Introduction
The problem with many modern refractors is that whilst many of them are fitted with high quality engineered focusers, often with precision dual speed 1:10 fine focus adjustments; The suitability for electronic focuser fitment seems to have been a second thought; the result being a lack of standardisation. In the best cases, threads with blanking grub screws are provided on the underside of the focuser housing which hopefully will line up with slots in a motor-focuser attachment bracket.
The same criticism can be made of the design of electronic focusers in the sense that there is no standardisation of the design of the bracket for attachment of the motor-focuser to the scope focuser. This is a chicken and egg situation. Without standardisation of scope focuser housing, there can be no easy standardisation of motor-focuser attachment brackets.
Some motor-focuser manufacturers produce brackets for specific telescopes, but there is a plethora of telescopes such that it would be impractical to manufacture brackets specifically for all of them. Some scopes can accept the bracket of a motor-focuser with no problems. One such example is the William Optics Zenithstar 81 APO that will accept a ZWO EAF bracket with no modifications to scope or bracket.
In this study we are preparing a Stella Mira 66 ED f/6 APO to work as a test scope for our Software AstroDMx Capture in combination initially with the SVBONY SV405CC 14 bit, cooled OSC CMOS camera and focused with a ZWO EAF.
We found that the focuser bracket can be firmly attached to the scope by making two modifications to the bracket and no modifications to the scope. This involves drilling out a hole in the central elongated gap in the bracket to allow the wider of the supplied attachment screws to pass through to a suitable grub-screw blanked thread on the scope focuser housing. The second modification to the bracket involves using a hacksaw to cut through an outside rail and then using a file to remove a small amount of aluminium to accommodate the rather large focus lock knob on the scope.
The modifications needed for the bracket
The bracket is symmetrical, so it is possible to see the portion that has been removed by looking at the other side of the bracket.
Close-up of the fitted bracket and EAF
The large focus lock knob has been easily accommodated by this simple modification.
The ZWO EAF fitted to the Stella Mira 66 ED f/6 APO refractor
Of course, the focus lock is not engaged from now on because the EAF will effectively lock the focuser in whatever position the focuser is.
The rest of the rig comprises a x1 field flattener and an Altair 2” magnetic filter holder plus the camera and spacers. Fine adjustment of the spacing will be made when the scope goes into use.
In conclusion, the ZWO EAF bracket was easy to modify to enable the Stella Mira 66ED APO refractor focuser to accept it with no modifications to the scope.
