Why not build a large primary mirror from several

Reading about adaptive optics tonight, I was wondering why more people dont use several smaller primary mirrors focused onto the same spot, giving effectivly more light being intorduced onto the secondary reflector. do you cet some strange artifacts, or do you run into constructive/destructive interference???

thanks
-Ben

Reading about adaptive optics tonight, I was wondering why more people dont use several smaller primary mirrors focused onto the same spot, giving effectivly more light being intorduced onto the secondary reflector. do you cet some strange artifacts, or do you run into constructive/destructive interference???

thanks
-Ben
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Making a multiple mirrror telescope is not an easy task. Each mirror must be an off axis section of a parabola. This is a daunting task, and in the amateur world, normally requires fabrication of a single large mirror which is then cut into pieces which then must be heald in precise alignment. The creates big problems and is really only potentially an advantage if one is doing adaptive optics.


Adaptive Optics is complicated and expensive, with the need for fast servo's and computational power plus the laser for adaptive optics... On top of that, imagine dealing with the thermal problems associated with all that electronics...


Such systems are very expensive and for the amateur with a big scope looking at deep sky targets the problem is not normally the seeing but rather light pollution and/or light gathering. So adaptive optics really isn't a big help and adds greatly to the complication and expense. It is my understanding that AO is only applicable to very small fields of view.

For planetary targets, the stacking techniques using webcams are producing remarkable images, those these are not visual.

There was a project known as the Multiple Mirror Telescope, I vaguely remember this, I believe they ended up going to a single mirror in the end.

In the world of research, ground based telescopes with adaptive optics are competing against the extreme expense of the spaced based systems. They have the advantages of larger possible aperture while getting around some of the problems associated with imaging through the atmosphere. For imaging extra-solar planets where the field of view is extremely small, this offers real promise.

For those of us who pay with our own dollars and setup our equipment every evening, it is not much help for a lot of money....

Jon Isaacs

In the amature space about the only practical designs I see out there are binoculars - two primaries, two secondaries. check out Bruce Sayre's stuff at http://www.foothill.net/~sayre/index.htm, he's a real engineer and has a bino 22" that looks really cool.

Steve Swayze also built a 12.5" bino system. JMI inc sells a 6", 10" and then some crazy big stuff commercially.

Hello;
There are several such telescopes, though they are more commonly known as segmented mirror telescopes. Telescopes like Keck, the Hobby-Ebery Telescope, the clone of Keck being built in the Canary Islands, all have segmented primary mirrors. When properly phased, such a mirror acts much like one large mirror, i.e. the theoretical resolution is as large as if the segmented mirror was a single mirror. This requires that the alignment between the segments be to optical tolerances, i.e. a fraction of a wave. One main drawback of such mirrors, at least for infrared astronomy, is that the emissivity of the mirror is much higher than for a monolithic mirror due to the gaps between the segments. The Large Biinocular Telescope will be an imaging interferometer, i.e. it'll combine the two beams from the two telescopes on the focal plane to get the angular resolution of a larger telescope in one direction at a time.
As far as I know, no amateur has attempted a segmented mirror. For telescopes like Keck, each segment is part of an off axis hyperboloid, and there are 6 different mirror figures among the 36 segments. For HET, the segments are all spherical making a large spherical segmented mirror (with the ensuing spherical aberration). Making a pair of off axis paraboloids isn't easy. For Keck, they used stressed polishing to get the proper figure. An then there's the alignment issue. The mirror support structure must be quite stiff. In the case of Keck, the actuators push on the segments one a second or so, and they need to phase the mirror at least once a night, To do this they have a wavefront sensor and some sophisticated software to automatically align the segments to eachother.
To do this manually, in an amateur telescope, would be challenging to say the least. Typical collimation is much too crude; you'd need much finer control of the mirror alignments. As mentioned above, making several off axis paraboloids isn't easy. It's just as easy to make one large paraboloid in a thin mirror, since off axis paraboloids are typically cut out of such a mirror. One could use several spherical mirrors to make effectively make a larger spherical mirror, but then you'd have to contend with a whole lot os spherical aberration. Even if it could work on paper, it would be impractical as an amateur telescope.

Cheers
Mike Connelley