FAQ about Telescopes

Frequently Asked Questions FAQ about telescopes, a technical note prepared by NIPON SCOPE & OPTICS


1. How to calculate magnification (or power) of a Nipon scope?

2. What is the maximum magnification power a telescope or spotting scope can achieve?

3. How to decide the smallest eyepiece a scope can use?

4. How much magnification do I really need to get a good image?

5. How to calculate the field of view of your scope?

6. What’s the difference between angled and straight eyepieces?

7. Should I go for zoom or fixed power scopes?

8. Why an image seen through a Finder Scope is often upside-down?

9. How to align the finder scope with the main scope?

10. What are the Bak4 and Bk-7 prisms? Is there a difference when they are used in scopes and in binoculars?

11. What is a Barlow Lens?

12. What are advantages and disadvantages of using a Barlow lens?

13. A recent communication about Nipon 700×60 telescope and 20-60×70 spotting scope

14. I have got the Nipon 350×70 telescope. All is fine, superb telescope, but the one thing that I can’t work out is how the finder scope fits on the main body of the telescope. Please could you advise?

15. I shoot old military guns at distances up to 1000 metres and need to see the bullet holes in the paper targets. Will the Nipon 350×70 scope allow me to do this nice and clearly?

16. Recent user feedback about Nipon 350×70 refractor scope for Archery

17. Could you please recommend a better telescope for looking at stars, the moon etc.?

1. How to calculate magnification or power of a telescope?

(1). Identify the focal length of the scope. This is often marked on the body of the scope or it should be given in the user manual. For example, the Nipon trophy MC800x80 scope’s focal length is 800mm.

(2). Find the focal length of the eyepiece. For example, for the PL16, PL26 and PL32 eyepieces, their focal lengths are 16mm, 26mm and 32mm, respectively.

(3). Divide the focal length of the scope by the focal length of the eyepiece to get the magnification of the scope. For example, if you wish to use a 16mm eyepiece on the 800×80 scope model, the magnification of the scope will become: 800/16=50x.

It is desirable to have a range of eyepieces with different focal lengths to allow viewing over a range of magnification levels.

Please keep in mind that, as a fundamental law of optics, at higher magnification powers an image will always become dimmer and less sharp. With every doubling of magnification you lose about half the image brightness and 3/4 of the image sharpness.

Therefore, it is best to begin viewing with the lowest power eyepiece (with longest focal length, eg. 32mm of the above example) or with the zoom eyepiece being adjusted to its lowest power level. This will provide the widest true field of view which will make target finding and centring much easier. After you have located an object, you can switch to a higher power eyepiece (with smaller focal length) to see 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, or for a zoom eyepiece, zoom out. In general, a small but well-resolved image will show more detail and provide a more enjoyable view than a dim and fuzzy, over-magnified image.

Please refer to “How to calculate the field of view” for more related information.

2. What is the maximum magnification power a telescope or spotting scope can achieve?

As described above, the higher the magnification level, the dimmer an image becomes. There is thus a golden rule for the maximum usable magnification (Mmax) of any scope: Mmax=2 x D (mm)

Where: D is objective lens diameter measured in mm.

For example, for the Nipon trophy MC800x80 compact zoom scope, its objective lens is 80mm, so the maximum usable magnification power of this scope is 2×80=160 times.

3. How to decide the smallest eyepiece a telescope can use?

Based on the Mmax value as calculated above, you can decide the minimum eyepiece focal length that you may wish to use for a scope. The equation is:
f = F / Mmax

Where: f is the minimum eyepiece focal length (mm); F is the focal length of the scope; Mmax is the maximum usable magnification of the scope.

For the Nipon trophy MC800x80 scope, for example, the minimum eyepiece which can be used is: f=800/160=5 mm. This would provide about 160x power which is nice for viewing Mars or resolving the rings of Saturn. To resolve Saturn’s rings, you may only need about 30x power, but to see them more clearly, higher magnifications are required.

4. How much magnification do I really need to get a good image?

If the visual condition is good (eg. on nights when the sky is clear and stable), the top usable magnification for a 60mm (2.4 inch objective lens) scope will be around 2×60=120x. This is more than enough to see the rings of Saturn, cloud belts on Jupiter and many star clusters and nebulae. So, 60mm scopes have been the standard size for a wide range of applications. For example the Nipon 15-46×60 Spotting Scope.

With a 70-80mm scope, however, the amount of light it gathers is about 55-65% more than a 60mm scope. This improves image quality especially when viewing at high powers and under low light. A selection of these models includes Nipon 25-75×70 spotting scope, 350×70 refractor scope, and 800×80 compact scope. In general, larger scopes outperform smaller ones, but they also tend to be bigger and heavier, and more expensive.

