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Would this work?

pipnina

By pipnina
in DIY Astronomer

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pipnina

pipnina

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Recently, I was thinking about how to remove the annoying defect you find in newtonian reflectors where the spider vanes cause bright objects to have spikes coming out from them and obscuring the view of the object you want to see.

This image demonstrates the annoyance of the effect:

http://upload.wikimedia.org/wikipedia/commons/9/97/Sunset_Glow_in_Orion.jpg

As you can see, Hubble's spider vanes are causing the bright star to make a white streak which obscures the view of the nebula NGC 2023.

My design acts removes the effect completely and also makes observed images brighter as the secondary mirror does not obstruct light any more.

The only disadvantages I can think of are increased weight, size and cost (Due to the primary mirror needing to be elongated.

If anyone can think of an example where this has already been used (Or if they want to try an make it themselves), that would also be great. I'd love to see it.

http://i.imgur.com/mPIuj5Q.jpg <-- A rough outline of how the design is laid out.

It removes the spider vanes from obstructing the primary mirror's view of targets and creating unsightly artifacts and reducing brightness.

~An entrigued pipnina.

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Kropster

Kropster

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Any form of elongation would upset somebody doing AP, but may be fine for visual.

From the original post comments on spikey star pictures, AP seems the main aim..... and I think most would rather have the diffraction spikes than oval stars.

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Ruud

Ruud

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Designs for curved vanes:

post-38669-0-33383900-1420215223_thumb.p
When the sum of the arcs of the vanes is a multiple of 180 degrees, the diffraction pattern becomes an even halo around the star. The shorter the total length of the arcs, the less diffraction there will be.
This does not remove diffraction, but it makes it less obvious and less unattractive. The vanes need to be made rather precisely though, to get an even spread of the light. Difficult with only slightly curved arcs (3 vanes of 60 degrees).
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pipnina

pipnina

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Off axis optics cause elongated stars. That doesn't need to be too bad, though.

Here is a review of an oblique Newtonian.

"Telescope Optics - Evaluation and Design",   by Harrie Rutten and Martin van Venrooij, has a very informative chapter on these kind of designs.

If only I could create a way to test this stuff in a computer.

The example you linked to just moved the secondary to the edge of the tube. It didn't create a new tube and have an oval mirror to compensate. Not sure if this makes a difference, though.

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Peter Drew

Peter Drew

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I think the cost of an oval mirror large enough to satisfy the projected design would make it a non starter for most people. The angle of incidence still equals the angle of reflection so a deviation large enough to avoid a complete standard secondary holder would require a mirror of long major axis. Off axis mirrors such as found in Herschellian unobstructed telescopes are usually very long focal ratio to minimise astigmatism. Off axis paraboloids are usually employed but these are astronomically expensive.  :smiley:

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Ajohn

Ajohn

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This links sums up what has to be done to make the OP's idea work

http://www.dgmoptics.com/oa_what_is.htm

and it still isn't that straight forwards. Notice how long the F ratio's are. That's due to the way images deteriorate off axis more and more as the F ratio gets shorter/faster. I think that the above link is taking liberties really - probably because they sell them.

One simpler way of looking at the problem is using a purely spherical mirror. That can be tilted around it's centre of curvature and be in the same position as the surface of a bigger mirror would be if it had the same radius.  A paraboloid wont do that.  A spherical 8in F9 mirror will meet Rayleigh's criterion without any figuring at all. The problem is that if this is tilted so that the centre of the view is at the edge of an 8in tube the rays look like they are coming from part of an F4.5 mirror which in practice they are. An normal 8in scope made with this mirror would be pretty good - it wont be at F4.5. In practice more tilt is needed to get the field of view outside of the tube.

If unobstructed reflecting telescopes is searched or tilted component telescopes lots of ideas can be found. They tend to be aimed at small field of view work and often have relatively slow F ratio's. One has spectacular resolution over a very wide field until it's realised that the mirror positions have to seriously limit the field of view.  They have lots of catches including the need for accurate mirror radii and very careful setting up.

The design looks like it has a mirror support anyway - that will result in 2 spikes. Might be worth mentioning that 3 supports would generate 6 that's why 4 are usually used. 1 will generate 2. I did own a scope that used an oval tube to support the mirror and the focuser.  The far side of the oval was on the far side of the 2ndry mirror and formed it's support. That minimises the effect as well as the diffraction spikes are spread out over an area just as they are the wavy vane way.  An oval is a lot easier to do.

John

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Ajohn

Ajohn

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That's been done but it can get expensive and really needs coating. Texereau did it on his own cassegrain but it was a narrow field scope with a long focal length to get the image scale up to a size to suit film. I'd guess that a plain flat plate would mess up the optics on anything with a wide field of view. Even something just wide enough to cover the moon might have problems.

John

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beamer3.6m

beamer3.6m

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What about the secondary mirror being held up by a 'pole' which runs from the centre of the primary mirror.

If I understand correctly the centre of the primary mirror is not seen anyway hence why the centering spot can be drawn on etc without degrading imaging quality.

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Zakalwe

Zakalwe

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What about the secondary mirror being held up by a 'pole' which runs from the centre of the primary mirror.

If I understand correctly the centre of the primary mirror is not seen anyway hence why the centering spot can be drawn on etc without degrading imaging quality.

Think about that for a moment

The pole would have to be incredibly stiff to resist vibrations. Can you think of any material that can hold a mirror at, say, a metres distance without moving?

How would you collimate?

The pole would obstruct light from the lower edge of the secondary, so the final image would have a shadow through it.

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Zakalwe

Zakalwe

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Any design is always a compromise between sometimes many competing advantages/disadvantages.

Newtonians give big aperture, no chromatic aberration, and can be very fast. But have diffraction effects (even a curved spider will cause diffraction artefacts, they are just spread around the star).

SCTs give long focal length in a compact format, but are slow, can suffer from mirror flop and are slow to cool.

Refractors give no diffraction artefacts, but are expensive and very expensive once you get into short focal lengths and triplet ddesign to remove chromatic aberration.

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Scott

Scott

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Think about that for a moment

The pole would have to be incredibly stiff to resist vibrations. Can you think of any material that can hold a mirror at, say, a metres distance without moving?

How would you collimate?

The pole would obstruct light from the lower edge of the secondary, so the final image would have a shadow through it.

but you could just brace the pole with 4 "spiderlike" veins  :evil: . Sorry but Ijust gotta say it.....frac?

sorry, ignore me....the meds are wearing off :D

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pipnina

pipnina

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I thought:

Look at my original design. Replace the primary mirror with a plain diagonal.

Now replace where the secondary is with a normal, parabolic primary mirror reflecting back down through a hole in the diagonal.

How does that sound?

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Zakalwe

Zakalwe

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I thought:

Look at my original design. Replace the primary mirror with a plain diagonal.

Now replace where the secondary is with a normal, parabolic primary mirror reflecting back down through a hole in the diagonal.

How does that sound?

Whats your experience of optical design?

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