I think I may have a problem with my mount dec drive chain.

[Stargazer33]

Last night I actually managed to get out with clear skies, no wind and no twinkly little stars (seeing was, as far as I could tell with my limited knowledge, good!).

I set everything up: Advanced CG-5 GT mount, C8 OTA with Revelation R&P dual-speed focuser & Canon 1100D. Guiding was done by a piggybacked Celestron Travelscope 70 and an Orion Starshoot AutoGuider through PHD.

Imaging M57 (Ring Nebula) to the East PHD was guiding on the RA axis okay - wavy trace on the graph but well within the middle section. However the dec trace went straight for only a few seconds or so and then took a dive off the bottom of the graph, never to return.

As a result of this I couldn't image for longer than 90 seconds without the start of startrails; and that is guided!

I am sure that I have read on this forum that people are getting 120 second subs on the CG-5 GT without guiding.

I did a polar align with the Celestron polarscope - the one with the plough and Cassiopeia asterisms. I then did a 2 star alignment and 1 calibration star but I didn't do a drift align.

I am wondering if my set-up is too heavy for the mount. I had to add a couple of 1 kg lead shot dive weights to my counterbalance shaft to balance the imaging set-up. I normally have to run with both the large and small weights anyway, to balance the OTA, focuser, barlow and eps.

Has anyone got any ideas of things that I can look for or try, to get my dec drive chain working better?

Thanks in advance.

[IanL]

Try this guide (The CG5-GT is basically the same mount as the SW EQ5 if you are looking for more tips):

http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

- The spec seems to suggest maximum capacity of about 15Kg not including counterweights, so you shouldn't be seeing any slippage up to to that weight (assuming you have reasonable balance).

- Weigh all your kit to see what the total is, if you are adding extra weights already, you may be overdoing it (don't know what counterweights are is supplied out of the box though, so maybe not).

- Next check that your Dec clutch isn't loose (should be possible to tighten it up if it is).

- Check that you haven't switched off Dec guiding

- Follow the instructions in the guide to check that guiding commands are being acted on in Dec.

Not that you will necessarily get good guiding if you are near the maximum capacity; for low-end mounts you'd be more likely to succeed with a weight of about half the max (7 or 8Kg), You may find the long focal length is also a challenge, especially with an SCT using a guidescope.

[Stargazer33]

Thanks for the suggestions Ian.

I stripped down my dec mech last night and found three things - the cog on the dec motor hung over the edge of the worm drive end cog by about 2-3mm; there was a very small piece of brass in the worm drive end cog teeth (looks like it came from the cog itself as one of the teeth had a bit missing on the end - could be why the drive cog had been set out of line.); lastly, the worm drive was very loosely engaged into the dec drive which meant a lot of backlash.

I have now tightened the worm drive to the dec drive; removed the piece of brass from the end drive cogs & cleaned the teeth up; aligned the motor drive cog with the worm drive end cog so that they are both flush on the outside face.

Finished too late last night to give it a test so I will try tonight. I am intending on just putting my guide scope on the mount and seeing how long it will track a star. I will then put my full imaging rig on and see how much that effects the guiding.

I will post how I get on.

[IanL]

Did you have the mount from new or is it second hand? I know these mounts are not built to the highest standards, but I'd hope that was a result of the previous owner rather than how it came from the factory. I haven't had the need (nor the courage) to strip my mount down, so good luck!

[Stargazer33]

Hi Ian,

Yes it was a perviously owned mount. It was well packed when I received it, but it has obviously had some problem in the past to break off part of one of the drive cog teeth!

I tried it out last night with just my guide scope on the mount (Celestron TravelScope 70) and Orion StarShoot AutoGuider. After an initial glitch with the dec trace now shooting off the top of the graph; I played around with the dec aggressive adjustment and managed to get the line travelling horizontally, albeit like a really dodgy heart monitor trace!

I turned the guiding off and the star drifted very slowly in the shot. Much less and a lot slower than it was drifting before.

This was with just a rough magnetic alignment. I didn't do the polarscope alignment as it was still too light and I didn't do the mount alignment routine letalone a drift align so I think I have solved the problem! Yipee!

I do however need to try it with my full imaging rig. I will weigh the complete rig and see how much over the ideal weight I am!

[Stargazer33]

Err ... I think I spoke too soon!

First off, I found out the other night that I had been looking at the RA drive and not the dec drive. Second the mount is way overloaded. The CG-5 has a recommended imaging maxium weight of 7 kg and my rig runs at 9.2 kg, close to the maximum recommended weight for visual at 10 kg.

On looking at the dec drive I found a lot of play in the gearing so I have now tightened that up too. I think I have gone too far with each drive however as I have done a mock star align and the motors do not sound smooth but the pitch goes up and down as though there are tight and loose spots on the movement.

Looks like I am going to have to take it apart again and back them off slightly.

[Stargazer33]

Err ... I think I spoke too soon!

First off, I found out the other night that I had been looking at the RA drive and not the dec drive. Second the mount is way overloaded. The CG-5 has a recommended imaging maxium weight of 7 kg and my rig runs at 9.2 kg, close to the maximum recommended weight for visual at 10 kg.

On looking at the dec drive I found a lot of play in the gearing so I have now tightened that up too. I think I have gone too far with each drive however as I have done a mock star align and the motors do not sound smooth but the pitch goes up and down as though there are tight and loose spots on the movement.

Looks like I am going to have to take it apart again and back them off slightly.