ATM Forum Message Forums: Atm · Astrophotography · Blackholes · Blackholes2 · CCD · Celestron · Domes · Education Eyepieces · Meade · Misc. · God and Science · SETI · Software · UFO · XEphem
 Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...The Space and Astronomy Agora RE: Tom? Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response ToPosted by tom@digitalmoon.net on September 15, 2000 05:52:31 UTC

perhaps i should just list some charicteristics for you . lets assume that you had a bare 12 foot long 6 inch (standard,sch 40) pipe pier sticking up out of your concrete block. if you applied an 850 pound load sideways at the top of the pipe, it would deflect 1 inch, and cause a bending stress at the base of the pipe of 14,000 pounds per squair inch stress, about 1/2 of the yield stress of the pipe. unless you are into some sort of astronomical bondage, you can see that the pipe alone is strong enough for about anything that you are likely to do, also quite stiff. the rotation at the top of the pier would be about 0.6 degrees. if you could suddenly let go of the end it would vibrate at a natural frequency of about 1.7 vibrations per second, well below the threshold of human hearing, about 25 cycles is the lowest that we can hear. now, take the same pier and apply a 100 foot pound torque at the top of the pier (equivalent to hanging a 50 pound weight on a 2 foot cantilever sticking out sideways from the top of the pier). the angular rotation of the top of the pier would be 0.0117 degrees(42.22 seconds of arc). notice that this is the change due to the application of a new weight. and if you were looking through the telescope with crosshairs on one side of jupiter, the crosshairs would move roughly to the other side of jupiter. in actual practice, you are unlikely to be making these chnges while photographing, and the change would go largely unnoticed while viewing. with these parameters defined, you can now make your own calculations concerning the changes that you might make while taking a long exposure. remember, you will be making guiding corrections an order of magnitude larger than this anyway.