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Orion Telescopes

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Beginner
*2nd* Orion Apex 90mm Maksutov-Cassegrain Telescope
This product is a 'Second'. Learn more
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  • Nifty little 90mm (3.5") aperture Maksutov-Cassegrain telescope provides long focal length performance in a conveniently compact instrument
  • 90mm aperture and 1250mm focal length Maksutov-Cassegrain optics for fantastic celestial and terrestrial views, especially at higher powers
  • An ideal telescope for scanning the solar system for views of the Moon and planets, as well as globular star clusters and some bright nebulas
  • Includes a 25mm Sirius Plossl eyepiece, 6x20 correct-image finder scope for accurate aiming during the day or at night, 45-degree correct-image diagonal, soft carry-case, and Starry Night astronomy software
  • Does not include tripod or mount - either can be purchased separately


Item #  79820
  • Best for viewing
    Lunar & planetary
  • Best for imaging
    Lunar & planetary
  • User level
    Beginner
  • Optical design
    Maksutov-Cassegrain
  • Finder scope magnification
    6x
  • Optical diameter
    90mm
  • Finder scope lens diameter
    20mm
  • Focal length
    1250mm
  • Focal ratio
    f/13.9
  • Eyepieces
    Sirius Plossl 25.0mm (1.25")
  • Magnification with included eyepieces
    50x
  • Resolving power
    1.29arc*sec
  • Lowest useful magnification
    13x
  • Highest useful magnification
    180x
  • Highest theoretical magnification
    180x
  • Limiting stellar magnitude
    12.5
  • Optical quality
    Diffraction limited
  • Finder scope
    6x20 Correct-image
  • Focuser
    Internal
  • Diagonal degrees
    1.25" 45° Prism Correct-image
  • Secondary mirror obstruction
    29mm
  • Secondary mirror obstruction by diameter
    32%
  • Secondary mirror obstruction by area
    10%
  • Mirror coatings/over-coatings
    Aluminum & Silicon Dioxide
  • Mount type
    Optical Tube without Mount
  • Astro-imaging capability
    Lunar & planetary
  • Tube material
    Aluminum
  • Length of optical tube
    10.0 in.
  • Weight, optical tube
    3.7 lbs.
  • Weight, fully assembled
    3.6 lbs.
  • Additional included accessories
    Other included items are not always the same between a new and a second. Seconds may be older versions of the new product, or may be customer returns where accessory substitutions have been made.
  • Other features
    2nds are covered by Orion's Satisfaction Guarantee
  • Warranty
    One year

Orders received by noon Pacific Time for in-stock items ship the same business day. Orders received after noon will ship the next business day. When an item is not in-stock we will ship it as soon as it becomes available. Typically in-stock items will ship first and backordered items will follow as soon as they are available. You have the option in check out to request that your order ship complete, if you'd prefer.

A per-item shipping charge (in addition to the standard shipping and handling charge) applies to this product due to its size and weight. This charge varies based on the shipping method.

Standard Delivery: 0.00
3 Day Delivery: 5.00
2 Day Delivery: 5.00
Next Day Delivery: $10.00

Can I collimate a Schmidt-Cassegrain with a Collimating eyepiece?
Yes. If you use a star diagonal with your telescope, as is common for Schmidt-Cassegrain telescopes, remove it before inserting the Collimating Eyepiece. The eyepiece should be inserted to a depth such that the bottom edge of the eyepiece tube appears slightly wider than the outer edge of the secondary mirror when you look through the sight hole. Tighten the thumbscrew on the focuser tube to secure the Collimating Eyepiece in place. Now, you are ready to proceed with collimation.

Collimation is best performed using a “star test,” but reasonable collimation can be achieved with the collimating Eyepiece. There is only one collimation adjustment for Schmidt-Cassegrains: the tilt of the secondary mirror. Insert the Collimating Eyepiece directly into the visual back of the telescope. The shadow of the secondary mirror will appear as a dark circle near the middle of the field-of-view. Adjust the three Allen-head screws located in the center of the front corrector plate to center the secondary mirror on the crosshairs. Do not loosen the screws more than two turns, or the secondary mirror could fall off its mount! Even a 1/10th turn will generate a lot of movement. Likewise, do not adjust the screw in the middle of the secondary mirror cell. It holds the mirror in place. Final fine-tuning of collimation on reflectors or Schmidt-Cassegrains can be done with a “star test” using a high-power eyepiece. The collimating eyepiece is a preferred tool for Schmidt-Cassegrain collimation.

