Focusing with a True Knife-edge Accessory
True knife-edge focusing can be a very difficult process for the novice. The main reason is that those trying this approach do not know what to look for in the light pattern they see and easily whisk by the focus position without noticing it. Once you have used our regular ronchi screens to practice your focusing, however, using a real knife edge becomes an easy process since it trains your eye.
The best way to look at a knife-edge is as if it were a ronchi screen with only one bar. Using our high resolution ronchi screens with 300 lines/inch, the number of bars is highly visible and it is easy to focus until the light pattern is free of bars. With essentially only one bar appearing with a knife-edge, however, you do not have this luxury. If you focus too quickly, the light pattern will go by without notice. This is why knife-edge focusing always sounds easy, but is very difficult to master without some viewing practice.
The key to focusing a knife edge, of course, is to take your time and choose the very brightest star possible to focus on. Remember the objective is to set the infinity position, and what is good for that brightest star is also valid for any other celestrial object. Infinity is infinity. Also, since you can not see a trend toward correct focus as you can with ronchi screens, it is important that you be fairly close to focus to begin with. If you begin too far out of focus, it will be easier to miss the light pattern when it flashes by. So, when using a knife edge, the best approach is to begin by doing a normal pre-focus through your SLR viewfinder or, in the case of CCD use, by obtaining a basic focus first on your screen. When this basic focus is obtained, replace your camera with the Series IV with the True Knife installed.
The knife-edge screen itself looks like a half-moon, with one clear see-through surface and the other half dark. As you focus the dark side will appear to flip from one side to the other, (eg. left to right, up to down) depending on which side of focus you are on. When you are on one side of perfect focus, for example, the dark side may appear to be to the left. When you are on the other side of perfect focus, the dark side may appear to be to the right. Neither of these positions indicate the exact focal point. What you are looking for is that 'sweet spot' in between these positions. This spot will appear to be dancing around with dark mixed with light. This indicates that the knife-edge is actually 'splitting' the focal point.
The words 'left' and 'right' above could as well be replaced with ' above/below' , 'upper left/lower right'. etc. since the orientation of the actual ronchi bars (knife-edge) depends on the orientation of the telescope axis itself. In practical terms, this positioning does not matter much. However, there is one special case which can cause some problems. This is where the bars on the ronchi screen (or bar on the knife-edge) are perfectly parallel with your horizontal axis. Since they are parallel, you may inadvertently find yourself perpetually on top of a dark bar (or directly on top of a clear space) and the pattern does not change as you continue to focus. This can be maddening until you realize what is happening. The simple solution, of course, is to turn your declination knob a bit to get off of the stationary bar. There really is no need to reposition the ronchi screen or knife-edge within the Stiletto assembly itself.
At the point where the knife-edge is actually 'splitting' the focal plane, you should not have to go to focusing extremes to see the light pattern flip as it appears across the knife edge. A very short press of your electric focuser button in either direction should reveal the flash of light you are looking for. Once the light pattern appears to be constantly flickering as illustrated above, you are in focus. Having said this, slow flickering is better than fast flickering. The more rapid the flickering at the focal plane, the worse upper atmospheric turbulence is likely to be, and thus more difficult to get a sharp image.
Finally, to test that you are at the proper focus position, turn your declination knob back and forth a bit. When in proper focus, the shimmering light pattern will disappear and then suddenly reappear as you turn the knob. You are now in focus and can attach your regular camera.
What You See When In Focus
What you actually see at the point of focus depends on s
everal factors - atmospheric conditions, the f/ratio of your telescope, and the optics of your telescope itself.
If the light pattern above is jiggling or wobbling as indicated by a dancing black knife-edge (or bars on the ronchi screen), this might indicate that there is undue turbulence in the atmosphere. In this case, you might find it difficult to take decent pictures that night and prefer to wait for better sky conditions.
The optics of your telescope also determine what you will see at the point of focus. Ideally under good conditions, the area next to the knife edge (or space between the ronchi bars) will appear white. It is not unusual, however, for this area to appear in various shades of gray instead. In this case, micro focus until you have found the point where the lightest shade of gray appears. That will be the optimal point of focus.
Keep in mind also that your telescope's f/ ratio plays a role in what you will see. For fast f/ratios below about 4, the small depth of field makes the focus point much more visible, but also easier to pass by. At higher f/ ratios the depth of field increases and allows for a slightly broader focusing range, especially when using the ronchi screen.
Since both knife edges and ronchi screens are commonly used to test the quality of telescope optics, the procedure above may also present some anomalies. Basically, both the knife edge and ronchi bar edges should appear to be straight. Any curvature in the bars may indicate that your optics are non-optimal. Before jumping to rash conclusions, however, and concluding that your equipment is suspect, you should realize that perfect field curvature is impossible to attain. All instruments including the most expensive refractors and mirror systems show some degree of imperfection. It is not unusual at very low f/ratios with Newtonians and camera lenses to see very curved lines instead of straight ones. The focusing procedure is still the same - focus until the curves disappear.
Please refer to the Series IV Instruction Manual for an explanation of the physics behind knife-edge focusing techniques.
