Skywatcher focuser probs

matt · #1 03-10-2005, 04:43 PM

Hi guys. Very happy to join the IIS community

I've got a Skywatcher 8 inch f5 and am generally happy with it, now that I've learnt to collimate! But I've noticed something very irritating with its R&P focuser. There seems to be some kind of "grabbing" of the focuser tube the moment i turn the focus wheels one way or the other. It causes the focus tube to jump up or down slightly. Is this what's referred to as image shifting?? It's particularly noticeable when I've got the laser collimator in and I turn the focus wheels backwards or forwards. All of a sudden my well-aligned optics appear to go all over the shop!

Is there a fix for this problem, or do I buy a better focus unit???

Any help would be much appreciated

Matt

davidpretorius · #2 03-10-2005, 05:21 PM

welcome, Matt!

I have a 10" f5 gso dob. I have a crayford, so i won't be much help with your r&p, suffice to say that everyone that has commented has said it is the best upgrade you can do to go from a r&p to a crayford!

there are lot of guys here who should be able to help more.

Some great imagers in canberra, so ask on this forum and i am sure help will be nearby!

gbeal · #3 03-10-2005, 05:26 PM

Not sure what you mean, is the movement of the focus tube proper stiff, or loose/very loose?
Sometimes if stiff, a total dismantle, and re-lube can work. I agree with Davo though, maybe time to try a Crayford, although there is nothing really wrong with the R&P design. Mostly it is the glue they use as lubricant. Get a local knowledgeable astro type to look at it.

matt · #4 03-10-2005, 06:26 PM

Thanks guys. Appreciate your feedback.

gbeal ... in reply to your question, I've tried varying the tension knob to see if it makes much difference. Sadly ... only a little. The problem, as I've indicated, is directly related to the turning of the focusing knobs. The instant I begin turning them .. the tube (which moves in and out - the focusing tube itself) moves a little up and down as I rack the focuser in and out. It's a small but perceptible wiggle. How can I describe this? It's like a see-sawing action. When looking through a cheshire you can actually see the tube make a slight upwards movement when you turn the wheels one way ... and a downwards movement when you go the other. It's like the movement your hand makes when you wave at someone ... only not quite so much. It's as if someone were actually moving the tube up and down as you are looking down it.

Does that make any sense. Anyone??????

Cheers

asimov (John) · #5 03-10-2005, 06:31 PM

I have the standard R&P focuser on my refractor. Rather than get a new one I stripped it & re-lined it with that teflon tape where the draw-tube runs so there was very little side-play. Cleaned out the 'glue' & replaced that. Also re-shimmed the pinion so there was no back-lash, but not so tight it was 'cogging'. I still get slight image shift, but it's not too bad..It's much better than what it was.

janoskiss (Steve H) · #6 03-10-2005, 06:32 PM

Typical cheap rack & pinion focuser problem. My GSO has it too. One thing you can try besides the tension adjustment is tightening the four little screws under the pinion wheel. The ones shown in the photo below.

http://www.bintel.com.au/Bintel2focuser.jpg

asimov (John) · #7 03-10-2005, 06:44 PM

The problem your describing is called cogging I think? Pinion's too tight against the rack on the draw-tube.

matt · #8 03-10-2005, 07:07 PM

Cheers guys.

janoskiss: I'll look for those screws under the pinion wheel and have a play to see if that makes a diffference.

asimov. thanks for your suggestion re: the pinion being too tight. My knowledge of the R&P mechanism isn't too flash. will janoskiss' suggested adjustments take care of what you're suggesting?

Also, where would I get teflon tape and how do you celan out 'glue" and replace it and replace it with what glue?

And as for re-shimming!!!!!! LOL

RAJAH235 · #9 03-10-2005, 07:30 PM

Matt, Welcome.

Your description is perfect. Remove the four screws as in pic. Remove shaft & take out the focuser tube. Look inside to make sure that the teflon slides are flat & not twisted or doubled up somehow. The teflon & tube must be clean & dry. Do not lubricate with anything. Before you put the focuser back together, make sure that the tube slides easily/smoothly with no sticking action. Clean the glue/grease off the R&P, & check for missing teeth. Re-assemble,(I used a light smear of vaseline on both parts), & carefully adjust the screws to just apply enough tension so that there is zero backlash. IF the tube still rocks up & down, then you will have to repack it with new thinner teflon, till there is none. I had to do this when I made my electric focuser. Now I have zero image shift & no backlash.
As already said, if you're not sure how to, then ask someone from 'your' Canberra Astronomical Society to help.
HTH.

