Backlash

[Demonperformer]

I have just started looking at this issue with my SLT mount and it’s time for me to demonstrate my monumental ignorance again. A copy of this bit of the instruction manual is attached.

My current settings [which are what they were when I got the mount] are:

Azimuth: positive and negative both set to ‘25’

Altitude: positive and negative both set to ‘50’

I mostly do imaging, for which it states a ‘higher value may be necessary’ [compared to 20-50 which is ‘best for most visual observing’]. It then talks about backlash compensation being applied ‘when a direction arrow is released and the direction moved is opposite to the direction of travel’.

Now I am confused (nothing new there, then). When I am imaging, I tend not to press direction arrows at all (in either direction) as I have found this tends to increase the amount of trailing in the resulting photo (not the desired result:)).

(1) So why do I need backlash compensation at all when I am imaging, let alone needing a ‘higher value’?

And, assuming that I do:

(2) What practical difference should I notice in my photos between having it set to an optimal value, compared to a less-than-optimal value? [The instructions say I ‘need to experiment’ to find the best value, which is fine, but it will be useful to know what I should be looking for to see if the result is closer or further away from this optimal value].

Thanks.

[rfdesigner]

"So why do I need backlash compensation at all when I am imaging, let alone needing a ‘higher value’?"

If you are correcting in both directions on the Declination axis you need backlash compensation.... most people tend to intentionally misalign a tiny bit so they have to keep turning the DEC a smidge to keep it aligned then they set the guider to only correct in one direction on the Dec, end result: no backlash issues

hope this helps

Derek

[Demonperformer]

Thanks, Derek.

The 102 mount is alt-az, so I don't have a declination axis. I can now see why it would be required if I had an autoguider, which would be constantly making adjustments in all directions to keep objects to within the nearest pixel, but I don't have anything that sophisticated at the moment. Surely, when it is simply tracking, it is moving in the azimuth axis, in the same direction, all the time, so without a change of direction the backlash figure would be irrelevant? I guess that the altitude axis changes direction once, when the object crosses the meridian, but before that it is moving constantly 'upwards' and after that it is moving constantly 'downwards'.

Or am I still missing something blindingly obvious?

[ollypenrice]

I would have thought that just being slightly out of balance on the Alt axis would entirely eliminate the issue.

Surely your instructions are discussing the mount when used on a wedge?

Olly

[Demonperformer]

Olly

Well it doesn't actually say that, and earlier in the manual where it talks about the various tracking modes on the handset, it says "The Nexstar SLT series are exclusively Alt-Az mounted telescopes and do not require equatorial tracking".

However, I like your thought about unbalancing the scope - easy to do and if it eliminates the issue I can get onto thinking of other problems - umm, I mean ways to improve;)

Thanks.

[dph1nm]

I have imaged on a Nexstar SLT mount. Never touched the backlash. Mind you, I did find I was limited to about 30sec exposures with 4" pixels. Personally I doubt this would improve much no matter what you did to the backlash (and the field rotation will get you if you improve it too much!).

NigelM

[rfdesigner]

Ah.. I'd missed it was an Alt az, mind out for image rotation if you want to image. Fine for short shots like planets, likely to be difficult to deal with on longer subs. (I'm assuming I'm talking teaching you to suck eggs here)

When I started out with my old Tal1M and the 5.5" piggy backed, hand guiding, I found that misbalancing didn't really help as the dec would be fine being pushed against gravity, but would slip and sn@tch when unwinding, minor misalignemnet to guarentee pushing only fixed the problem. With an Alt Az I'm not so sure.. it might work well, with a good mount it might work very well... everyone seems to do things slightly differently.

Derek

[Glider]

Think you may have picked up the general instruction addendum for the hand controller where this section doesn't apply to your mount.

[ollypenrice]

Think you may have picked up the general instruction addendum for the hand controller where this section doesn't apply to your mount.

That would explain it nicely! I couldn't fathom backlash in alt-az...

Olly

[DoctorD]

Hi

Backlash is the slack that is inherent in any gear system and is seen in Alt Az and Equatorial mounts.

