BACK BAY AMATEUR ASTRONOMERS

Observing Techniques

There are many methods to find celestial objects, here are a couple of hints on how to SEE them once you locate them.

Averted Vision: This is by far the most powerful technique in the amateur astronomer’s arsenal. It relies on the fact that the human eye has two distinct mechanisms at the base of the retina for detecting light. Known as Rods and Cones they differ in their response and location. The Rods are more sensitive to dim light and are located away from the center of the eye. This is why you can perceive fainter objects by looking away rather than directly at an object. To see fainter, place the object between your nose and the center of view. With practice, you can learn to detect objects up to several magnitudes dimmer than with “direct” vision.

Jiggle the scope: Faint objects are often easier to see when you can introduce movement. By shaking or gently jiggling the scope, you can more easily pick out small differences in light level and distinguish faint diffuse objects from their background.

Compare: When you are sure that you are in the right area of a diffuse object, move the scope to a location that you re sure is away from the object. “Learn” the level of blackness that the sky exhibits and then move back to the object. The difference in background brightness will stand out and you can trace the outline of objects that a moment before were invisible to you.

Magnification: Higher magnification enhances the contrast between a diffuse object and the background sky. But because higher magnifications bring with it smaller fields of view, you’ll have to know where you are looking.

Filters: some objects are greatly enhanced by filters that select for certain wavelengths of light and block the transmission of other wavelengths. It helps to know your subject. Emission Nebulae and planetaries are enhanced by nebula (light pollution) filters. Reflection nebula, galaxies and globular clusters are not. The most versatile filters are the Lumicon OIII, and UHC and the Orion Ultrablock filters. Wide band filters such as the Orion Skyglow and the Lumicon Deep Sky are good for increasing the general contrast of the sky and are worth a try with any object in light polluted skies.

Dark Adaption. Last but far from least. Maintaining or improving dark adaption will do more than any other technique in increasing your ability to see faint. Avoid light! Even your red light degrades your dark adaption, keep it as low as possible and use it sparingly. To see really faint try covering your head with a dark cloth for several minutes before looking in the eyepiece.

The prevailing wisdom in the amateur community is that cleaning of your scope’s primary mirror should be done only rarely. An accumulation of dust will make very little difference in the performance of your scope and every time you handle your primary optics you risk disaster.

Granting the above statement as true doesn’t mean that you should be afraid to clean your mirror when the need arises. Here’s some advice that might help.

Use only pure distilled water, and lots of it. You’ll need a clean spray bottle, keep one just for this purpose. And you’ll need to mix a very diluted soap solution. I can’t emphasize enough how little soap you should use. One very tiny drop of concentrated dish washing liquid to a couple of quarts of distilled water is all you need. Have 100% cotton pads standing by, it’s best to buy the sterile cotton that comes in a roll and cut it into pads.

With most scopes you will have to remove the mirror cell from the scope. You should mark the cell and the tube to assist re-assembly in the same orientation later. It is a good idea to wear latex gloves in case you can’t keep your fingers from contacting the mirror. Scopes vary, but the mirror cell is usually secured in the tube with three or four screws that enter radially from the circumference of the tube. Remove the fasteners and slide the mirror cell (mirror and supporting back) from the tube.

Support the mirror nearly vertical. Use the clean spray bottle filled with pure distilled water to spray the mirror, wetting it thoroughly. Get any visible solid matter off the mirror this way. Lay the mirror flat and pour the soap mixture over the mirror. Let it pool up in the center of the mirror and use the cotton to gently swirl it around. Then rinse with copious amounts of pure distilled water. Don’t skimp on the rinse water, more is better. Stand it back up and dry the mirror with the cotton. Be sure to change the cotton frequently and use gentle, short strokes. If you notice colored streaks, you’ve left soap film on the mirror; rinse it again.

Now would be a great time to “spot” your mirror to facilitate collimation.

After your mirror is dry, reinstall it in your tube. You should then collimate your optics, which is another subject for another time. Keep your tube covered when stored to reduce the dust accumulation (but don’t cover a dew soaked mirror until it dries).

 "Spotting" Your Primary Mirror

Placing a spot in the center of your primary mirror is the best way to facilitate collimation of your Newtonian or Dob. Don’t worry about degrading the image, the center of the mirror is in the shadow of the secondary and doesn’t contribute to the image.

