Pole length

I am about to cut my poles. I know the Kriege-Berry book says to put the mirror box and upper cage on a table and focus on an object over a mile away. I do not have that luxury. Can you safely determine the length of the poles by subtracting the distance of the racked in focuser to the secondary to the bottom of the upper cage and the distance from the primary to the top of the mirror box from the focal length. To that add the distance of the pole seats to the top of the mirror box?
It would look like this:
81.75" - 11.50" - 4.25" - 13.75" + 3" making the pole length of an 18" f4.5 my scope 55.25" Is this the best way or is there a better way. The only other way I can do the Kriege way is to build a stand to prop up the mirror box and slide the upper cage on a couple of boards to focus on Polaris. To me this seems cumbersome and dangerous. I want to get this right so I don't have to buy 8 more poles to try again.

Vince,

I have just recently completed this step of the process with my f/5 16". Like you, I didn't have a convenient 1 mile view since my property is surrounded by 70' trees. I have a low profile focuser, and that combined with my variety of eyepiece brands, barlows, and glasses/no-glasses viewing left only 1/4" on either side of my drawtube. Therefore, I opted to use the accurate but cumbersome process of building a sliding rack from scrap 2x4s that would hold my secondary cage.

The rack could slide on holes drilled through the rack that fit over 4 of my uncut 6' truss tubes. By sliding the rack holding the cage, I could bring the zenith stars to focus. The bottom of the truss tubes were firmly pocketed into a similar non-sliding rack that held the mirror box pointed at the zenith. With this set up I was able to use small plastic spring clamps (99 cent bin at the hardware store) clamped to the poles to hold the cage in place at focus, try out all eyepiece permutations, then measure the mirror box - cage separation. I took care when building the racks that the uncut tubes were equidistant from the central axis of the scope, then measured the separation at each pole to arrive at an average.

I took Kreig and Barry's advice and cut the poles 2" long. After installing the tube clamps on the mirror box, I assembled the scope with the overly long tubes and went back outside. Nothing came to focus; but I quickly determined I was OUTSIDE of focus (WHEW!). Three rounds of trimming 1", then 3/4" then 1/4" brought me right back to my original measurement and spot-on focus with the drawtube and collimation bolts set at their mid-points.

Sure it's cumbersome, but reassuringly accurate (plus you get a sneak-peak prior to first light)!
Using the actual components eliminates the "OOPS, I forgot to include the focuser board thickness" type of errors I always seem to run across when just measuring.

If I were to use the measure-only method I think I would cut the tubes 4" long and whittle them down 1" at a time until you can get something in focus, then measure exactly what to trim to put you where you want to be.

Mark Eschbacher

"The hurrieder I go, the further behind I get."

Vince,
It sounds like you've already gotten some good suggestions, but this may also help. I'm putting together a 10" truss, and used a couple of different methods to get the length nailed on stars. Here are a couple of pictures. Hope this helps!

Evan

Attached Image:

and here is the more crude method...

Attached Image:

You guys- I see a subject line like this my mind sees a golden opportunity- and he wants his poles cut to boot!

Jess Tauber

I for one have had success cutting poles by measurement alone. I used the basic formula you did except I added an adequate amount of focus travel to focus all of my eyepieces. In order to do this I looked through my eyepieces and found the one that focused at the shortest distance (for me, since they are not parfocal, I think it was the 27mm panoptic), put it in the focuser and racked it out a 1/4 inch for breathing room. When I took the eyepiece out I used this as the minimum focuser height. As it turns out the remaining out travel will focus all of my eyepieces.

After getting this number I measured from there to the inner ring of the upper cage to set the height of the focal plane including focuser board and shims to level the focuser. Then I added the radius of the upper cage and the distance from the center of the focuser to the bottom of the cage....

You could try focusing on a nearer object and then correct for the focus shift. With an 18" f4.5 mirror, your focal length is about 74". Say you go to the football field of your local high school and focus on an object 100 yards (3600 inches) away: The focal plane would be displaced by 74 * 74 / 3600 = 1.52". So, you'll have to move the upper cage 1.5" closer to the primary to focus on the stars.

I hope this helps.

Thanks for all the tips and advice.
I made a second set of blocks and drilled the hole all the way through.
This way I can get the length right on. I feel a lot better this way than setting the secondary on a couple of boards clamped to a couple of ladders and trying to get it the right distance in the dark.
If you knew me you would understand that would be an (expensive) accident waiting to happen. That was the way I had planned on before posting here.
I also will use a miter saw to cut to cut the poles.
Astromart is a great resource