Converting your GSO Single Speed Crayford Focuser to 1:10 Dual Speed
Text and photos by Anjal Sharma. Copyright 2008 Agena AstroProducts.
This conversion kit allows you to easily convert your single speed Crayford focuser on your GSO scope into a precision 1:10 micro dual speed Crayford focuser. If your scope has a reasonably fast focal ratio, say f/7 or lower, the depth of critical focus is extremely shallow and becomes even shorter for very fast focal ratios. Although the single speed Crayford focuser allows you to achieve critical focus for visual applications, it may not be precise enough to achieve perfect pin-point focus for imaging applications. This conversion kit will allow you to greatly extend the functionality of the Crayford focuser unit and enable very precise focusing during imaging as well as for visual use.
Please note that this kit can only be used to convert the newer GSO single speed Crayford focusers which have a small set-screw sitting between the tension adjustment and focus lock thumbscrews on the underside of the focuser. Older versions of GSO’s single speed focusers which lack this little screw nestled between the two larger ones cannot be converted to dual speed.
The photos below show a dual speed upgrade being performed on a reflector Crayford focuser. The process is exactly the same for refractor focusers.
1. Assemble the parts together on a flat surface. You will require your single speed Crayford focuser, a Philips head screwdriver, a 2mm Allen wrench and the 1:10 dual speed focuser knob with shaft, as shown in the following figure:
2. Use the Allen wrench to loosen and remove the retaining screw from the existing single speed focus wheel. The figure below shows the retaining screw being removed from the left hand side focus wheel – this will allow placing the dual speed focuser knob on the right hand side (note that because of the asymmetrical design of the microfocuser plastic housing, you cannot put the microfocuser assembly on the left side in the photo below. It must go on the right.):
3. Remove the focus wheel by gently but firmly pulling it off the shaft as shown and set it aside:
Note: There is a small plastic part which securely guides the placement of the focuser shaft within the focuser body. This part is recessed within the focuser skirt as shown, and may come loose when the focus wheel and shaft assembly are removed in the previous step. If this happens, please remove the focuser drawtube and place this part back in the recessed hollow as shown – FAILURE TO DO SO WILL COMPROMISE FOCUSER FUNCTION:
4. There is a small plastic ring around the shaft to help guide smooth focus shaft rotational action during use. Gently tip the focuser to the left to remove this ring as shown and set it aside:
5. Remove the remaining focuser wheel and focuser shaft assembly by gently but firmly pulling the focuser wheel straight out as shown – store this assembly elsewhere in case you want to re-convert back to single speed in the future:
6. At this point, the partially disassembled focuser will resemble the picture below:
Unscrew and remove the locking and tension adjustment screws and use the Philips head screwdriver to loosen and remove the central screw.
7. Gather the conversion parts as shown in the figure below. Also, it is EXTREMELY helpful to turn the focuser drawtube slightly towards the dual focus knob side, so that the shaft can be easily inserted and the drawtube will naturally turn back to the optimum squared on position after insertion.
8. Insert the focuser shaft on the dual speed focuser part into the appropriate port on the focuser body as shown. Ensure that the insertion is done gently, but firmly with even pressure applied and the shaft does not bend during insertion. Also, note that the focuser drawtube begins to turn back to its optimum squared on position as the shaft is inserted and becomes fully squared on with the tension and focus lock screw holes when the shaft is fully seated in position.
Important Note: While doing this, try not to grip the microfocuser assembly by the knobs (black or silver); grip the plastic housing instead. If you need to get a better grip, put two fingers on either side of the silver focus knob on the microfocuser and push it in gently with even pressure. Don't force it though. If you meet resistance, remove it and try again. Applying too much pressure can result in damage to the microfocuser itself and/or scratches on the inner drawtube.
Insert the plastic roller onto the focuser shaft as shown:
10.Insert the focuser knob removed in step 3 back onto the shaft and push it in completely as shown:
11. Fasten the retaining screw onto the focuser knob using the Allen wrench as shown:
12. Fasten the remaining screws as shown:
The focuser tension adjustment screw can be tightened or loosened to obtain the desired stiffness in the focus action. We recommend that this adjustment be done incrementally and the focus action (both coarse and fine) be tested after each incremental adjustment to ensure that the focuser turns smoothly and accurately with the right (for you, the user) amount of tension and zero backlash.
Congratulations!! You’re done, and your completed dual speed Crayford focuser will look similar to that shown in the picture below: