|
|
Thread Tools | Display Modes |
#131
|
|||
|
|||
Straight vs. Curved Vanes (WAS Long focal lenght newtonians
anyone know where You can get the Focuser and diagonal mount set?
I know several of our club members have scopes with just *One* support holding the secondary and they have very Sweet views through them. No diffraction spikes of any kind split close doubles beautifully Awesome views of nebula. I have been toying with the idea of crafting such a mount for my 6" Newt as the three heavy wire vanes defract something *Awful* when looking at something very bright, like Mars or a high mag star. makes splitting doubles a pain if they are to close. 8*\ -- Clear and Dark Skies for all! _________________________ -------------------------------------------------- Tim Longwell Black River Astronomical Society http://junior.apk.net/~arstar50/BlackRiver.index.html _________________________ -------------------------------------------------- "David" wrote in message ... Bryan Greer wrote: ... In this case, the single-arch design represents a 42% reduction in overall diffraction compared to the 4-vane, and 22% less diffraction than even the 3-vane. The material thickness can be kept the same as the straight-vane spiders for small and medium sized reflectors if you're allowed to get away from a parallel-edged arch. A better idea is to think of the simple cantilever anchored at one end, and with the load applied to the free end. The mechanical engineers out there might remember that the material in a cantilever is uniformly stressed if the anchored end is wider, and it tapers down to a point at the free end where the load is applied. This is another way of saying the material is utilized in the most efficient manner. You can think of the single arch design as two cantilevers (albeit bent ones), with each one anchored to the tube wall, and meeting at the hub of the secondary. Hello Bryan, when will Protostar offer such vane ? Thanks, David |
#132
|
|||
|
|||
Straight vs. Curved Vanes (WAS Long focal lenght newtonians
The
mechanical engineers out there might remember that the material in a cantilever is uniformly stressed if the anchored end is wider, and it tapers down to a point at the free end where the load is applied. Lots of issues with cantilevers like vibration modes, stiffness, adjustability. I know Del Johnson has a single strut mount but for rigidity and stiffness, the standard vaned spider is hard to beat. Cantilevers not only depend on being rigid themselves but the tube also must be stiff. I still like the music wire spider using guitar tuners and 6 wires..... jon |
#133
|
|||
|
|||
Straight vs. Curved Vanes (WAS Long focal lenght newtonians
Jon Isaacs wrote:
Lots of issues with cantilevers like vibration modes, stiffness, adjustability. I know Del Johnson has a single strut mount but for rigidity and stiffness, the standard vaned spider is hard to beat. Cantilevers not only depend on being rigid themselves but the tube also must be stiff. I still like the music wire spider using guitar tuners and 6 wires..... jon Hi Jon, Absolutely, and there are two considerations with secondary support designs. The structural and optical aspects both need to be looked at. As is typical in life, they pull you in different directions. If you solely focus on structural requirements, the straight 3-vane design is the most efficient in terms of use of material. Another way of saying this is that you will get the most rigidity with the fewest number of vanes in this configuration. As we all know though, any straight edges will produce concentrated diffraction effects (i.e., spikes). If you only consider the optical effects, the best support is no support at all. Unfortunately, rigidity is rather poor. Curved supports are an attempt to blend the opposing optical and mechanical needs. A reasonable approach is to set a threshold for rigidity where it is deemed sufficient. For visual observing, two criteria need to be met. First, you don't want the static weight of the secondary mirror to cause excessive deflection in the support. This will cause the collimation to change as the telescope is pointed to different altitudes. Secondly, the support should not excessively resonate. This latter issue could be solved by simply beefing up the supports, but that results in an increase in overall diffraction. (That is the mistake many curved support designs make.) A better way to address vibration is to incorporate dampening elements into the design. Finally, none of this will make much difference at all in the lunar/planetary detail you'll see. Secondary supports need to be outrageously thick before they impact the MTF in a significant way. This all falls in the category of aesthetics. Some don't mind diffraction spikes, while others are bothered by them. Sincerely, Bryan Greer |
#134
|
|||
|
|||
Straight vs. Curved Vanes (WAS Long focal lenght newtonians
On 10/16/03 00:44 +0900, Bryan Greer wrote:
[ snip lots o' good stuff ] Finally, none of this will make much difference at all in the lunar/planetary detail you'll see. Secondary supports need to be outrageously thick before they impact the MTF in a significant way. This all falls in the category of aesthetics. Some don't mind diffraction spikes, while others are bothered by them. This has turned out to be a most informative thread. Thanks, everybody, for the great input. I'll stick with the straight 3-vane design of the stock spider. It'll be easier to build and collimate, and I doubt I'll be bothered by the diffraction spikes. When I see them in photographs, I find them rather aesthetically pleasing. Cheers, trane -- //------------------------------------------------------------ // Trane Francks Tokyo, Japan // Practice random kindness and senseless acts of beauty. // http://mp3.com/trane_francks/ |
#135
|
|||
|
|||
Long focal lenght newtonians as planetary scopes
"Larry Brown" wrote in message ... My 8-inch f/8 Newtonian blows away every other scope at star parties on the planets. This includes 4 inch apos, and every configuration of reflector you can think of, including a 25-inch obsession. SCT's don't even come close. All of the Newtonians yield sharper and more contrasty images than any of the SCT''s regardless of size. I've never compared it to an 8-inch apo, but I have compared it to 8, 11, and 16-inch achromatic refractors. Just my experience. Larry Brown http://home.fuse.net/astronomy I have an 8" f8 too which I bought second hand. Long after he had my money and all the sales pitches were done, the previous owner told me "Don't let aperture fever rule you. This scope gives better views of the planets, moon, and the Messier object than I ever got from my StarMaster 11" or my Obsession 15" scopes." I was surprised by that. Ed |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
NASA Finds Ocean Water on Mars - Long John Silver's Gives America Free Giant Shrimp To Celebrate | Ron | Astronomy Misc | 0 | March 25th 04 05:25 PM |
How to determine binoculars focal lenght? | Benoit Morrissette | Amateur Astronomy | 5 | September 3rd 03 01:03 AM |
OA Newtonian's are the best darn scopes... | Mike Fitterman | Amateur Astronomy | 95 | August 24th 03 02:55 PM |
Convex mirror focal lenght | André P. | Amateur Astronomy | 2 | August 8th 03 08:50 PM |