A Question of Balance

[Thalestris24]

Hi all

I always set my Heq5/150pds up according to Dion's excellent Astronomyshed video. But I'm never quite sure how much imbalance to introduce for imaging. Should it be very slight or quite obvious? I'm wondering because I've been having guiding problems - the dreaded eggy stars It tracks ok without guiding for 60s but then it all falls apart with PHD2, even starting to go off with just 120s subs. It was previously ok but I removed the ST80 I had attached to the tube rings so rebalanced everything yesterday before trying to do some imaging last night. I think maybe I was too well balanced? Problems seemed mainly in DEC but also, to some extent, in RA so wondering if this is a symptom of DEC backlash?

I've complicated matters by the installation of a coma corrector (last night was supposed to be a test of that!) and a LP Filter. However, I could tell by the guiding graph that things were not going so well...

Here's a couple of examples:

(8 x 120s)

(2 x 360s):

The 360s subs were supposed to be the lower part of the Eastern Veil, so not many stars - no sign of the nebula either!

Any advice much appreciated!

Thanks

Louise

[Davey-T]

Not sure how much truth there is in the out of balance theory, may just be an astro urban legend, or perhaps depend on the mount, I did experiment once and ended up with so much weight that it refused to calibrate to the North in PHD but it still tried to make North and South guiding corrections when running, same in East and West

Sorry not much help

Dave

[Uplooker]

 It should be very slight. Just enough to ensure that the motors are having to effectively push up hill. It is help keep the gear teeth properly meshed.

Ian

[michael.h.f.wilkinson]

The veil is very large, and has low surface brightness. I am not sure, but would you not need much longer total integration time to capture that?

[Thalestris24]

The veil is very large, and has low surface brightness. I am not sure, but would you not need much longer total integration time to capture that?

Hi Michael

Yeah I realise that though thought there might have been a slight hint. I stopped after 2 subs because of the eggy stars - my reason for posting!

Thanks

Louise

[Davey-T]

 It should be very slight. Just enough to ensure that the motors are having to effectively push up hill. It is help keep the gear teeth properly meshed.

Ian

But if it's not Polar aligned it will still switch guiding directions in Dec  and introduce backlash, better to skew the PA so it only corrects in one direction against the imbalance.

Dave

[Thalestris24]

Thanks for everyone's input. I suppose I'll just have to suck it and see next time!

Cheers

Louise

[Guest]

I have my set up balanced perfectly with all the gear on the scope, so the slightest extra weight either way will make a big difference, BUT I do not have the cable loom connected when balancing, so when that is added and connected to the gear, that is the difference in balance to keep the gears tight. It only makes a difference with front to back balance, though on my set up, the top and bottom of the OTA are still perfectly balanced, with guide scope piggybacked and weights underneath.

I have the meade LX90 8" and can get perfect round stars on 20 min subs.

Hope that helps

MM

[Thalestris24]

I have my set up balanced perfectly with all the gear on the scope, so the slightest extra weight either way will make a big difference, BUT I do not have the cable loom connected when balancing, so when that is added and connected to the gear, that is the difference in balance to keep the gears tight. It only makes a difference with front to back balance, though on my set up, the top and bottom of the OTA are still perfectly balanced, with guide scope piggybacked and weights underneath.

I have the meade LX90 8" and can get perfect round stars on 20 min subs.

Hope that helps

MM

Hi

Ok - thanks. I don't have a cable loom as such - I have all the cables attached to the tripod so they don't contribute any weight really.  I find it takes quite a bit to 'unbalance' my 150pds otherwise it just sits there (with the clutches off). So it's either obviously unbalanced or it isn't!

Cheers

Louise

[Guest]

If you can release the clutch, and it just sits there, it sounds to me as if it is perfectly balanced, that's how mine is, but I only have to give the slightest push one way or the other and it will move a couple of inches and stop, and the same the other way, when I add the wires, it very slowly moves in the one direction, which I was told is what you want, very slightly imbalanced in one direction .

Regards

MM

[Thalestris24]

If you can release the clutch, and it just sits there, it sounds to me as if it is perfectly balanced, that's how mine is, but I only have to give the slightest push one way or the other and it will move a couple of inches and stop, and the same the other way, when I add the wires, it very slowly moves in the one direction, which I was told is what you want, very slightly imbalanced in one direction .

Regards

MM

Hi

Ok. It sounds like my setup is maybe a bit 'stiff'. Hard to tell without something to compare with!

Thanks

Louise

[Alien 13]

Hi

 

Ok. It sounds like my setup is maybe a bit 'stiff'. Hard to tell without something to compare with!

 

Thanks

 

Louise

 

Hi Louise

I am not sure if there are any similarities with your mount but the RA setting circle is designed to introduce a lot of drag and balance can often be difficult unless you untighten the locking screw so its free to rotate.

Alan

[ollypenrice]

Running out of balance certainly isn't an urban legend, it is a very poweful technique. In the N hemisphere it is the east which may want to be slightly heavy. Very slightly! Remember to reverse the imbalance after the flip.

Oscillation in Dec, due to backlash, is hard to deal with via balance because, when the scope is on the zenith, a front-back imbalance has no effect and the effect it has elsewhere varies with the elevation. You need a right-left Dec imbalance on the zenith. A better weapon is slight polar misalignment which will cause all the Dec corrections to go the same way. When this is the case you can set your autopguide programme to send corrections only in that direction. Disable the other one. This reduces a vicious circle of corrections which generate an oscillation. It can be very, very effective but you need to suss out which direction to disable. Being too thick to work from theory I use trial and error! Oh, and reverse after the flip, once again.