You will have to remove the mirror cell from the scope. You should mark the cell and the tube to assist re-assembly in the same orientation later. It is a good idea to wear latex gloves in case you can’t keep your fingers from contacting the mirror. Scopes vary, but the mirror cell is usually secured in the tube with three or four screws that enter radially from the circumference of the tube. Remove the fasteners and slide the mirror cell (mirror and supporting back) from the tube.

Using a compass, draw a circle on newspaper the same diameter as your mirror and cut it out. Use a pencil to slightly enlarge the pinpoint that the compass left in the center of your circle. Gently lay the newspaper disk on the mirror and align the edges. Use a magic marker to mark the center of the mirror through the hole in the paper, then remove the paper. Place a loose leaf reinforcement ring on the mirror centered on your mark. You can either blacken the inner hole of the reinforcement ring and then remove it, or blacken the ring and leave it on. Both methods work equally well. Use an indelible marker.

Replace the mirror in its original orientation. You are now ready to collimate.

The popular SCT telescope has a closed optical tube assembly that helps to keep the primary mirror dust free longer. The “corrector plate” or front lens of the scope, however, is a “dust magnet.” Still, dust will effect your scope’s performance very little and you shouldn’t be cleaning your corrector too often. When you do clean it, some simple precautions will help you to avoid disaster.

The best way to keep your optics clean is to keep them covered when not in use, but NEVER cover a dew soaked corrector plate until it dries. When returning the scope from outdoors, check to insure its dry and cover it outside before bringing it into the warm house or car (where condensation can quickly form). Keep your rear cell sealed as well to prevent the introduction of dust borne air into the optical tube assembly (OTA).

Forced air is the best way to remove dust from your corrector plate. But, be wary of the canned compressed air, as it can sometimes propel liquid or particles at your scope. If you use these, don’t shake the can, hold it upright, test it on your hand first and don’t allow the spray to continue too long. I use a simple squeeze bulb to force air onto my corrector plate. Point the scope down and force air toward the corrector at a sharp angle, be careful not to actually contact the glass with the bulb.

If you need a liquid to remove stubborn material try using pure distilled water. Dampen a clean cotton ball with the water and use another to dry it. There is disagreement among sources as to the proper motion to use when cleaning a corrector plate so as to not damage the delicate coatings. I use the Celestron recommended method of swiping radially outward from the center obstruction toward the edge of the plate. Be careful not to let liquid reach the edge of the plate where it might enter the tube. Use gentle, uniform strokes. Others recommend a circular swirling motion. Check your manufacturer’s recommendation. If all else fails, you can use a very diluted soap solution using a tiny drop of dish washing liquid in lots of distilled water or a commercially prepared lens cleaning solution.

Cleaning the primary of an SCT is not for the faint of heart. You will have to remove your corrector plate. The corrector is secured to the tube with a retainer ring that is fastened with screws. There are usually shims under the retainer, or under the corrector itself. It is important to mark these shims and the corrector plate. The corrector plate is designed to offset inherent aberrations in the spherical mirror and the orientation is factory set for optimum correction. Great care should be taken to insure that the corrector is reinstalled properly. Note also that the edges of the glass are thin in a Schmidt corrector and can easily be crushed or cracked by too much (or uneven) pressure from the securing screws. Once access is gained to the primary it can be cleaned with the same methods used for the corrector outlined above. Dust can be removed from the optical tube with a small vacuum cleaner. Removing the primary from the OTA is not recommended.

Collimation of an SCT is accomplished by adjusting the secondary mirror (three screws on the front of the obstruction in the middle of the corrector). But that is another subject.

Collimation Basics

The purpose of collimation is to align all of the optical elements of your telescope so that the optical axes line up. The collimation requirements of different scopes vary; a truss tube Dob must be collimated before every use, a refractor might never need realignment.

Most reflectors are of the Newtonian design where a parabolic primary is combined with a flat diagonal to collect light and direct it sideways to a focuser where an eyepiece then magnifies the image. Collimation aligns the primary, diagonal and focuser.

The procedure below uses a set of collimation tools that are available from several sources. The Tectron tools I use can be purchased for about $100. The set contains a “Sight tube” with cross hairs, a “peep sight” eyepiece called a “chesire” and an autocollimator.

