What's wrong with a Crayford Focusers?

[Rogering1]

Hi Guys,

Need your input

I've been researching the dual speed GSO Crayford Focusers to replace my cold war (gun metal colour) R&P for my GSO Dob, but there appears to be a lot of loathing about mid range Crayfords & with William optics moving towards R&P's...........

Forums seem to be split some people saying it was the best modification they have made & others saying they are hideous and should be avoided.

What can goes wrong with mid range Crayfords?

Note: I found 2 different GSO Crayford Dual Focusers models:(neither were low profile)

one was "linear bearing" used for SC's (I think) it had 1 adjustment screw I was told the linear bearing model was for heavy lateral motion on the SC's

The second model looked at the standard unit but with 2 adjustment screws.

[nightfisher]

I have found a crayford to be a big improvement over standard R &P focusers

[coulthamst]

All focusers whether R&P or Crayford benefit from careful and accurate setting up to suit the range of equipment the user intends to carry on that focuser.. There are good setting up instructional advice sites on the web. Those who attack this or that focuser type have usually expected perfection "out of the box"and thus been dissatisfied.

[Peter Drew]

Nothing wrong with the Crayford principle, it's the means for adjustment that can let them down.

[JamesF]

I have crayford style focusers on several of the scopes I tend to use only for visual observing and they're very pleasant to use. There are those however who find that if you're imaging and have a few kilos of camera, filter wheel plus filters and perhaps an OAG and guide camera hanging off the focuser, particularly as you get towards the zenith then the design doesn't work so well because they can slip even when they're locked. In those circumstances it's entirely possible that a rack and pinion focuser can work better.

I think it's fair to say though that really cheaply manufactured focusers are probably going to be nasty whatever the design.

James

[John]

I've used good and not so good crayfords and rack and pinions. Knowing how to adjust them helps get good performance out of them as does excellent mechanical construction. One or two of the cheaper crayfords look very impressive externally but are not well made on the inside, I've found.

Even the much maligned standard Skywatcher rack & pinions can be made reasonably smooth and positive for visual observing with a re-grease and some careful adjustments.

[JamesF]

I wonder if it's easier to get a crayford wrong? I guess you can be fairly sloppy with the engineering of a rack and pinion and it will still work ok-ish, but a crayford is heavily reliant on the quality of the roller, the machining of the face it rolls on and the tensioning bearings.

James

[John]

I think the problem may have been that we (the amateur astronomy community) decided a few years back that R&P focusers were rather crude and we wanted crayfords and we wanted them cheap. The Chinese pricked up their ears and duly obliged, as they tend to do, with glossy, relatively low cost crayfords. Most scopes are supplied with these now but you still hear about many who replace those with better quality crayfords so perhaps things have not really moved forward ?.

[sologuitarist61]

I must admit that I want a Crayford focuser to aid with perfect focus (10:1) and have been put off by some comments.

Has anyway bought one of those German ones that use a rail to take the strain as opposed to ball bearings? The TS variety is not much more expensive than the SW model and looks a good piece of kit.

[JamesF]

Isn't a Baader Steeltrak basically a crayford? They're reasonably well spoken of, I believe.

James

[John]

Isn't a Baader Steeltrak basically a crayford? They're reasonably well spoken of, I believe.

James

Thats my understanding too. I think it uses bearings but they run against the steel plates set into the drawtube rather than the anodised drawtube surface as is the case with most crayford designs.

[sologuitarist61]

Anybody got one who can give us a small review on how good they are?

[JamesF]

Gina was asking about replacement focusers recently and bought one on the strength of the responses I believe. I'll try to find the thread.

