"Unbalancing" the Mounting for photography

[Bizibilder]

I've heard on several occasions, in both threads here and elseware as well as books, of the technique of deliberately putting the mount "out of balance" to aid tracking when taking photographs on a driven or guided mount. I believe "east heavy" is the rule

I understand that this is to gently coerse the gears to remain fully in mesh. (If it makes a difference I have a newtonian reflector on a german equatorial mount).

My question is "How much out of balance and which direction ?"

Thanks in anticipation...

[barkis]

On a mount like the EQ6 for example, if your scope was on the West side of the mount, and imaging in the East, the counter weights should be slightly heavier than the imaging setup.

Imaging in the West, and the scope on the East side, the scope should be heavier. Only sufficient to keep the drive pushing the RA around in both cases.

In all cases, the drive should be deemed to be pushing the the mount, and the weight distributed accordingly.

You have to be wary when the weight bias crosses the meridian though. The RA could fall away from mesh, and would spoil the exposure.

Hope I've got that right.

Ron.

[Bizibilder]

Thank you Barkis - that makes it clear - as long as that's the right way round

[ollypenrice]

Yes, the east must always be heavy. As for how much, it seems to vary. I look at the guide trace and if my error values are displeasing I move the counterweight. A sure sign of backlash is two sets of guide hit 'clusters'. AstroArt is good for shownig this phenomenon. It arises when the mesh has fallen from one side of the gear to the other so some of the guide images are east heavy and some are west heavy but few lie in between. The see saw is, as it were, up or down.

I keep the camera end a little heavy, too.

Olly

[Bizibilder]

Thank you. Is your "camera end" for a refractor? If so what do I do with a newt? (sensible answer please - about the newt!!)

[Euan]

I'm going to put the cat among the pigeons and say I don't agree with this idea

If your mount has so much backlash that you need to mess with the weighting then that should be addressed directly instead of trying to outsmart it. You could easily offset it to the point where you are doing more harm than good

I'm all for mount modification via hypertuning and better worm adjustment than trying something like this. I've done the hypertuning on my EQ6 and it does make a difference, but the thing that makes the biggest difference by a huge margin is the accurate meshing of the worm gears, which can be done without any stripping down on the Skywatcher EQ mounts and can go a long way to alleviate the problem you are tying to sort out (that's if you have one of these mounts?)

All IMHO of course

[Bizibilder]

Accepted, Euan, But there is always a case for SLIGHTLY pre-loading a gear train to avoid "rattling" as no gear train is perfect. Your point about "how far to go" is valid - I suspect a few ounces is all we are talking about here. I would tend to ensure that when balancing the scope - which will never be perfect at all positions, under normal circumstances, I would ensure that there was NO west "loading" and that the residual imbalance in the set-up was deliberately "east heavy". Hope I'm making sense writing these thoughts!!

PS I'm talking about an HEQ5 mount.

[ollypenrice]

I'd like to know more about Euan's adjustments (hint!) but my own 'unbalancing' is empirical in that I unbalance as little as possible but watch the guide trace. There is no doubt that with the right tweak on the counterweight I can get my tracking accuracy far better than with perfect balance. I do this almost every night. The same is true of my expensive Takahashi mount. Even that is quite susceptible to balance/out of balance, though less so than my EQ sixes. In an ideal world we'd have expensive anti backlash gears in two halves and spring loaded apart to absorb free play. Or maybe friction drives. (But not ASA direct drives in the light of even more expensive local failures!!)

And sorry, yes, I use refractors and so camera end is the 'low end'. With a Newt I gather the best camera position may be in line with the counterweight bar and on the mount side, not the outboard side, but I am not a Newt user for imaging.

Please Euan, more on these adjustments!

Olly

[Euan]

Please Euan, more on these adjustments!

The Hypertuning isn't very well documented online, the best way to see it is the Astro-Rubylith DVD

HyperTune® DVD for the Orion Atlas EQ-G - Rubylith | Saves Your Night Vision

They also sell the spacers and the digital calipers you need to do it

The worm adjustment (which IMO is by far the best thing you can do to an EQ6) is documented in quite a few places, although regardless of how many different descriptions I've read of it, I never really understood it until I started to play with the mount. The best I've read by far is Astro Baby's:

EQ6/Atlas - Worm Adjustment

The only thing I could add to Astro Baby's is that if you take out the control panel and both stepper motors, you can get the mesh tighter by pushing the motor gears around manually with your finger and feeling for the point where the set screw adjustment starts to lock up the gears. Just try and get it as tight as possible before hitting this point. The hypertune will help with this slightly, as it will help with the height difference between the worm gear and axis gear.

I have played with this quite and lot and compared the guiding performance and amazingly, the stepper motors have such good torque that they will still drive even though it feels like it has locked up when you turn it with your finger. It does make the guiding jumpier though, so best to be just short of this.

My guiding RMS with my EQ6 is down to 0.8 arc secs, which is 0.1-0.3 pixels on my QHY5 with Orion ST80 (I have no idea if this is good or not, but it's the best I've ever had), my imaging CCD resolves at 2.3 arc secs per pixel so it's easily tight enough.

[Euan]

lease Euan, more on these adjustments!

I put a bit about this on my site as well, I forgot to mention the stepper motor backlash adjustment is also quite a good thing to do

Modifications Photon Junkie

[Bizibilder]

Euan - I've just had a look at that link and see that you suggest using tin-foil to "mesh" gears correctly. May I suggest another, possibly better, material? Its been used for years by clockmakers (my other hobby!) engineers and instrument makers. Good old Rizla cigarette papers are 1.5 thou thick (0.0015" or 0.04mm) and will "run" through a pair of gears without splitting. Ali foil has been known to split and leave a trace between the gears - enough to cause a problem when the gears are run!

[Euan]

Euan - I've just had a look at that link and see that you suggest using tin-foil to "mesh" gears correctly. May I suggest another, possibly better, material? Its been used for years by clockmakers (my other hobby!) engineers and instrument makers. Good old Rizla cigarette papers are 1.5 thou thick (0.0015" or 0.04mm) and will "run" through a pair of gears without splitting. Ali foil has been known to split and leave a trace between the gears - enough to cause a problem when the gears are run!

Good idea! The tolerances are so small that I had to double up the foil sheet, with one it would bind them but two was fine, I should have measured it with the calipers it must still be really really thin

[Bizibilder]

It can vary - "Chicken" is about 1thou, "Xmas Turkey" can be up to 4 thou