newsletter of the WCAC.
Critical Collimation for Planetary Observing
by Philip Hoyle
Have you ever wondered why your reflecting telescope didnt perform as well as a refractor of even half the aperture? Do you think that perhaps the main mirror in your telescope doesnt quite measure up to others of equal size that youve looked through? Do you always seem to be blaming the seeing conditions for poor images even when someone right next to you thinks the seeing is pretty good? Chances are that the only thing wrong with your reflecting telescope is that it needs to be collimated.
Collimation is the alignment of the optics in your telescope. If the optics are not properly aligned, they cannot bring starlight to an accurate focus. Refractor telescopes are permanently collimated at the factory and therefore should never require collimation. In general, reflector telescopes are prone to go out of collimation, especially when carried in your car.
Lets face it, if you own a reflecting telescope of either the Newtonian or Schmidt-Cassegrain types, you should know how to collimate it accurately. Just like a guitar player tunes up his or her guitar before every performance, you should at least check the collimation of your telescope every time you use it. Just like an out-of-tune guitar makes bad music, a telescope out of collimation is going to deliver bad images every time.
When observing the planets, collimation is extremely critical. When youre observing deep sky objects, collimation is important, but since most of what you are looking at is faint and fuzzy anyway, you might not notice the difference quite as much as you would in planetary observing. When youre doing planetary observing, most of the time, you want the highest magnification the seeing conditions will allow. This is what makes the details really begin to show. It is also the reason precise collimation becomes absolutely critical.
When precisely collimated, even a 4.5 inch Newtonian reflector is capable of showing the Great Red Spot on Jupiter and the Cassini division in Saturns rings in average Ohio seeing conditions. Under good to excellent seeing conditions, a 10-inch telescope is capable of showing:
Intricate detail in Jupiters multiple cloud belts,
Jupiters moons as disks, not points of light,
Saturns A, B and C rings,
Saturns Encke Division,
Saturns cloud belts,
Multiple details on the surface of Mars.
It is worth noting here that although precise collimation is a requirement for showing all the details your telescope has to offer, seeing conditions will also limit the detail you will be able to see. The seeing conditions become more important as the aperture of the telescope increases. Therefore to get everything out of a large aperture reflecting telescope, you need precise collimation AND good seeing conditions.
Here are the basics for getting a Schmidt-Cassegrain telescope properly collimated (Figures 1 through 6 show what you might see through the eyepiece.):
1. Collimate the scope according to the owners manual. (I.e. get the shadow of the secondary in the center of a defocused star image.) This involves turning the mounting screws of the secondary mirror.
2. Let the scope cool to ambient temperature. (I usually stick to low power views of deep space stuff while I'm waiting.)
3. Getting back to collimating.. Center a relatively bright star in the eyepiece and defocus the star. Make sure the star is in the center of the field of view.
4. At this point, you shouldn't notice much difference from the initial collimation. But, here's the first trick...Slowly bring the star to ALMOST focus. If the star is brighter on one side then the other, or looks kind of like the fan of a comet, you've got more adjustments to make. Slowly adjust the collimating screws, only 1/4 to 1/8 of a turn, until you get the brightness of the star even in each direction.
5. Now try to get closer to focus and see if the brightness of the star is still even. If not, adjust some more.
6. Repeat this procedure until you get close enough to true focus that you can't see any collimation error.
7. Now hears the second trick...Put in an eyepiece that will give you a magnification of 60 to 70 times the aperture in inches.
8. Repeat the above process. However, note that you will have to turn the screws even less and that when you turn the screws, the star will move around in the field of view, possibly even leaving the field of view. You will then have to re-center the star.
9. When it is properly collimated, assuming the seeing is relatively steady, you should now be able to see the classic diffraction pattern of a bright center spot surrounded by a tight ring.
Figure 1: Low power, unfocused star in a telescope that is not even close in collimation.
Figure 2: Low power, unfocused star in a telescope that is at least close in collimation.
Figure 3: This is what a star might look like in a telescope that is in focus, but not collimated.
Figure 4: This is what a star might look like in a high power eyepiece when the telescope is just out of focus and slightly out of collimation.
Figure 5: This is what a star might look like in a high power eyepiece when the telescope is just out of focus, but properly collimated.
Figure 6: This is what a star might look like in a very high power eyepiece when the telescope is properly collimated.
Letting the scope cool to ambient temperature is an important step in critical collimation. Once I was adjusting the collimation of my Schmidt-Cassegrain telescope attempting to get good views of Jupiter and Saturn in very humid conditions. The corrector plate of my telescope kept getting covered with dew. I removed the dew with a 12-volt hairdryer-style dew remover and continued trying to collimate my scope. I finally gave up in frustration after realizing that the temperature change of the corrector plate was continuously changing the collimation of my scope! By the time the corrector plate had come to steady-state conditions, it had covered with dew again. I have since purchased the type of dew remover that consists of a battery powered heating strip that wraps around the front of the telescope tube. Although this does not allow the scope to come to ambient conditions, it does allow it to come to steady-state conditions. In other words, the amount of heat going into the telescope is equal to the amount of heat lost and the corrector plate does not have minute changes in size that cause the optics to go ever-so-slightly out of alignment.
Collimating Newtonian telescopes is slightly more complicated than collimating a Schmidt-Cassegrain. This is because there are two mirrors that have to be accurately aligned. Also, collimation of Newtonian telescopes is usually done in the daytime when it is easier to see the mirrors and screws. To collimate at Newtonian:
1. Take the eyepiece out of the focuser. You will be looking down the focuser to check all of the alignments.
2. Make sure that the secondary mirror is properly centered beneath the focuser. If not, use the adjustments on the support spider to center it. Depending on your secondary support, you may need to re-center the secondary mirror.
3. After the secondary is centered under the focuser, make sure that it is aimed in the proper direction. The main mirror of the telescope should be centered in the reflection of the secondary mirror.
4. Once the main telescope mirror is centered in the secondary, all adjustments will be with the support screws under the main mirror. Adjust the main mirror until the reflection of the secondary mirror is centered in the main mirror.
5. When everything is properly aligned, you should be able to see a reflection of your eye; first off the secondary mirror, then off the primary mirror and again off the secondary mirror.
Figures 7 through 10 show what the view through the focuser might look like when performing these collimation steps.
Figure 7: This is what the view down the focuser might look like when the secondary mirror is not centered under the focuser. Centering the secondary is the first step in collimating a Newtonian.
Figure 8: In this sketch, the secondary has been centered, but now needs to be accurately pointed. Depending on your secondary support, you may need to re-center the secondary mirror.
Figure 9: In this sketch, the secondary is now properly aligned. The primary mirror now needs to be collimated. This is done by adjusting the three support screws behind the primary mirror.
Figure 10: The primary and secondary mirrors are now properly aligned. You should be able to see a reflection of your eye.
When performing these adjustments on a Newtonian telescope it is important to keep your eye centered over the focuser. This can be difficult to do. A collimating eyepiece can be a big help here. Orion offers one for less than $50.00 and it is probably a good investment if you own a Newtonian telescope. If your budget is limited, I have also heard of people using a plastic film canister with a hole drilled in the bottom to perform the same function, but I have never tried this myself.
After the alignment is done in the daytime and you get your scope out under the stars, it is probably going to be worth it to try a star test on the collimation. To do this, just follow the same procedures for collimating a Schmidt-Cassegrain. However, instead of adjusting the secondary mirror, adjust the primary by turning the support screws.
Whether you own a Schmidt-Cassegrain or an Newtonain telescope, try these tips. If youve never tried them before, chances are that your scope will perform just as youd always hoped it would.