SW mount extensions

Merlin66 (Ken) · #1 12-02-2019, 09:21 AM

As users will be aware, the 200mm extension piece uses three M5 bolts top and bottom to fit the EQ tripod/ EQ head.
When mounted, the bottom bolts are held securely with the central bolt tension.
The top bolts however only locate in the EQ head with no longitudinal pressure to "lock" them in the slightly oversized holes.

The reason for mentioning this is that I use the extension on the top of a permanent pier.....

The last few sessions have given me poor guiding and poor GOTO...
Upon checking the PA (which when last checked a year ago was within 10 arc sec) showed the polar axis had dropped 1.5 degree

Unbelievable!!

Trace the issue to the head droop on the top of the extension. The mount holds a fair load - C11+spectroscope+electronic finder+ 4 cameras and 25Kg counterweights.
It seems impossible to "pre-load" the top bolts to give more rigidity.
The mount has now been realigned and the PHD and GOTO issues have gone away.

JA · #2 12-02-2019, 10:11 AM

Quote:

Originally Posted by Merlin66

Trace the issue to the head droop on the top of the extension. The mount holds a fair load - C11+spectroscope+electronic finder+ 4 cameras and 25Kg counterweights.
It seems impossible to "pre-load" the top bolts to give more rigidity.

Hi M,

It's difficult to say without seeing the extension, as I'm not sure ther may??? be a few types. In any event, perhaps you could take out some of movement associated with the oversized holes by wrapping the bolts /shimming the holes with brass shim or something similar or rebore etc holes or perhaps try using a countersunk sockethead capscrew instead of the conventional sockethead capscrew used. this could be tried with or without a countersunk (modified) recess. Better yet might be to redrill 3 new holes on the extension post to a very tight fit

Best
JA

Merlin66 (Ken) · #3 12-02-2019, 10:39 AM

All the SW HEQ5 and NEQ6 extensions seem to be the same design.
May be enlarging the holes and having a tapered enlarged section on the bolts would tighten things up.
At this stage I’m assuming it will find a “rest” position under load and become stable.

Will need to check PA regularly to verify.

JA · #4 12-02-2019, 10:44 AM

Countersinking the holes and using a countersunk socket head capscrew would be easier and/or redrill 3 new holes on the extension post to a very tight fit OR leave the existing 3 screws in place whilst redrilling 3 new holes at 120deg apart and drill and tap those holes, WITHOUT redrilling outside wall of the post, thereby providing NO clearance in the screw holes on the extension post

Best
JA

Merlin66 (Ken) · #5 12-02-2019, 11:15 AM

JA,
A couple of good ideas there....
If pain persists I’ll certainly consider them.
Thanks.

Merlin66 (Ken) · #6 12-02-2019, 12:22 PM

Another thing is that the EQ head bottom boss is a VERY poor fit in the extension tube.

JA · #7 12-02-2019, 12:36 PM

Quote:

Originally Posted by Merlin66

Another thing is that the EQ head bottom boss is a VERY poor fit in the extension tube.

If by poor you mean loose fit due to a slightly undersized boss diameter or oversized mating hole, then you could pack it out with brass shim or, dare I say it: tape of appropriate quality and durability.

Best
JA

Merlin66 (Ken) · #8 12-02-2019, 02:05 PM

Undersized boss diameter.
Any packing would have to be pretty solid.

JA · #9 12-02-2019, 02:13 PM

Quote:

Originally Posted by Merlin66

Undersized boss diameter.
Any packing would have to be pretty solid.

Depending on how undersized the boss is relative to the mating hole then use brass shim or even a large thin walled brass or copper tube split in half or thirds (and teased in to shape) to suitably make up the gap. It depends if it is several millimeters or under one mm. Something like a very short (maybe 30-40mm) length of diameter 32mm, 40mm or 50mm copper pipe/tube would possibly work. It has a wall thickness of around 1.2mm.

EDIT: having now seen the underside of the mount extension headpiece, I would now not recommend using tube (perhaps too difficult for piece). Simply fashion a strip of copper of appropriate thickness to take up the gap, possibly even 3 pieces, at the screw boss protrusions on the underside of the mount extension headpiece. You may even want to fix these in place with adhesive like araldite - It would depend on the surface contour and whether one used 3 separate pieces or one ring as a copper packer.

Best
JA

Ukastronomer (Jeremy) · #10 12-02-2019, 07:20 PM

Images ?

Merlin66 (Ken) · #11 12-02-2019, 08:58 PM

I don't have detailed close-ups of the extension but you can clearly see it in these images.

traveller (Bo) · #12 12-02-2019, 10:12 PM

I am a bit confused Ken.
Isn't there a central retaining bolt with a large star knob that secures the top part of the pillar to the underside of the mount?
Shouldn't that tension be enough to hold the mount to the pillar and the three M5 screws to secure it further?

Bo

Merlin66 (Ken) · #13 13-02-2019, 08:37 AM

Bo,
I understand the confusion...
The EQ head assembly is in two parts....
The head, which would normally fit onto the tripod and a bottom part which effectively replaces the "top" of the tripod. As is the case on the tripod these two are held together by the large central bolt (in the extension replaced by a new bolt and thrust washer).
This interface provides the azimuth adjustment (See fig3)
The base section of this assembly then mounts to the top of the extension tube using the three M5 screws. (Fig8)

The slop I'm seeing is between this base and the tube...
Hope this helps.

JA · #14 13-02-2019, 09:25 AM

or .....

Click here to load the video

Best
JA

The_bluester (Paul) · #15 13-02-2019, 09:31 AM

I had never really looked at the design of these before, I was considering one but I reckon I might give it a miss and just wait until I can put in a pier. The design looks to be very compromised in the name of cost and simplicity to me. Movement is inevitable. I can think of a few different ways to design it but it would cost more and take more setting up. I think the best way to fit them would be to fit it all up, put the payload on the mount and then slightly loosen the socket screws at the top of the extension to allow it to drop to a loaded position before doing them back up firm.

On the plus side Ken, I reckon that it should settle to a location and more or less stay there unless it gets a shock load in a different direction.

Merlin66 (Ken) · #16 13-02-2019, 09:46 AM

JA,
Thanks for that....
Step five is the problem area....
Paul,
That's what I'm hoping for.....