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Can Somebody Explain This Guide Graph?


AstroAndy
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Hi all

I was wondering whether someone has an answer to a problem which is vexing me, namely below guide graph.

To explain: I am normally pretty much on the nose with guiding (my best yet is 30min exposures, single subs). Lately, I have been having a spike in my dec. graph, about every 8th square.

I do not think it has to do w/ the values, they have served me well in the past. I do not have PEC (thinking that autoguiding takes care of that; is that correct?). Axes have been tightened against loose play and backlash (enough?). Lithium grease was used to grease the gears. 2-star and drift alignment (2 iteration AZ, 2 iterations long) have been done w/ EQAlign so polar alignment is on the nose (when the graph doesn't spike, it's flat, except when it compensates for the spike). RA axis is east heavy, telescope front heavy and balanced, mount well levelled and stable. Basically, I have done everything I could think of.

Target was M33, to the South East, PHD guiding w/ 9x50 finder, SW 200P EQ5 (PE on that abt. 11 min.)

#rd and 4th graphs are after a forced recalibration.

If somebody has any ideas, that would be appreciated (I have screenshots of the brain values I use, too, if they're needed for analysis).

Andy

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I suspect that you have a tight spot in the gear train - have you adjusted the gears? If so they may be too tight (even at only one point in their revolution. I would suggest undoing the gears and re-mashing them with a single slip of cigarette paper between the meshing teeth, then gently remove the paper and you will find the gears have just enough backlash for the problem to be solved. (Rizla green papers are 1 "thou" thick - 1/1000 inch).

The other possibility is that your guide scope is "moving" - possible a tight or caught cable somewhere?

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Thanks, Earl and Roger, for your suggestions, I'll check into that.

@ Roger, I suspect it's the gears, I have indeed adjusted them, however, being very careful, I thought I'd done it right (the backlash clearing goes fast now). I ajusted the gars in the mount, and refitted the motors. How tight should the scope gears be (when moving it by hand knob?) Should they be easily movable, or medium hard (obviiously too hard is not an option)?

Andy

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First off I think you can forget anything to do with RA. That means it isn't connected with PE, PEC or counterweight balance or anything in the RA drive, ie the primary spur gears or the worm and wheel. This is a Dec problem so I think there are two possibilities.

1) the Dec is oscillating because of free play in the drives. That's to say the Dec payload is resting on one side of mesh and a guide command is sending it off to the other side. You say you are running front heavy. So, 1)Try running camera end heavy instead. 2) Try Dec guiding in only one direction, not both. (I do this on one of our mounts otherwise it oscillates.) 3) Adjust the worm and wheel for less play and try putting Dec into true balance.

2) For some reason the mount is getting big and unwanted corrections on a regularly timed basis. Again try disabling one of the two guide directions. A slight polar misalignment can help here to ensure that the only necessary corrections all go the same way. If you have the right choice of out-of-balance in Dec this should work OK.

The thing about guiding is that these things happen even when you haven't changed anything!!!

Olly

Edit; Just a thought but you don't have a cable snatching and releasing, or similar. Worth a look, at least.

Edited by ollypenrice
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Hi guys

Thanks for all your advice, I shall try all, or some, in combination, and see what happens.

@ Roger, w/ scope gears, I mean the declination axix and knob, to which the dec. drivee wheel has been fitted (I probably shouldn't have said scope gears, as the scope doesn't have any).

@ Olly Cable snagging should not be a problem, and thanks for your detailed answer.

Andy

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