Astronomical Telescope Questions And Answers


Astronomical Telescope Questions

There is something very special about looking at the night sky through a telescope. It is an amazing experience to see the planets and stars up close. But how to optimize your astronomical telescope and stargazing experience? This article answers all your FAQs.

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How do I convert my aperture size in inches to metric?

Multiply the inches by 25.4. ex. 4.5″*25.4=114mm


How do I know my scope F value?

Divide the focal length by the aperture size in mm. Ex. 900mm/114=f8


What does the Focal ratio (f-ratio)mean?

The ratio of a telescope’s focal length to its aperture. Short focal ratios (f/5, f/4.5) produce wide fields of view and small image scales, while long focal lengths produce narrower fields of view and larger image scales.


How do I collimate my refractor?

Remove the dew cover from the front of your telescope and gaze through it. A threaded ring holds the pair of lenses in place in the cell. Three screws positioned 120 degrees apart hold this cell in place. The bigger Phillips head screws keep the cell in place, while the smaller, hidden Allen screws press on a ledge at the front of the tube, allowing the cell to tilt slightly due to stress against the Phillips screws. The objective is to alternately loosen and tighten each until you get a circular star picture. There are many collimation devices available. Your eyepiece and the North Star are the greatest. The Big Dipper is the simplest method to locate the North Star. Draw an imagined line across the bowl of the Big Dipper’s two terminal stars. Polaris is the first star you come to along this route. It is better if your telescope is not polar oriented for this purpose; instead, aim the mount head directly east or west. This is due to a blind zone surrounding the pole on German Equatorial Mounts. Also, if you have a motor drive connected to the mount, turn it off. To obtain Polaris, use your lowest power (biggest number eyepiece) and center it using your slow motion settings. Now, while keeping the picture centered, switch to your next higher-power eyepiece. The in-focus star picture will have a dazzling central point, a somewhat fainter inner ring, and a barely visible outer ring. If it doesn’t look like this, or you can’t get it to focus, do this: pull out your star diagonal and stare at the picture slightly out of focus to evaluate the deflection. When you bring the focus out, a typical off-collimation picture will have a bright spot off to one side. The true procedure is to slightly loosen the pair on the deflection side, slacken the Allen head screws, and then tighten the Phillips head screws against them. After bringing the star picture to the center of the eyepiece, double-check it. If your picture is growing worse, switch directions or loosen the other two Allen screws slightly. You’re done after you get a circular star image. It is beneficial to have a companion assist in collimation. While you gaze through the eyepiece, have your spouse adjust the screws according to your instructions.


How do I collimate my Newtonian telescope?

a. Proper collimation A correctly collimated (aligned) mirror system provides the clearest possible pictures. When the main and diagonal mirrors are slanted, the focused image falls directly into the center of the focuser drawtube. These mirror tilt changes are accomplished using the diagonal assembly and main mirror cell and will be covered in more detail later. Look down the focuser drawtube with the eyepiece removed to evaluate the mirror collimation. The reflections of the main mirror, diagonal mirror, spider vanes, and your eye will be framed by the edge of the focuser drawtube. All of these reflections will seem concentric if properly aligned (i.e. centered). Any departure from the concentric reflections necessitates changes to the diagonal assembly and/or the main mirror cell. a. Spider vane alterations If the diagonal mirror is off-center inside the drawtube, release the spider vane adjustment/lock knobs on the outer surface of the main tube and slide the whole diagonal assembly up or down the tube along the slotted holes until the diagonal mirror is centered. The thread is one of the spider vane adjustment/lock knobs while unthreading the other if the diagonal mirror is above or below the center inside the drawtube. Make just two knob adjustments at a time until the diagonal mirror is in the drawtube. When the spider vane is in the proper position. b. Diagonal holder modifications If the diagonal mirror is centered in the drawtube but the primary mirror is only partially visible in the reflection, unthread the three diagonal tilt screws slightly so that you can rotate the diagonal holder from side to side by grasping the diagonal holder hand and rotating until the primary mirror is as centered in the diagonal mirror’s reflection as possible. When you’ve found the optimal position, screw in the three diagonal tilt screws to secure the rotating position. Then, if required, make adjustments to these three screws to modify the diagonal mirror’s tilt angle until the full main mirror is seen centered inside the diagonal mirror reflection. c. Adjustments to the primary mirror If the diagonal mirror and its reflection look centered inside the drawtube, yet the reflection of your eye and its reflection appear off-center, the main mirror tilt screws of the primary mirror cell must be adjusted. These principal tilt screws are placed at the bottom end of the main tube, behind the primary mirror. To adjust the main mirror tilt screws, remove the three hex-head primary mirror cell locking screws located adjacent to each primary mirror tilt screw several twists. Turn the principal mirror tilt screws by trial and error until you gain a feel for which direction to turn each screw to center your eye’s reflection. Once centered, relock the tilt-angle adjustment by turning the three hex-head main mirror cell locking screws.


How do I center the secondary mirror?

A simple sight tube may be used to center the secondary mirror. A sight tube is a simple tube that fits tightly into your focuser. I use a 1.25″ drain pipe extension that can be found at any hardware shop. Insert the sight tube into the focuser until the secondary mirror is visible with a tiny space all around. Take care not to bump against the mirror. Your secondary is centered if the mirror lines in the tube are parallel. If not, release the spider vanes, center the mirror, and tighten it again. You must now adjust the main mirror.


How do I clean my primary mirror?

