Pier Head Modifications for EQ8

[Gina]

I started posting about this in another thread but as this is specific to the EQ8 I thought a new thread more appropriate.

First, a bit of history...  I built my pier and pier-head assembly to suit my NEQ6 mount.  Then I built the observatory around it but made a mistake with the height which meant my pier was about 8" lower than I intended.  I used it at this height for a while.  Here is a diagram and photos of the arrangement.

     

Two mild steel discs 250mm diameter x 10mm thick and one aluminium plate 200mm square and 5mm thick were used.  To take the mount needed a 30mm hole in the top steel plate and a 60mm hole in the aluminium to accept the 60mm x 5mm high protrusion on the mount base.  The 30mm part protrudes about 14mm below the 60mm part so an additional 5mm depth was needed.  The 60mm disc I cut out of the ali plate made a very convenient washer to add this depth.  Then the mount was secured with an M12 bolt and M12 "penny" washer.

To allow access to tighten/loosen this central bolt (needed for PA) just sufficient space was provided to get an open ended spanner in by using M16 washers on the M16 bolts connecting the plates.  Although later found to be unnecessary, the mount plate was levelled using the nuts on the threaded rods that protrude from the concrete (1m long and bent at the bottom to provide secure fixing in the concrete).  I used M16 washers rather than nuts as I thought the extra area would add stability.  I think this whole system was heavily over-engineered for an NEQ6 mount but I'm glad now as I'm upgrading the mount and adding a lot of extra weight.

[Gina]

To raise the mount by 8" and allow viewing down to the horizon in the east and south I bought an EQ6 mount extension second hand (from a member here as I recall) that meant I no longer needed the ali plate as the extension mounting systen was slightly different from the mount itself.

This is the new arrangement and what I have now. As the second hand extension lacked the long top bolt, I used the bolt from the NEQ6 tripod with a pile of washers

 

[Gina]

Now to the problem of fitting the EQ8.  There is just room inside the M16 bolts and washers to accommodate the 180mm diameter of the EQ8 but the hole in the steel plate is 30mm as required originally for the NEQ6 mount whereas the EQ8 requires a 12mm hole and M12 bolt to act as pivot when doing the PA azimuth adjustment.  Pity holes can only be opened out to a larger size and not reduced

Probably best solution would be to buy a new mild steel disc to replace the current one but the source I used before seems to have dried up.  So I think I'll go for the second best option of adding a plate on top with a 12mm hole in the centre. Aluminium plates like I used before for the NEQ6 are available as are other thicknesses.  I can get 2mm, 3mm, 4mm, 5mm, 8mm or 10mm.  I'll need a washer to bring the bolt head below the steel plate for access so I guess the ali top plate will only be acting as a PA pivot and nothing else.  I thinking that fairly thin ali would do - easier to machine and cheaper.  Maybe 3 or 4mm.