5. How to calculate the field of view of your telescope?

(1). Find out the value of the apparent field of view of the eyepiece. Every eyepiece has its own value of the apparent field of view and this value is supplied from the manufacturer. For the optical eyepieces PL16, PL26 & PL32 as mentioned in the above example, their apparent field of view is 52 degrees.

(2). Find the value of magnification. This is calculated by dividing the focal length of your scope by the focal length of the eyepiece. For the Nipon 800×80 scope (focal length=800mm) with a 16mm eyepiece, for example, its magnification will be: 800/16=50.

(3). Divide the value of the apparent field of view by the value of magnification. For the above example, 52/50=1.04 degrees. This is the value of the field of view for this scope using this eyepiece.

6. What’s the difference between angled and straight eyepieces?

The Nipon 12-36×50 scope comes in either straight (0 degree, NIPON12-36x50B) or 45 degree angled (NIPON12-36×50) eyepiece designs. Straight scopes used to be the norm of these scopes, but angled scopes seem to have taken over for a majority of users nowadays.

With a straight eyepiece design, you can view a target in line with the central line of the scope. It is thus convenient to locate and track a target, especially when the target or the viewer is moving.

Angled scopes can better accommodate people of different heights, and they seem to be easier to use for digi-scoping (eg. to mount a digital camera). Nevertheless, either option is a personal choice, and both options are available from this store.

7. Should I go for zoom or fixed power scopes?

Most Nipon scopes are available with zoom or fixed power eyepieces which can vary from 9x to 80x. Scopes with zoom eyepieces have become popular in recent years especially for many bird watching. It’s easier to find your object at low power (which gives a wider field of view) and then zoom in to see more details. This gives you much convenience and saves you trouble changing eyepieces for different magnifications.

However, it should be noted that even though zooms are optically very sharp, they have a relatively narrower field of view as compared with fixed power eyepieces at the same magnification level. Looking through a scope with a fixed wide angle eyepiece (eg. 32mm) is a joy with a bright and wide visual field! If you put two scopes together, one with a 20-60 zoom set at 30x power, the other with a 30x fixed power eyepiece, the difference is striking. You can actually get a wider field of view in the 30x fixed power eyepiece than you will with the zoom eyepiece even when the zoom is set at 20x.

In addition, at the higher end of the zoom power, the image tends to become fuzzy or blurred. This is normal because the amount of light which comes through the objective lens remains the same regardless of the zoom levels. As the image is magnified, the amount of information that contained within the same area of the image is reduced. Therefore, to get a good quality image, either zoom out, or use a fixed power eyepiece.

That’s why we have provided a set of fixed power eyepieces as optional accessories in our store, with 16mm, 26mm and 32mm focal lengths. They can add greater values to your scope.

8. Why an image seen through a Finder Scope is often upside down?

A “finder scope” is included in some telescope packages, such as the NIPON 350×70 and NIPON 700×60 telescopes. A finder scope is a small telescope that is attached to the main telescope. Its purpose is to aid in aiming the main telescope toward objects of interests such as a particular star. A finder scope is built with low magnification (eg. 2x, 3x, 5x etc.) but with a wide field of view (5 degrees or more), thus allowing you to see more of the sky than you can through the main telescope. Therefore, the finder scope enables you to locate a star more easily and centre it on the crosshairs, you can then view more details through the main telescope.

Beginners are often surprised that the image in a standard finder scope is upside-down when viewing objects on earth. That’s normal for any refractor used without a correction prism. For most astronomical observation, it makes little difference if an object is seen upside-down or at an otherwise odd angle because there is no “right side up” in space, since all you are trying to do is to centre the object on the crosshairs so that you can view it through the main telescope.

A relatively new type of finder scope is known as a reflex sight, or “Red dot finder scope”. It is a non-magnifying device that displays a red, LED-lit bull’s-eye pattern or red dot in the center of the field of view. The red dot appears superimposed on the sky, showing exactly where the telescope is pointed (once the finder scope and the main telescope are properly aligned, of course).

9. How to align the finder scope with the main telescope?

The finder scope only works to its purchase when it has been aligned with the main scope, so the two are aiming at exactly the same spot. This is easy to do.

It is more convenient to align the finder scope while it’s still light outside. A good time to do it is after the telescope has been set up but before the Sunset.

First, put a low-power eyepiece (with long focal length) in the eyepiece holder (focuser) of the main telescope. Looking into the eyepiece, centre a distant object in the field of view – the top of a telephone pole, a treetop, or a chimney on a house. The object should be at least a quarter-mile away. Now look through the finder scope and see if the object appears in the centre of the finder’s field of view (where the crosshairs intersect). If it does not, use the adjustment screws on the finder scope bracket to adjust the aim of the finder until the object is centred. Then, look back into the telescope eyepiece and make sure the object is still centred there as well. If so, you’re done. If it is not, repeat the procedure, being careful not to move the main telescope while you’re adjusting the finder scope positioning.