Cleaning the telescope’s mirrors should be done just once every other year or less often. When not in use, cover the telescope with the dust cover to prevent dust from building on the mirrors. Mirror coatings may be scratched by improper cleaning, therefore the fewer times you have to clean the mirrors, the better. Small bits of dust or flecks of paint have almost little influence on the telescope’s visual performance. Your telescope’s big main mirror and elliptical secondary mirror are aluminized on the front surface and over-coated with strong silicon dioxide to prevent oxidation. These coatings often endure for many years before needing to be re-coated. Remove the secondary mirror from the telescope first. Hold the secondary mirror holder still while twisting the central Phillips-head screw. There is a spring between the secondary mirror holder and the Phillips head screw, so proceed with caution. Make certain that it does not fall into the optical tube and strike the main mirror. Handle the mirror using its holder; avoid touching the mirror’s surface. Then, for cleaning the main mirror, repeat the steps outlined below. Remove the mirror cell from the telescope and the mirror from the mirror cell gently to clean the main mirror. Do not use your fingertips to contact the mirror’s surface. Carefully lift the mirror by the edges. Place the mirror, face up, on top of a clean soft cloth. Fill a clean sink with room-temperature water, a few drops of mild liquid dishwashing detergent, and, if available, a capful of rubbing alcohol. Submerge the mirror (aluminized face up) in the water for a few minutes (or hours if it’s really unclean). Wipe the mirror with clean cotton balls, using little pressure and stroking in a straight line across the surface. For each wipe of the mirror, use one ball. The mirror should then be rinsed with lukewarm water. Before drying, tilt the mirror to a 45o angle and pour distilled water over it. This will keep any dissolved particulates from tap water from lingering on the mirror. Any particles on the surface may be gently swabbed with a succession of cotton balls, each used just once. Dry the mirror with a spray of air (a “blower bulb” works well) or with the corner of a paper towel to remove any stray drips of water. A clean surface will repel water. Cover the mirror surface with Kleenex and place it in a warm place to dry before returning it to the mirror cell and telescope.


How do I perform a star test?

After you have completed the collimation, you should verify the alignment accuracy on a star. Point the telescope’s 25mm eyepiece onto a reasonably bright (second or third magnitude) star, then center the star picture in the telescope’s field of view. With the star in the middle, use the following method: Slowly blur the star picture until one or more rings are evident around the core disc. If the collimation was done properly, the central star disk and rings should be concentric circles, with a black point dead center inside the out-of-focus star disk (this is the shadow of the secondary mirror), A telescope that is not correctly oriented will exhibit elongated circles with an off-center black shadow. If the out-of-focus star disk seems extended, the main mirror must be adjusted. Move the telescope until the star image is at the edge of the eyepiece’s field of vision. The out-of-focus star disk picture will shift across the eyepiece field as you make changes to the main mirror tilt screws. Select one of the three main mirror tilt screws to center the star disk picture in the eyepiece field. Rep this procedure as needed until the out-of-focus star disk shows as concentric circles when the star disk picture is in the middle of the eyepiece field. The star testing patterns are shown below. It is critical to remember that your scope has cooled down and that the viewing conditions are excellent, otherwise, you may get a misleading result.


How do I polar align a telescope?

Release the Azimuth lock on the Azimuth base, allowing the complete telescope-with-mounting to be turned horizontally. Turn the telescope until the polar axis is pointing directly north. As a precise reference for due North, use a compass or identify Polaris, the North Star. If required, adjust the heights of the three tripod legs to level the mount. Check a road map or atlas to determine the latitude of your viewing point. Release the latitude lock and tilt the telescope mount using the latitude adjustment knob until the pointer on the latitude scale reflects the right latitude of your viewing position. Tighten the latitude lock again. If steps (1) through (3) were completed accurately, your telescope is now well-aligned to the North Celestial Pole for visual observations. Once the mount has been polar-aligned, the latitude angle does not need to be altered again until you relocate to a new geographical area (i.e. a different latitude). The sole polar alignment process that must be performed each time the telescope is used is to orient the polar axis due North, as indicated in step (1) above.


How do I balance an equatorially mounted telescope?

To move smoothly on its mechanical axes, the telescope must first be balanced around the two telescope axes: the polar axis and the Declination axis. All polar-aligned telescope movements occur by moving along these two axes, either independently or concurrently. Follow the steps below to fine-tune the telescope’s balance: Turn the telescope so that the counterweight shaft is parallel to the ground and loosen the R.A. lock (horizontal). Slide the counterweight down the counterweight shaft until the telescope does not tend to slip down in either direction. Then, tighten the counterweight lock knob to secure the counterweight in place. Close the R.A. lock and open the Declination lock. The telescope may now freely rotate around the Declination axis. Loosen the cradle ring lock knobs so that the main tube in the cradle rings moves up and down freely. Place the main tube in the cradle rings until it is rotationally balanced around the Declination axis. Re-secure the knobs. Both axis of the telescope is now correctly balanced.


How do I align the finderscope?

Take your telescope outside in the daytime and aim it toward a distant object (NOT the Sun, never stare at the Sun). It has the potential to permanently blind you.) In the eyepiece, center the item. Adjust the finderscope to center the crosshairs on the item. To fine-tune your scope, go outside after dark and aim it at any brilliant star. Begin by using your lowest power eyepiece. Repeat the first operation, then go through your eyepieces until you reach your maximum power eyepiece. The finderscope and telescope should be exactly aligned.


What is collimation?

The alignment of the main mirror to the secondary mirror is referred to as collimating a telescope. This guarantees that as much light as possible is collected by the telescope. Giving you the greatest picture possible from your telescope.

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