The first step, and one that should only have to be accomplished once, is to “square the focuser” to the optical tube. The centerline of the focuser must be perpendicular to the centerline of the optical axis of the primary, so it is assumed that the focuser must be perpendicular to the center of the tube. One way to do this is to remove the secondary mirror from the spider assembly and replace it with threaded rod that will extend down into the axis of the tube. Adjust the spider so that this rod is centered between each spoke. Then sighting through the focuser, use the rod to mark a vertical line on the tube opposite the focuser. Insert a chesire (peep sight) eyepiece into the focuser with the pinhole inside and flush with the inner diameter of the tube. Measure the distance from the top of the tube to the peepsight. Then mark the same dimension on the vertical line opposite the focuser and highlight the intersecting point. Remove the threaded rod and the chesire and install a cross hair sight tube in the focuser. Shim or adjust the focuser to align the crosshairs on the mark you made across the tube.

Next reinstall the diagonal (secondary) mirror. (We assume that no offset will be imposed on the secondary mirror). Slide the sight tube in or out until it is just larger than the apparent diameter of the secondary. Rotate the secondary until it appears round when viewed through the sight tube. Move the secondary toward or away from the primary (adjust up or down), keeping its image round, until it is centered in the sight tube. (Ignore all of the reflected images) adjust the diagonal until its round shape is centered in an annular ring (the wall of the sight tube). Now look at the reflected image of the primary in the secondary. Tilt the secondary until the image of the primary is centered. Most secondary holders have three screws to adjust the tilt of the diagonal mirror. Be careful, they often also secure the holder to the spider and if loosened together, will allow the mirror to fall.

The last step is to adjust the primary. Put the chesire eyepiece into the focuser and find the center spot or ring that you placed on your mirror previously. Your focuser should be at mid travel for this adjustment. Use the collimation screws to tilt your primary to center the pinhole over your mirror’s center dot.

Collimation can be checked by using the “star test.” Well-collimated optics will focus stars to pinpoints (if “seeing” allows). Allow time for your mirror to cool to ambient temperature and select a moderately bright star (about third magnitude) that is high off the horizon. Keep the image in the center of the field. The star should “snap” into sharp focus from both sides of “out of focus.” The shadow of your secondary should be centered in the ‘out of focus’ star. Use a moderately high power eyepiece (one that yields a magnification equal to 15 to 20 times your scope’s aperture in inches) and be sure the image is in the center of the field. “Touch up” your collimation to achieve the best possible star image. At High power a star should focus to a sharp point with faint concentric rings around it, the rings should appear round.

Collimating the Schmidt Cassegrain Telescope

Collimating an SCT is very easy in principal and quite difficult in practice. Fortunately, these scopes hold collimation quite well and if treated gently may go years without requiring adjustment. But they are very sensitive to small misalignments and benefit greatly from precise optical collimation.

The SCT is collimated by adjusting the secondary mirror which is mounted in the middle of the corrector plate and constitutes the secondary obstruction. There are three adjusting screws located 120 degrees apart on the front of the secondary obstruction. Often, there is a plastic cap that must be pried off to expose the screws.

Take care in adjusting these screws. To move the mirror requires loosening one side while tightening the other. Be careful not to loosen all three at once as the mirror can come loose and fall into the tube with catastrophic results.

Sct’s are collimated using a star image, normally a fairly bright star half way to the zenith is a good candidate. Position yourself where you can turn the screws while observing the image. Unfortunately, this procedure is best accomplished without a diagonal making it quite difficult to do alone. But it is better to accomplish it with a diagonal than not at all. Do course adjustments with the star defocused, adjusting to center the shadow of the secondary in the image and make the surrounding rings concentric and circular. Check the in-focus star image for a sharp pinpoint when you think you’ve achieved collimation. The star should “snap” into focus from both sides (inside and outside) of focus.

Some have reported success with an “artificial star” for collimating during the day. You might try a sparkling object like a Christmas tree ornament if you have a sufficiently large test area. Hang the ornament as far away as possible and use the reflection as the point source of light.

Dealing With Dew

Dew is formed on optics when the temperature of the glass falls below the “dew point” of the air around it. So fighting dew is a matter of keeping your optics warmer than the surrounding air. The two methods used to do this are to 1. Limit the exposure of the optics to the atmosphere and 2. Apply heat to the glass.

