Telescope Eyepieces - link for beginners ?

[Chubster]

Not sure if good or not but sure some of the more experienced guys here will let us know, good reading though

http://observers.org/beginner/eyepieces.freeman.html

[Naz]

Good find there Beadell, useful primer and reminder.

naz

[radioactive]

excellent reading BD thanks for sharing the link with us

[The Warthog]

I like his rules for picking out your first eps. One I worked out for myself is that your optimum ep will be equal in mm to your f/ratio. Therefore, f/8 scope, 8mm ep. Put a 2x Barlow in, and you are at maximum practical power.

[FLO]

Nice find Beadell,

Thats an awful lot of primer for something aimed at the beginner.

There is one part that I have trouble understanding:

Not enough light. When you look at any particular object, the amount of its light entering the front of your telescope is fixed. If you are looking at an extended object, like the surface of the Moon, or a planet, or a galaxy or nebula, then as you increase magnification, that light is spread out over an ever-greater area of the retina of your eye, so the image looks dimmer and dimmer. Spread it out too much, and it will become too dim to see at all. What's more, various kinds of fine or low-contrast detail become hard to see before the object itself vanishes. If the object is dim to begin with, then the right magnification to see detail of a given size and contrast will be different, and almost certainly lower, than for a bright object. You will have to try many magnifications to see which one works best.

I am not saying that he is wrong, its just that I am not familiar with his explanation.

Perhaps someone can shed some light (pun intended ) on this?

[The Warthog]

That's a lot of words for a fairly simple concept, I think. If you increase magnification, you spread out the available photons over a larger area, therefore the image appears dimmer. I have come across this, dramatically, using zoom binoculars at twilight, when there isn't a lot of light to go around.

[MartinB]

I think it's wrong Steve. I think it should read "The light reaching your retina has been spread over an ever greater area. "

[OXO]

It's a mix of the two isnt it?

James

[FLO]

I have difficulty getting my head around this.

Increasing magnification reduces exit-pupil diameter which means light is spread over less of the eye retina.... I think :?

[Naz]

I have difficulty getting my head around this.

Increasing magnification reduces exit-pupil diameter which means light is spread over less of the eye retina.... I think :?

Yes, but you are magnifing a fixed amount of light, spreading it over a larger area.

For example,Take a f2, 50mm camera lens, diameter of the lens is 25mm. Now take a f4, 100mm lens, diameter is still 25mm, but the image is twice as big as the 50mm lens, thats how tele lenses work, the amount of light coming in is the same, but it has to illuminate an area twice as large.

If the object is for instance Jupiter, and its image in the 50mm lens was 1mm diameter, for the sake of argument. in the 100mm lens, Jupiters image would be twice as large at 2mm diameter. With both images being circles, the area illuminated by the 50mm lens would be 0.785 sq mm, the image from the 100mm lens would illuminate an area of 3.14 sq mm. With the 100mm lenses image being twice the size of the 50mm. but illuminates an area of four times. Therefore the the image is fainter

Your eyepiece acts in the same way, the amount of light available is controlled by the aperture of your scope, the more you magnify it the fainter it becomes, the exit pupil can not put back what ain't there.

What happens when you fit a barlow and the shortest ep you have, especially on a faint object, does the object not becomes fainter?

naz

[Chubster]

Well thats cleared that up :? :?

:)

[FLO]

Yes, but you are magnifing a fixed amount of light, spreading it over a larger area.

For example,Take a f2, 50mm camera lens, diameter of the lens is 25mm. Now take a f4, 100mm lens, diameter is still 25mm, but the image is twice as big as the 50mm lens, thats how tele lenses work, the amount of light coming in is the same, but it has to illuminate an area twice as large.

If the object is for instance Jupiter, and its image in the 50mm lens was 1mm diameter, for the sake of argument. in the 100mm lens, Jupiters image would be twice as large at 2mm diameter. With both images being circles, the area illuminated by the 50mm lens would be 0.785 sq mm, the image from the 100mm lens would illuminate an area of 3.14 sq mm. With the 100mm lenses image being twice the size of the 50mm. but illuminates an area of four times. Therefore the the image is fainter

Your eyepiece acts in the same way, the amount of light available is controlled by the aperture of your scope, the more you magnify it the fainter it becomes, the exit pupil can not put back what ain't there.

What happens when you fit a barlow and the shortest ep you have, especially on a faint object, does the object not becomes fainter?

naz

Good post Naz and I agree - anyone who has printed thier own film will have seen what you describe on the enlarger baseboard. However, you have covered the reason behind the drop in brightness. With regards to how much of the retina is illuminated, that must depend on the diameter of exit-pupil - the straightened beam of light that enters the eye.

I think Martin might have got it when he said "I think it should read 'The light reaching your retina has been spread over an ever greater area'."

Possibly :?

[GazOC]

The guys explanation isn't the best. But in a nutshell, a fixed number of photons hits your telescope lens/mirror from an object and the bigger area you attempt to spread (ie magnify) them over, the fainter the image will become.

Every time magnification is doubled the image will become 4 times fainter.