Crayford on an SCT - How much of a pain is it?

[michaelmorris]

As many of you will have read in previous threads, I'm currently exploring my options for providing fine focussing on a tight budget for my 8" LX200 Classic. At the moment I'm leaning towards buying a JMI Motofocus to attach to the main focussing knob in conjunction with a Peterson EZ focus conversion kit and FocusMax autofocussing software when imaging. Whilst this set up should reduce image shift, it will not be eliminated and may stay problematic.

The obvious answer is a Crayford focuser mounted on the rear cell of the LX200, but this will cause problems as I will not have enough clearance to use the scope near the pole (about 15-20 degrees radius around Polaris). My perception is that this would be a major pain in the ~*$; but is this perception correct?

So, calling all you who have a Crayford mounted on an 8" LX200 Classic (or similar); what do you think?

[anthony]

My WO 2 speed on the back of my Lx200 8" is abolutely great. Fine focus, focus lock, rotateable and NO imge shift.

Is it on a wedge and is this to help imaging?

Ant

[michaelmorris]

My WO 2 speed on the back of my Lx200 8" is abolutely great. Fine focus, focus lock, rotateable and NO imge shift.

Is it on a wedge and is this to help imaging?

Yes it's on a wedge and yes I mainly want it for imaging. As you will see from my original post, I guessed that a Crayford would be great for focussing; but, assuming you have it on a fork amount and assuming it causes clearance problems, how do you cope with the clearance problem?

[anthony]

Mine used to be on a wedge. I took the OTA off the forks and put it on a EQ mount.

When I had it in alt az and eq mounted on the forks I always found that when pointed to zenith or (when eq mounted) polar regions it was a pain in the .......... because of clearance issues. I was using an Mx7c camera which has a reasonably long barrel.

Not having an wedge mounted Lx in front of me it is hard to say. For me, north was my worst horizon for imaging and I was quite happily pointing south where it was not an issue and balance was easier. Now that I am chasing after M81 M82 in the winter months I can imagine it being a real pain.

Personally I would go for the WO 2 speed SCT crayford. It is solid and easy with no image shift as you are not moving the primary at all. It locks and holds quite a load very well. Overcoming problems 1) dont image where you can't (not really helpful as a suggestion) or 2) take it off and go back to normal 3) swop to an eq mount - (but why disturb so a nice set up and you might run out of room with an eq mount)

The other focusing option you mentioned I have no experience of. However, I have cables coming out of my ears in my set up which I am keeping as simple as possible. The more you have, the more that can go wrong. They can affect balance, they cause snags when guiding and slewing. However, I have looked at your, (superb), website and observatory set up and I am sure you would integrate that flawlessly. I shall pop down later today and measure up the distance the focusser adds to the set up. Remember to allow for - FR, camera, filter wheel etc in the imaging train.

You are welcome to have a look at it if you want. I am just off Jn 14 of the M5.

all the best

Anthony

[lx200gps]

Michael,

If you are considering buying the WO two-speed focuser then I should alert you a potential problem you will experience when attempting to marry it to a Meade SCT. In order to keep this message very brief I will not go into details except to say the focuser doesn’t fit properly. For more information, and a lively discussion with the regime at WO regarding this problem, you should refer to the WO Yahoo site. Search the site for “LBC8556” or “Alan Sickling” and you should find some interesting comments. This all kicked off again (it originally started last May when I posted a question about the focuser) around July and carried on through August. By all means have a look and draw your own conclusions. I believe Alan, who has a lifetime of experience in engineering, posted a couple of photos in the files section of the group which you may wish to consult.

Here’s a quote from an e-mail Alan sent to me around that time;

"Firstly, you are quite correct about the threaded SCT attachment ring on my 2” WO focuser, on two counts.

As you predicted, the ring IS finished in silver. To be specific, it has a very smooth satin-finish, hard-anodised surface, as you described. So it certainly is the same model as you got (before you modded it).

Secondly, I have now had time to try the fit on the OTA back, and just as with yours – IT DOESN’T! The specifics are that the male thread length on the OTA rear cell projects ~12mm out from the flat rear face of the cell. Unfortunately, WO makes the mating female thread on the inside of its connection ring only ~7mm deep. This causes the focuser attachment ring to become thread-bound before it beds down properly on the rear cell face. It leaves a gap between the face of the rear cell and the front of the focuser of around 5mm! For a proper fit with maximal rigidity for all the junk to be attached behind the focuser, these two surfaces should be firmly in contact.

Now WO advertises this focuser unit as being specifically for use with SCT’s. I wonder if they would care to specify which SCT designs this WILL fit correctly? (None as far as I am aware!)

I agree with you - how could they have got this so wrong?"

For those who do not wish to research this then the bottom line is that because the focuser does not butt up snugly against the visual back, the unit’s rigidity is compromised and could pose a problem when other astro gear is attached to it.

Something else that was discovered was that the inner barrel of the WO focuser was not concentric with the outer barrel. And as for the famous WO customer care??? Don’t get me started!!

Best wishes

Stan

[FLO]

I will not pretend to be an expert in this field (I am doing my best) but I have just added the Moonlite range of focusers to Auntie FLO's portfolio which most definitely do work:

http://tinyurl.com/yjsed9

I shall be listing them on the website later this month and prices will be very reasonable - none of the £=$ nonsense!

Hope that helps

[Martin 2]

If you fit one of these to a sct what happens with the normal focuser, doesn't it interfear.

M2

[FLO]

The original focuser works by moving the Primary mirror whereas the replacement focuser racks the eyepiece in/out - they don't clash. (The LX200 also has a mirror/focus lock which removes any possibility of it moving).

[michaelmorris]

Steve

The 8" LX200 Classic does not have the facility to add a mirror lock (unlike the 10" and 12").

I'm afraid the Moonlite focusser fails on two counts, it will still leave me with clearance issues and it's beyond my budget. I've got a maximum budget of £180 for a motorised focusser or £100 for a manual one which has the facility to a retrofit a motor.

I might consider spending a small amount more if the focusser will fit both the LX200 and Orion 80ED with ease. The motorised JMI Crayfords are beyond my budget.

[FLO]

Ok