Why don't mirrored telescopes have the light crossover before going to the eyepiece?

[ELS]

I haven't seen any design which does this, have the light cross over before hitting the eyepiece... It would seem like that would allow you to have a smaller secondary mirror, as well as allow you to use shorter focal length primary mirror.
Does it cause some wacky diffraction problems or what? Or does it have a specific name for it?

Edited August 4 by ELS

[bosun21]

Your crossover point is the focal point. That's where your eyepiece would need to be to achieve focus. This is directly obstructing the primary mirror in your drawing.

Edited August 4 by bosun21

[Gfamily]

1 hour ago, bosun21 said:

Your crossover point is the focal point. That's where your eyepiece would need to be to achieve focus. This is directly obstructing the primary mirror in your drawing.

Correct, though technically, your eyepiece would need to be its own focal length from that point to be usable.

So a 5mm ep would be 5mm from the 'cross over' point.

And if you were using a camera, the sensor would need to be positioned exactly there.

But regardless, the eyepiece and the camera would obstruct the light before it reached the primary mirror.

[rl]

You could use a relay lens (probably a pair of achromats back-to-back) to move the focal plane outside the tube but the  you risk introducing other aberrations. Not worth it just for a smaller secondary..and you still have the spider diffraction.

 

[ELS]

I get it now. Also makes sense why it wouldn't be common, since it would introduce all the problems of lenses, and would be better to avoid.

[Louis D]

The only design I know of similar to what you are asking for is the Gregorian telescope:

[Xilman]

13 hours ago, rl said:

You could use a relay lens (probably a pair of achromats back-to-back) to move the focal plane outside the tube but the  you risk introducing other aberrations. Not worth it just for a smaller secondary..and you still have the spider diffraction.

 

The Dilworth does just that and does not have a spider diffraction because it is essentially a Cassegrain with purely spherical optical surfaces. The relay lenses correct for spherical aberration.

The reason I know this? My main telescope is a 0.4m Dilworth. The diagram at https://www.telescope-optics.net/miscellaneous_optics.htm is extremely close to that of my scope; I wonder if it was taken from Larossa's design drawings.

[Louis D]

7 hours ago, Xilman said:

The Dilworth does just that and does not have a spider diffraction because it is essentially a Cassegrain with purely spherical optical surfaces. The relay lenses correct for spherical aberration.

The reason I know this? My main telescope is a 0.4m Dilworth. The diagram at https://www.telescope-optics.net/miscellaneous_optics.htm is extremely close to that of my scope; I wonder if it was taken from Larossa's design drawings.

The Dilworth looks a lot like a Gregorian with an added relay/corrector lens assembly, assuming the primary and secondary are spherical.

[Xilman]

46 minutes ago, Louis D said:

The Dilworth looks a lot like a Gregorian with an added relay/corrector lens assembly, assuming the primary and secondary are spherical.

The major difference is that in the Gregorian the light reaches its focus from the primary before reaching the secondary and is then brought to a second focus by the concave secondary beyond the position of the primary. That makes for a markedly longer OTA of a Gregorian than a Cassegrain. The Dilworth is more like a Cassegrain in that the first focus is located after the light reaches the secondary and is then brought to a second focus by a series of relay lenses. My 0.4m f/6.5 ( focal length of 2.614m) Dilworth is only about 1.5m long, as can be seen here: http://www.astropalma.com/equipment.html Further, all the surfaces are spherical, which makes construction markedly easier in some respects.

The principal  downside of the Dilworth in my experience is that the transfer lenses inevitably scatter some light. Alignment and collimation difficulties are only experienced once, when the telescope is first made, because it is a completely enclosed design and needs recollimation about as often as a properly built refractor. The flat window at the front keeps out all the dust, spiders, gecko droppings, etc.

This image of Sycorax, Uranus XVII, shows a little of what I mean. Uranus was perhaps half a degree outside the field of view but, there again, the planet was about 15 magnitudes brighter -- a factor of about a million. http://www.astropalma.com/Projects/Satellites/sycorax.html

Doubtless if all the relevant surfaces were scrupulously cleaned the scattered light could be reduced.

Edited August 5 by Xilman
Fix minor tpo.

[Louis D]

2 hours ago, Xilman said:

The major difference is that in the Gregorian the light reaches its focus from the primary before reaching the secondary and is then brought to a second focus by the concave secondary beyond the position of the primary.

I totally missed that.  I was genuinely perplexed as to how the Gregorian could bring the image to a second focus.  It does this as you say using the secondary because the primary focus occurs before the light reaches the secondary.

For the Dilworth, I suppose very high quality multicoatings need to be applied to all window and lens surfaces to try to cut down on scatter and reflections.  That, and very high polish standards for the glass itself.  Even then, there's only so much that can be done for the situation you highlighted.

[Xilman]

25 minutes ago, Louis D said:

I totally missed that.  I was genuinely perplexed as to how the Gregorian could bring the image to a second focus.  It does this as you say using the secondary because the primary focus occurs before the light reaches the secondary.

For the Dilworth, I suppose very high quality multicoatings need to be applied to all window and lens surfaces to try to cut down on scatter and reflections.  That, and very high polish standards for the glass itself.  Even then, there's only so much that can be done for the situation you highlighted.

Indeed.  But no more important than the many lens surfaces in a modern eyepiece. Mine is used solely for imaging, so I don't have the latter problem.

Edited August 5 by Xilman
s/importan/important/