James

[JamesF]

There are a couple:

http://stargazerslounge.com/topic/157667-baader-steeltrack-v-moonlite-focuser/

http://stargazerslounge.com/topic/157425-baader-steeltrack-and-sw-fffr-for-evostar-80-ed-ds-pro/

James

[sologuitarist61]

Thanks James

[faulksy]

Thats my understanding too. I think it uses bearings but they run against the steel plates set into the drawtube rather than the anodised drawtube surface as is the case with most crayford designs.

also all the rolling/bearing surfaces are stainless instead of the body itself so a lot less friction so they just glide like they should. as you said john

[ollypenrice]

I detest Crayfords. I was one of the first to rail against them on here and I think that since I did so my position has moved from lunatic fringe to mainstream! I'm an imager and in visual use Crayfords are sometimes fine. If the focus drifts it isn't the end of the world, you just put it back, and the action is nice and smooth. In imaging, if the focuser drifts, your subs are scrap. And drift they do. Sometimes they hurtle out of focus rather than drift.

Cheap Crayfords really are abominanble and people all over every forum in the world are throwing the basic 'as supplied' models in the dustbin. The one on my Lunt is going in the bin ASAP. It doesn't work. A thread on here confirms that others are doing likewise. William optics have produced some of the worst in history. Mind you, it seems that they may have got their new R and P wrong as well but the jury is out on that.

We use a Steeltrack (yes, it is just a Crayford) on Yves' 14 inch ODK and I recently conceded on Gina's thread that it is coping with the big full frame CCD camera and 2 inch filterwheel with Off Axis Guider. Famous last words. No sooner said than it slipped two nights ago and messed up our run. Gina, I hope yours will be fine and your payload is a fraction of ours. Right now I have just refocused it because after an hour it had drifted. This is on an F6.8 scope so the focal plane is relatively deep. We are working near the zenith which will not be irrelevant. I will be up all night checking it, something which would be entirely unnecessary with my Takahashi and TEC (Feathertouch) R and P focusers which, so far as I can recal, have never budged. (And the Tak is sometimes working at F3.9.)

I'm not that fussed over discussions as to whether or not Crayfords could work. I am interested only in the fact that they frequently don't work. I do this for a living. I don't want to mess my guests about and I want to make my kit entrely Crayford free for just that reason. (OK, not the big Dob because the Crayford is visual only and is always horizontal.)

Am I ranting? You'd be ranting if you'd just spent all night awake for no good reason!

Whoooo, that's better!!!! Crayfords, get 'em off and keep 'em off.

Olly

[Stu]

..........and breathe

Having just experienced the crayford on my WO 66 flopping all the way out when pointing at the zenith with a DSLR on it, I know a tiny bit of the frustration you face regularly.

There's still something lovely and smooth about them for visual work though.....

Stu

[Steve Ward]

Dear Olly ,

Glad you got it off your chest , but please remember that not all , or rather not many of us out here can afford to spend £ 350.00 ! ! on a focuser

Steve.

[ollypenrice]

Dear Olly ,

Glad you got it off your chest , but please remember that not all , or rather not many of us out here can afford to spend £ 350.00 ! ! on a focuser

Steve.

Oh, I do understand that but you don't have to spend a bomb. I have a TeleVue Pronto with TV rack and pinion and the whole scope cost about £200 second hand. The focuser is great.

The OP asked what was wrong with Crayfords and my answer was that they slip when used for imaging. It is just one of those things that this kit is expensive, but to lose a night's data on an imaging rig is annoying and that is what you will do with cheap Crayfords. I cannot for the life of me see why an R and P has to be vastly more expensive. I also think that the reaction against Crayfords is going to see more R and Ps coming through. Crayfords may be cheap but since so many get replaced by upgrades are they, in the end, really cheap? Or are they, in reality, a rip-off?

Olly

[nightfisher]

I think we are starting to hit the nail on the head now, the cheaper crayfords are generally ok for visual observing, but not up to the task of imaging work.