Can the Laser Collimator be used on Schmidt-Cassegrains and Maksutov Cassegrains?
No. The Laser Collimator doesn’t reflect off the primary, so it won’t work to collimate the primary for a Schmidt-Cass or Mak-Cass. It can be used to collimate the secondary mirror for a Schmidt-Cass.

How do I align a finder scope?
Before you use the finder scope, it must be precisely aligned with the telescope so they both point to exactly the same spot. Alignment is easiest to do in daylight, rather than at night under the stars. First, insert a low power telescope eyepiece (a 25mm eyepiece will work great) into the telescope’s focuser. Then point the telescope at a discrete object such as the top of a telephone pole or a street sign that is at least a quarter-mile away. Position the telescope so the target object appears in the very center of the field of view when you look into the eyepiece. Now look through the finder scope. Is the object centered on the finder scope’s crosshairs? If not, hopefully it will be visible somewhere in the field of view, so only small turns of the finder scope bracket’s alignment thumb screws will be needed. Otherwise you’ll have to make larger turns to the alignment thumb screws to redirect the aim of the finder scope. Use the alignment thumb screws to center the object on the crosshairs of the finder scope. Then look again into the telescope’s eyepiece and see if it is still centered there too. If it isn’t, repeat the entire process, making sure not to move the telescope while adjusting the alignment of the finder scope. Finder scopes can come out of alignment during transport or when removed from the telescope, so check its alignment before each observing session.

Can the finder scope crosshairs be adjusted?
Yes, but before taking this on, regardless of the orientation, the intersection of the crosshairs marks the center and that’s what important. However, should you feel the need to change the orientation of the finder scope’s crosshairs; you can do so by carefully rotating the finder scope in its bracket. Loosen the adjustment screws or pull on the tensioner (depending on the model) and rotate the finder scope tube in the bracket until the crosshairs are oriented the way you want. You should not need to rotate the finder scope tube more than 1/4 of a turn. .

For right-angle finder scopes, unthread the eyepiece to re-orient the crosshairs; gently turn the eyepiece until the crosshairs are oriented as you wish. You should not need to rotate the eyepiece more than 1/4 of a turn to do this. This may leave you with a loose eyepiece. If so, you can add an o-ring or shim to tighten it at the new orientation.

How do I calculate the magnification (power) of a telescope?
To calculate the magnification, or power, of a telescope with an eyepiece, simply divide the focal length of the telescope by the focal length of the eyepiece. Magnification = telescope focal length ÷ eyepiece focal length. For example, the Orion Apex 90mm Maksutov Telescope, which has a focal length of 1250mm, used in combination with the supplied 25mm eyepiece, yields a power of: 1250 ÷ 25 = 50x.
It is desirable to have a range of telescope eyepieces of different focal lengths to allow viewing over a range of magnifications. It is not uncommon for an observer to own five or more eyepieces. Orion offers many different eyepieces of varying focal lengths.

Every telescope has a theoretical limit of power of about 50x per inch of aperture (i.e. 180x for the Orion Apex 90mm). Atmospheric conditions will limit the usefullness of magnification and cause views to become blurred. Claims of higher power by some telescope manufacturers are a misleading advertising gimmick and should be dismissed. Keep in mind that at higher powers, an image will always be dimmer and less sharp (this is a fundamental law of optics). With every doubling of magnification you lose half the image brightness and three-fourths of the image sharpness. The steadiness of the air (the “seeing”) can also limit how much magnification an image can tolerate. Always start viewing with your lowest-power (longest focal length) eyepiece in the telescope. It’s best to begin observing with the lowest-power eyepiece, because it will typically provide the widest true field of view, which will make finding and centering objects much easier After you have located and centered an object, you can try switching to a higher-power eyepiece to ferret out more detail, if atmospheric conditions permit. If the image you see is not crisp and steady, reduce the magnification by switching to a longer focal length eyepiece. As a general rule, a small but well-resolved image will show more detail and provide a more enjoyable view than a dim and fuzzy, over-magnified image.

What are practical focal lengths to have for eyepieces for my telescope?
To determine what telescope eyepieces you need to get powers in a particular range with your telescope, see our Learning Center article: How to choose Telescope Eyepieces

Why do Orion telescopes have less power than the telescope at department stores?

