Shares and cherishes the gorgeous night sky, the everyone’s property.
The exit pupil is the image of the objective that is formed by the eyepiece. It’s where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope’s focal ratio or you can divide aperture by magnification.
Fe: Focal length of eyepiece
f/# (N): Focal ratio of telescope
D: Diameter of aperture (mm)
An important factor to consider a eyepiece is eye relief. This is the distance between the viewer’s eyeball and the eye lens of the eyepiece.
The eye-relief distance is fixed by the eyepiece design and is normally rather exact. A person who wear glasses may have problem to get the full field of view as he cannot reach the correct eye relief point. If his eyes are outside the correct eye relief distance, the field of view is reduced and the reduction depends on the individual and the design of the eyepiece.
I would like to know the field of view I can archieve with my scope if I am going to buy an eyepiece, especially a wide angle eyepiece. Apply the formula below, you can get a rough figure of the true field of view (TFOV). Why is it just a rough figure? There are still several factors to be taken account but I think this is good enough. There is a more precise formula to do the same work.
The apparent FOV (AFOV) of an eyepiece is provided by the manufacturer. To know the magnification, please refer to Calculate Eyepiece Magnification.
Apparent Field of View of Several Eyepiece Designs
Orthoscopic - 43º
Plossl - 50º
Radian - 60º
Panaoptic - 68º
Nagler - 82º
This is very important to every astronomers but easy to learn. Calculating the magnification always helps you to know the current situation and get the best view of the objects. For every celestial objects, there are always the most suitable views (magnification) for it. Let’s back to the topic, I shall discuss the most suitable views in the coming days.
Today, the Malaysia Meade Distributor, Uncle Looi told me how do an illunimated reticle work and why was it invented.
Long exposure astrophotography will always suffer from some imperfect tracking because even the most precise man-made gears have some minute imperfections in them, the mounts can never be perfectly rigid (they are invariably flexible to some degree) and the atmosphere itself can cause positional shifts in images.
Continue reading ‘How do reticle eyepiece maintain the guiding?’
Light gathering ability is the most major factor when considering a telescope. Large power, ie. high magnification, is not always encouraging. Moreover, over-magnifying the objects will cause diffraction as it’s over the diffraction limit of the telescope. Why? The term, light loss is always linked to this.
For any telescope entrance aperture, the brightness intensity seen at the eyepiece is inversely proportional to the square of the magnification. This is the natural “square law”.
If you own a telescope, you must know what is a Barlow lens. It’s invented by Peter Barlow (1776-1862), an English writer on pure and applied mathematics. Barlow lens is a concave lens which is place between the primary mirror of telescope and eyepiece to increase the magnification of the eyepiece which is currently used.
Continue reading ‘What Is A Barlow Lens?’
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