Need to exchange a helical focuser for a rack and pinion one

[rodentraiser]

I'm just totally jazzed. I just bought an 8" f/6 Orion Deep Space Explorer (Dobsonian). Yeah, it's an old scope, but from what I understand, the mirrors on the older scopes were made by Discovery. I hope that would include this scope. At any rate, it needs two things to get it to viewing status. One is a right angle finder scope (on the way) and the other is something much more difficult.

This scope has a helical focuser and I would like to replace that with a rack and pinion focuser. My problem is the racked in height. The last time we did this on an Odyssey, replacing the helical focuser with a rack and pinion one, we changed the focal point of the scope and had to move the mirror up. Then we had to move the "altitude bearings" to rebalance the scope. I would really like to not have to do all that again.

I really don't want to put a lot of money into this, so I would only like to get a 1.25" focuser, not a 2" one. I do have a club locally where someone may be able to help me with this, but I was curious if you guys had any suggestions at all. Having had a scope with a helical focuser already, I'm not exactly jumping up and down for joy to use another one.

 

[Drakkith]

How did you change the focal point of the scope? That's set by the focal length of the primary mirror. Or are you saying that new focuser was too large and couldn't be moved inward far enough to get to the focus?

 

[rodentraiser]

Sorry, you are right. It has to do with not being able to move the focuser in enough to focus. We did have to move the mirror up to accommodate that new rack and pinion focuser on my old scope, though. That scope was an f/4.5, I believe. The thing is, this scope is currently an f/6 and I'd like to keep it an f/6. So I think I may have to get used to having a helical focuser again. I did find a focuser with a racked in height of 2", but it's one that takes 2" eyepieces (I know I could get an adapter) and the cost is more money than I want to spend. Let me put that another way. After replacing a Wil Tirion Deluxe 2000 star atlas, a portable table, a flashlight with a red filter, and buying a new lawn chair, basically, I'm tapped out. LOL Fortunately, I never loaned out my old eyepieces or filters, or I'd probably be replacing those too.

 

[Drakkith]

Moving the mirror isn't going to do anything to the f-ratio. That's determined solely by the curvature and diameter of the primary mirror (for a Newtonian).

Regarding the focuser, the first thing I'd do is take it outside, find something in the very, very far distance to focus on with it, and then measure how far out the focuser is from its minimum position. Then you can measure how far out from the tube wall that point is. Your new focuser needs to have a fully racked-in height of at least this much. Well, I think so at least. I don't have too much experience modifying my scopes. I've replaced a focuser on one of mine a few years ago, but I don't remember much about it, lol. If you want to make sure, I'd hit up the cloudynights equipment discussion forum and ask those guys. They should be much more trustworthy than myself. Here's the link: http://www.cloudynights.com/forum/61-equipment-discussions/

 

[rodentraiser]

Well, I know replacing a helical focuser on a Dobsonian with a rack and pinion focuser is going to change something. I will try the link you gave me. Thank you, Drakkith.

I have run into a small problem, however. I finally got around to taking the mirror out of the scope a couple days ago and cleaned it, and I think I have the scope pretty well collimated. The only problem I had was when I turned the collimating screws too far and the mirror fell right off the screws in the tube! I've never even heard of that happening. Anyway, I have that taken care of, and then I went to line up the finder scope. Surprise! There's no glass in the eyepiece of the finder scope! I never really did check that before I bought the scope because I had already known I was going to replace the straight through with a right angle. I've ordered that and it's on its way, but it's coming from Germany and it won't be here till the end of the month. *sigh*

 

[Drakkith]

Surprise! There's no glass in the eyepiece of the finder scope!

Gah!