There are many sources of information about how to fight dew and many clever devices for warming or shielding optics have been developed. Our own Kent Blackwell is the author of an S&T article that describes one way to build an anti-dew heating system. There are commercial products and home made systems aplenty. This article is to make you aware of the fact that, if you are going to observe here in Tidewater, you must deal with dew or limit your observing to the time it takes dew to form and send you packing.

There are methods to remove dew once it has been formed, but most will agree that dew prevention is better than dew removal. The first line of defense against dew is the dew shield. By extending the telescope tube beyond the objective of a refractor or corrector plate of a compound telescope we delay the formation of dew by slowing the cooling of the glass. This method can also be applied to the Telrad or finder scope. The dew shade should be kept flat black on the inside to reduce reflections, benefits from lining with a moisture absorbent material such as felt and should not be so long as to cause vignetting of the optical path. Dew shields are a MUST for SCTs, their corrector plates are very exposed and radiate their heat rapidly!

Applying heat to optics is a controversial subject. Optical performance is very dependent on temperature equilibrium. Even slight temperature variations can deform the shape of a mirror or lens and induce aberrations that degrade the image. But once dew forms, observation ends, so pick your poison.

Every glass surface is subject to dewing. Keeping eyepieces covered, or in your pockets will prevent them from forming dew. If they fog from your breath or proximity to your face just fan them to facilitate evaporation (in this case, your body has warmed the air near the surface of the glass so rather than warming the glass you need to cool the air). Once in the focuser, keep dew from forming on eyepieces by applying heat, or keeping a cover on them when you’re away from the scope.

An anti-dew heating system can be purchased (Kendrick Astro Instruments is a popular source) or fabricated. They run on battery power, and should provide just enough gentle heat to keep exposed optics above ambient temperature. I use a 12 volt battery to power heat ropes or resistors on my secondary mirror, telrad, finder objective, finder eyepiece and the eyepiece in my focuser. My SCT has a heated dew shield. (from Roger W. Tuthill Inc.)

Once dew forms, it can be removed with a hair dryer. There are battery powered devices that serve this purpose well. Another trick is to turn your optics to the ground, they will clear eventually. Both of these responses offer only temporary relief, however, unless they are warmed your optics will dew up again.

Polar Alignment

A telescope that is on an equatorial mount must be placed parallel to the Earth’s axis of rotation to realize its full potential, a process that is called “polar alignment”. Once aligned to the pole, this type of telescope mount can track a star’s diurnal motion with the movement of just one axis.

There are two common designs for equatorial mounts; the German Equatorial Mount and the Fork Mount. They both operate from the same principle. The “polar axis” is tilted by an angle equal to the observer’s latitude and then pointed at the celestial pole. We are fortunate in the Northern Hemisphere to have a “pole star”, Polaris, that is very close to the north celestial pole. It is so close in fact, that for casual observing, just aiming the polar axis at Polaris will suffice. But if you intend to use your mechanical setting circles to find objects or if you will be attempting to do guided photography, you need a more accurate alignment.

Some commercial telescopes come equipped with devices meant to facilitate polar alignment. It might be a sight that is offset from the polar axis of the scope by the same amount that Polaris differs from the true pole. The sight is “set” for the date and time so that when it is centered on Polaris, the scope it is attached to will be aimed precisely at the pole. Another method is an etched reticle that can be used in a finder scope and that has a pattern that when aligned with Polaris (or a circumpolar asterism) places the scope’s polar axis parallel to the pole. In any case, truly accurate polar alignment will require refinement by a more exact process such as “declination drift”.

Declination (or star) drift is a tedious process that only need be accomplished if accurate alignment is a must. With your right ascension drive running, center a star on the celestial equator and on the meridian (as near as possible to due south and zero degrees declination) and watch for drift. If the star drifts south, the polar axis is too far east. If the star drifts north, the polar axis is too far west (in the eyepiece, with the drive off, drift is from east to west, north is the direction 90 degrees clockwise from drift). Adjust as necessary until there is no drift north or south. Repeat the procedure with a star 20 degrees above the eastern horizon. If the star drifts south the polar axis is too low. if the star drifts north, the polar axis is too high. Adjust the position of the polar axis until there is no drift on either star. If your mount is “square” and the ground firm, this method can provide near perfect alignment.

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