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MortonH
21-05-2014, 12:40 PM
A strange thing happened the other night with the focuser on my SV80ED. It's one of the standard Chinese 2" Crayfords and is just over two years old.

Basically, I took the scope out of the case and attached it to my mount. When I turned the focus knobs the focuser was rough and "grinding". It was the same with the coarse and fine focus knobs. It's particularly strange as it was fine when I last used it about 10 days ago.

I removed the focuser, checked for anything obvious (nothing) and put it back together. I did check the little grub screws around the shaft and adjusted them slightly to make sure they were all evenly tightened.

The coarse focus knobs are a lot smoother now but not perfect. However, the fine focus knob seems to turn ok for about a third of a turn then it hits a "bump", i.e. it stiffens slightly then "falls" over the bump before feeling normal again. It's doing this approx 2-3 times per rotation of the fine focus knob.

I'm at a complete loss as to how this has happened. The scope hasn't been dropped or mistreated in any way. In fact, I treat my scopes with kid gloves at all times.

I've emailed Stellarvue for advice but thought I'd throw it open to the many experts here as well. Maybe a trip to Bintel's technicians is required?

Cheers

Morton

killswitch
21-05-2014, 01:05 PM
Hey Morton

I had to fix up my slipping drawtube recently and found different screws do different things:

Yours might be similar to mine, if so have a look at this:

http://www.teleskop-austria.at/information/serv-just-hu/pdf/Adjusting_Crayford_focuser.pd (http://www.teleskop-austria.at/information/serv-just-hu/pdf/Adjusting_Crayford_focuser.pdf)

MortonH
21-05-2014, 01:37 PM
Thanks, Ed. I'll have another tinker tonight.

AG Hybrid
21-05-2014, 02:58 PM
Pictures?

MortonH
21-05-2014, 03:58 PM
Before or after I take it apart and truly wreck it???

Steffen
21-05-2014, 04:08 PM
Before and after ;)

Cheers
Steffen.

dannat
21-05-2014, 04:08 PM
i prefer rack & pinion to crayford -needs to made of decent material though

MortonH
21-05-2014, 04:31 PM
Agree. Stellarvue scopes now come standard with a 2.5" R&P focuser. I have one on my SV110ED and it's excellent.

If I can't fix the Crayford I might splash out and upgrade the SV80ED to match.

traveller
22-05-2014, 11:41 AM
Is the focuser the same diameter as the standard ED80? You may need to tap some extra holes to accommodate? :shrug::shrug::shrug:
Bo

LewisM
22-05-2014, 06:37 PM
Every Chinese Crayford I IMPROVED by adding shrink tube to the pinion shaft - gave the thing grip and actually holds more than a fly's weight.

I DETEST Crayfords - any flavour. Whilst Feathertouch are the best, I still found they slip enough.

R&P all the way, especially Vixen's dual speed version (which I prefer significantly to Takahashi's)

Try the shrink tube Morton. It works. Not as smooth as a standard Crayford, but at least it will hold something and actually move when it is supposed to.

MortonH
22-05-2014, 06:44 PM
What on earth is shrink tube???

FlashDrive
22-05-2014, 06:55 PM
You can buy it from any JayCar , Dick Smith Electronic Stores ..etc

It is very small ' dia ' tubing used in Electronics for covering wiring ... such as ' two wires soldered together ' ....it is ' shrink wrapped ' around the exposed wire to insulate it from causing shorts......

I've used it in Electronics myself ......used a cigarette lighter to gently ' shrink ' the tubing....

It comes in different sizes and coloured coded.

Flash.......

nebulosity.
22-05-2014, 07:29 PM
Heat shrink is a great idea Lewis! Will give it a go.

Cheers
Jo

LewisM
22-05-2014, 07:43 PM
If you get a tight diameter shrink tube, apply some vegetable oil to the pinion shaft before so it slides on easy. Then heat gently with a cigarette lighter or a hair dryer.

MortonH
22-05-2014, 08:08 PM
I think the Stellarvue is narrower. I considered replacing it with a cheap GSO model but not sure it would work without some sort of adapter.

According to the Stellarvue website, if I'm reading it correctly, their new R&P models have an 88.5mm screw thread, which is about 3.5". Not sure if any others focusers would fit.

traveller
23-05-2014, 10:06 AM
Interesting tip Lewis re shirk tubing, I must give it a try. Just a question, would the veg oil not eventually dissolve the plastic tubing?
I had a single speed crayford on my ED80 and have since changed it to a R&P as it holds the DSLR better (IMO).
Bo

MortonH
26-05-2014, 08:34 PM
For those that asked for pictures, here you go. Stellarvue asked me to send them so they can attempt to troubleshoot.

I also have some more info:

Ignoring the fine focus knob, I can get a reasonably smooth setting on the coarse focus knobs, but even here there is a mild 'sticking' once every turn. The fine focus continues to 'bump' three times per turn.

