My refractor R&P focuser tweaks

[spaceboy]

Hi All

I finally got around to stripping the R&P focuser on my newly acquired C100ED. Anyone who has a Chinese refractor will no doubt have a R&P focuser attached to it and will already know they're not the best. As with most things mass produced there is room for improvements. Ones that can cost a fortune ie: fitting an aftermarket Crayford or carrying out simple modifications. In an ideal world I would have preferred the focuser upgrade but due to the cost of adapters and flanges on top of an expensive focuser I decided to go with making some modifications. Although similar to Astro_baby’s tune up guide http://www.astro-baby.com/Skywatcher%20Focuser%20Tune%20up/Skywatcher%20Focuser%20Tune-up.htm I planned to carry out further changes to help smooth out the movement.

The strip down is straight for ward enough with the only difference to AB’s guide being that there are two small grub screws next to the tension thumbscrew on top of the focuser that need to be slackened of first. Once I had the separate components I cleaned the old grease of with soapy water. While the grease had obviously been change at some point for a better substitute over the black tar usually found on Chinese refractors I decided it best not to mix my new grease with the old grease in case the two didn’t mix well. Next I turned my attention to the focuser body. Inside you will find two white corrugated strips which space the draw tube away from the focuser wall at the bottom and at the top is a black plastic strip. I removed ( you may need to use acetone or similar to remove the adhesive) the two white strips and replaced with a combination of Unibond exterior double sided foam and Teflon tape. The Teflon tape is very thin and if used by itself would not allow the correct fit and spacing of the draw tube. The Teflon tape is a adhered to the DS foam, cut to size and then applied to where the white plastic strips were removed. For the black plastic strip a piece of the Teflon tape can be applied directly.

Next is a case of re-assembly and adjustment of the 4 retaining screws for the pinion and the 2 top grub screws that adjust the pressure of the black strip. It is advised that you use a refractor collimation EP http://spacealberta.com/equipment/refractor/collimate.htm at this point as you need to make sure that although the draw tube is properly adjusted at both ends of focus that the collimation is not lost yet it still allows for a smooth focusing movement. It’s a fine balancing act and this is where you need to take your time and make small adjustments.

Needless to say collimation is the all important factor but due to the slight give in the DS foam and the reduced friction of the Teflon tape you can actually get the draw tube nice and snug with out making the movement excessively stiff. OK at the end of the day it is a R&P focuser and because of this there will always be backlash on inward and outward focus and equatorial mount users will on occasion experience image slop depending on the orientation of the scope but the improvements are well worth the time taken to do. The total cost is £1.99 for http://www.ebay.co.uk/itm/PTFE-Teflon-tape-mouse-gaming-/170689434341?pt=UK_Computing_ComputerComponents_MouseMats&hash=item27bde212e5 , £2 (from most cycle shops) for http://www.ebay.co.uk/itm/TF2-Lithium-Wheel-Bearing-Grease-Bike-Lubricant-40g-/150619793877 £4.71 for http://www.homebase.co.uk/webapp/wcs/stores/servlet/ProductDisplay?langId=110&storeId=10151&partNumber=680819 I'm sure there are cheaper double sided tapes out there but I find this useful for all sorts of things plus it sticks like **** to a stick.

My next mod was not so much for improving the movement of the focuser but more an attempt to save my diagonal from being slowly marred to death by thumbscrew pimples. I removed the 2” thumbscrew retaining ring from the focuser draw tube and set about making it into a compression ring fitting. As I have a hobby lathe this was pretty straight forward for me to do. I had a spare extension tube that I pinched the brass ring out of and made the necessary adjustment to the focuser ring. A new thumbscrew hole had to be drill and tapped so that it was central to the compression ring which now left the original thumbscrew holes. These were made tidy by threading in a couple of black grub screws that I’d got from a previous mod. A quick go over with a black sharpie on the machined parts and hey presto a 2” compression ring to securely hold my diagonal.

All in all I am happy with the improvements. I would prefer a DS focuser but hey beggars can’t be choosers.

[nightfisher]

jobs a good un nick, is that your own lathe i spy in the pics?

[spaceboy]

Thanks Nighfisher. I have a couple of tools at my disposal the lathe being one of them. They are a handy piece of kit and has found a lot of use recently for doing different mods on my scopes. This time it has saved me a few quid ScopeStuff sourcing an alternative.

A QUICK UPDATE:

I was able to get the refractor out under the stars last night and the compression ring mod worked an absolute charm. I was kind of hoping the focuser was going to be a pain to use as I wanted to justify the expense of fitting a crayford but the action was so much better it just doesn't seem worth the cost for what little amount of time I plan to use the scope. I think for lunar observing the focuser will be more than adequate. The Teflon is doing it's job perfectly. The draw tube is now held firmly in place with "as far" no noticeable slop while at the same time it glides almost effortlessly across the tape. The only let down it the actual R&P itself needing approx 1/8 of a turn before engaging the opposite focus.

I think with the C100ED it is also a matter of keeping a level head. I'm guessing that the intension of Celestron was to offer an optically superb refractor to the less fortunate of us (budget range so to speak). The compromise here was going to have to be the fixtures and fittings to compete against Sky-Watchers 100ED. My thinking is that Celestron made it such a difficult refractor to upgrade to allow the C100ED to remain an affordable refractor for the duration of it's observing days. If it was easy to upgrade then I'm assuming most first time buyers would have upgraded the focuser for a better crayford. When it came to selling the scope the added value of the crayford would have bumped the second hand costs way over what they are now and the likes of me would still have had to dream of owning an ED refractor. My point is that the C100ED is an affordable refractor S/H because of the R&P focuser. If I was to slap on a DS focuser the C100ED would no longer be offering me what it was sold as.... basically a poor mans ED refractor. If this was the case I may as well have gone out and purchased the bells and whistles SW version.

The SW DS crayford's are simply put, a superb piece of kit that is both functional and oozes quality. The optics of the C100ED are well worthy of the upgrade but upon reflection I can't help but think it would defeat the object of having the C100ED in the first place.......... an affordable ED refractor.

With a little TLC a R&P focuser can be improved on hugely with little expense and with no real DIY skills. I'm not the first to have carried out these sorts of modifications but I hope in sharing my experience others may benefit for making similar adjustments to their budget focuser's be it on a refractor or reflector.

[nightfisher]

Thats a well phrased summary and seems very apt, i hear on some scope`s r and p focus is the in thing

[spaceboy]

Apparently there are some engineered R&P focuser's that are far superior to the mass produced crayfords. I'm sure the price of them are far superior also If some thing comes up at the right price I'd jump on it as the scope would benefit from the improvement a crayford would make but until then the R&P is much better than it was so that will have to do.