How To Calculate Magnification Of Camera On Telescope

This article reviews some of the absurd claims and outright lies regarding magnification sometimes seen on the packaging or advertising of depression-end telescopes. Nosotros'll review what magnification is used for, how yous calculate it, and what are its practical limits. You'll see it's not a pregnant factor in distinguishing 1 telescope from another, and you'll understand how that misleading claim on the toy telescope is justified.

Lies Toy Telescopes Tell Y'all

You've probably seen shiny telescope boxes in department stores, toy stores, camera stores, and on eBay, with colourful infinite pictures, and boldly proclaiming the "ability" of the telescope. 450-power and 675-ability are peculiarly common claims.

They're lying.

Ok, they're not lying in a mathematical sense. As we'll see in a moment, they have correctly multiplied ii numbers together to become the answer 675.

The lie is the implication that you care; that it means something; that it is a marking of quality; that information technology is a reason to buy that telescope. Information technology would be similar a box containing a clock saying "Purchase this clock! It has over 3500 seconds in each hr!"

Customers who do buy such a telescope (oftentimes every bit a gift for a child) will never be able to use the claimed high magnification. And fifty-fifty if they could, they never would, because using such high magnification requires other weather and equipment not available to most mortals. Most such scopes stop up not being used at all, or being used merely at the lowest available ability.

What'south Magnification For Anyhow?

Magnification is important, though, correct? Your telescope's job is to make things bigger so you can come across them?

Actually, no.

Or at least not always. Many of the things we like to look at (like nebulae and galaxies) are really not small, they are dim. Your telescope'due south job isn't so much to make them bigger as to make them brighter. You lot need some magnification — like xx to 100 power — only you lot don't demand a lot.

An exception is viewing planets, like Saturn and Jupiter. They are already bright, just they are small-scale, and magnification is needed. Simply what you lot need and can use is 200 to 300 power, non 600. More than on this in a moment.

How To Calculate Magnification

Computing magnification is uncomplicated.

Your telescope'south primary tube has a "focal length", which is essentially the distance the lite travels from the offset lens or mirror to the eyepiece. It'due south usually measured in millimetres, and a number like 1000mm is common. (Pocket-size refractors tend to be in the 400 – 1000mm range; entry-level reflectors in the chiliad – 2000mm range; and Schmidt-Cassegrains in the 1500 – 3000mm range.) It will be specified in your telescope's manual and may exist marked on the lens.

You too put an eyepiece in your telescope, and the eyepiece also has a focal length. It will be clearly marked on the end or the side of the eyepiece. Numbers between 3mm and 30mm are common for eyepieces.

Magnification is just

So, in the examples shown in these photos, a 816mm telescope fitted with a 25mm eyepiece gives a magnification of

816 / 25 = 33 ability.

If nosotros replaced the eyepiece with a 4mm eyepiece in the same telescope, we would go

816 / four = 204 power.

Change the eyepiece and you lot change the magnification — it is not a fixed property of the telescope. Annotation that smaller eyepiece numbers give larger magnifications.

Yous can too get improver optical devices chosen Barlow Lenses that act equally amplifiers. They multiply the magnification past an corporeality marked on the lens. For case, that 816mm telescope, 4mm eyepiece, and a "2x Barlow", gives a magnification of

(816 / 4) x ii = 408 power.

But there'due south a catch. That telescope would non produce useful results at such loftier magnification. Its main lens is 102 mm4 inches in diameter, and its useful magnification is limited to most 200 to 250 times. Allow'southward hash out that.

Limits of Magnification

As yous can encounter, any telescope tin can be fabricated to produce whatever magnification merely by mixing eyepieces and Barlow lenses. Nevertheless, there is a limit to the amount of magnification a given telescope tin can usefully provide. Across this limit, the prototype will be blurry — like trying to zoom in a digital photo beyond what the resolution of the photographic camera supports.

The limit of useful magnification is a function of the bore (not the focal length) of the lens. This limit is complex, but a simple rule of pollex that works in many circumstances is that the maximum magnification a given telescope can usefully provide is about 2 ability per millimetrel power per inch of diameter. So a telescope with 100 mmfour inches diameter might be able to produce around 200 power magnification before going fuzzy.

There is a second limit, too. The fact that we have air around the states, turbulent, churning, and grit- and moisture-filled, likewise limits magnification. Beyond about 400 to 500 power, no small telescope located on the ground produces articulate views. That'southward why observatories are on mountains, or in infinite.

Finally, there is a 3rd limit for many people: the stability of your mount will limit what magnification you tin can utilize. Budget-priced telescopes are often sold on lightweight mounts that can't concord the telescope completely steady. At depression magnifications this doesn't thing, and may not fifty-fifty be noticed; but at high magnifications, the small vibrations of an unstable mount can brand it impossible to discover your target, focus, or discover.

The Toy Telescope Revisited

The department-shop telescope that introduced this commodity has about 76 mm3 inches of discontinuity, so its maximum useable magnification is near 76mm 10 23″ x 50 = 150 power. Y'all might get 200 ability out of information technology under ideal conditions. 675? Non a chance. Only mountaintop observatories, or the dozen-foot-loftier, multi-g-dollar, monster scopes used past very serious amateurs, will do that.

