Monday, August 18, 2014

Fixing Collimation on the SV4

In the previous post, I wrote about how images taken at an earlier session showed collimation/centering errors.

I picked up a laser collimation tool to help with the exercise of centering the focuser on the SV4 refractor.  The focuser has been removed from the scope several times over the last year or for flocking.

At the time I tested, I noted that the reflected dot from the laser collimator was offset by a significant amount.  With some judicious tweaking, I was able to keep the dot centered as the grub screws were tightened.  To seal the final adjustment, I ran a tiny bead of blue locktite along the seam to act as a removable glue.

Also while the focuser was off the scope, I removed the flocking paper and painted the internals with the flat black paint.  This worked better for the other metal parts and doesn't leave fuzzy bits to cause problems later.

After the focuser was centered, I figured that I'd try to identify issues of tilt with the DSLR.

I've read the method that Starlight Xpress suggests to fix tilt on their cameras.  The idea is to put the camera into a rotatable system, point a laser at the sensor and then note the position of the reflected spot as the camera is rotated.  If the sensor is tilted, the spot will wobble and wander as the camera is rotated.  Minimal movement will show a sensor that is perpendicular to the axis of rotation.

So I made a jig from a collection of stuff including the flattener, t-ring, and 2 inch drawtube extender with a set of 3-point rings for a 50mm finder.

Once I had the camera in place, I lined up the laser pointer.

Reflections of the laser were shined on a far wall, about 12 feet distant.  The initial test showed wobble of at least 6 inches.  After shimming, the deflection was reduced to less than 1 inch.  Not too bad from just a little collection of parts!

Thursday, August 7, 2014

Centering/Collimation Error

While reviewing the frames from the M8/M20 project, I noticed that all had a region in the upper left that was blurred.  The stars were streaked outwards and didn't look good at all.

So I ran CCDI on an example and this is what it shows:

When I did some measuring, it appears that there could be two things happening here:
Tilt and Centering.

Tilt makes sense because I've noticed the way that focus gets better when the camera shifts position.  I'd shimmed the camera body to avoid this whole issue.

Also, Centering makes sense, too, since I've had the focuser off the telescope several times.

So I put a collimation tool on the scope.  When using the Feathertouch-supplied 2 inch to 1.25 inch adapter, there was a slight deflection of the reflected laser.  I loosened the focuser and shifted it to show a centered dot.  What's annoying is that this centering wasn't uniform for the adapter - when rotated, the reflected dot wanders.

Tilt is easy to remedy.  I'll have to shim the camera adapter by wedging small bits of metal in the flange.

The final challenge will be to test and get some results from stars.