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Frequently Asked Questions

We have compliled a list of questions we are often asked by amature astronomers interested in purchasing a telescope.

  1. How do I know the maximum power at which I can use my telescope?

    The theoretical power of any telescopes is 50-60 x per inch of aperture. However, under average sky conditions, the practical power recommended to be used is 20-30 x per inch of aperture. To determine the magnification of your telescope with a certain eyepiece, divide the focal length of the telescope by the focal length of the eyepiece used. You can then get the magnification per inch of aperture by dividing the result by the aperture and compare it with the recommended range.

  2. Is it a good idea to purchase a telescope from a department store?

    Usually not. The telescopes generally sold at department stores have unrealistic focal ratios, shaky mounts and small apertures. You will usually see them only accepting .96" eyepieces. Stay away from telescopes that advertise power (usually department store scopes) as it is not a relative parameter to go by when purchasing a telescope. Finally, think of the knowledge level of the staff who is selling you this telescope, based on its power. Thatís why The Sky Plus customers value the personalized attention and expert advice they receive from us when making a purchase.

  3. I have a go-to 3" telescope with the focal ratio of f/14. Why canít I see all 14,000 objects in the database?

    Many of the objects listed in the database are deep sky objects which are rather dim and require a larger aperture (at least 5") to be observed. In addition, the focal ratio of f/14 is not conducive for viewing at low magnification which is the appropriate magnification for observing dim objects.

  4. What is focal length?

    Focal length of a telescope is the distance from the primary mirror or objective to the eyepiece.

  5. How are eyepieces classified by their magnifications?

    The higher the number (focal length) of the eyepiece, the lower its magnification is. Similarly, the lower the number of the eyepiece, the higher its magnification is. For example, a 6mm eyepiece has higher magnification than a 32mm eyepiece.

  6. What does apparent field of a telescope refer to?

    The apparent field of view is the angular diameter (in degrees) of the circle of light that the eye sees. All Celestron Plossl eyepieces have excellent apparent fields (46 to 52 degrees). Meade eyepieces such as the Wide Angle, Super Wide Angle and Ultra Wide Angle eyepieces also have excellent apparent fields that range from 67 degrees to 84 degrees.

  7. What is the difference between SMA and Plossl eyepieces in terms of performance?

    SMA eyepieces provide high quality images at medium to high magnification. At low magnification, the image around the edge of the apparent field tends to be slightly elongated. With an apparent field of 52 degrees and eye relief of 4mm to 14 mm, Celestron SMAs are great for planetary viewing. Since planetary viewing does not require the same sharpness out to the edge as does deep sky viewing, the SMA eyepieces are great low-cost solution for planet lovers. Plossl eyepieces, on the other hand, provide excellent sharp image out to the edge of the apparent field. Most Meade Super Plossl eyepieces have apparent field of view of 52 degrees and exhibit high contrast throughout the field. Plossls are ideal for observing any object but they tend to be slightly more expensive than SMAs. Plossl is the best choice for viewing at lower power.

  8. Can I use my 40 mm Plossl eyepiece for a low magnification eyepiece on my 6" Newtonian?

    It is not recommended to use any eyepiece lower than 32mm on telescopes with aperture of smaller than 8". If 40mm is used as a low magnification eyepiece for a 6" telescope, you are risking seeing the secondary mirror. So for a good low magnification view on a 6" Newtonian you should use a 32mm eyepiece.

  9. I do not have much time to spend viewing. Should I buy a computerized telescope?

    Computerized telescopes make it much easier to find objects with the telescope. However, you still need a certain knowledge of the sky in order to orient yourself between the guide stars (required input for operation of the computer). We feel that learning the sky to a point is the first step towards appreciation of astronomy.

  10. What role does focal ratio play in observing at different magnifications?

    Telescopes with high focal ratios produce great planetary images as they "maximize" magnification of even the most powerful eyepieces. However, you need to be careful not to magnify the object beyond the realistic range for the sky conditions since magnification decreases surface brightness. While telescopes with high focal ratios are primarily planetary telescopes, those with lower focal ratios allow us to view deep sky objects better. These ratios "minimize" magnification of even the least powerful eyepieces and, with sufficient aperture can produce bright, wide images of deep sky objects.

Please remember to review our advice on selecting a telescope!

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