EyepiecesEtc.com Becomes Howie Glatter Dealer!
Posted 22 March 2013 - 04:49 PM
EyepiecesEtc.com has now become a Howie Glatter collimator dealer,
and we will be stocking all of his collimator products.
Collimators and accessories are on order and we should have them before too long.
All products went live on the site today and may be pre-ordered.
Otherwise, keep an eye on the site for when stock shows up.
Posted 23 March 2013 - 09:40 AM
Posted 23 March 2013 - 10:40 AM
Posted 23 March 2013 - 11:15 AM
The biggest problem in collimating commercial SCTs is that the corrector plate, primary mirror, and back axis are not adjustable, and the secondary has no centering adjustment. Only secondary mirror angular alignment is user-adjustable.
Additionally, in some of these telescopes there is excessive clearance between the bearing surface on the outside of the baffle tube and the sliding outer baffle that the primary is mounted on. This allows the angular alignment of the primary to shift when the direction of focus adjustment is changed, and when the scope tube crosses the meridian when tracking the sky. Newer versions incorporate a primary mirror lock, but if you don't have this feature you should always finish your focusing adjustments and perform collimation by rotating the focus screw counterclockwise, so that the mirror will be pushed upwards, and won’t settle backward against screw backlash.
If all the non-user-adjustable alignments were correctly and permanently set at the factory, and there was no primary mirror shift, the telescope could be reasonably well collimated by adjusting the secondary so that the beam from a laser collimator folds back on itself and retraces its path back to the laser aperture, which would set the secondary square with the optical axis. However, because of possible or probable misalignments in the non-adjustable elements, the best setting of the secondary may actually be tipped with respect to the optical axis. Tipping the secondary can partially (but never perfectly) compensate aberrations induced by misalignments in the non-adjustable elements.
<b>The best use of a laser collimator with SCTs is not to perform an initial collimation, but to measure and record the optimal secondary adjustment, once it has been obtained using the star test.</b> Once recorded, the optimal adjustment can be reproduced with the laser collimator alone whenever required.
The star test adjustment, done under good seeing conditions, is the best method of finding the optimal secondary adjustment. It need be done only once. The secondary is adjusted to produce the most compact image of a star at high power in the center of the field. The star test (and subsequent re-collimation with the laser collimator) should be done without using a diagonal unless the diagonal is known to be accurately collimated itself. As soon as the secondary adjustment is judged as good as possible, the eyepiece is removed and the single beam collimator inserted in the back.
It is important that the collimator be mounted and clamped in the scope identically each time, so that the registration in the holder will be the same. The laser beam reflects from the secondary and returns back to impact the face of the collimator. The location of the beam impact is a measurement of the setting of the secondary at that moment. Take careful note of the beam impact position so that you can re-collimate the secondary any time that it becomes necessary by adjusting the secondary to move the reflected beam impact to the same spot. You can place a mark on the face of the collimator at the exact location, but then the collimator must be inserted in the telescope back in the exact same rotational orientation each time.
The beam impact can be seen from the front of the telescope, looking through the corrector plate, by double reflection in the primary and secondary. It may be difficult to see the collimator face because the view is restricted by the baffles. If so, Glatter produces a side cut-out adapter tube that makes it easy to see the collimator face at the back of the scope.