L.
ps. The 'GLUE' we refer to, is the original grease that they use. It may as well be glue, cos it ain't grease.

L. This just wipes off with turps/metho.
pps. Ask CAS members re; thin teflon tape. This can be stuck on with thin double sided adhesive tape.

westsky · #10 03-10-2005, 07:36 PM

Matt, the glue is the strange grease Synta use , it is only found behind the plate which is held on by those four screws.Replace it with white lithium grease.
Those screws are used to adjust the tension on the R&P, too tight and it will tend to grab
Behind the plate is for want of a better word a flat spring which puts pressure on the R&P
have a play to see if you can reduce the drag, I managed to get the drag out of mine.
Cheers
David.

asimov (John) · #11 03-10-2005, 07:47 PM

Try Steve's idea 1st. Try having the 4 screws a bit tighter, & note the focuser movement. If it's too tight, loosen the screws & try the movement again.

The 'glue' I'm referring to is the grease the manufacturer uses, it's really thick stuff. I got rid of all that gunk & replaced it with some high quality automotive grease. No doubt someone else could suggest something better.

IF the pinion is too tight against the rack, loosening those 4 screws will help to a point. But if this is the cause of the trouble, it's the housing where the pinion shaft runs on that's the problem. I had to put shims between the shaft & the housing to get some clearance between the R&P to fix mine, but not too far otherwise you end up with too much back-lash. I used very thin shim paper (actually made of brass) from an engineering W/shop.

asimov (John) · #12 03-10-2005, 07:50 PM

BTW Try googling around to find some R&P fixes etc with diagrams.

zipdrive (Dave) · #13 03-10-2005, 08:04 PM

there are two very small grub screws next to the focuser lock, try tensioning that a little if all else fails. you'll need an allen key. i think i have the same reflector as you and that's how i got rid of the rocking..

goodluck!

matt · #14 03-10-2005, 08:17 PM

Hey zipdrive.

That's amazing. I didn't even know those grub screws were there. You're right. They are tiny. I'm very grateful to everyone for the advice. I've got about 63 different remedies to try ... and you can bet I'll give 'em all a shot!! LOL

This forum is outstanding. So much help so quickly.

I guess that's what it's all about. Getting stuck in and finding out what works for you and your scope? If my short experience so far has taught me anything it's that very rarely is anyone completely right or wrong. It's all very circumstancial and there are many shades of grey!!!

So much depends on the individual scope or equipment and the way it's being used. By the way zipdrive .. how has your scope (8 inch f5) performed recently as far as observing mars? I'm still struggling to see anything more than a small featureless orange blob LOL

Isn't it only a month away from a fairly good opposition!

asimov (John) · #15 03-10-2005, 08:36 PM

I'd forgotten about those 2 allen screws! My focuser was unlucky enough not to have them lol.

westsky · #16 03-10-2005, 09:50 PM

Hi Asimov, just be carefull using automotive grease on the chinese scopes, they use a soft aluminum which can react to petrochemical based grease's, white lithium is the preferred choice as it won't eat the aluminum away.

cheers
david.

I forgot about the two grub screws as well

Mick (Michael) · #17 03-10-2005, 10:32 PM

Hello and welcome Matt, lots of help here.

zipdrive (Dave) · #18 04-10-2005, 01:36 AM

Quote:

Originally Posted by matt

By the way zipdrive .. how has your scope (8 inch f5) performed recently as far as observing mars? I'm still struggling to see anything more than a small featureless orange blob LOL

Isn't it only a month away from a fairly good opposition!

I too see it as an orange blob only

, a barlow does help a bit though. I use it for mainly taking shots at DSO's and it seems to do ok..
have a look at my gallery below:
http://members.westnet.com.au/dahn/astro

i guess you can get an indication of what the scope can do in the hands of a total noob like me..

iceman (Mike) · #19 04-10-2005, 07:22 AM

Hi matt!

to the forum!

How did you hear about us?

Another option is a crayford focuser, though i'm not sure how "plug and play" they are on the skywatcher - it depends on the size of the OTA compared with the GSO tube, but they should be fairly similar.

There's a recent thread discussing crayford options here.

matt · #20 04-10-2005, 08:31 AM

Hi Iceman

Thanks for the welcome.

Happy to be prat of the IIS community. I was referred here by a number of kind astrofolk including mick who's been very helpful at the astronomy online website. I was also told about it by a friend at the Canberra AStronomical Society which I recently joined.

IIS gets some pretty favourable mentions when you "shop around" the various societies and websites. So everyone's onto it!!!!! LOL

I'm gonna give the good people at Andrews a call today to see what they've got on offer as a potential solution but I'm keen to have a crack at doing it m'self first. Let's see how far we can go with what I've got before I splash out more of my hard earned

By the way ... where do I get teflon sheets etc for the focuser????