Olly

[Thalestris24]

Hi Louise

I am not sure if there are any similarities with your mount but the RA setting circle is designed to introduce a lot of drag and balance can often be difficult unless you untighten the locking screw so its free to rotate.

Alan

Hi Alan

My scope/mount is kindof in pieces at the moment - awaiting a replacement azimuth peg so that I can set up a pedestal. Anyway, with just the mount on the tripod and the clutches off, it's noticeably stiffer in DEC than in RA. I can rotate it with one hand but RA is much free-er. Loosening the DEC setting circle makes no difference. How to tell if this is normal? Nothing to compare with! I'd have thought that both axes would feel the same with the clutches off?

Thanks

Louise

[Lonestar70]

Hi Louise,

It sounds like your DEC axis bearings may have been tightened a little too much when the mount was assembled.

It is quite easy to slacken this though.

See Astro Baby's HEQ5 rebuild to see how to do this.

http://www.astro-baby.com/heq5-rebuild/heq5-d2.htm

It amounts to undoing the locking grub screws on the counterweight shaft housing and unscrewing the housing a little... then re-tighten the locking grub screws.

You will probably only need to slacken it about 1/8th - 1/4 turn... so you would not need to dismantle the mount to achieve this... just remember to undo the counterwight shaft clamp first.

Hope this helps.

Best regards.

Sandy.

[Thalestris24]

Hi Sandy

Ok - will give it a go!

Thanks

Louise

[Thalestris24]

Hi Sandy

"It amounts to undoing the locking grub screws on the counterweight shaft housing and unscrewing the housing a little... then re-tighten the locking grub screws."

Um, I can't undo the grub screws I have to use the long axis of the allen key to reach them so not much leverage... Any tips? If I can't do the adjustment do you think that, as it is, it's affecting the DEC performance?

Thanks

Louise

[Lonestar70]

Hi Louise,

Yes they can be a bit tight to undo... I tend to use a length of small bore steel tube over the short end of the alan key to get extra leverage... a pair of pliers could be used to the same effect.

If the bearings are a little too tight then small PHD correction values may not be enough to actually move the DEC axis by the required amount... as a result, PHD would issue a larger correction value, possibly leading to an overshoot... the subsequent correction in the opposite direction would have much the same result... so YES tight bearings will effect guiding performance.

The other possibility is that the grease (loose term) in the bearings is either non-existant or lumpy... requiring a total dismantle of DEC axis and cleaning/re-greasing of the bearings...

hopefully a small adjustment will be enough to solve your problem.

Keep Happy.

Best regards.

Sandy.

[Thalestris24]

Hi Sandy

Well I did it! The first screw loosened with a loud crack! The others then didn't need much effort. After loosening the counterweight collar and re-tightening the grub screws, the DEC axis is maybe a little looser but still feels nothing like the RA axis. It didn't make any difference to loosen the counterweight collar a lot. I hope that doesn't mean I have to go through the worm/motor engagement adjustments... There doesn't seem to be any binding - it just feels sort of 'heavy', if you know what I mean.

Thanks

Louise

[Lonestar70]

Hi Sandy

Well I did it! The first screw loosened with a loud crack! The others then didn't need much effort. After loosening the counterweight collar and re-tightening the grub screws, the DEC axis is maybe a little looser but still feels nothing like the RA axis. It didn't make any difference to loosen the counterweight collar a lot. I hope that doesn't mean I have to go through the worm/motor engagement adjustments... There doesn't seem to be any binding - it just feels sort of 'heavy', if you know what I mean.

Thanks

Louise

Hi Louise,

Loosening the collar to far will result in end float in the DEC shaft... about 1/8th - 1/4 turn should be more than enough to ensure the tapered lower bearing is not jammed to tight in it's trackway. 

As to how it feels... this can be difficult to judge manually especially with no scope etc mounted... just turning the puck by hand will always feel somewhat stiffer than the RA axis mainly due to the fact that the RA axis has the mass of the head attached.

Try mounting your ST 80  with your DSLR attached and see how that feels.

The worm gear/motor engagement would not effect this feel as it is disconnected by the DEC clutch... I would only mess with this if your PHD graph suggested a lot of periodic error was present.

I would try the new setting under dynamic conditions using PHD and see if it still shows the problem.

It may also pay you to slightly unbalance the scope in DEC make it a little North heavy (= camera heavy with a refractor).

And a little East heavy in RA.

When I use my ED 80 I set the DEC balance so that the camera end very slowly drops when the clutch is released.

For the 150PDS it is set back end heavy (primary mirror end)... this ensures corrections are always in the same direction regardless of OTA type... it also helps if the focuser/camera is on the mount side of the OTA tube.

I adjust the RA balance such that whichever part ( Scope or Counter weights) is on the EAST side then this side slowly drops when the RA clutch is released.

Balancing in this way gets round most of any backlash errors and can help a lot with guiding.

However, as Olly has stated DEC balance can be a real pain the closer to zenith you go, where the only action is to slightly mis-align the polar axis.

Hope this helps.

Best regards.

Sandy.

[Thalestris24]

Hi again!

Ok - sounds like very good advice! I'll give it a go when I can get everything back together. I've been trying to change to a pedestal (from Orion Optics) but the adapter they supplied won't properly fit because they use a hex peg which hits the inside of the casing . They sent me a couple of alternatives in the post but neither will do the job... It might be possible to file one down though I'd need a file to do it! OO are suggesting that my Heq5 is different to others which I find hard to believe! Why can't it be straightforward and simple?? Sigh.

Anyway, thanks for your help and advice.

Cheers

Louise