Remote focussing for MN190

[Gina]

Gina you should open a scope repair shop! Next time anything of mine breaks I'm coming to you first

I think I might just have enough work with repairing and making my own gear. ATM I seem to be more short of time than money and I'm certainly short of that, though DIY about the place helps with repair/decorating bills

[fwm891]

That's a neat little motor, Francis. It's quite a low voltage / high current spec compared to the 12V ones we used on the focusers, but still within the current capacity of the EasyDriver if I remember correctly.

What's the application on the OAG?

Adrian

Hi Adrian - I want to add remote focus to the lodestar. Most of the time the focus stays matching the 383 but just occasionally it drifts defocusing the guide star. Both PhD and AA5 still guide OK on the defocused image but the graphs are tighter when the stars are well focused. This is a wish list job ATM so may not progress very far!

Gina: this is the new (and old) pressure plate I made for my focuser (Altair Astro 8 inch f4) its very similar to the MN190 plate. As you can see the new one has a pair of small bearings on an adjoining shaft. The pressure is applied to the shaft then via the bearings directly to the draw tube. The original tension screw is not affected as this adjusts the roller pressure. The new shaft and bearings sit midway between the 4 existing bearings where the original lock screw was.

New and old pressure plates

In position in the focuser.

[SnakeyJ]

Francis, another nice bit of engineering to get the very best from the stock focuser.

[opticalpath]

Hi Adrian - I want to add remote focus to the lodestar. Most of the time the focus stays matching the 383 but just occasionally it drifts defocusing the guide star.

A nice refinement, Francis. I haven't noticed a particular problem with slightly out of shape guide stars, but then I haven't tried refocusing to see if I get an improvement either!

Once you make the guide camera movable for focusing, do you think there's any risk of flexure creeping in? That would be my only worry - but I'm paranoid about it and like to have everything clamped tight!

Adrian

[Gina]

Gina: this is the new (and old) pressure plate I made for my focuser (Altair Astro 8 inch f4) its very similar to the MN190 plate. As you can see the new one has a pair of small bearings on an adjoining shaft. The pressure is applied to the shaft then via the bearings directly to the draw tube. The original tension screw is not affected as this adjusts the roller pressure. The new shaft and bearings sit midway between the 4 existing bearings where the original lock screw was.

I think I must be a bit thick this morning as I don't understand how that works. I can see that the new bearings may reduce movement in the drawtube but I can't see how they improve the Crayford. That depends on the pressure applied to the focus shaft and contact between that and the flat on the drawtube.

[fwm891]

I think I must be a bit thick this morning as I don't understand how that works. I can see that the new bearings may reduce movement in the drawtube but I can't see how they improve the Crayford. That depends on the pressure applied to the focus shaft and contact between that and the flat on the drawtube.

Gina - my explanation I expect.

Looking at your photo of the focuser parts, mine like yours has a curved plate with a bearing at each corner and that is fixed to the inside of the focuser body and these bearings sit against the round side of the draw tube.

On the other side of the focuser housing sits the pressure plate that provides the loading to the adjustment shaft against the flat area of the draw tube with a tension screw and locking screw. Mine is like yours.

What I have done:

I have provided an independent pressure point directly opposite the curved plate with the 4 bearings. This new pressure point although situated within the pressure plate, it doesn't touch the pressure plate, and has no effect on the normal operation of the pressure plate in providing pressure to the adjustment shaft.

The two small bearings act directly on the flat surface of the draw tube applying pressure on the draw tube and forcing it against the curved bearing plate, so the draw tube is now fully supported but still free to move as it is held in position by the 4 bearings on the curved plate and the two bearings pushing against the draw tube flat face.

You now only need to apply enough pressure on the adjustment shaft as is needed to move the draw tube and its camera load up and down.

I have positioned the two small bearings and their shaft within the pressure plate to stop the short shaft from rotating during operation.

Still not a good description, but think of it as though the pressure plate was no longer there. The draw tube would be held in place between the 6 bearings with all the required force to support the draw tube and its load being applied by the 2 small bearings via a new loading screw where the original lock screw was situated - brass screw in photo:

The silver coloured screw is the tension screw for the pressure plate.

[Gina]

Thank you Francis So I was right then, mostly The new bearings provide extra support to the drawtube to prevent movement against the Crayford pressure plate which might not necessarily be tight enough to hold the drawtube against the 4 main bearings when the imaging rig weight is pulling it against the Crayford rather than the 4 main bearings. This could happen when the scope is pointing towards the west. The Crayford is unaffected - that's what confused me, I thought you were imporoving the Crayford operation.

[fwm891]

... I thought you were imporoving the Crayford operation.

I hope I was/am by taking all the unessessary loading off the adjustment shaft and transferring it to the focuser body. Loading on the adjustment shaft need only be weighted to overcome the camera assemblies and slippage

[Gina]

Good point Francis

I now have the replacement bearing and just about to put it in the focuser.

[Gina]

Bearing replaced and everything back together Took the opportunity to smooth off some roughness on the Crayford flat on the drawtube. The focussing feels nice and smooth now

[fwm891]

Gina - if you really have problems with slippage stick a layer of very fine wet-n-dry < #800 on the flat surface - that will provide a little 'bite' if needed. I have wrapped some around the shaft on my little crayford on the OAG for that reason...

[Gina]

Thanks for the suggestion Francis - if I get a problem still I'll try that

[Gina]

Replacement focuser bearing.

[Chris]

Bearing replaced and everything back together Took the opportunity to smooth off some roughness on the Crayford flat on the drawtube. The focussing feels nice and smooth now

I've heard that smoothing the Crayford flat is a worthwhile thing to do as they often end up running on a burr from the factory, I'm not sure why these are not deburred during the manufacture process but I guess its not difficult to DIY. I know Dion uses a wet stone on his Youtube vid for this mod did you follow this method or do it another way Gina?

Chris

[Gina]

I've heard that smoothing the Crayford flat is a worthwhile thing to do as they often end up running on a burr from the factory, I'm not sure why these are not deburred during the manufacture process but I guess its not difficult to DIY. I know Dion uses a wet stone on his Youtube vid for this mod did you follow this method or do it another way Gina?

Chris

I just used a small piece of find sandpaper to smooth off the burrs - not as drastic as a wet stone. Yes, I've seen that video The burrs weren't from the manufacture but due to the shaft turning on the flat without moving it, I think. It seems to be working fine now. I had a few minutes between clouds last night and the remote focussing was working fine.

With the main focus correct I was getting nice stars in the OAG - 3 at one stage. It calibrated in 6-7 steps on the brightest with 3.5s integration time but it wouldn't guide after that. I think it was probably cloud causing the problem - the clear spells were pretty short. Looking better for tonight so fingers crossed

[Gina]

I think I now have all the bits required to build the Arduino control box. As I said, this will control both the focuser and dew heater. I also have nichrome wire and sleeving to make up the dew heater and that is well under way. As this thread is particularly about remote focussing I think I'll start a new thread to cover the dew heater and it's control.

[fwm891]

I think I'll start a new thread to cover the dew heater and it's control.

How many threads have you got going now Gina?

[Gina]

How many threads have you got going now Gina?

No idea Francis - too many probably But people seem to be interested in my activities and hopefully some of my ideas are useful to others