LX200 Startup Position - slight error

[rogerg (Roger)]

G'day all,

As part of my continuing efforts to improve the automation of my equipment:

Whenever I start up my LX200 it's slightly off the position where it thinks it's pointing. It varies how much, but something in the order of a few degrees, mostly in RA.

I follow a fairly strict routine of turning it off as soon as it parks at it's home position and then turning it off ASAP such that it doesn't move in RA, or in the case of automation parking it, setting it to land mode immediately and then turning it off later.

My theory has been that he sooner I turn it off after a park or land-mode action the more accurate it's startup position will be.

Yet I'm still a few degrees off.

The LX200 does a slight move upon startup to test it's RA/DEC motors, but that amount is not significant in the difference seen, and it seems to correctly know it's moved and in what direction during that test anyhow.

Any ideas?

Thanks,
Roger.

[Omaroo (Chris Malikoff)]

I can't remember where I read it Roger - but I was under the impression that LX200's don't park properly, or the park "logic" doesn't work, when in equatorial mode - only in AltAz. This may only be true of the LX200GPS with Autostar II though - I'm not sure.

[mick pinner]

if in POLAR mode l shut it down in any position and do a 1 star alignment the next night and it's realigned, no need for a park position.

[rogerg (Roger)]

The "Park" position is simply the position it assumes it's at when the telescope starts up, for the case of the classic LX. So the "park" used is to put it at 0 alt and 0 az (0 dec, current RA). That's what TheSky does for me. It does work in EQ and AltAz modes.

I normally would do a one star alignment when I start off a night's session, what I'm wanting to do is avoid doing that one star alignment - to be able to immediately do a plate-solve and start observing without me being present.

I think I'm going to have to do some testing of the exact errors and where they are.

Thanks,
Roger.

[Omaroo (Chris Malikoff)]

Originally Posted by rogerg

The "Park" position is simply the position it assumes it's at when the telescope starts up, for the case of the classic LX. So the "park" used is to put it at 0 alt and 0 az (0 dec, current RA).

I've just done some googling it appears that the firmware on the Autostar II does have a problem after the scope is parked via the handset if the scope is in equatorial (polar) mode. You select "Park" through the utility menu and it moves, as you say, to RA/Dec 0,0 or home position - and when the scope starts up again apparently after the GPS re-aquires and gets a fix on time and date, the pointing accuracy is way off. It should, buy all rights, be bang-on. There is a fix coming I believe.

So - it's a LX200GPS prob, not classic.

At the end of all that - if you have a Classic, how is the scope meant to know, automagically, what time and date it is without a GPS - or do you have one retro-fitted? A one-star alignment on a known star is what the scope relies upon to get its bearings back - so I'm not sure that what you want is actually achievable without a GPS....

[rogerg (Roger)]

Originally Posted by Omaroo

At the end of all that - if you have a Classic, how is the scope meant to know, automagically, what time and date it is without a GPS - or do you have one retro-fitted? A one-star alignment on a known star is what the scope relies upon to get its bearings back - so I'm not sure that what you want is actually achievable without a GPS....

The scope knows what time & date it is between startups, and it assumes when started up that it's at 0,0.... I don't see why it'd need to have the date & time updated from a GPS or anything else?

I don't consider that it loses it's bearings as such, it just assumes it's at point 0,0 upon startup so if it's pointing at 0,0 accurately upon startup, it knows it's pointing in the right place..

It seems from discussion on the LX200 group that the concensus is my PC time and LX200 time don't match closely enough, or aren't accurate enough, or there is too much delay between me parking the scope at 0,0 and turning it off. I'm going to do some testing on those theories.

[Omaroo (Chris Malikoff)]

Roger are you saying that the scope

1) has a built-in clock which is reasonably accurate, and that the "parked at" time is stored within the logic at the precise moment it finishes slewing to home position...

2) that you want the scope, once parked and turned off to keep on tracking time with that clock...

and

3) want the scope to power up, know the time and that it's at 0,0 home position, and then slew to an object without doing any further alignment at the start of the new session?

I gather that it has SmartDrive - so how do you get past the SmartDrive calibration test on startup? Wouldn't that affect the post-startup position, which is why the scope usually wants to do a 1-star align?

I'm sorry if I sound daft, but having a GPS version I'm not sure how the older models were meant to do this. I assume that Meade developed the GPS version so that people could do what you are trying to do, and refer to real time rather than clock time.

[rogerg (Roger)]

Originally Posted by Omaroo

1) has a built-in clock which is reasonably accurate

Correct. I find I need to update the clock once every few months, usually correcting it by about 30 seconds.

Originally Posted by Omaroo

2) that you want the scope, once parked and turned off to keep on tracking time with that clock

That's right, the clock keeps ticking. It doesn't keep "calculating tracking" or anything, it just keeps the time 'accurate'.

Originally Posted by Omaroo

3) want the scope to power up, know the time and that it's at 0,0 home position, and then slew to an object without doing any further alignment that session?

Yeap. I'm expecting to do a plate solve upon startup (to allow for some innacuracy in pointing) but at the moment it's too far out (up to a couple of degrees I think) for the plate solve to be successful.

Originally Posted by Omaroo

I gather that it has SmartDrive - so how do you intend to get past the SmartDrive calibration test on startup? Wouldn't that affect the post-startup position?

Yeap, that does move it a little. I don't think that is a large contributing factor - the movement is relatively small, the kind of movement I'd expect a plate solve to be able to handle. If not, I have considered writing a little application to, after the test, counter-act the movement done by the test, but that might be a bit hit and miss on the accuracy.

Originally Posted by Omaroo

I'm sorry if I sound daft, but having a GPS version I'm not sure how the older models did this.

If you sound daft then I sound daft . I have some reasonable understanding of the situation but it's still a bit like the blind leading the blind

Originally Posted by Omaroo

I assume that Meade the GPS version so that people could do what you are trying to do.

To tell you the truth, I have never understood the use of the GPS in LX200GPS. I haven't delved in to it deeply to work out what the advantage is.

I first assumed it was implemented so you didn't have to align - it'd start up and know exactly what angle of north it's pointing at. Then I found out they hadn't done that and star alignments were still needed. Time - yeah, sure, they'd probably get the time from the GPS, but the LX200 classic's quartz clock isn't that inaccurate to desperately require GPS. It's kind of off topic to this discussion, but I don't understand what the GPS gives the LX200 that the classic doesn't have, other than gimmic.

[Omaroo (Chris Malikoff)]

Originally Posted by rogerg

I don't understand what the GPS gives the LX200 that the classic doesn't have, other than gimmic.

Yeah - we digress, but the GPS gives the machine a precise, and I mean precise, latitude to within 5m. Time is derived from the almanac exchange with the GPS satellites and is accurate to atomic time standards. That's a LOT better than what an on-board clock can maintain I guess. Combine the two and overall accuracy is far better - if your scope is moved around a lot. It's not as though GPS technology is overly expensive - so why not use it? I don't think it's a gimmick on these scopes - it's just good to have - you don't have to go feeding in the latitude every time it starts up. If you're in a new area (unlikely for you since I guess you're on a permanent mount) you don't have to go looking up latitudes either. Makes star parties and travel a bit easier.

You still have to do an alignment - even after it's found north and levelled itself.