When the distant object is centred in both the main telescope and the finder scope, the finder scope has been properly aligned and ready to use. Verifying the finder’s alignment should be one of the steps you go through each time you set up for an observing session.

10. What are the Bak4 and Bk-7 prisms? Is there a difference when they are used in scopes and in binoculars?

Both Bak4 and Bk-7 are common types of optical glass used in prisms that fold the light path inside the scopes and binoculars. “Bak” represents “Barium light Crown” glass, while “Bk” represents “Borosilicate Crown” glass.

In general terms, Bak4 prisms are more expensive and are considered better because they give a smooth, rounded “exit pupil”, due to a slightly higher level of refraction than Bk-7. Bk-7 prisms are also good quality, but brightness falls off slightly at the edge of the field compared to Bak4, so they create a slightly square exit pupil. Sharpness and clarity aren’t affected by using BK-7 and only the outer edges of the exit pupil are shaded (blurred). If your eye pupil is closed to a size that fits within the unshadowed diamond shape inside the exit pupil there is no loss of light at all with BK-7 prisms compared to Bak4.

However, BaK4 is preferable to BK-7 as a prism glass only when the focal ratio of the objective lens falls below about f/5. Virtually all scopes have focal ratios above f/5, so there’s no disadvantage in using BK-7 prisms. It achieves total internal reflection just as well as BaK4. In some telescopes it’s actually a slightly better choice because a prism made with BK-7 has a little less spherical overcorrection and chromatic aberration at blue/violet wavelengths than BaK4.

For binoculars, their objective lenses are mostly around f/4 or less, so BaK4 is preferred for their prisms to achieve total internal reflection at the edges of the fast f/4 light cone. Whist the binoculars with the Bk-7 prism would look no difference to the Bak4 in daylight condition, as the light levels drop, and the eye pupil expends, you start to observe the effects of the shaded regions as the image quality drops and becomes prone to chromatic aberration around the periphery of the image.

11. What is a Barlow lens?

A Barlow lens can multiply the power of the eyepiece by a specified magnification factor. For example a 2x Barlow lens will double the magnification and a 3x Barlow will increase the magnification by 3 times. A Barlow lens is placed between the objective lens and the eyepiece.

The magnification/amplification factor of a Barlow is a function of its position in relation to the eyepiece and the objective lens. This factor can be increased by increasing its separation from the eyepiece using an extension tube. There are compact shorter tube Barlow lenses and longer tube Barlow lenses, both can achieve the marked magnification results.

Conventional Barlow lenses (eg. this 2x Barlow) have a small magnification lens mounted inside a plastic or metal tube which is inserted into telescope’s focuser. Eyepieces are then inserted into the Barlow tube. A relatively newer design is now available in the form of a compact Barlow lens which can be attached to the end of the eyepiece. This ensures a perfect alignment between the centre of the Barlow lens and the eyepiece. In digi-scoping, it also serves the purpose of adjusting the image focal point and enables the camera to get a focused image through a wide range of telescopes. See more descriptions here.

12. What are advantages and disadvantages of using a Barlow lens?

Some major advantages:
Achieve higher magnifications. Provide flexibility of magnification levels with your existing eyepieces. For example, if you have 2 PLOSSL eyepieces with 16mm and 26mm focal length and use them on a telescope with 800mm focal length, you have a 800/16=50x and 800/26=31x magnification levels respectively. With a 2x Barlow lens, you will get 100x and 62x magnifications as well. Increase eye relief (distance of exit pupil from eye lens). Many eyepieces have an eye relief which is directly related to its focal length. For example, the eye relief of a Plossl is 0.73 x its focal length. Therefore, for these eyepieces, there will be a greater eye relief with a Barlow than without one.

Disadvantage: a major disadvantage of adding a Barlow is a slightly decreased brightness in the produced image.

13. I am interested in the 700×60 Refractor Astronomical Telescope (with 28x to 315x magnification), and the Nipon 20-60×70 spotting scope. Is there a need to have both and can a camera be fitted to each one?

Reply from Nipon Support: The 700×60 refractor telescope is designed for astronomical observations with high power, while the Nipon 20-60×70 zoom scope is a spotting scope which is mainly used for viewing objects on land. It can take a little time to set up the 700×60 scope so it is best to be kept in the same place if possible, while the 20-60×70 scope is very easy to set up so you can take it around and set it up in seconds. So you may need both of them if you have got the interests in both aspects.

Both scopes can be fitted with a digital camera (using a universal adaptor for example) or with a digital eyepiece. Please do not hesitate to contact us should you need further information.

Question: Thank you for getting back to me. Do you sell the connections (adaptors) I would need? My camera is a Nikon D60.

