Fitting JMI mini Crayford to TAL 2M

[jamespels]

The R&P focuser on my TAL has been knackered for some time so I decided to invest in a new Crayford style focuser in spring. After a lot of hunting around, I settled on JMI's mini Crayford ("RCF-mini1", here: http://www.jimsmobile.com/buy_rcf.htm). I have the 3.75" base plate (perfect fit) and the extended draw-tube (more on this a bit later). Total cost is $148. I also added a motor controller (half way down this page: http://www.jimsmobile.com/buy_focuser_acc.htm). Total cost was a little under £200 shipped to the UK, but BEWARE VAT & DUTY - I am debating a £70 fee with HMC&E at the moment (should be about £50 - 20% VAT and 5% duty).

This is a lovely wet weekend so I decided this morning to fit the new focuser.

[jamespels]

First off, I removed the primary and secondary mirrors and put them somewhere safe. Then I took off the old focuser.

Having done this, I could put the new focuser base-plate up to the OTA:

The first joy here was that, while the base-plate has a quoted min radius of 3.75", it fits the TAL OTA perfectly without needing to prop up the outer edges (positioning screws are available for this - more later on them).

That just left the four mounting holes and the additional cut-out for the draw-tube assembly (the original R&P has a round hole; the JMI needs a key-hole shaped opening). These all needed to be marked out and cut.

In this pic, you can see the marked lines; the black bits above the lines are the finder scope mount; the two smaller holes are the existing mounting holes for the original R&P; the larger hole is the mounting hole for the spider.

With these checked, and checked again, it was time to cut out the key-hole and drill the mounting holes:

Note: I started out with a junior hack-saw and finished off with a Dremel router. Hence the rounded off "nick" in the hole in the next shot. The four mounting holes are a couple of mils larger than really needed - I wanted to make sure I have plenty of "wiggle room" later on.

[jamespels]

Next step - de-burr and polish off the new cuts and holes, remove the swarf, clean down and vacuum the OTA inside and out along with the surrounding area.

Moving on, first thing to do is try the base-plate on for size:

JMI provide four very long screws (about 2.5cm / 1") to mount the focuser. Following this trial fit, I found I could cut them down to about 1cm, giving a good three threads extra beyond the nut when tight with minimal protrusion into the OTA (I THINK the screws do not extend as far as the mirror outer edge so they should not cause any refraction issues - the cut ends will all be painted black though) . For anyone doing the same - tighten the nut all the way onto the screw, then cut - then you can unscrew the nut to straighten out the thread).

One more trial fit, and make sure there was no muck at all left in the OTA, and it was time to fit the focuser for real. For this, the base-plate needs to be fitted back onto the focuser before attachment.

Next are two pics showing the OTA interior with the draw tube all the way out and all the way in. Note the cut ends of the mounting screws.

I have just reassembled the OTA and popped it back onto the mount so pics of the next stage - aligning the focuser - to follow.

[Spikey]

Nice job so far James

[nightfisher]

Nice tidy job james, this year i WILL have a look through it at PSP, just a thought, how about flocking the inside of the tube

[jamespels]

Cheers both,

Jules - it's not clear from the pics but TAL score the inside of the tube and paint it matt black - the inside is very rough (as my knuckles will confirm!).

When I paint the ends of the screws, I will probably touch up the interior of the OTA too but I don't think flocking will achieve too much.

J.

[AndyH]

All great stuff !!

Looking forward to the next part.

Andy.

[jamespels]

Well that was fun! First time I've ever collimated the scope (bought a Cheshire just fro the job) so I had the joy of working out what was focuser alignment and what was collimation

Reasonably straightforward but first thing to note is the focuser tube is NOT exactly square. This means I had to use the adjustment grub-screws in the base to align it correctly:

Note the silver grub-screws outside the main mount screws: these contact the OTA surface and allow you to independently set the height of each corner. I had to lift the front end quite a bit, and one side a little, too. As a result, I only just had enough thread to play with on one of the mount screws, illustrated below (previous post edited to reflect this EDIT: cannot edit older posts for some reason! If a mod sees this, please PM me).

After a lot of shenanigans and reference to Astrobaby's collimation guide, I finally got there:

I think it fits in well with the general TAL look and feel - I may get round to sealing under the base-plate with something black but really not sure if there is any benefit to this.

[jamespels]

So, what do I think of the focuser and what would I do differently?

Regarding the focuser, I like it a lot, especially at the price. It it simple to adjust and the motor drive works brilliantly. The motor drive is lovely - simple controller box with "in" and "out" buttons. The focuser itself is single speed but the motor drive has two speed settings (starts very slow then speeds up after about a second) so very fine adjustment is a doddle.

Now for the negatives, though none of them are major in any way.

1. the draw-tube itself - the TAL has a lovely split-ring inside the draw-tube that stops the EPs from getting marked; this one has a tapped hole in the side and you hold the EP in place with a bolt through it. In fairness to JMI, they do offer an upgraded draw-tube (see the links earlier in the thread) and they also supply a plastic EP set screw. I am going to resolve this be chopping the JMI draw-tube down to match the TAL one and threading the outside of it to take the TAL attachments. I guess the local engineering shop will charge me about £25 for the privilege so this rally is not a biggie.

2. While the motor drive is really nice, it is secured onto the focuser using two screws at one edge; it has a sprung hinge to hold the drive against a cog on the adjustment axle and a simple thumb-screw to lift the drive clear. This makes it really simple and fool-proof, but results in the metal end of the thumb-screw rubbing the paint off the focuser body.

3. Adjusting the base-plate was a pain. A little more effort in the build of the focuser would ensure it is true, but then this is reflected in the cost (try getting s motorised moonlighjt or fethertouch for under £200!). Ultimately, the build is solid snd it's not going to move anywhere so I really cannot complain that it took me a couple of hours to set up right.

What would I do differently? Not much - but I wouldn't cut the mount screws until I had it fully adjusted! I VERY nearly ran out of thread on one screw...

[Moonshane]

great job James. re the set screw, you could always use a nylon thumbscrew? if you can get the Tal fitting moved over for a few quid though....

[AndyH]

God, I have a bizzare techno crush about knurled thumbwheels. They just look the part !!

[Gina]

Good looking mod - hope it gives you good service

[jamespels]

Heh - the knurled knobs swung it for me

Shane, I thought about a nylon screw but I thing it will wear out too quickly in a metal thread that I am likely to use quite a lot. Not too fussed about it but a nylon-tipped screw or a s/steel rubbing strip would solve the problem neatly. I guess my point is this is an irritation but doesn't really detract from the focuser. If you mean the EP lock screw, JMI have provided a plastic screw to avoid damaging the EPs. Will definitely get it tapped to take the TAL bits & bobs though.

Gina, thanks! Looking forward to getting the scope out if we get a clear night again this year... Have the Baader solar film ready for Tuesday morning just in case.

[Gina]

A clear night...??? Wassat!!!!! I have Baader solar filter too but I'm afraid I'm not holding my breath