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

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Discussion Overview

The discussion revolves around the challenges of replacing a helical focuser with a rack and pinion focuser on an 8" f/6 Dobsonian telescope. Participants explore the implications of such a modification, including potential changes to the focal point and the overall setup of the telescope. The conversation touches on practical concerns, experiences with similar modifications, and the need for specific measurements to ensure compatibility.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant expresses excitement about acquiring an 8" f/6 Dobsonian but is concerned about replacing the helical focuser with a rack and pinion due to previous experiences that required adjustments to the mirror and altitude bearings.
  • Another participant questions how the focal point was altered, suggesting it may relate to the size of the new focuser rather than the focal length of the primary mirror.
  • A later reply clarifies that the issue was not being able to move the focuser inward enough to achieve focus, leading to the need to adjust the mirror height in a previous scope.
  • Participants discuss the importance of maintaining the f-ratio of the telescope and the implications of moving the mirror on that ratio.
  • One participant suggests measuring the current focuser's position to determine the required specifications for the new focuser, while admitting limited experience with such modifications.
  • Another participant shares a personal anecdote about cleaning the mirror and encountering unexpected issues with the finder scope, highlighting the challenges of telescope maintenance.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of replacing the focuser, with differing views on how it may affect the telescope's performance and setup. The discussion remains unresolved regarding the best approach to the modification.

Contextual Notes

Participants express uncertainty about the necessary measurements for the new focuser and the potential need for additional adjustments. There is also mention of financial constraints affecting the choice of focuser.

Who May Find This Useful

Individuals interested in telescope modifications, particularly those considering replacing focusers on Dobsonian telescopes, may find this discussion relevant.

rodentraiser
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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.
 
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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?
 
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.
 
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/
 
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*
 
rodentraiser said:
Surprise! There's no glass in the eyepiece of the finder scope!

Gah!
 

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