Get get autofocus to work


#21

I also have the focus cube… Works very well.

Make sure you set the compensation “IN” to 60 in the SGP focuser “other” settings.

I also use the hardware backlash in the Pegasus settings set to 120. But the most important setting is the above “IN” compensation.

You can test this during the day with a toothpick and some tape… Put the toothpick on your focuser knob like a clock then move the focuser around 100 steps at a time x5 times then move it back x5 times. The toothpick should be pointing to the same location each time!

Goodluck :slight_smile:


#22

No. All moves IN do not need or invoke the backlash.
All moves OUT do need and must invoke the backlash.
I think your confusion about this is your assumption that there is backlash in both directions, IN and OUT. In general this is not true for telescopes, and is definitely not true for the SGP implementation of the backlash routine. This is easy to see if you watch the moves the focuser makes as you rack in and out to different positions. It will behave exactly as I have outlined above.
Here are the key points:
—For direction set to IN
-------All moves IN do not apply the backlash
-------All moves OUT first move out by your desired amount + backlash, then moves IN by the backlash.

There is a reason the SGP backlash routine only operates in one direction. That reason is, generally speaking, it is only needed in one direction on telescopes. That reason is the camera is always LOWER than the telescope. Therefore the weight of the camera assembly is always pulling down on the focuser. This means when the focuser is pulling the camera closer it is pulling against gravity. Consecutive racks IN (ie. closer to the telescope and against gravity) will not experience any backlash, assuming after the initial large rack out you rack back in enough to take out the backlash.

All of which is why SGP’s focus routine starts by racking OUT your requested intervals + backlash, then racks IN to remove the backlash, then only racks IN for all your position sample focuses. Almost the entire focus routine in doing only IN racks.


#23

A little more elaboration is in order.
If your focuser is really tight, tight enough to resist the pull of gravity on your camera assembly, you may very well have actual backlash both racking IN and racking OUT.

But the key point here, is that SGP only supports backlash compensation in one direction. You must choose which direction you want the backlash to be applied. SGP then applies that backlash for all moves in that direction, and does not apply it for any moves in the other direction. This applies to all moves: focusing, filter changes, or manual moves.

If you set direction to IN, SGP will apply the backlash on racks OUT, and vice versa. Terminology seems backward to me.


#24

Exactly - you are describing two different things here. “Slippage” due to gravity is not backlash. And yes, on my focusers, they all have tension adjustment that prevents slippage, so gravity is not playing a part here.

Backlash can occur due to slop in the gears in the motor that moves the focuser, or by slop in the focuser mechanism itself (e.g. in a rack and pinion setup).

And I’ll state it again: The backlash due to that slop does not have an inherent direction - it can occur in either direction, depending on which way you last moved the focuser. (i.e. if you rack the focuser in, then any backlash will happen next time you move out, and vice versa)

Anyway, it seems we’re not going to agree on this point so let’s agree to disagree and move on :slight_smile:

All the best!

Steve


#25

Sorry should have uodated this thread! I put in a backlash of 120 and it now works a treat… seems it was not high enough all along. Although 120 seems a lot the focuser is very fine and has a range of about 50,000 so 120 is not that much in reality.


#26

We actually are not disagreeing here. We are talking about two different things.

I agree completely with you that inherent backlash can easily be present in both directions, with the obvious bias from gravity, which may or may not be adequately dealt with.

What my concern is that folks think in terms of backlash compensation correctly so that they can use SGP to correctly compensate for backlash. To do so, one needs to understand that the inherent direction I am referring to is the fact that SGP only supports one direction of backlash compensation, specified by the “direction” setting for backlash. So backlash compensation with SGP is only going to occur in one direction, regardless of whether it results from gravity slippage, gear slop, or several other reasons. If folks think that backlash compensation is going to occur in both directions, they are going to waist a lot of time getting this feature to work for them.


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