jmi
Senior Member
Posts: 60
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Post by jmi on Sept 2, 2006 12:47:32 GMT -5
Anybody just joining us, see GeneW's "New Computer" thread. Hopefully we can move the discussion out to here... I will endeavour to get some pictures ASAP, but I have no digital here so will have to be on film - hopefully by next weekend with the wind behind me, or sooner in B+W. The roof has to be off unless I had a really wide lens, and it's wind and intermittent rain here. Pity I didn't take the opportunity last week but I was testing the drive system then. I would say there is a lot more in the way of images taken with telescopes on my web page, but unfortunately the disk that most of it lives on, and in particular all the images, is not working right now (died in the middle of backups too, as usual). This is the trouble with being a part-time system admin, and being cursed this week  |
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jmi
Senior Member
Posts: 60
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Post by jmi on Sept 4, 2006 15:33:42 GMT -5
Okay, as promised... these are really quite bad, sorry - did them very (too) quickly during my coffee break, on the last few frames of a roll I nearly finished over the weekend.  View from the back of the building, the primary mirror is in the middle of the rectangular section (which is attached to the mounting), you can see the secondary at the top, and the third mirror is at the bottom. There seems to be a lot of lens flare here (yes, I was using a lens hood).  The CCD, mounted at the focus, we're looking up from the third mirror. The focus is roughly in the middle of the primary mirror, looking down. I must have wobbled here, rather slow shutter speed...  This is the "polar wedge" part of the mounting, with the drive motor visible (this is new - what I was doing), along with a couple of boards where the various wiring is collected together. Wonder what the milk bottle is doing down there. I'll try to get some more (and better) pictures later. |
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Post by John Parry on Sept 4, 2006 15:38:55 GMT -5
Whoo...!
I was right - a serious piece of kit. Think we need a sketch here Jonathan!
Regards - John
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jmi
Senior Member
Posts: 60
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Post by jmi on Sept 4, 2006 16:45:30 GMT -5
Just put together a quick one (looking from the same direction):  I'll have to do a mount diagram later (or google on fork equatorial mountings might turn up something I suppose). Rough diameters of the mirrors are: M1 20 inches M2 12 inches M3 14 inches (on a 16 inch blank) ie. a lot of glass here (well, actually it's Zerodur, which has much lower thermal expansion). |
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Post by nikonbob on Sept 4, 2006 18:07:34 GMT -5
Very interesting kit and project to be working on. Thanks for the photos and diagrams.
Bob
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Post by John Parry on Sept 5, 2006 7:08:18 GMT -5
Intriguing! The ultimate piece of kit for a Camera Collector!
OK - I can see how it works (assuming that M1 has a big hole in it to take the light from M2 and the camera/ccd arrangement inside that). But what does it give you, over a more conventional arrangement? Don't you lose a massive amount of light because of 'the hole'? Or was it a labour of love on Mr Willstrop's part - built because he could?
Sorry! If you hang around on here you'll find that basically I'm an ignorant but nosy person!
Regards - John
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Post by greyhoundman on Sept 5, 2006 7:21:47 GMT -5
The use of multiple mirrors allows a long effective focal length in a more compact package.
Any loss of light is small, due to the very effective mirror coatings.
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jmi
Senior Member
Posts: 60
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Post by jmi on Sept 5, 2006 15:13:39 GMT -5
This particular three mirror design is optimised to give a very wide field of view.
For a simple paraboloidal shape of mirror, the images of a source at infinity are perfect on-axis, but off-axis suffer from coma and astigmatism. This typically limits the usable field with images of acceptable quality to 0.5 degree or less. The problem can be fixed with corrector lens systems, placed close to the focus. Typically a three-element system is used, which needs aspherics - ie. expensive, and since lenses were used this introduces chromatic aberration. The three element lens will give a 1 degree field or thereabouts.
The next level up is the Ritchey-Chretien design, which uses two hyperboloids. This is corrected for coma, but astigmatism limits the usable field to about 0.75 degree or so. The primary mirror can also be used in isolation, which gives aberrated images all over the field, but with corrector lenses (which are easier than the ones for the paraboloid) a 2 degree field is obtainable (eg. the Anglo Australian Telescope prime focus corrector).
The three-mirror design corrects both aberrations to an acceptable level (0.33 arcsec or so - atmospheric seeing means this limit is rarely reached) over a 5 degree field. No corrector lenses meaning no chromatic aberration. It is essentially an all mirrors version of the Schmidt camera, which is a spherical mirror and full-aperture aspheric corrector plate system.
There are a few snags with the three-mirror telescope: (a) the mirrors are very difficult to figure, and align correctly due to the strongly aspheric shapes and the speed of the system, (b) the focus is inside the telescope tube, a disadvantage shared with the Schmidt but not a conventional prime or cassegrain as in the first two cases, however here it's even more inaccessible being in the middle of the primary mirror, (c) the focal plane is curved, also a problem with the Schmidt.
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Post by John Parry on Sept 5, 2006 16:18:03 GMT -5
Thats me told!
Seem to remember reading about the 'Sky Survey' using a large Schmidt (on Palomar?), but that was in the days of film. The curved focal plane must give you a problem with the CCD, or can you correct for it electronically?
Regards - John
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jmi
Senior Member
Posts: 60
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Post by jmi on Sept 7, 2006 13:25:07 GMT -5
The CCD is so small that the curvature is not normally noticeable, especially since the collimation on the optics is not too good right now. For a larger chip we would have to install a lens to flatten it out.
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