EQ8 overhaul

[peter_4059 (Peter)]

I've decided to strip, clean and overhaul my EQ8 after five years of use. While it spends most of its life in an observatory, those cheap steel bolts do not last and rust was starting to set in on the tripod bolting.

I purchased a number of upgrade kits from Shibumi Astro Engineering in Ireland and they arrived a week ago. After digesting the instructions, I have started work on the mount. The scope will include:
1. Replacement of most of the carbon steel bolting with stainless on both the tripod and the mount.
2. Replacement of the deep groove 608 bearings with angular contact 708A bearings.
3. Replacement of the DEC drive coupling with a stiffer option.
4. Replacement of the 80XL RA timing belt and pulleys with a 2GT system with Fairloc clamping systems.
5. Replacement of the DEC weather seal to prevent moisture ingress.

Tony from Shibumi has done a good job documenting how the mount comes apart including some quite detailed assembly drawings.

https://shibumi-astroengineering.com



I'll post pictures as the project progresses..here are a couple showing the kits and the tripod with new bolting.

[Ryderscope (Rodney)]

Hmmm, most interesting Peter. I’ve had my EQ8 operating in my obs for 8 years now and it has been working very well. A couple of adjustments to tighten the clutches and remove some backlash is all that I have done. I will be looking forward to see how you go with your upgrade.

Clear skies,
Rodney

[peter_4059 (Peter)]

Good to hear your mount is humming along. Mine makes a trip to astrofest each year so it gets some outside time and the humidity here is probably worse than your location. Have you measured the periodic error in your mount?

[Ryderscope (Rodney)]

Originally Posted by peter_4059

Have you measured the periodic error in your mount?

Measuring PE with PHD2 gives me a reading that usually sits just under +/- 8”.

[peter_4059 (Peter)]

Hi Rodney - I realised why you arn't concerned about the state of the bolting on your tripod - you are using a fixed pier!

I've done quite a lot of analysis of my mounts PE and while it is pretty good, there are a few peaks on the frequency spectra between 2 and 5 times the worm period that are most likely related to the worm bearings. The stock bearings are 608ZZ deep groove (same as used in roller skates) and this bearing design is intended for an axial load. The worm bearings in a EQ mount are seeing a thrust load and angular contact bearings are more suited to this application. There is also a blip at 18 times the worm period that is related to the timing belt teeth meshing. The actual PE of the worm in my case is 3.5 a-s peak to peak based on measurement over 25 worm cycles unguided.

I'm hoping my planned upgrades reduce the amplitude of the peaks from 2-5 times the worm period as these can be tricky to guide out. The belt teeth meshing can be improved by tightening the timing belt however this also puts load on the worm.

[Ryderscope (Rodney)]

Peter, whilst my PE is running at about the specification for the mount, it does not cause me any issues as I am running around the 0.5” RMS guiding with PHD2. This is with my Tak TSA120 with reducer for a focal length of 675mm. With these metrics the mount is performing fine.

My only bug bear with my mount is the DEC backlash. When I measured it I recorded a 6 second backlash using PHD. I managed to adjust this down to about 3 seconds but cannot get it down below that as the amount of backlash varies as it moves around the DEC axis. Having said that, PHD handles this quite well and I don’t think that it is adversely affecting my guiding.