Reply: With the further information that you have provided about the type of camera that you wish to use for digi-scoping, here are some technical aspects for you to consider:

For your Nikon D60 camera, there are two options to connect it to the scope: one is to use a camera adapter which is specially designed for Nikon cameras (view this Nikon DSLR camera adapter); the other option is to use a universal camera adapter with extended camera stand which can not only connect this Nikon camera, but also other types of cameras to telescopes.

With the first option, you replace the camera’s lens with the Nikon camera adapter and then connect the adapter to the scope’s 1.25″ eyepiece holder. With the 2nd option, you do not need to remove your camera’s lens, but attach the camera to the scope’s eyepiece by using the universal adapter.

However, for the images through the telescope to be visible from the camera’s large lens, you would need to use a larger eyepiece on the scope. We recommend a 32mm eyepiece as a minimum. This large 32mm eyepiece can be directly inserted into the eyepiece holder of the 700×60 telescope, but for the 20-60×70 zoom scope, you would need to replace its zoom eyepiece with a specially made eyepiece adapter so that other types of eyepieces such as the 32mm eyepiece can be attached to this scope.

14. I have recently bought the Nipon 350×70 telescope from yourselves. All is fine, superb telescope, but the one thing that I can’t work out is how the finder scope fits on the main body of the telescope. Please could you advise?

There is a small finderscope stand in this telescope package with a tube which is linked to a hinge. The base of this hinge can be inserted into the small slot which can be seen slightly to the right of the compass, between the telescope body and the rubber cover. You can then fit the finderscope to this stand, as shown in the photos below.

15. I shoot old military guns at distances up to 1000 metres and need to see the bullet holes in the paper targets. Will the Nipon 350×70 scope allow me to do this nice and clearly?

To answer this question, we need to establish the level of magnification that is required in order to see a small target such as a bullet hole over that distance.

Someone with ‘normal’ 20/20 or 6/6 vision (visual acuity) is just able to decipher a letter (eg. E) that subtends a visual angle of 5 minutes of arc (5′) at the eye. What this means is that if you draw a line from the top of a 20/20 letter (E) to the eye and another line from the bottom of the letter to the eye, the size of the angle at the intersection of these two lines at the eye is 5′ of arc. It does not matter how far away something is from the eye, as long as it subtends an angle of 5′ of arc at the eye, then a person with 20/20 visual acuity will just be able to distinguish what it is.

For shooting range up to 1000 metres, the bullet diameter is assumed to be about 0.45″ or 11.43mm. If we know how far an individual with 20/20 vision can see an 11.43mm bullet hole with naked eye, we can then work out how many times the same target should be brought closer from 1000 metres (i.e., times of magnification).

Here is a calculation on how far one can see this 11.43mm target, where:

The bullet hole’s visual angle subtended at the eye is 5′ of arc (5 minutes of arc), one-half of which is 2.5′ of arc (this is to form a right angle by the line of sight and the plane of the target);

d is the distance along the line of sight, from the eye to the target, and

h is one-half the height of the 20/20 letter in mm.

The simple trigonometry is calculated as:

(1). 2.5′ of arc / 60=0.04167 degrees

(2). Tangent 0.04167 degrees=h/d=5.72mm/d (note: 11.43/2=5.72)

(3). d=5.72mm/0.00072=7944mm=7.944m

This means that an individual with normal vision will be able to read a letter with 11.43mm height (i.e., to identify the direction of letter E) at about 8 metres. In fact, to see a round bullet hole is much easier than reading a letter. A field test has indicated that an 11mm white dot on black background (or black on white) can be seen by people with normal vision at 10m or slightly further. In other words, if the same target is placed 1000m away, it needs to be magnified (or ‘brought closer’) 1000/10=100 times.

For the Nipon 350×70 scope, with the K9mm eyepiece and 3x Barlow lens included, it can achieve a 120x magnification which is within the power required for this purpose.

If a target is located at 150 yards (140m), with a 0.22″ (5.69mm) bullet, the required scope magnification can be calculated as: 140/(5.69/2/0.00072/1000)=35x

It needs to be understood that magnification is only one basic aspect which needs to be considered in this example. There are other factors that also play important role in target observation, such as the size of the objective lens and optical coatings of the scope, which affect image clarity.

16. Recent user feedback about using the Nipon 350×70 Refractor scope for Archery, with an additional 40mm eyepiece:

Tried the scope out today and it is really fantastic, I can even read at over 200 feet the small letters on the target through the 40mm eyepiece even without the added Barlow, so I am very well pleased and would recommend this scope to any archer.

17. Could you please tell me what would be a better telescope for looking at stars, the moon, etc.?

For stargazing/astronomical observations, we recommend the telescopes that are specially designed for astronomy, such as NIPON 600×50 refractor astronomy telescope. The NIPON 25-125×92 powerful spotting scope can also be used for astronomical observations.

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