You notice it when you change direction of any one of the axis and can been seen as a delay in responding to the input.

Backlash compensation works by driving the axis in the opposite direction enough to take up the slack and then start moving the axis. Some mounts specify this as an absolute value in arc minutes and seconds, others as a compensation factor like the Nexstar mount.

It's more important for imaging when using guiding, because the guide input will be constantly sending corrections to the mount and any delay in responding to these may cause problems such as oscillating back and forth slightly.

As suggested you can slightly unbalance the scope so it is always leaning in one direction (actually you need to do this on both axis - difficult in AZ mode, easier in equatorial mode).

You are unlikely to see any benefit in tracking unless you are using a guide scope. You may see benefit in positioning accuracy and response to manual inputs.

Play with the values and see what happens - just make a note of the default settings in case it all goes pear shaped.

Hope this helps.

Paul

[Demonperformer]

Thanks for all the input guys. I understand what is going on a lot better now.

With my current setup it is probably not worth messing around too much, except ...

As a supplementary question, I am wondering if the backlash settings have anything to do with a weird behaviour the mount exhibits:

The mount aligns ok, then, whenever I GOTO an object, it centres the object and when it has finished does a quick 'drop' in altitude, to put the object almost at the bottom of the eyepiece. A 10-count on the down arrow (moving the object up in the eyepiece) at speed 5 restores it to the central position (after the further obligatory 'drop' when I release the button). This happens with such consistency that I am now used to what I have to do. It does not apply to pressing any of the other arrow buttons. Comments like "Backlash compensation works by driving the axis in the opposite direction" makes me wonder if the problem is caused by one (or more) of the backlash settings?

Any thoughts?

Thanks

[Spikey]

Intersting that the image drops. Is the OTA balanced in the mount?

[brantuk]

If you have settings for "approach" then that's also worth looking into and calibrating - helps to keep the gears engaged constantly

[Demonperformer]

Thanks guys.

Quick trip back to the manual ... There is a 'Goto Approach' setting, which says that when a camera is used (which I do most of the time), it should be set to negative, so that 'the telescope always approaches the scope from the opposite direction'. Have no idea what it is set to atm, but will certainly be worth checking next time.

So, if I set that to negative, the object will appear to move up in the e/p as it is approached using GOTO, so I will therefore need to align the scope with the object approaching the centre from below.

While I am at it, I should check the azimuth one as well. Presumably, positive in azimuth would be clockwise and negative would be anticlockwise ... although I have found nothing in the manual to confirm that.

[Demonperformer]

A big THANK YOU to Brantuk , for pointing me in the direction of the Approach settings.

Checked them at the start of the session: alt was set to negative and azm was set to positive. Thought that was ok, and aligned stars from proper directions and when I did my GOTO to the first object of the evening ... DROP. Oh well ... nothing new there then.

However, as I was taking my first set of lights, I started having problems with excessive drift left to right across the screen, resulting in lots of trailing. Because I had been thinking about these things, I realized that the azm drive was always moving left to right as it tracked objects across the sky, so I really ought to have the approach button set to negative. So, quick interrupt of the photo sequence and change approach azm setting to negative. The result was both amazing and immediate. Yes, there was still a bit of movement between each sub, but when Als Reticle was centred over a star (my usual way of checking for 'drift'), with the preview set to full screen, the total drift for the rest of the set was small enough that the star never left the centre square! I have never known this mount to track that well before. Which is not to say that there are not subs with trailing visible in one direction or another, but certainly the overall trailing was dramatically reduced.

Not only that, when I moved onto my second (and final) object of the evening, it happily slewed to the object and ... NO drop:hello2: There the object was, right in the middle of the screen, with no adjustment necessary. Although why a change to the azm setting should affect a weird effect in the alt direction is beyond me.

Clearly, further tests are necessary before I can be absolutely sure that the problem has permanently disappeared and that the results above were not just a fluke for some reason. But initial results seem promising, so thank you, B, for the pointer.