Off centre declination balance?

[Martyn87]

Bit of a weird issue here, I've been having trouble guiding and PHD2 was complaining about declination balance (amongst other things), so I've followed a YouTube tutorial to free up the DEC axis a bit. This has exposed that I was indeed out of balance, but not that it's more sensitive, I'm having a weird issue, the video below might explain better than I can. Any suggestions what might be going on here?

[Elp]

What's that thing sticking out the back and left (if scope is in front of you pointing north), I think that's your issue. Take it off and you might find the scope balances as expected. You'd need to place something on the saddle or near the front right of the scope to balance it as is by the looks of it.

Also a minor issue, cables can also cause imbalance.

[Martyn87]

Thanks @Elp, the cabling is a bit of a mess as if had this in bits so many times today trying to figure it out. The bits sticking out if the back are EAF, OAG etc. I had wondered if these were the issue so I've printed a new bracket to move it more towards the centre but it seems to have made no difference. I think I'm going to have to strip everything off and check.

It seems odd that it seems to balance fine through most of the axis, other than when it gets too close to that balance point. 

[Elp]

I get it with my C6 because I use a 30mm GS on one side and the asiair on the other, it's biased on the GS side, I usually just leave it if the rest of the rotational Dec range it seems settled to not move.

When I'm imaging f6.3 reduced however when it's back heavy, I do put a vixen clamp under the dovetail near the front and put CW into the clamp to balance.

IOptron specifically have a z axis balance kit for this exact purpose as a lot of their mounts have a threaded hole on the side of the saddle to connect it to, little help for your mount though.

[Martyn87]

So I've stripped everything off now except the OTA and it's landing in exactly the same place (when balanced in the rest of the rotation). If I move the balance back far enough, it sits where I'd expect it to. Is it maybe the focusing mechanism? 

I think even with the imbalance, it's better than it was before, so I think I'll have to do more testing to see if its still an issue before I mess too much more. 

[Elp]

Very odd. Could be the saddle knobs but I highly doubt theyd cause an issue as it'll be a design problem of the mount if that were the case. You could try using just a dovetail bar on it to eliminate this as a potential cause but as stated, highly doubt it.

Handling my OTA, it doesn't feel off balance to suggest the focuser could be the issue, but it is offset to one side.

Edited September 2 by Elp

[Martyn87]

15 minutes ago, Elp said:

... It is offset to one side.

 

That was my thought too. It does 'hang' with the focuser side at the lowest point so it sort of makes sense. Now I've reattached everything on the back, it is worse, so it's obviously not perfectly balanced at the back which is hardly a surprise. The losmandy bar is mounted in the same place as the original dovetail, I only replaced it as I couldn't move the scope far enough forward to get balance on the vixen dovetail.

Ill see how it goes, and if it's an issue I'll find a way to move some weight to the other side. Thanks @Elp

On the plus side, at least I've eliminated any stiction in the dec axis! 

Edited September 2 by Martyn87

[michael8554]

"PHD2 was complaining about declination balance "

I've never seen PHD2 complaining about Dec Balance, how would it know ?

Perhaps you mean Dec Backlash ?

If you do have Dec Backlash, then Dec unbalanced may hrlp !

Michael

[Martyn87]

3 hours ago, michael8554 said:

"PHD2 was complaining about declination balance "

I've never seen PHD2 complaining about Dec Balance, how would it know ?

Perhaps you mean Dec Backlash ?

If you do have Dec Backlash, then Dec unbalanced may hrlp !

Michael

Thanks @michael8554. I don't remember exactly what the error itself was, but dec balance was definitely mentioned in the description. This is the only reference I can find in the PHS2 documentation 

"the GA (guiding Assistant) will try to compute an accurate estimate of the backlash amount, corrected for Declination drift.  This won't be done if the mount never established a consistent rate of south movement that was at least 90% of the measured rate moving north  That situation usually indicates binding in the Dec axis or substantial imbalance, in which case a simple estimate of backlash will be inaccurate and probably irrelevant."

[Martyn87]

Looking at my calibration graph, I do have a number of South points that are a long way from where they should be: 

[Elp]

That is not a good calibration, it should be near 90 degrees to each other, something is causing movement but you already know that.

[Martyn87]

Yep definitely not good. I think I'm right in saying that all north/south and east/West points should be somewhere along the dec/RA axis', at 90 degrees to each other, but the position on the graph is just related to camera rotation? 

I think I've solved the balance issue now, although it does mean I'm temporarily back to manual focusing while I wait for some longer belts. 

[Elp]

Yes as long as the RA and Dec calibration is 90 degrees you're fine.

[Martyn87]

On 02/09/2024 at 19:56, Elp said:

Very odd. Could be the saddle knobs but I highly doubt theyd cause an issue as it'll be a design problem of the mount if that were the case. You could try using just a dovetail bar on it to eliminate this as a potential cause but as stated, highly doubt it.

Handling my OTA, it doesn't feel off balance to suggest the focuser could be the issue, but it is offset to one side.

@Elp believe it or not, I think you've hit the nail on the head with the saddle knobs! I've had literally everything off, had the OTA back on its original dovetail, still not balanced, and I figured, while I'm here, let me just check...

I've now been able to balance it perfectly (only with the OTA, still putting everything back together) with the help of my make fix counterweight made up of some M10 washers and a velcro strap. Time for some stick on weights I guess! 

[Elp]

11 hours ago, Martyn87 said:

saddle knobs

Are those standard knobs, if so I'd message the retailer/manufacturer and ask them whether they deem this acceptable because I don't. My ioptron gem28 is super smooth too in Dec, but certainly doesn't move at all when declutched.