Sunday 17th Jan

[glowjet]

Andy, vignetting is the physical blockage of the light path from the primary optic to the point of focus, if present, vignetting prevents the use of the telescopes full light-gathering capability and will contribute to darkened edges around the field of view, often a problem for imagers when using various optical components in the light path, to increase focal length. There is a review of the TeleVue 2.5 Powermate on the Cloudy Nights web site, look for the eye pieces section under barlows.

John.

[glowjet]

Try U.K.Astronomy Buy & Sell.

[Andros]

Andy, vignetting is the physical blockage of the light path from the primary optic to the point of focus, if present, vignetting prevents the use of the telescopes full light-gathering capability and will contribute to darkened edges around the field of view, often a problem for imagers when using various optical components in the light path, to increase focal length. There is a review of the TeleVue 2.5 Powermate on the Cloudy Nights web site, look for the eye pieces section under barlows.

John.

Ok sort of get you, sounds like more advance stuff, im reading the primer on "Understanding and choosing eyepeices" at the mo in the tutorials, so still learning the basics.

One thing that fascinates me though is why you cant see the secondary mirror through the ep, or when i put my hand in front of my scope i cant see it, I guess its to do with the way the primary mirror is shaped, how the hell does that work? Or am i getting ahead of my self again?

[glowjet]

Your ep is designed to be placed at the position of focus where the cone of light from the main mirror via the secondary comes to a point, you will not see any structure of the mirror only the reflection, putting your hand in front of the scope will cause the image to darken as you block the light path entering the scope tube. During any cloudy spell I would suggest you read up on the basics of how telescopes, ep`s and other bits and pieces associated with Astronomy, work. Its great to be enthusiastic but things can be a little overpowering at times and difficult to get your head round, with Astronomy patience is a virtue, but well worth it in the end.

John.

[Andros]

Your ep is designed to be placed at the position of focus where the cone of light from the main mirror via the secondary comes to a point, you will not see any structure of the mirror only the reflection, putting your hand in front of the scope will cause the image to darken as you block the light path entering the scope tube. During any cloudy spell I would suggest you read up on the basics of how telescopes, ep`s and other bits and pieces associated with Astronomy, work. Its great to be enthusiastic but things can be a little overpowering at times and difficult to get your head round, with Astronomy patience is a virtue, but well worth it in the end.

John.

What i'm trying to ask which probably never came over too clear is how does a scope manage to eliminate the image of the secondary mirror, seeing as it(secondary mirror) is in front of the primary. I apologise in advance if you already answered the question, and i'll definately research it some more.

[pvaz]

To create an image you need light. The scope doesn't eliminate the image, what happens is that you don't have light illuminating the inside of the spider (secondary mirror holding structure) so for the primary it looks black, so it isn't reflected. The mirrors reflect light, not dark shadows.

The effect of blocking the entrance with your hand only reduces the aperture, because it reduces the amount of light that gets in.

[yesyes]

A star (for example) shines its light on the whole surface of the primary mirror, not just one small spot. The curvature of the primary mirror focuses all that light to one point (the focal point).

So by putting your hand in front of the scope you're just blocking some of this light and through the eyepiece you still see the rest of the light, i.e. it gets dimmer. The same applies to the secondary mirror.

Hope that helps understanding it...

[Andros]

To create an image you need light. The scope doesn't eliminate the image, what happens is that you don't have light illuminating the inside of the spider (secondary mirror holding structure) so for the primary it looks black, so it isn't reflected. The mirrors reflect light, not dark shadows.

The effect of blocking the entrance with your hand only reduces the aperture, because it reduces the amount of light that gets in.

Ok im with you, nice one thanks, the mirror reflects light and not dark shadows..bit obvious really, like trying to see your reflection from a mirror in a dark room. So when you have your hand in front of the scope, because there is no light coming from it, it wont reflect, as with the spider, it just blocks some light from getting in, (reduction of aperture). Im actually amazed by that.

Thats enlightening! thanks again.

[Andros]

A star (for example) shines its light on the whole surface of the primary mirror, not just one small spot. The curvature of the primary mirror focuses all that light to one point (the focal point).

So by putting your hand in front of the scope you're just blocking some of this light and through the eyepiece you still see the rest of the light, i.e. it gets dimmer. The same applies to the secondary mirror.

Hope that helps understanding it...

Yes thats cleared it up! should have paid more attention in school really, thanks for all your help. Nice to know how things work!