Looking carefully at the part that contains the bearings it looks to me as if the axis of rotation is slightly out of alignment, which would seem to explain the grinding issues. I can't see any way to adjust the alignment and have not tried to do so. There is nothing that is obviously 'bent'.

Anyone know if Bintel can deal with this? I forgot to call them today.

killswitch
26-05-2014, 08:53 PM
Is there any chance the bearings are being pinched by a retaining nut/washer that's on too tight?

MortonH
26-05-2014, 09:03 PM
Possibly, but I can't see anything that looks adjustable beyond what I've already done.

The other question is how this happened, as the focuser was working fine before. The only thing I can come up with is that I store the telescope case on its side rather than flat, and it seems the way I put the scope in may have been putting weight on the fine focus knob. So maybe over time something has shifted. Or, worst case scenario, something is now bent. :sadeyes:

killswitch
26-05-2014, 09:11 PM
Having a closer look, are the bearings supposed to be sticking out like that? It looks like it popped out of the housing.

MortonH
26-05-2014, 09:17 PM
Good question. I don't know!

MattT
27-05-2014, 08:13 AM
Morton your focuser looks like the GSO types of which I have 5.....so having admitted that here is what I think.
The ball bearings on the fine focus are supposed to look like that. I have found the grindy thing happens every now and then with my GSO focuser's too. The grinding sound problem was solved by unscrewing the grub screw and moving the knobs out of the housing and the fine knob away from the corse knob giving them all a tiny bit of wiggle room.
Works for me, only discovered after pulling apart cleaning etc. I guess the tolerances GSO focusers are built to is good but not great. By contrast a Feather Touch on my Newt stored in the same place as my two refractors never has this problem.
Might work for yours :question:
Matt

MortonH
27-05-2014, 10:26 AM
Hi Matt.

In the last picture the fine focus knob is the only thing attached and it still grinds.

MattT
27-05-2014, 06:43 PM
Three times every revolution must mean something that the 3 balls roll over. No dirt, grit or depressed insect that decided to end it all in the housing?
Thats all I can think of. Get rid of the grease and replace it with a small amount of Lithium grease.
Otherwise…a Feather Touch?
Matt

MortonH
27-05-2014, 07:48 PM
Yeah, reckon it's the bearings. Can't see any foreign matter but still have the feeling that the alignment is off a bit. Waiting for Stellarvue to reply - the owner is at a star party and won't be back for two more days.

Feather Touch isn't in the budget, but the excellent Stellarvue 2.5" focuser would be around $330 delivered, so we'll see.

nebulosity.
27-05-2014, 07:50 PM
Small bumps when turning ball bearings usually means one (or more) of the balls inside the bearing has a dent in it, this can easily happen if the centre part of the bearing has been nocked sideways somehow, which dents the balls.

Have had a lot of grief with this before when building focusers.

Jo

MortonH
27-05-2014, 08:06 PM
Is is possible to replace the dented ball in this scenario?

MortonH
28-05-2014, 09:50 AM
Stellarvue have replied and commented that my pictures seem to show a lack of grease. They suggest the first step is to add some new grease and move it around to see if that helps. However, they also agree that the rotation axis looks to be slightly out of alignment and may need some attention.

Anyone know the best option for someone to check the focuser? Bintel or somewhere else?

MortonH
01-06-2014, 12:25 PM
After discussing with Bintel it seems this issue is fairly common with cheap Crayfords and is a result of minor damage to the rotating pin and/or the bearings. Apparently the cost of trying to fix it wouldn't be viable, assuming I could even find someone to attempt it.

So I'm on the lookout for a replacement. Wonder what the chances are that someone has upgraded their Stellarvue focuser and kept the old one? Time to to post a Wanted ad...

MortonH
03-06-2014, 07:29 PM
Have decided to order the Stellarvue 2.5" R&P focuser. My scope is out of warranty but they're giving me a discount :)

Will be putting the old one up for sale if anyone is interested in trying to fix it, or just using it as is. It still works, just not as nicely as it should.

MrB
04-06-2014, 12:09 PM
Hi Morton, just noticed this thread.
I bought a used SV70 here (described as good condition) with exactly the same issue.
I noticed the drawtube has been crushed in many places by the bearings, indicating that it has been overtightened. Nothing can be done about that but a new drawtube.
The 10:1 knob (actually closer to 11:1 from memory) has the exact same issue as yours and I couldn't see the cause either.
I gave up and replaced it with a Moonlite, never looked back.

MortonH
04-06-2014, 12:14 PM
Sounds like these standard focusers aren't built to last, especially if you start hanging heavy imaging equipment on them (which I never have!).

I did strongly consider a Moonlite but they're another $100+ over the SV model. Plus I would have had to choose a colour and then live with that decision forever! ;)

MrB
04-06-2014, 12:42 PM
Ah yeah I was fortunate in that a used Moonlite popped up here for a great price. I machined up an adaptor to suit which saved me about $100 I think. Not as nice finish as the Moonlite adaptor but perfectly functional, including rotation adjustment.