May 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - May 1999 Issue

The Telescope Nut
by Jeff Baldwin

Star Testing

Star Testing is a method of evaluating a telescope mirror by how the star image appears inside and outside of focus. As strange and inaccurate as that sounds, it is the ultimate method of testing the mirror, collimation, atmosphere, mirror cell, tube currents and many other 'filters' that impurify the image. Here's the deal.

We pick a star that doesn't move much but is bright enough to use on an uncoated mirror. Polaris usually works well. If it isn't bright enough for you, pick another star, but be prepared to track it.

When the star light is coming towards the telescope, the wave-fronts are in planar sheets. They are actually spherical, but the centers of curvature of these spheres are at the star, and as far as the telescope is concerned, that is an infinite distance, making the wave-fronts planar. The wave-front enters the telescope and reflects off the paraboloidal mirror. When the wave-front is leaving the mirror, it is no longer planar, rather it is spherical with the focus at the center of curvature of that sphere. That makes sense when you think that all the light is going to hit the focus, so all the light rays are heading towards the focus, the light rays are perpendicular to their wave-fronts, and the speed of light is the same for all the photons. The only way that can all happen is if the wave-fronts are spherical with the focus at the center of curvature. I have a mathematical proof for all this -- holler at me if you want it, I'm not going to print it here.

So, let's put an eyepiece on the telescope and focus a star. Boing, a dot. Or is it? At very low power it is a dot. (Because of the wave nature of light it is actually a disk with rings around it; we'll hit that stuff in a future article.) Let's defocus the star. It now becomes a disk with a dark disk in the middle. Why? The light from the mirror starts out as a wave-front that is as wide as the mirror, converges toward the dot getting smaller along the way, and we are capturing the light before it becomes a dot, so it is a round disk. If the mirror were square, it would have been a square light bundle, not a disk. If the mirror is perfect, then the light distribution throughout the disk would have been uniform, since the wave-front coming from the mirror is spherical and we are sampling a tiny part of it. If I had defocused outward instead of inward, I would see the same disk, only because the light was diverging from the dot instead of converging toward it. The dark disk in the middle is the shadow of the secondary mirror.

OK, let's put a little itty bitty deformation on the mirror somewhere. Now the entire mirror is reflecting a spherical wave-front to the focus except for that particular deformed spot. The light from that spot is going in slightly the wrong direction, and the wave-front is no longer spherical. When you defocus the star you can see the wave-front is not uniformly lit, and we call this spherical aberration. When you defocus outside of focus, the aberration has crossed over to the opposite side from when you were inside of focus. If the deformation caused an area of darkening on this out-of-focus disk when we were inside of focus, that was because the light rays were rearranged so that this area was missed by the wave-front. After the light crosses the focus it may cause an extra bundling of light at that area instead of a lack of light bundling. I'm careful here because each optical error makes a different visual error, not all are dark on one side, light on the other.

When we star test we usually start out with a low power eyepiece. This makes you feel good about your mirror, because a low power eyepiece way out-of-focus shows the least errors, and your mirror looks pretty good. So let's start out with an 8" f/6 mirror and a 25mm eyepiece. This makes about 48 power. Inside and outside of focus looks pretty good when the star is way out-of-focus. So, let's bring it out-of-focus in and out just a hair instead of a lot. Both images should be exactly the same. The light density in any region on the out-of-focus star inside of focus should be exactly the same as outside of focus. The disk may not be uniformly lit because of diffraction effects, but the light density per region should be the same in and out. The size of the secondary mirror shadow should be the same. There should be the same small dot at the center of the secondary mirror shadow. The edge should have the same edge sharpness. If all these are true, then you have just passed the first iteration of the star test.

Now let's use a higher power eyepiece. That will allow us to look at the out-of-focus star the same size only from a smaller and closer sample of the image. What I mean is that when we used a low power eyepiece we could look at the out-of-focus star when it was 2 mm across, and now we can look at it when it is only 0.5 mm across. On the first one, when we made it as big as a bb at 3 feet we saw the star a certain distance out-of-focus. Now with the higher power eyepiece, when we make the out-of-focus star the size of a bb at 3 feet, we are looking at the light bundle much closer to the focus and can see high precision in optical errors.

This is important because most mirrors are going to look fine when the star is way out-of-focus, but only high precision mirrors are going to pass this test at extreme powers. When the mirror has an optical defect that is viewed in the star test, the optician can work the mirror until that defect can no longer be seen. When it has been repaired and no error is visible, the optician then moves to a high power eyepiece, at which time he can usually see a defect to work out. This should be continued until there is no visible error using an eyepiece that is at least as powerful as the most powerful eyepiece you will ever use in the field. If you plan on using a 4.8 Nagler eyepiece in the field, then you might consider star testing not only with the 4.8 Nagler, but even with a 2.5 power Barlow lens and the 4.8 Nagler. If there is no optical defect using a 4.8 Nagler and a 2.5 power Barlow, then using the 4.8 Nagler alone would show no defects. This is a qualitative test rather than a quantitative test, but it is the best test there is. Period.

Next month we'll finish out star testing with discussion of the various problems that can be detected, as shown in the chart. These images are from:

"Star Testing Astronomical Telescopes"
by Richard Suiter

Stay tuned.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.