10" Dob Problems?

I just purchased my 10" and already made one modification. I replaced the particle-board mirror cell with a University Optics. I've tryed to focus on far away trees with a 40mm Plossl, and can't get a sharp image. I thought I measured correctly when I installed the new cell, but my thoughts are going down.

How far are the secondary and primary faces supposed to be apart? If I did miscalculate, what would be easier:
moving the focuser and secondary, or the primary?

It's a 10 inch, f/4.5 mirror.

Thanks for any replies!
-Adam Back

Definately easier to move the mirror but first you have to figure out which way. When the eyepiece is closest to focus, where is the focuser, all the way in or out. If out, then the mirror needs to be moved further back in the tube. First try to use the adjuster knobs/nuts to move the mirror further back against the springs. If you are close already that may do it. Otherwise you will have to drill new holes. Try to guess how much more to move the mirror and add maybe 1/4" to be sure.

Walt

It is much easier to move a primary--you don't have to cut holes in the tube to move the focuser.

Before moving anything--see how much difference there is in the height of the mirror above the mounting holes on the new cell compared to the old cell. (By mounting holes, I mean wherever the cell meets the tube.) That is the distance you need to move the new cell.

By the way--try starting out with new collimation. Sometimes there's enough travel to get things to work.

Alex

What do you mean when you say the trees are not sharp? Are they completely out of focus? If they approach being in focus, and then get worse as you continue moving the focuser in the same direction, the issue is not focus, but image sharpness. I suspect you do mean you actually can not reach focus.

If the focus is down below the focuser top, find a 1.25" tube that will slide down inside the focuser. Cut a disc of thin paper to fit inside the bottom of the tube. Aim the scope at the Moon, and slide the tube into the focuser until the Moon forms a sharp image on the paper. Now that you know where the focal plane is, you can figure out how far to move the mirror to push the focal plane out where you need it.

If the focus is outside the tube, you can reverse the tube so the paper disk is on the end sticking out of the focuser, and slide it out until the Moon forms a nice image on the paper.

IMHO, this method beats the heck out of trying to measure everything and guess where the focal plane is.

Adam,

I was not thinking.

Don't do anything until you try to focus on something at infinity (like the moon or the stars). Even things a quarter of a mile away are not at infinity.

Later in the thread, you mention that you cannot rack the focuser all the way out far enough to bring it to focus.

Try this: Rack the focuser all the way in, and then 1/4 inch back out. (Less than a quarter inch if you do not have much travel in the focuser--more if you have lots of travel.) Insert an eyepiece. From what you have said--it won't focus when fully inserted. Loosen the setscrew and pull the eyepiece out while looking through it. When you get to focus, set the setscrew. (Be sure you are focusing on the moon or stars--not trees or anything terrestrial. Measure the space between the end of the focuser and the stop on the eyepeice. THis will tell you how much you have to move the primary to get the system to work.

By the way--try out your various eyepeices. WHick one focuses closest? WHich one furthest away? What happens to the focus position when you use a Barlow or COma Corrector? All this should be taken into account before you decide where to put your primary.

Alex