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An experiment you might like to try to determine the ota's sensitivity to focal plane tilt as a function of mechanical deformation:
This assumes you have access to a laser collimator?
1) Orient the ota such that the ota is pointing at the horizon and the focuser draw tube is horizontal (parallel to the ground)
2) Insert the mpcc into the draw tube of the focuser.
3) Tie a piece of string around the end of the mpcc.
4) Hang a weight off the piece of string that approximates that of your camera (plastic juice container filled with the appropriate amount of water for example)
5) Insert the laser into the focuser and turn it on. (leave it turned on until the end of the experiment)
6) Use a whiteboard marker to place a spot on your primary mirror that coincides with the laser spot. (don't be too concerned at this stage if the laser doesn't point at the optical axis of the primary - this is not a collimation exercise)
7) Cut the string with a pair of scissors and allow the container to fall to the ground being careful not to apply a force moment perpendicular to the string.
8) Inspect the primary mirror... is the laser still pointing at the whiteboard marker spot? If not, measure the displacement (in mm)
9) Repeat the experiment with the ota now pointing at the zenith and the focuser horizontal.
10) You can turn the laser off now.
Here's how to derive your focal plane tilt from the above steps:
Take your focal length (800mm) and divide by 57.3 (or 360/2xPi if you want to be precise) this gives you your image scale. ie)14mm per degree.
If the laser dot shifted 7mm in the above experiment, you know that you will experience +/- 0.5 degrees of focal plane tilt as a worst case example.
So... assuming the worst case example; +0, -1 degree of focal plane tilt and focus derived in one corner of the ccd (with a diagonal of 40mm) the opposite corner will be out of focus by an amount equal to:
The chip diagonal (or half diagonal if you focused on the field centre) divided by the primary focal length, multiplied by the laser deviation measured in steps 1 through 9 above.... Or, sine of 0.5 degrees x ccd diagonal if you prefer trig.
ie) 40 / 800 x (+/-) 7 = +/- 0.35mm
To find the extent to which your star images will increase in size for this focus error, simply divide +/- 0.35mm by your focal ratio (f/4) and multiply by 1000 to get the answer in microns
ie) +/- 87.5 microns. From this you can see that for every mm of deviation in the laser spot, you will increase the blur circles of your star images at the field corners by 12.5 microns.
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