Where did the number 675 come from?

That telescope has a focal length of 900mm. It comes with 20mm and 4mm eyepieces, and a 3x Barlow. The smallest eyepiece produces the highest magnification. 900 / iv = 225 power. Add the 3x Barlow and you lot become 225 x iii = 675 power. This is a very common combination, specifically fix up to allow these misleading magnification claims. 450-power is another common one. That's a 900mm telescope, a 4mm eyepiece, and a 2x Barlow.

On the other paw, those telescopes are commonly 76 mmiii inches or 100 mm4 inches aperture. The better one, 100 mm, would be practiced for a maximum of about two x 100 = 200 power. Perhaps 250 on rare occasions. Challenge 675 or 450 power is true in the sense that the arithmetic is right, but information technology's misleading and, in my opinion, fraudulent.

Needless to say I wouldn't recommend anyone buy such a telescope. More than that — I tend to have potent feelings most beingness lied to. Personally, I won't purchase from a manufacturer that makes telescopes with such claims (which rules out even some mid- and loftier- end manufacturers), and I prefer not to purchase from a store that sells them, although that'southward non ever practical. (If they're willing to lie to me about that product, why should I trust them on other topics?)

How to Control Magnification

As we've seen, there are 3 ways to command the magnification of your system:

1. The focal length of the telescope. Longer gives higher magnification. This is a fixed attribute of the telescope — you lot can't vary it. Focal lengths are usually betwixt 300mm and 3000mm. For a beginner, something around 600 to 1500mm is a good choice.
2. The focal length of the eyepiece. You lot will want several eyepieces, to give depression, moderate, and high magnification.
3. Amplifiers like Barlow lenses. A adept quality 2x Barlow is like doubling the population of your eyepiece collection, provided information technology doesn't push the magnification of your highest-power eyepieces beyond the limits of your telescope.

Depending on the focal length of the telescope, a typical beginner might like to starting time with something like two eyepieces and a Barlow.

For instance, let's imagine you had a 150 mmhalf dozen-inch scope with a focal length of 1200mm. The maximum magnification of your 150 mm scope would be most 2 * 150= 300 power, which you would get with a 4mm eyepiece. However, you might find it better to start with something like a 25mm eyepiece, a 10mm eyepiece, and a 2X Barlow. Y'all and then become four useful magnifications with only 3 pieces:

25mm lone	1200 / 25 = 48 power
25mm and 2x Barlow	48 10 2 = 96 power
10mm alone	1200 / x = 120 power
10mm and 2x Barlow	120 x 2 = 240 power

The lower powers will be great for large nebulae and clusters, and the higher powers volition be skillful for the Moon, Saturn, and Jupiter.

Magnification versus Brightness

Photographers with single-lens reflex cameras know that, when they double the focal length of their lens, the image gets dimmer and so they have to double the exposure likewise.

The same is true with your telescope. When yous enhance the magnification, the image gets dimmer. This means that selecting the correct magnification to view a given object will frequently involve making merchandise-offs.

For example, suppose you are observing M57, the Band Nebula. It'due south interesting, simply it's quite small. At moderate power, you'll run across the tiny wispy smoke band. Should you raise the magnification to go a ameliorate wait?

Doing and so will make the nebula bigger, but it will likewise make it dimmer. Whether this is better or worse for observing will depend on you and on observing conditions.

Personally, I tend to keep such objects smaller and brighter for observing, while others find magnification adds detail and that the dimmer view is still acceptable.

Reducing Magnification

Strange as information technology may seem, you may even find yourself wishing you had less magnification at sure times. For example, certain objects (e.g. M31 — the Andromeda Galaxy; M45 — the Pleiades) may be too large to fit in your field of view, even with your lowest power eyepiece. This is especially a problem with mid-sized and large Schmidt-Cassegrain telescopes, which tend to take very long focal lengths.

In such circumstances, you may wish to reduce the magnification of your telescope. You tin can do this by adding an accessory called a Focal Reducer. This is an extra lens which acts like the reverse of a Barlow lens — it reduces the magnification of the telescope – eyepiece set by a fixed factor, usually something similar 1.4 times.

What Magnification to Use

Which magnification you lot should utilise at a given moment depends on many factors including the size of the object y'all are observing, its brightness, the surface detail, the aperture of your telescopic, the stability of your mountain, and the observing conditions. I tin't remember of whatever style to give y'all specific guidelines, except that I virtually often find myself using around 100 power when observing star clusters and dim objects like nebulae, and around 200-250 power when observing bright planets. If I had larger telescopes with greater resolving power (greater discontinuity) I'd probably apply more, only not much more.

Nevertheless, I recommend you should always start with your everyman magnification, or at least something on the low end. This will help in finding the object and observing information technology against the context of the background stars. Then you tin gradually increase the magnification on objects where you think it will amend the view.

Source: http://themcdonalds.net/understanding-magnification/

Posted by: burkhalternobs1952.blogspot.com

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