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bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
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Time to estimate my truss pole lenghts. I wanted to ask what the group considers 'best practice' in a couple of areas.
1. Do you set the collimation bolts all the way up or all the way down when measuring the distance from the front of th mirror box to the top of the mirror.
2. Do you measure to th bottom of the curve of the mirror, or just use the edge thickness?
3. I always assumed you measured to the center of the secondary mirror when determining the length of the light path, but I've since read some things that make it seem others are doing something different. What has worked for you?
4. Is the quarter inch above the focuser rulr in K/B enough?
5. Is 1" of extra length on the tube length too much to allow for final adjustments with the assembled optics? How close do you cut them to the calculated length?
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Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
Oberwerk 20x100 binocs
Meade AR5
Meade LX10 8" SCT
All sold to a good home
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MitchAlsup
super member
Reged: 08/31/09
Posts: 186
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B/K (the earlier book--all I have) does not say where the mirror cell is initialized. I would assume it would be in the middle.
B/K sets the mirror box horizontal on a platform (table) to image something on the horizon. Then using the front perch of a ladder to hold the upper cage assembly, moves the UCA back and forth until focus is achieved.
Poles can then be measured and cut.
See attached image. Note weight on ladder.
Edited by MitchAlsup (10/28/09 08:23 PM)
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bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
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Thanks. I've got the K/B book, but that method seems cumbersome to me. I can't think of a good reason the pole length cant be honed in to within an inch by math, then focus testing done with the assembly....assembled. There will of course be some iterataion, but you can point the thing up in the sky and use stars to focus test as opposed to something that may or may not actually be at infinity focus on the ground.
-Brian
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Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
Oberwerk 20x100 binocs
Meade AR5
Meade LX10 8" SCT
All sold to a good home
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Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1624
Loc: Under the SE Michigan lightdom...
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On a clear night I just sat the mirror box on the patio pointed at the zenith and used a ladder with reinforced arms that I fashioned to hold the UTA over the mirror. I measured from the top of the mirror to the field stop of my longest EP(26 nag) in the focuser. I only took the mirror box cover off to see if stars would focus. I had to adjust the arms once to get the stars above focused and used that plus 1/2" for my starting point of the truss poles. Had to cut them down that 1/2" I added plus a little more to get them just right.
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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df_2112
member
Reged: 11/28/07
Posts: 25
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I think the best practice is to overestimate with the rough cutting. You can't add anything to your expensive investment once you cut them too short! With that in mind:
1. Rack the bolts all the way forward.
2. I use the back of the mirror and add the edge thickness.
3. Use the center of the secondary as that will put you in the center of the axis of the focuser.
4. Depends on your eyepieces. If you have a wide range in focus and need the in travel you will likely need more. I use 1/2 of the focuser travel as a starting point.
5. Generally once you set the box and upper cage up square and on axis to determine the exact angle of the trusses and calculate an average length 1" addition should be enough.
With my setup, I require just under 1" of travel to use both the 31 Nagler and 8 Ethos Eyepieces. So I attempt to set the focal plane to just focus the 8E (my EP requiring the most back focus) and then trim in small increments to give the 31 about 1/8 of in travel. Generally I need to cut between 1/2 to 1" off of the final length...which is a pain, but comforting knowing I won't need to buy any 'extra' trusses.
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Homebuilt Scopes/ATM:
12.5" f/5 Walnut and Maple Dob - 2005
16" f/4.5 Cherry and Maple Dob - 2009
17.5" f/4.4 Walnut and Maple Dob (in progress)
Refigured more mirrors than I can recall.
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