Why Doesn't the Secondary Mirror in a Cassegrain Telescope Cause a Blind Spot?

[pegasusherd]

Hello,

I have a question regarding a Cassegrain style telescope (or any other "obscured" type telescope, for that matter).

Can someone explain (in detail) why the small secondary mirror out in front of the large primary mirror does not cause some sort of a "blind spot" in the center if we use a Cassegrain reflector for a telescope? I understand that some of the incoming rays are blocked by the small secondary mirror out front, but I am confused as to why this doesn't translate into some sort of a visible "blind spot" in the center of the image we're looking at...

Any explanations would be very much appreciated -- Thanks!

 

[mgb_phys]

The rays form the object that your telescope focusses are coming in paralell, so no particular ray represents a single point in the image. By having a secondary in the way some of these rays are blocked - but all this does is reduce the total brightness.
It's like having an iris in a camera lens, it reduces the brightness but again no point on the image corresponds to a particular point on the entrance of the lens

 

[turbo]

The rays form the object that your telescope focusses are coming in paralell, so no particular ray represents a single point in the image. By having a secondary in the way some of these rays are blocked - but all this does is reduce the total brightness.
It's like having an iris in a camera lens, it reduces the brightness but again no point on the image corresponds to a particular point on the entrance of the lens

This is a good explanation of an ideal approximation, as long as the central obstruction is small. If the central obstruction is larger, the distortions are greater. For decades, amateur telescope makers (not a whoo-whoo fringe group by any means) have tinkered with larger and smaller secondary mirrors, secondaries that are offset from the center, etc, to try to minimize even these minimal effects.

 

[pegasusherd]

Two comments on the above:

-You say no one ray represents a specific point on the object. I think this concept may be the one I'm having an issue with. So, why not?

-Second, in addition to rays hitting the reflector coming in parallel, I'd imagine you'd also have some rays coming in at an angle. What effect does this serve? Maybe this just increases the brightness?

Thanks for the responses -- I'm trying to get as best of an understanding on this topic as I can. I'm obviously not an optics guy ;-)

 

[Creator]

This is a good explanation of an ideal approximation, as long as the central obstruction is small. If the central obstruction is larger, the distortions are greater. For decades, amateur telescope makers (not a whoo-whoo fringe group by any means) have tinkered with larger and smaller secondary mirrors, secondaries that are offset from the center, etc, to try to minimize even these minimal effects.

Here's good blurb emphasizing that point...both the importance in design of the secondary size, its shape, corrector lenses, its resultant flat focal surface vs. spherical surface...and its application to wide field vs. high mag. and in visual vs. photometrics/CCD imaging.

http://images.google.com/imgres?img...and+diagram&start=180&ndsp=20&um=1&hl=en&sa=N



...

 

[russ_watters]

Two comments on the above:

-You say no one ray represents a specific point on the object. I think this concept may be the one I'm having an issue with. So, why not?

All objects emit light in all directions, so every point on an object will send rays of light to every point on the telescope mirror.

-Second, in addition to rays hitting the reflector coming in parallel, I'd imagine you'd also have some rays coming in at an angle. What effect does this serve? Maybe this just increases the brightness?

For all intents and purposes, the rays of light end up parallel because of the distance.

 

[Andy Resnick]

Hello,
<snip>
Can someone explain (in detail) why the small secondary mirror out in front of the large primary mirror does not cause some sort of a "blind spot" in the center if we use a Cassegrain reflector for a telescope?
<snip>

In brief, the location of the secondary is not conjugate to any image plane. The secondary, as pointed out by others, does affect the total throughput (sensitivity) of the telescope. The spider (the hardware that holds the secondary in place) can create interesting 'spikey' diffraction patterns in the image, tho- the design of a spider is complicated due to the need to be high-strength and occupy as little space as possible.

Two comments on the above:

-You say no one ray represents a specific point on the object. I think this concept may be the one I'm having an issue with. So, why not?

-Second, in addition to rays hitting the reflector coming in parallel, I'd imagine you'd also have some rays coming in at an angle. What effect does this serve? Maybe this just increases the brightness?

<snip>

There's some mixing of concepts here- the light emitted from a distant object and the light collected by the telescope with a non-zero field of view. Since the stars are (approximated as) infinitely far away, the light from a particular star enters our optical system as a plane wave, which is "parallel rays" in geometrical optics. Stars appear as points (or Airy discs) to us not because there is a single ray that intercepts our optical system, but because the wavefront crossing the entrance pupil is flat. Stars located at different places then have ray bundles oriented in different directions- in the physical optics picture, the plane wave is tilted in the direction of the star.

Now for the brightness- the reason telescopes have such giant primary mirrors is to get a brighter image- more colecting area. The wavefront is still a plane (neglecting effects for air turbulence), but more light is collected. Having a non-zero field of view is a little different, and is determined by the eyepiece, rather than the primary mirror. here's a calculator showing the ranges of common eyepieces:

http://www.csgnetwork.com/telefov.html

 

[pegasusherd]

Hello everyone -- thanks for your answers -- I think I now understand the issues.

Primarily, I think that the part I was missing is that each point emits rays in all directions hitting every point of the mirror. Considering that one fact, it all makes sense.

Thanks!