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redbase

Slow motion declination issue on CG-3 equatorial mount

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Hello Everyone,

I am a beginner, and have been reading a lot of articles and watching a lot of videos about different telescopes, mounts, etc.  A used Celestron Astromaster 130EQ telescope became available locally for a really good price, so I bought it.   It has a CG-3 equatorial mount.  When I brought it home I noticed I could not turn the slow motion declination knob.  The  DEC clutch knob located on the side of the mount is secure, so the slow motion should move.  When I looked at it closer, I think it might be bent somehow, preventing correct turning.  For better or worse, I removed the knob connected to the DEC adjustment and unscrewed it from the mount.  Attached are pictures.  Once I removed it, I could turn DEC spindle but noticed it was warped.  If I back it out all the way, I can no longer screw.  How much are we supposed to be able to turn the slow adjustment knob?  I've looked for articles, but could not really find anything about how exactly the DEC slow motion works.  For RA, I can clearly see a gear which turns.   I also noticed that the metal pointer is against the DEC clutch knob.  Could this also prevent it from turning correctly?  Thanks.

 

 

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I don't know your specific mount but it seems similar to the skywatcher EQ2. That spindle being bent isn't going to help as the threads are probably going to bind as you wind it to/fro. On my EQ2 the DEC adjustment is limited and under the plate I can see a lever that moved to/fro driven by the SloMo spindle. It has a limit to travel by the housing that contains it (see pic)

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Not sure how easy it would be to straighten the spindle, you might get lucky or it may snap of course. Heating it may help prevent it snapping but add a challenge to handling unless you've a vice and blowtorch. 

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@DaveL59  Thank you for sharing your picture.  My mount is slightly different.  In your picture, what is the silver bolt pointed downward called? I have this too, and it is still attached.  It doesn't connect to the piece I removed.  I am also afraid I may have ruined the mount.  Do you know if this is so? 

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2 hours ago, redbase said:

@DaveL59  Thank you for sharing your picture.  My mount is slightly different.  In your picture, what is the silver bolt pointed downward called? I have this too, and it is still attached.  It doesn't connect to the piece I removed.  I am also afraid I may have ruined the mount.  Do you know if this is so? 

The top silver part is the DEC SloMo drive that the cable attaches to. The bottom one is the socket that the shaft from the SloMo goes into and acts like a bearing so it runs parallel. I've not had mine apart so far so can't say if there's a spring or bush inside of if it's just a single piece. We'd need a pic of yours to have any idea if parts are missing etc. What does the rest of the DEC gear look like?

If you can straighten it then maybe it'll work again as that bend will make the thread and bar bind against the component it acts against and when backed off probably the mount will just turn in DEC if the threads aren't engaging against the mechanism inside.

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@DaveL59 I have attached two more pictures.  The first one shows the location where I unscrewed it from the mount.  The scond one shows the underside of the mount.  The upside down T-shaped plastic part that points about 1:30 touched the round end of the piece I removed.  On the other side of this T-shaped plastic piece is another silver bolt looking part that has a spring the end that faces outward.  This also looks to be bent from the picture.

mount2.jpg

mount3.jpg

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ok I expect the remaining parts look tilted because they are leaning against the lever indicated with the red line

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That lever is acted on by the SloMo screw and allows the mount to be turned as required. The spring in the bolt part will impart movement in one direction as you unscrew the control and by screwing the control inward it moves in the other direction giving the DEC adjustment, if that makes sense. Hopefully all you do need to do is to get that first part straightened up tho that may not be easy to achieve and not damage the thread. Depends on how willing you are to try. Perhaps if you remove the screw and silver tube that remain you'll be able to see how they work with the bent part and if lucky will give you a means to protect the thread while you try to ease the part straight. Worth doing anyway just to establish that no other parts are damaged.

Not sure you can buy just that part but you might find an EQ2 used on astroboot and be able to do a straight swap, but check that it's the same and would fit your tripod before buying.

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@DaveL59  Thank you for the advice.  It makes sense the spring loaded bolt puts pressure from the other side of the lever.  I think it is logical this lever should be free to be moved around since the spindle on the other side is now missing but it does not move at all no matter how far I unscrew the other spring loaded spindle.   The other spindle I removed is now somehow to long completely screw back into it's original location.  I can back try unscrewing the other spring loaded spindle all the way but don't know what that would do the DEC control.   

This weekend I took the scope outside for viewing and was able to use the RA slow motion control.  I can possibly see how the DEC slow motion would come in handy when I am trying to locate a target in the sky.  I struggled to locate anything other than the moon and I was using the lowest power eyepiece I have which is 17mm.  The red dot finder is aligned correctly but I have read the RDF on these scopes is pretty poor, so I think if I get another RDF or another eyepiece with a wide FOV, I might have better luck.

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With both parts removed and the DEC clutch locked are you able to move the scope on the DEC axis by hand? It should move around 10 degrees or thereabouts I think and with everything removed should do that quite freely. You should see that lever also moving in sync with the scope. Be careful not to get your fingers in the DEC control tho while doing this so you don't get injured. If that doesn't work then it is somehow jammed and may need more dismantling. On assembly you'd need to have the bolt end slackened off so that you can refit the cable control side fully, then gradually tighten the bolt end to get things snug, just enough for the control to still turn smoothly. 

I wonder if somewhere in its previous life it's taken a big knock that caused the pin to bend and then the bolt was tightened further inward to make up the slack. Or perhaps the bolt end was tightened hard while the DEC axle was at the extreme end as in your pic.

As for the eyepieces, yes 17mm is maybe a bit much, a 25mm would be better or perhaps a 32mm for a wider FoV. Can't answer on your RDF, the ones I have are SW or non-celestron and work fine, but there are better, Telrad for example, or perhaps fit an optical finderscope of decent quality which you can always move to another scope when you upgrade.

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@DaveL59  I was able to get both parts of the slow motion DEC assembly and install them both on the mount without any difficulty.  I know have about 20 degrees DEC movement.   Once both spindles were out of the mount I was able to turn the lever smoothly.  Thank you for your help in showing how this all works together!  I agree in that it appears the telescope mount took a big knock in its previous life.  I am looking forward to trying it out soon.  I have used it about 6 times now, looking at planets Jupiter, Saturn, Mars.  I ordered a 32mm eyepeice and that seemed to help with finding objects in the sky more easily.  No matter how well I align it with the stock RDF, it is still off a bit.   I know the slow motion DEC assembly will help me fine tune the location.  I guess the next step is to upgrade the RDF to a Telrad or other such device, and purchase a sky atlas so I can look for other deep sky objects!

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that's great news and will make life easier for sure 🙂

For the RDF, is it just not able to adjust far enough in a particular direction? You may be able to tweak that by shimming the underside of the foot with a bit of tape at the front/back if its a vertical adjust you need, or loosen the fixing screws in the finder base and giving the base a tweak in the required direction and then tighten the screws. Do this with the tube horizontal or aimed slightly downward tho in case anything drops free so as not not to damage the mirror.

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@DaveL59  The RDF on the telescope itself is the old style which has 2 pieces of clear circular plastic (front/back) with concentric circles that have to be aligned along with the faint red dot.  It has very limited amount of adjustment that one can do and no real way to shim it without making it to where it no longer attaches to the telescope properly.  The RDF is advertised as "permanently mounted".  

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ah ok, yeah seen pics of those. Replacement sounds the best way but as I recall you may need to drill the OTA or use double-sided tape

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