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Why aren’t Celestron C6 focusers as good as their older C’s?

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#1 bmwscopeguy

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Posted 07 January 2024 - 01:15 PM

Anyone that has used an older C5, knows that the focusers were….special. The C6’s, while seemingly universally excellent optically, have a fairly stiff and inconsistent focus feel. Having just found myself the coincidental owner of both scopes, I set about to find out why.

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#2 MarkMittlesteadt

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Posted 07 January 2024 - 01:30 PM

I've owned so many Maks and SCT's over the years and they've always had a stiff(er) mechanism. I now own a C5, and at first I thought the focus knob was too loose. But it was just in comparison to the others in the past. I recall one C6 I had that was exactly as you described...the rubber cover was just pushed too tightly against the back of the OTA. I just pulled it back slightly, but trimming an 1/8" off would be a good, permanent fix.

 

My C5's focus knob operates very smooth and easy. 


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#3 EdFromNH

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Posted 07 January 2024 - 02:06 PM

I just pulled it back slightly, but trimming an 1/8" off would be a good, permanent fix.

Exactly what OP did in the article.  Great minds think alike.


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#4 Bob Brockway

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Posted 08 January 2024 - 07:16 PM

Excellent article. My guess for the rubber to metal contact would be for sealing out moisture. In the article you said to trim it off as opposed to just backing it out to maintain as much rubber to shaft contact as possible. Whether you trim off an 1/8" or back it out 1/8" without trimming it off you will have the same amount of rubber to shaft contact. Now, if only it was that easy to improve the R&P focuser on my ST80...maybe your next article (hint hint). 



#5 bmwscopeguy

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Posted 09 January 2024 - 03:03 PM

Excellent article. My guess for the rubber to metal contact would be for sealing out moisture. In the article you said to trim it off as opposed to just backing it out to maintain as much rubber to shaft contact as possible. Whether you trim off an 1/8" or back it out 1/8" without trimming it off you will have the same amount of rubber to shaft contact. Now, if only it was that easy to improve the R&P focuser on my ST80...maybe your next article (hint hint). 

Hi Bob, I don't know if the internal bore of the rubber cladding is tapered or not. Just in case it narrows towards the closed end, better to have it fully seated. Not that there's a lot of force to be reckoned with in either case.

 

With regards to your ST80 R&P focuser - I am assuming your issue is stiff focusing. Most issues come down to either the focus tube being too tight in the focuser bore, or the pinion shaft bearing too hard against the rack. Most import focuser bores typically had two low friction plastic pads/sheets at 120* and 240* with the pinion itself providing the 3rd point of contact.

 

This pinion is typically held in place by a U shaped metal part that bears against both sides of the pinion shaft. The U bracket hold-down screws can be tightened or loosened to press the pinion against the rack. Try backing them off a bit to see if this eases the motion. Also check to see if the rack, pinion and the shafts where they contact the U bracket have some lube on them.

 

If you can get at them without completely dissembling the focuser, try smearing a bit of grease on the friction pads too. You can always wipe off unwanted grease on the outside after. You only need a thin film.

 

These are easy and incremental improvements. If none of them help, then the problem may be that there are 3 sets of friction pads that are a bit too thick and are creating too much friction of the focuser tube... This would require a complete disassembly of the focuser (not a biggie BTW) and a light sanding of the bore friction pads to increase the internal diameter a tiny bit..

 

Hope this helps.



#6 RichA

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Posted 12 January 2024 - 06:03 PM

There are many examples of bad engineering out there.  Oddly some of it has nothing to do with cost-cutting, it's just bad.  Like rubber feet used on tripods which adds in most cases to instability.  Lets face it;  top-flight engineers go into academiia, government, aerospace, vehicle design, heavy equipment, not mass market telescopes.


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#7 MarkMittlesteadt

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Posted 12 January 2024 - 07:34 PM

There are many examples of bad engineering out there. Oddly some of it has nothing to do with cost-cutting, it's just bad. Like rubber feet used on tripods which adds in most cases to instability. Lets face it; top-flight engineers go into academiia, government, aerospace, vehicle design, heavy equipment, not mass market telescopes.


Most engineering is done by accountants.
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#8 RichA

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Posted 13 January 2024 - 06:49 PM

Most engineering is done by accountants.

Partly.  One major failure point of electronics (LX200 owners, take note) is using electrolytic capacitors that are at the bare minimum of volt tolerance for a circuit.  It's done for cost. 



#9 CharLakeAstro

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Posted 13 January 2024 - 06:52 PM

Great article Malcolm



#10 StevenBellavia

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Posted 17 January 2024 - 12:28 PM

For years, I thought something was going wrong internally, until I too discovered it was the rubber, and only very recently, when I removed it to install a ZWO EAF electronic focuser.  I am glad you shared this.


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#11 Old Speckled Hen

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Posted 19 January 2024 - 11:43 AM

You could always add some springs running on Thin Needle Roller Thrust Bearings to pretension any movement out of the system without adding any stiction/friction effects...

 

  shopping?q=tbn:ANd9GcQPiBhs0s2wChYQLqJvf

 

I'm sure there is a thread or something about it....


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#12 ANM

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Posted 22 January 2024 - 04:18 PM

Hey Malcolm,

Thank you for posting this article!

I have fought my 6SE trying to adjust the 3 screws in the bearing retainer plate to no avail. I trimmed the knob, cutting off a bit more than you did, and then used a belt sander to chamfer the end. Now the focuser feels just like my C5!

Thanks again!




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