a plastic 35mm film can and punch a small hole (less than 1/16" diameter and as round as possible) in the lid. Re-install the lid on the film can and tape it to a small flashlight (the lid towards the flashlight). Then just insert the film can into the focuser and rack the focuser in/out till the lid is at the focal plane. All of this will result in a collimated light beam being shone at the camcorder. To initially get the system looking at the light dot, use low power zoom. Once you have the light dot in the FOV, use as much magnification as possible (I used a 200mm telephoto lens afocally projected onto my camcorder objective lens with the camcorder set at a zoom of 6x).

	Altitude Trunion adjuster. The bolts holding the trunion to the trunion-box are loosened and the nuts on the adjuster bolt allow the OTA to move "fore & aft" to adjust the side to side misalignment
	The small flashlight and 35mm film canister used for a pinhole light source. Punch a small round hole in the lid and measure its diameter as accurately as possible. Cut the end from the canister, re-install the lid, and tape the flashlight to the lid.
	Photo of the flashlight/film canister inserted into the focuser. Adjust the focuser till the lid/pinhole is at the focal plane.
	The adjustable hard stop on the inside of the front panel of the rockerbox. (a carriage bolt in a wooden plug, secured to the rockerbox via a t-nut and jam-nuts/washers)

As the scope is rotated in Azimuth, the small dot of light will describe a circle on the TV screen. Pick 4 "cardinal" points with respect to the ground board. to use as reference (i.e. call them N, S, E, and W, or 0, 90, 180,and 270). The idea is that you will adjust one of the altitude sectors fore and aft along the "X" axis of the OTA, then rotate the rockerbox to see if that made the circle smaller. Ultimately it will collapse into a line from a circle when the side to side tilt of the OTA’s "X" axis is finally made orthogonal to the Altitude axis (and perpendicular to the Azimuth axis). The next step is to adjust the altitude hard stop on the front of the rockerbox that limits the OTA altitude to 90 degrees. Adjust it in the same manner as for the altitude sector adjustment till the motion due to the tube not being parallel to the azimuth axis in altitude collapses to nothing.

Video on TV screen showing the reflected image of the secondary mirror and the lighted dot from the pinhole.

By going back and iterating the adjustments once or twice, and by using as much magnification as possible, you will end up with the dot probably describing a small figure 8. This is due to any small astigmatism in the metal plate on the bottom of the rocker box that forms the bearing surface and any "noise" in the bearings or bearing surface.

The actual angular errors can be measured quite accurately, especially if you use high magnification. The size of the dot (actually the subtended angle) can be calculated by measuring the actual size of the pinhole in the film can lid and using the formula: (P/F)x(206.265)=D where P is the diameter of the pin hole, F=focal length of the Primary Mirror, D=arc sec that the Dot image subtends. Then, the distance the dot wanders from the "center" of the figure it describes can be measured in terms of dot diameters on the TV screen, and then multiplied by the calculated angular size of the dot.