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.
Exit pupil (mm)= Fe ÷ f/# (N) | D ÷ magnification
Fe: Focal length of eyepiece
f/# (N): Focal ratio of telescope
D: Diameter of aperture (mm)
For reflector telescopes, it’s best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.
For the full field of view to be seen, the exit pupil should correspond with the dilation of the dark-adapted pupil, which is between 5 and 7mm. The dialation of our dark adapted pupil has to be larger than the exit pupil to see the full field of view. An exit pupil larger than 7mm means that some of the light will be lost outside of the eye’s entrance pupil, no matter what the distance of the eye from the eyepiece.
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.
TFOV = AFOV of Eyepiece / Magnification
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.
The formula is very easy.
Magnificaiton= Fo / Fe
Fo: Focal length of telescope
Fe: Focal length of eyepiece
Let’ say, I am now using a f=2000mm telescope with f=26mm eyepiece. The magnification value will be 2000mm/26mm= 77X. Therefore, I am now having a 77X magnification.
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.
All these contribute to periodic and non-periodic shifts in the celestial object’s position that can be seen in the eyepiece, and if it is not immediately “corrected” by gentle tracking steering, they will blur the captured image. The reticle allows one to see the minor shifts and to make the needed corrective actions.
“Guiding” is one of the most important and difficult skills needed in long exposure astrophotography. These days computer guiding by CCD has made this chore much less challenging.
In order to use this eyepiece when imaging, you need to get an off-axis guider. If not, you can only either use the reticle eyepiece or imager at the same time.
It seems that this is a good gadget to invest. It can also help you to do the alignment more precisely as there is a crosshair for you to center the star in the eyepiece.Due to the rising of the autoguiding chip built in the CCD camera or autoguiding CCD camera, people are not buying this anymore.
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”.
When you double the magnification, you get one quarter the brightness. ie, 1/2 squared.
When you triple the magnification, you get one ninth the brightness. ie 1/3 squared
When you half the magnification, you the brightness increases four times. ie 2 squared.
You can work out the relative brightnesses between any pair of eyepieces by doing the simple arithmetic.
FAQ
>>> Is there a light loss if I use a barlow lens?
Same square law applies. A 2x Barlow will result in a quarter the brightness because you have 1/2 squared it.
>>> Does a eyepiece connected with a barlow lens offer the same brightness as the doubled magnification eyepiece above?
Nearly the same. There is a slight bit more loss because of the extra number of lenses in the Barlow that the light must pass through.
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