In many ways, our new Hybrid Knife-Edge screen is the best of both worls when it comes to pinpoint focusing. The hybrid screen is made up of two 150 LPI ronchi screens placed at right angles to each other, along with a clear circular knife-edge area in the center of the screen. Unlike a pure knife-edge design, however, the mesh of ronchi screens allows the user to easily know when he is approaching focus. The bars on the grid grow larger and fewer in number as the focal plane is reached. At the point where the grid disappears altogether, the circular k
nife-edge may be used, if desired, to precisely cut the focal plane from any direction.
The Hybrid Knife-Edge screen is manufactured of BK7 glass with black chromium evaporated on the surface to form the pattern. The screen is also interchangeable with other screens from STI.
The hybrid screen comes installed in our Series IV Standard model, and is included along with our high resolution 300 LPI screen in the Series IV Pro Edition model. The hybrid screen is also available separately in our online order section for previous Stiletto ow
ners who wish to add it.
This new screen outperforms and replaces our earlier True Knife-edge design, and works best at f/ratios above f/5. The hybrid screen is especially indicated as a one-step focusing solution for use with high-quality refractors.
The Focusing Procedure with the Hybrid Screen
First, center a bright star and attach the focuser as would a camera. The out-of- focus star illuminates the ronchi grid (2) and central clear area (1). Rack your telescope’s focuser in and out watching the grid lines grow in size as the star comes closer into focus. When a single line fills the view the star is nearly in exact focus.
At this point, switch to the central circular clear area. Slowly moving the telescope declination control, move the star across the knife-edge boundary (3). Using this technique, when a star is out of focus either in or outside of exact focus, the image will move across your view. When the star nears exact focus the image changes to a gradual shading, entering on one or the other side of the field. When the star is at exact focus you will not be able to distinguish from which direction the star enters the field since the central area gradually brightens without a hint of movement in any direction. When this occurs, you are in perfect focus. At this point, lock down your telescope’s focuser to prevent any inadvertent movement and procede with retraining your scope and attaching your camera.
