Star Testing


There are a lot of pages out there on star testing, and a great book by Richard Suiter called “Star Testing Astronomical Telescopes”. It’s awesome, and it’s the official reference on the subject.

Basically, when you are focusing a star, the perfect telescope under perfect skies using a perfect eyepiece would have the wave front of light approaching the focus is perfect spheres and then leaving the focus in perfect spheres, exactly the same but opposite in concavity front and back. Therefore, if you defocus inside and defocus outside, they would look exactly the same.  When the star is focused, it should have an Airy disk at the center surrounded by usually 2 or 3 diffraction rings. 84% of the light should be in the disk and 16% should be in the surrounding rings. The worse the optics are, the less light is in the disk and the more light is in the rings. We measure mirrors using the Strehl ratio, which states how much of the light that is supposed to be in the Airy disk is actually in it. 99% would be pretty good. Most mirrors are probably at about 15% or less. Diffraction-limited optics have 80%. Good ATM projects usually have about 90%. Great ATM projects have 98% or more. After considering the issues above, anything else would be spherical aberration and you’d be able to detect it and understand where in the optic or system this defect originated.

I like to look at the out of focus star when it is about the size of a ping pong ball about eight feet away. If this is done with a low power eyepiece, then you are not examining the out of focus star very close to focus and some of the defects may not show themselves. If you use a high power eyepiece, then you are looking at the wave front as it nearly gets to the focus and optical defects are easier to spot. The lower the magnification, the better optics seem to be. Therefore, to examine defects, you would want to use the highest power you can get away with. If you can see spherical aberrations using a 25mm eyepiece, then the telescope’s defects would have to be magnificent, while subtle minor defects would only be viewable at higher powers. We usually star test 8” optics around 900X, and have star tested optics at over 2000X. Multiple Barlows is one way to get there.

The anomalies you can detect using the star test are spherical aberration, astigmatism, turned down edge, collimation, mirror stress from pinching cells, zones, heat currents in the tube, and probably others I can’t think of right now. One should confirm good collimation before doing an analytical star test since a poor collimation may in fact add to or eliminate aberrations you are looking for. There is one more area that causes optical errors, and that is your own eyeball. It and your retina are the last filters that can diminish the optical result, but as an ATMer, you can’t do much for them as you can your telescope.

Collimation ought to come first. When I collimate a Newtonian telescope, I begin by looking into the focuser and seeing if the basic alignment of the mirrors exist. If not, make it so. Then I move up to my laser collimator. The laser spot first needs to hit the secondary mirror at its center, or if it is a large aperture fast focal ratio telescope, be sure to incorporate the intentional offset. At this point you are positioning the location of the secondary mirror, not if it is hitting the primary mirror correctly. Once the focuser and the secondary mirror are positioned, then you can worry about if it is pointing in the right direction by adjusting it so that the spot now hits the center of the primary mirror. I use notebook ring hole protectors on my mirror at aim at. Once the focuser and secondary mirrors are established so that it now hits the center of the primary mirror, you can then adjust the primary mirror so that the return spot from the laser retraces its path onto the secondary mirror and back to the focuser. Once these laser paths are set up, you are fairly well collimated, but possibly not perfectly. My next iteration is to use a Cheshire eyepiece to zero it in. The next step is with a star at high magnifications. If there is no intentional offset, then you can defocus your star and see the secondary mirror shadow dead center to the out of focus star to perfect your collimation. If you have an intentional offset, you can account for it. Finally, coma is what a star image looks like when it is off axis. Coma makes stars look like little badmitten shuttlecocks which point toward the optical axis. If your stars have coma at the center, then the center of your field of view is not your optical axis, and slight adjustments in your primary mirror’s position is needed. Once dead center, if it has no coma and all other adjustments have been satisfied, you’re there. Keep in mind that when you adjust mirrors during a star test, the image of the star will move and you will have to relocate it. Do that by moving the scope, not be returning to bad collimation.

For Schmidt Cassegrain telescopes, point the scope at a star and cram about 800X on it, more if you can. Defocusing the star inside and outside should give you perfectly round disks with a perfectly centered and round secondary shadow. The smaller you can make them by getting close to focus, the better. Adjust the three screws on the secondary mirror to perfect this image. You will have to move the scope as you do this because adjusting the secondary mirror’s position will cause the star to move it the field of view, which is at really high power.

Now that you are collimated, you can perform star testing to determine optical, structural and atmospheric errors.

Astigmatism is easy to see. Instead of round stars blobs inside and outside of focus, you will see oblongness in and out of focus, rotated 90o from each other. Recollimating doesn’t fix this, reworking the mirror does. Don’t recollimate to fix this issue, you’ll only add errors, and they don’t cancel each other out. Sometimes structure on the telescope can cause issues. When I see an astigmatism, sometimes I rotate the primary mirror 90 degrees. If the astigmatism rotates with the mirror, your mirror is astigmatic, and if it doesn’t rotate with the mirror, then there is a structural issue causing it.

Turned down edge is easy to see. Outside of focus you will see an energy ring at the perimeter of the disk, and inside of focus you will see the disk is not distinct and that the edge may have a hairy appearance. An overcorrected outer zone will also show this effect, but without as much hair and probably with more energy in the ring.

Spherical aberrations that imply general under-correction or over-correction are easy to spot. Under-correction shows itself by having an energy ring at the disks perimeter inside of focus. Outside of focus, there appears to be higher energy in the inner part of the disk and lower energy in the outer part. For general over-correction, these are reversed.

Pinched optics from an over-tightened mirror cell usually causes a triangular shaped wave pattern about the star.

Rough mirrors and turbulent air have both have chaotic messy aberrations to the concentric rings of a defocused star. The main difference is that a rough mirror has stationary chaos, and turbulent air has dynamic chaos.

If you have a zonal trough, say at the 60% zone, then inside of focus, the star will show a bright ring at 60% on the disk surrounded by darker zoning, and outside of focus, the star will show a dark ring at 60% surrounded by brighter zoning.

Tube currents will show the out of focus star as having a flat area on the perimeter of the disk that flip flops when you go inside and outside of focus. When in focus, the star will show a streamer similar to a comet tail.

One thing you should note before throwing away your telescope after failing star testing. ALL telescopes fail the star test. There is no perfect telescope, atmospheric condition, eyepiece, eyeball, cornea, collimation, etc. All things are imperfect and they conspire with each other to screw things up. You can optimize your scope, you can work your optics so they are as good as you can make them, but there will always be things that goof up your star test. It is such a high order examination when done correctly that you cannot beat it and all telescopes eventually fail them.

OK, one more thing. It is considered a serious fax pas to star test somebody’s telescope without them asking you to do it. If you step up to look at M13 in a guy’s scope and start yacking about how the star test is making this or that happen on his scope, then you’re being a snob. However, if they ask you to analyze their telescope, then go for it, nicely, and show them what it is you are seeing and what it is supposed to show.

Here are some links to star testing that shows images of the test. [Richard Suiter’s site]