On the subject of the new WO R&P focuser, seems its okay, just a bit of user error

[Steve Ward]

Fair point Olly ,

Personally I've not suffered from slippage with the Crayfords on either my 200PDS or the ED80 ,

I'm guessing that because I only hang a 1000D off the back helps somewhat ( main reason I chose the Canon was for it's weight after some advice and a quick comp with the equivalent Nikon)

And I also use a USB controlled motor focuser that holds things in place , a £40 per scope upgrade and £70 for the controller covers the 200pds , ED80 and ST120 all for a lot less than one Moonlite ( without motor and laptop control - another £300 ish) and gives far more control than even the 1:11 by hand.

I appreciate that you are doing this at a much more advanced level than the majority of folks here and are hanging all sorts of kit off your scopes ( fantastic images by the way) I just think that the current trend to immediately upgrade the focuser on everything seems a little unnecessary for the average Joe.

Each to their own at the end of the day I suppose ,

Clear skies,

Steve.

[ollypenrice]

I was mulling this over during last night's run and had an idea for Crayfords. What is needed is to uncouple the locking mechanism (the lockscrews on Crayfords have little or no effect anyway) from the focuser mechanism. It might go like this;

A flat strip of carbon fibre is attached to the main tube and runs back alongside the drawtube. Near it's rear end the drawtube has a small clamp through which the other end of the carbon fibre strip passes. When the locksrew of this clamp is loose the carbon fibre strip is free to slide through it for focusing. Once focused you lightly screw down the clamp so it bites onto the carbon strip.

Why might this work better? Firstly the carbon is unaffected by thermal expansion/contraction which is vital. Secondly this locking mechanism does not press on the drawtube. Normal lockscrews do this and induce a small shift in focus and sometimes in the image, neither of which is what imagers want to happen. Even the R and P of our Tak shifts if you lock it, though it is never necessary to do so.

Taking the idea a step further, suppose the carbon strip were attached to the front of the OTA at the lens cell and the Crayford were adjusted to have relatively little inherent grip. (No difficulty in arranging that, it seems!) Now any expansion or contraction in the main tube could be resisted by the thermally stable carbon strip. In this version the strip would work sometimes in tension, as in the original idea, and sometimes in compression as the main tube contracted during the night. For this to work it would need to be a stiff strip. For the first one, which is probably a better idea, it could be small and light because it only has to offer minor resitance by being pulled.

I'll put a fiver on someone coming back and pointing to an example of the former idea which already exists. I'd like to see it get a trial.

Honestly, all those folks changing cheapo Crayfords are not doing it just to spend money. I have put up with my Lunt focuser because it is just about possible to use it and contrary to appearances in my kit list I am not even slightly well off! The kit in my sig allows me to make a living.

Really glad to hear that the WO R and P is OK because they made the right decison in going that way.

Olly

PS after one adjustment and a bit of lockscrew the Steeltrack held for the rest of the night.

[FLO]

I was one of the first to rail against them on here and I think that since I did so my position has moved from lunatic fringe to mainstream!

Ooooh no it hasn't (said in best pantomime voice)

Sales of Crayford focusers continue to rise.

Crayford, Helical and R&P focusers have their pros and cons but ultimately it is the build quality (price!) that determines performance. Your Feathertouch is one of the most expensive focusers on the market. The majority of members would find it difficult to justify the expense and in most instances it would be quite unnecessary. Consider also that at the low-mid price end of the market the Crayford design produces a better product than R&P. A cheap R&P is horrible! And if I am not mistaken the 80 & 115 Altair telescopes you have been endorsing are fitted with Crayford focusers. I assume you are pleased with them.

I agree most of the Crayford focusers supplied with low-mid price telescopes are not good for astrophotography so I understand why you and anyone else imaging with heavy equipment invest in an expensive R&P but the majority of astro-photographers are quite happy using mid-price Crayfords.

If you do find a range of mid-price R&P focusers that perform well please send details because there is probably a gap in the market.

Oh, and you are most definitely 'not' a lunatic

Steve

[The Sailor]

For me visual they are great but put a camera on and all hell breaks loose, my Craford has been a pain from day one, been seriously thinking of binning it .

Jim