http://calgary.rasc.ca/nova.htm
I was grateful to have the opportunity to utilize the dark sky site to observe the Orion Nebula (M42), Crab Nebula (M1) and a beautiful open cluster in Canis Major first documented by Aristotle in 320BC (M41).
This month we learned about observing deep-sky objects and an introduction to eyepieces for your telescope. We learned the 5 basic characteristics of eyepieces and how that information can be leveraged to form a complete eyepiece set.
Focal Length - The focal length is generally defined as the length required for light to converge to a certain point called the focal point. The shorter the focal length of an eyepiece for a telescope, the higher the magnification and the lower the field of view. When asked what power your telescope is, it all depends on the eyepiece you are using.
Field of View - Every eyepiece has an apparent field of view associated with it. The field of view depends on the optical design of the eyepiece. Eyepieces range in apparent field of view from 35° to some that are 100°. The true field of view that you see through the telescope is the apparent field of view divided by the magnification provided by that eyepiece. The magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. Most of my eyepieces are of the Plossl deisgn with a 50° apparent field of view. I have a couple eye pieces that are 82° and they are amazing. The apparent field of view is an important factor based on what you are looking for in your visual astronomy. Typically the higher field of view the more expensive the eyepiece. The TeleVue Ethos line with 100° apparent field of view could run you up $1000 each eyepiece. A luxury item indeed.
Barrel Diameter - Eyepieces come in two different diameters, 1.25" and 2". The wide angle eyepieces are typically of the 2" variety. Most modern telescopes come with a converter that allows you to use both 1.25" and 2" eyepieces.
Eye Relief - The eye relief of an eyepiece is simply the distance the eye must be from the eyepiece in order to view the whole field. This typically depends on the design of the eyepiece, so be sure to check the eye relief before making a final purchase. The common Plossl design's eye relief typically gets smaller as the focal length is reduced. This would force you to pretty much put your eye on the eyepiece at high powers. If you intend to wear glasses while you conduct your observing sessions it would be wise to choose designs of a minimum 15mm eye relief.
Coatings - Most modern eyepieces are very well coated at every air-glass surface. This improves the transmission of the light. The purpose of a telescope is to collect as much light as possible. A poorly constructed eyepiece with poor transmission will lose some of this light and defeat the purpose of a great telescope.
Some images of eyepieces are shown below.
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| TeleVue Ethos (100° AFOV, 21mm Focal Length, 15mm Eye Relief, 2" Diameter) |
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| TeleVue Nagler (82° AFOV, 31mm Focal Length, 19mm Eye Relief, 2" Diameter) |
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| TeleVue Panoptic (68° AFOV, 35mm Focal Length, 24mm Eye Relief, 2" Diameter) |
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| TeleVue Plossl (50° AFOV, 32mm Focal Length, 22mm Eye Relief, 1.25" Diameter) |
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| Celestron Luminos (82° AFOV, 23mm Focal Length, 20mm Eye Relief, 2" Diameter) |
My current set includes:
5mm Celestron X-Cel
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| 7mm Celestron Luminos |
| 10mm Celestron Plossl |
| 23mm Celestron Luminos |
| 25mm Celestron Plossl |
| 2x Celestron Barlow |
This set provides me with a good range of magnification. However, I am lacking in wide true field of view provided by higher apparent field of view (82°) and focal lengths (30-40mm).
It is very cloudy at the moment and may continue into the evening for the Rothney Astrophysical Observatory's Open House. However, it will still be happening and the lecture on the Antikythera Mechanism is worth attending.
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