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Testing Optics

We all know that various errors are likely to be present in mass-produced telescopes from Synta, Meade, and Celestron. Higher end scope makers do not have as many problems. The good APOs and some of the better SCTs, MAKs, and DOBs have good quality control. They test their optics, and only very slight errors are allowed in production (less than 1/8 wave and most of the time 1/10 wave or better). Don't think that a $10,000 scope is perfect. No scope is perfect because the nature of light will not allow for perfection. There are many variables, and when one part is made perfect the cost is at the loss of another part. It's give and take. You gain here but lose there. But we can get very close with exotic materials and complex design, at a high cost in man-hours and materials. Wonder why those good APOs cost so much? Now you know.

Let's start with the most common aberrations of low to mid-range scopes. COMA is caused by misaligned optics, and good collimation will cure it. It will make images look like comets or meteors, with a little tail to one side. ASTIGMATISM, which is common in the human eye, will cause the image to look asymmetrical in out-of-focus images. A good test is to defocus just a bit and see if the image is nice and round or egg-shaped. If it is egg-shaped, it will look the same on both sides of defocus, just turned around. If the inside focus is vertical, then outside will be horizontal. If it's at an angle, like 8 o'clock and 2 o'clock, then the other side of focus will be 10 o'clock and 4 o'clock. It just depends on the figure of the lens or mirror and where the astigmatism is. Poor collimation or using wide angle EPs and poor seeing can give a false result which might look like astigmatism, so use narrow field EPs and make sure your optics are collimated perfectly and the seeing is good. The amount of astigmatism can be calculated by how long the image is stretched or elongated before snapping to a symmetrical image. If it snaps into a symmetrical circle in the first 0.26mm of defocus, you have about 1 wave astigmatism. Defocus of 0.14mm is 1/2 wave or very close. (This is based on a 150mm at F8. As the F ratio gets faster, the defocus to one wave gets smaller. Slower F ratios get longer. If the scope was F10, then 1/2 wave would be 0.22mm defocus, and F6 would be 0.10mm.)

SPHERICAL errors. Lower spherical aberration (LSA) is more common and unfortunately worse. LSA will cause the image to shift light energy to one side of defocus, and the other side will be dimmer. Higher spherical aberration (HSA) will do the same but on the other side of focus from LSA. LSA is known as undercorrected and HSA is overcorrected. LSA is more damaging to images, and not much can be done to fix this problem except a corrector like the CHROMACOR, which will be corrected to compensate for LSA or HSA. If your scope is undercorrected the corrector will be overcorrected, resulting in a very well-corrected scope. TUBE CURRENTS are no big deal. It's the same as turbulence in the atmosphere, just in the scope tube. Let the scope cool down to the same temperature as the outside and they will be gone. TURNED DOWN EDGE refers to an area where the primary mirror slopes at the edge and is not figured true with the rest of the mirror. A small mask at the edge will fix this at a slight loss of aperture. A turned edge shows a defocused star image with a cloudy or smeared edge, along with a loss of crispness at the edge of image. ZONES, imperfections in the glass or figure of the lens or mirror, can make a scope useless. Zones can take on many forms: blobs, rings, or blurry spots that just won't go away. Slight zones can seem to clear up a bit as the scope cools down, but sometimes they may get worse, depending on the nature of the zone. Both mirrors and refractors lenses can suffer from this.

Take the time to get the collimation DEAD ON -- not close but RIGHT ON. I can't help but drill you about getting the collimation perfect. I MEAN PERFECT. It makes a big difference in the scope's use of every mm of aperture. A slight turn of a screw can make an image at high power go from a fuzzy blur or smear to a very shiny pinpoint with detail. For example, I was looking at the Ring Nebula and the central star was in and out and faint at that. After tweaking the collimation to "absolute dead on," which was just a touch from "good," the star was very plain to see and the rings and color were impressive.

Here's how I collimate a scope. First, I use a steel ball (around 2 to 4mm) set up outside in the sun about 80 feet from the scope. Use the sun's reflection on the ball as a star. A green filter will help with the image of the ball dancing around. Take a high power EP and start collimating on both sides of defocus. If you think it is perfect, it's not -- when you get outside under real skies, bright stars at high power will be a new task to make your ultra fine adjustments. I use a star like Deneb or Altair as they are not too bright (like Vega). I defocus around 1 to 1-1/2 wave and look for where the circle or disk wants to start out from. It is hard to see, but the image always wants to start out at one side or the other. We want to get the scope to go right to a circle from the smallest defocused point without pinching or squishing out one side. Then I make a micro adjustment and go to the other side of focus and see if it has gotten better or worse for that side. Remember what I said about variables and no free rides? Well, split the difference for the figure of your optics. Sometimes you get a perfect image on one side but just a tad off on the other, so find the best compromise to get all the scope can give for the errors that might be present in the optics. Favor the inside focus if possible; it is the preferred side and HSA is better than LSA. I adjust inside and check outside and repeat until I have the best pinpoint image I can get out, regardless of what the out-of-focus image might look like on one or both sides of focus. The key is to get a nice image at focus. That is what we use the scope for -- not to dial in a defocused image for collimating and have a less-than-optimum final image. Scopes with very good optics will almost always have near perfect out-of-focus image patterns on both sides, but those with less-than-very-good optics or multiple errors need to be adjusted to make the best of what they have. That might mean a very nice inside focus image and a slightly skewed outside focus image if that's what gives the best focused image. Always go for final image at focus, not the perfect defocused image for collimating.


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