Advertising claims for high magnification of 400X, 600X, etc., are very misleading. The practical limit is 50X per inch of aperture, or 120X for a typical 60mm telescope. Higher powers are useless, and serve only to fool the unwary into thinking that magnification is somehow related to quality of performance. It is not.

What is the best telescope for a beginner?
The “best scope” for anyone is highly subjective and varies based on the person who will be using the telescope. Their level of interest in the hobby, their aptitude for “the technical”, the level of investment that you want to make, and the ability to carry differing weights. For more detailed information on this topic see our Learning Center article: How to Choose a Telescope

How do I see the best detail on the surface of the Moon?
The Moon, with its rocky, cratered surface, is one of the easiest and most interesting subjects to observe with your telescope. The myriad craters, rilles, and jagged mountain formations offer endless fascination. The best time to observe the Moon is during a partial phase, that is, when the Moon is not full. During partial phases, shadows cast by crater walls and mountain peaks along the border between the dark and light portions of the lunar disk highlight the surface relief. A full Moon is too bright and devoid of surface shadows to yield a pleasing view. Try using an Orion Moon filter to dim the Moon when it is too bright; it simply threads onto the bottom of the eyepiece, you’ll see much more detail.

What will the planets look like through the telescope?
The planets don’t stay put like stars do (they don’t have fixed R.A. and Dec. coordinates), so you will need to refer to the Orion Star Chart on our website. Venus, Mars, Jupiter, and Saturn are among the brightest objects in the sky after the Sun and the Moon. All four of these planets are not normally visible in the sky at one time, but chances are one or two of them will be.

JUPITER: The largest planetJupiter, is a great subject to observe. You can see the disk of the giant planet and watch the ever-changing positions of its four largest moons, Io, Callisto, Europa, and Ganymede. If atmospheric conditions are good, you may be able to resolve thin cloud bands on the planet’s disk.

SATURN: The ringed planet is a breathtaking sight when it is well positioned. The tilt angle of the rings varies over a period of many years; sometimes they are seen edge-on, while at other times they are broadside and look like giant “ears” on each side of Saturn’s disk. A steady atmosphere (good seeing) is necessary for a good view. You may probably see a tiny, bright “star” close by; that’s Saturn’s brightest moon, Titan.

VENUS: At its brightest, Venus is the most luminous object in the sky, excluding the Sun and the Moon. It is so bright that sometimes it is visible to the naked eye during full daylight! Ironically, Venus appears as a thin crescent, not a full disk, when at its peak brightness. Because it is so close to the Sun, it never wanders too far from the morning or evening horizon. No surface markings can be seen on Venus, which is always shrouded in dense clouds. Sometimes using a color filter will lessen the glare of Venus and help you see the crescent.

MARS: If atmospheric conditions are good, you may be able to see some subtle surface detail on the Red Planet, possibly even the polar ice cap. Mars makes a close approach to Earth every two years; during those approaches its disk is larger and thus more favorable for viewing. For more detailed information on this topic see our Learning Center article: What Will You See Through a Telescope

What will a star look like through a telescope?
Stars will appear like twinkling points of light in the telescope. Even the largest telescopes cannot magnify stars to appear as anything more than points of light. You can, however, enjoy the different colors of the stars and locate many pretty double and multiple stars. The famous “Double-Double” in the constellation Lyra and the gorgeous two-color double star Albireo in Cygnus are favorites. Defocusing the image of a star slightly can help bring out its color. For more detailed information on this topic see our Learning Center article: Stars and Deep Sky Objects

What eyepiece should I use for terrestrial viewing?
For land viewing, it’s best to stick with low power eyepieces that yield a magnification under 100x. At higher powers, images rapidly lose sharpness and clarity due to “heat waves” caused by Sun-heated air. Remember to aim well clear of the Sun, unless the front of the telescope is fitted with a professionally made solar filter and the finder scope is removed or covered with foil or some other completely opaque material. Many Orion telescopes are capable of focusing on objects that are quite close, so you can view fine details of objects that are nearby. Try focusing on a flower or insect at close distance to enter a normally unseen microscopic world. Check the specifications on the product web page or instruction manual for your Orion scope.

