Adjustable Telescope Declination Counterweight With 0g - 500g Weight Options

As I'm doing an astrophotography session I often change the equipment on the back of the 'scope, so I'll have various combintions of camera, off-axis guider, Barlow lense or a focal reducer, each of which will cause the balance point in the 'scope's declination to change.

In order to make the mount's job as easy as possible I designed this conterweight system that will allow you to balance the 'scope on the mount with your lightest equipment attatched, then when you add heavier equipment to the scope you can move the counterwieght and / or change the weight in the holder to get back your perfect declination balance.

The counterweight should be 3D printed using whatever filament you have to hand (and matches your 'scope's colour scheme ;-)).

As the device relies on two captive nuts, one for the M8 clamping bolt and one for the M4 weight retaining bolt, you'll need to have the ability to either slice the model with colour changes in the g-code and a printer that can suspend the printing at the two required layers, or have the ability to pause the print at the two correct print heights in order to drop the captive nuts into their respective slots as the print proceeds.

1. 1 off, M8 50mm Bolt
2. 1 off, M8 Nut
3. 1 off, M4 100mm Bolt (or M4 threaded bar, at a pinch)
4. 1 off, M4 Nut
5. Calibration Weight Set, with 2 x 200g, 1 x 100g and 1 x 50g weights (50g not used in this project). Available from Amazon in the UK with product code B07YFYT9GC (just put that code into the Search box), for £15.49.
6. 3 off, 19.5mm x 9mm Compression Springs
7. 3D Printed Parts
8. Counterweight body
9. M8 Knob
10. M8 End Cap
11. M4 Knob
12. M4 Knob Securing Piece

Start off by printing the main weight enclosure piece using whatever filament you wish.

All 3D printed parts should be printed in the orientation they are in in their respective STL files.

As mentioned above, you'll need the printing to pause at two points in order for you to be able to drop the two captive nuts in place. So, if your 3D printer supports filament changes, load the STL file into your slicer, choose a fairly coarse layer height (I did mine at 0.28mm) and then slice it, not using any support material at all.

Once you've sliced the object, view the print at a gradually increasing height until the slot for the M8 nut gets closed over and add a filament change at that point. Using Pruse Slicr 2.3.3 and a 0.28mm layer height this happens for me at 18.12mm. Then keep moving the print view up until the slot for the M4 nut gets closed over and add a second filament change there. For me, that happens at 57.04mm.

Once you've re-sliced the model, chuck the file onto your 3D printer in whatever way you do so, then set it away. If all goes well, the printer should pause to allow a filament change, at which point you should drop your M8 nut into the slot. Reload your filament and continue the print until it stops again ready for insertion of the M4 nut. Pop the nut into its slot, then restart the print.

If your printer and / or slicer can't do pauses, then you'll need to either manually pause your print, or try to sneak the nuts in as the print proceeds, but I suspect that that will be almost impossible!

Once the print has finished you should be able to thread your M8 and M4 bolts through their respective slots to show that everything is lined up correctly.

Now, get your three 19mm x 9mm springs and bend the very top coil of each of them up using some needle-nosed pliers so that it looks like it does in the photograph. This allows the spring to engage in a threaded hole at the bottom of the counterwight body. Use a pair of needle-nosed pliers to position each spring (opened up end pointing towards the threaded hole) against its retaining threaded hole, then screw it in until it is firmly in place.

3D Print the M8 Knob and End Cap using a fairly dense fill percentage and at a medium layer height. I printed in PLA and used an 80% fill and a 0.2mm layer height. No supports are required.

The M8 Knob should be an interference fit over the head of your M8 bolt, so it might need to be hammered into place.

Thread the M8 bolt in from the non-dovetail side of the counterwight enclosure, then thread the End Cap onto the end of the bolt, using some superglue to keep it in place.

The recess in the body of the counterweight allows the end cap to be mostly out of the way when the counterweight is attached to the dovetail on your 'scope, in case there are any retaining bolts for the OTA that it needs to get past.

3D Print the M4 Knob and Retaining Piece using a fairly dense fill percentage and at a fine layer height. I printed in PLA and used an 80% fill and a 0.1mm layer height, as the two pieces are an inteference fit and need to be printed accurately in order for them to fit together properly.

I used Custom Supports in the slot in the knob, with only the two overhangs at the very outside of the slot and the overhang right close to where the bolt will protude having supports, with all automatic support capabilities switched off. This is so that the bare minimum of support material was printed. Once the print is completed, carefull remove all support material so that the slot for the bolt head is totally clear.

For the retaining peice I allowed automatic support material creation, then just chipped it off once the part was printed.

If all is well, the bolt head should slip into the slot in the knob, although it might need 'persuading' with a small hammer), and the shaft should be right against the end of the slot, then the retaining piece should be forced in after the bolt head, securing it in place.

At the start of your astrophotography session, add the lightest set of equipment you expect to use to your telescope, as well as all cabling, etc., and then balance it in declination and right ascension (Dec and RA) as you would normally.

When you change the equipment on your 'scope, adding more weight to the back of the 'scope, slide the counterweight onto the dovetail at the front of the 'scope and then add enough weight at the appropriate point on the dovetail for the declination axis in order to get your declination balance correct again, then tighten the M8 Knob to secure the counterweight in place.

The next time you change equipment, add or remove weights, and / or reposition the counterweight, to keep your Dec balance correct with the new equipment.

If there are no weights in the counterweight body, just unscrew and remove the M4 bolt, slide whichever weights you think are required into their appropriately sized slots, then push them down against their springs and slide the M4 bolt back into place, then screw it home. The springs stop the weights from moving as the mount moves the 'scope to point to different points in the sky!

I hope that this works as well for you as it does for me :-)