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Focussing problem


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#1 Chrischez

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Posted 18 April 2016 - 08:28 AM

Hi, I picked up a used Meade ds-2114 scope that was missing a few parts and in need of some repair. I figured it was a good project scope for me and something I could try some of the tweaks I'm not willing to try on my larger scopes. I removed all the parts from the optical tube, replaced a few missing screws, I center marked the primary mirror and flocked the tube with some cheap flocking material. The rack and pinion on the focuser (site tube) was shot so I had to replace it. I ordered a new focuser (whole assembly) but when I received it, it wasn't the right one. The base, although similar, was different and didn't sit flush against the tube when assembled. Rather than send it back, I figured since the only thing I really needed was the focusing tube itself, I would just swap that out with my original one. When I did, the rack and pinion didn't touch each other because the rack on the new tube was set just a little bit lower than the original. I removed a very small amount of plastic from each side if the focusing knob/wheel cradle allowing it to sit lower and reach the rack on the tube, worked fine. I then proceeded to collimate the scope. I did this without the corrector lens in place thinking it would be easier. I did my collimation with a Cheshire, site tube and laser and, although i struggled a bit, seem to have gotten things aligned pretty well. I then carefully slid the corrector lens back into the tube (I built up the side of the corrector lens with a little bit of electrical tape so it fit snugly but not so much that I had to force it). I tried the scope out last night but could not get it to focus. No matter what I tried to focus on and look at, everything was just a blur. I did manage to get the moon to a viewable point but even that wasn't very good. Jupiter I couldn't get to focus at all. I have read these scopes can be difficult to focus but I don't think this is difficulty, I think there's something off. I also read a previous post on here in which someone had a similar problem, turned out to be their collimation. Do you think it's my collimation or could it be the focuser?

#2 MistrBadgr

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Posted 19 April 2016 - 07:45 AM

I had an issue with a DS-2114 in my early days.  The problem ended up being that the mirror was ground perfectly, but with a focal length that was shorter than it was supposed to be.  The focuser simply could not get there.  I ended up getting the mirror refigured as a parabolic, instead of its normal spherical shape, and removed the booster lens all together.  It made a really nice rich field scope that I still have.

 

If you are moving the focuser in as far as it will go, then most likely the focal length of the primary is too short for the tube length.  You can cut off the rear end of the tube some to make up the difference.  I would try to cut off enough so that when the scope is focused, the silvery part of the draw tube just barely is exposed in the main tube or not exposed at all.

 

Hope this helps,

 

Bill Steen


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

#3 Chrischez

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Posted 20 April 2016 - 12:39 PM

Thanks Bill!! I will have to try tonight to see where the focuser ends up. I think when I tried using the scope the other night, there was still travel left in the focuser. I'm going to see if maybe my collimation isn't as good as I thought before I try anything out of the ordinary.

#4 Chrischez

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Posted 23 April 2016 - 07:16 AM

I had the scope out the other night after tweaking the collimation a bit. It made a big difference but I still had a hard time getting it to focus. I only had the scope out for literally a few minutes so I'm thinking, even though it's a small scope, maybe it didn't give it a good chance to cool down. I'll try again on the next clear night.

#5 MistrBadgr

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Posted 23 April 2016 - 10:25 AM

Thanks for the update!  Glad to here it was a little better.  Seeing does not adversely affect these small scope the way it does a bigger one, but it can still be a negative factor as well as cool down.  I am interested in what you find out.

 

Bill Steen


Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma




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