Observing hint: If the object is too close to focus. You may be able to use an extension tube that allows the eyepiece to move further back as you focus closer. Try lifting the eyepiece out of the holder as you look. If it focuses in about an inch or two, you can purchase an eyepiece extension tube. For more detailed information on this topic see our Learning Center article: Choosing Eyepieces

How do I clean any of the optical lenses?
Any quality optical lens cleaning tissue and optical lens cleaning fluid specifically designed for multi-coated optics can be used to clean the exposed lenses of your eyepieces or finder scope. Never use regular glass cleaner or cleaning fluid designed for eyeglasses. Before cleaning with fluid and tissue, blow any loose particles off the lens with a blower bulb or compressed air. Then apply some cleaning fluid to a tissue, never directly on the optics. Wipe the lens gently in a circular motion, then remove any excess fluid with a fresh lens tissue. Oily finger-prints and smudges may be removed using this method. Use caution; rubbing too hard may scratch the lens. On larger lenses, clean only a small area at a time, using a fresh lens tissue on each area. Never reuse tissues.

How do I Clean a Mak-Cass Lens?
Any quality optical lens cleaning tissue and optical lens cleaning fluid specifically designed for multi-coated optics can be used to clean the Apex’s front meniscus lens or exposed lenses of your eyepieces or finder scope. Never use regular glass cleaner or cleaning fluid designed for eyeglasses. Before cleaning with fluid and tissue, however, blow any loose particles off the lens with a blower bulb or compressed air. Then apply some cleaning fluid to a tissue, never directly on the optics. Wipe the lens gently in a circular motion, then remove any excess fluid with a fresh lens tissue. Oily fingerprints and smudges may be removed using this method. Use caution; rubbing too hard may scratch the lens. For the large surface of the meniscus lens, clean only a small area at a time, using a fresh lens tissue on each area. Never reuse tissues.

How do I focus my Mak-Cass?
Point the telescope so the front end is aimed in the general direction of an object you wish to view. When you first look in the eyepiece, the image you see may be fuzzy, or out of focus. If so, gently turn the focus knob with your fingers until the image becomes sharp. Go a little bit beyond sharp focus until the image just starts to blur again, then reverse the rotation of the knob, just to make sure you’ve hit the exact focus point. You will have to readjust the focus when aiming at subjects of varying distances for daytime viewing, or after changing eyepieces. If you have trouble focusing, rotate the focus knob counter-clockwise as far as it will go. Now look through the eyepiece while slowly rotating the focus knob clockwise. You should soon see the point at which focus is reached.

Hint: Telescopes with long focal lengths and lots of focus travel can be challenging to focus. Turn the focus knob gently and allow the turn to settle before adjusting further.

How do I do astro photography with an Apex Scope?
When coupled to a SLR camera, the Apex becomes a telephoto lens. For terrestrial or astronomical photography, you need only a T-ring for your specific camera model. The T-ring attaches to your camera and threads onto the Apex’s eyepiece adapter. Use the camera’s viewfinder to frame the picture. Use the telescope’s focuser to focus the image. You may want to consider using a remote shutter release instead of the shutter release on the camera; touching the camera can vibrate the system and blur the resulting photographic image on the film. Also, be sure to use a solid tripod. If you would like to change the orientation of the camera relative to the telescope, do so by first loosening the knurled ring located in front of the eyepiece adapter You can then rotate the camera (and eyepiece adapter) to the desired orientation. Retighten the knurled ring when done. Add a Universal Camera Adapter and you can shoot through the eyepiece for a magnified picture of planets.

Are there optical performance differences between the Apex and StarMax models?
The optical tube of the Apex and StarMax are identical. The Apex is optimized for daytime terrestrial use and the StarMax for astronomical viewing due to the Equatorial mount included with the StarMax models.

I recently purchased a solar filter for my telescope and can’t see anything with it. Any suggestions?
One of the problems with a solar filter on a telescope is that it’s a bit tricky to aim it at the sun. You can’t look through the finder to point the scope or you’ll cause injury to your eye. So, cap off or remove the finder. Also, because with the very dark filter on the front if the sun is slightly outside the field of view of the eyepiece you’ll see pitch blackness in the field. With the solar filter properly mounted, try looking at the shadow of the optical tube on the ground, move the tube until the shadow is at a minimum. You’ll be pointed at the sun, or at least close enough to find it with a little sweeping and a low-power eyepiece to bring it into view. It can be difficult, even with the shadow method. An other trick to try after you’ve got it close with the shadow if your still not having any luck getting the sun in the field...take the eyepiece out of the focuser. Then look into the focuser...you won’t see an image but when the sun gets close you’ll see a flicker of brightness coming through the mirrors. Then pop the eyepiece back in and you should have it.

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