EQ6 pointing accuracy

[lhansen (Lars)]

I'm new to the EQ6. PE seems OK +- 7 arc secs and well managed by using PEC but the pointing accuracy appears very poor. Ive done a couple of TPoint models (88 and 48 points) and the best Ive got so far is around 1500 arc secs - thats 25 arc minutes . My old EM200 did around 45 arc secs.

Polar alignmen is OK 1.5 arc min in azimouth and about 10 arc minutes on the elevation.


Ive fastened every nut and bolt on scope and mount.

Is there anything I should be looking out for?

[Astro_Bot]

Slipping gears? That's just a guess since I've yet to open mine up.

I've never achieved the advertised "up to 1 arc minute" GoTo accuracy, but if I pick decent alignment stars and take care with a 3-star alignment, I routinely get 5 to 10 arc minutes of pointing accuracy ... that's across the whole sky, so might be better using PAE in one specific region.

[lhansen (Lars)]

Its pretty weird. Ive been syncing on Hadar and the pointing accuracy when on the east side of the pier looking west seems really good (around 40-50 arc secs). Do a pier flip an all hell breaks loose - the pointing accuracy goes REALLY bad.

[Tandum (Robin)]

Hey Lars,
I've been running tpoint on my eq6 this week, mainly to ensure the dome is tracking the scope properly, I don't intend using it at all once I'm done.

I normally slew from park to a star, do a maxim pinpoint then sync and focus. Then slew to target and pinpoint/sync again. It is definitely a magical mystery tour after a flip but I find the pinpoint then sync method works for me there as well.

Also make sure eqmod is not creating it's own model and polluting the setup.

[lhansen (Lars)]

I have a weakness for CCDAP and unattended operation so getting the pointing accuracy right is a priority. I'm getting 3-4 minute unguided subs (with PEC) so there must be some way of getting the pointing accuracy improved. Are there any TPoint afficionados out there who can look at a TPoint model and tell whan the problem might be?

I read about cone error? Im wondering if that might be at play.

[Astro_Bot]

Ah, well, there you go. I'm just using the SynScan firmware and the standard tripod, and don't usually fuss with setting up the tripod too carefully. Don't mind me.

Cone error ought to be corrected for with 3-star alignment.

[Tandum (Robin)]

Most of the eq6 guys I know are using ccd commander and maxim for unattended capture. No need for tpoint with that combo.

[lhansen (Lars)]

Just had a first look at CCDCommander - it uses PinPoint for platesolving as does CCDAP, but you still have to be close for it to work as with CCDAP.

So I got one vote for slipping gears - i can check that out. Any other suggestions?

[Tandum (Robin)]

Can't comment Lars, I don't use either but I know the guys use ccdc in the field without tpoint and it works fine.
Maybe one of them will chip in here.

[gary]

Originally Posted by lhansen

So I got one vote for slipping gears - i can check that out. Any other suggestions?

Hi Lars,

You can discount slipping gears. It is not that. Otherwise it would have betrayed itself
before the flip.

Have you been sampling stars on both sides of the meridian? Doing a meridian
flip and sampling on the "other" side is absolutely essential.

When you examine the pointing data in the spread sheet interface, you will
see a column for the star's RA and a column for its Dec as well as a column
for the scope's RA and a column for its Dec.

Examine the Dec pointing data for the scope.
Are the absolute values of the Dec data always in the range 0 to 90 or do
you see some that are in the range 90 to 180 as well?

Once you create a model, you should not be re-syncing on anything as you will
invalidate the model.

Originally Posted by Astro_Bot

Cone error ought to be corrected for with 3-star alignment.

Alas, life would be simpler if this were true, but it is not.

By the way, "cone error" is a term Chinese mount makers coined in their instruction
manuals. On professional observatories, the preferred description for the Hour Angle
component of any Dec to Optical axes non-perpendicularity is usually Collimation
error in Hour Angle or CH for short. This is also the expression TPoint uses.

GEM's notoriously tend to suffer from CH and it is among the terms that
reverses its direction during a meridian flip. Hence my question to first check that the
pointing data being supplied to TPoint is correctly denoting when the mount
is flipped by the absolute range of Dec values going from 0 to 180 not just
0 to 90.

Once you can confirm that, there are several reasons why CH can commonly occur
but we can pick up the thread from there.

[lhansen (Lars)]

Thanks Gary

I'll check those values when I fire up the observatory computers and get back.

I'm using eqmod with the mount in conjunction with thesky6 and aag tpoint mapper.

Regards

Lars

[troypiggo (Troy)]

How good is your balance? I've had similar problems if things were s bit out of balance. And how much weight do you have on it? I noticed it more with 10in newt than little 500mm refractor.

[lhansen (Lars)]

Balance I thought was ok, but I will double check.

Weight - it's carrying a fairly heavy 6inch newt plus cameras auto guiders etc. Certainly not near the supposed (I don't know for sure) 25 kg weight limit of the mount.

Thanks for that

[lhansen (Lars)]

Just checked out the various things suggested:

Balance is good in RA, could be better in DEC

I checked the tpoint model numbers and the dec values range from 0 to 90 + and -.

I also looked at the error vectors and there is a strange pattern - Measurements made with the scope on the east side of the pier, looking west have error vectors that point due west, with the reverse pattern being evident on the west side of the pier. I interpret that as either, my pier could be rocking? I can check that - or the mount is not properly attached to forks of the mount base.

Any thoughts?

[Terry B]

I only used EQMOD to run mine but could usually position a star within a 20arcmin wide frame anywhere in the sky from anywhere else.
Backash makes a bit of difference but it still should point better than you are getting.

[gary]

Originally Posted by lhansen

I checked the tpoint model numbers and the dec values range from 0 to 90 + and -.

Lars, this is key and I would need you to clarify it further.

When you examine the column for the Dec values of where the scope
appears to be pointing, do any values appear whose absolute values (i.e.
ignore the +/- sign) are in the range 90 to 180?

The column for the Dec values of the star that you sampled will
always have absolute values in the range 0 to 90 but those for the mount
need to also supply information to TPoint that the OTA was flipped.

If you can attach the data I can have a quick look.

I also looked at the error vectors and there is a strange pattern - Measurements made with the scope on the east side of the pier, looking west have error vectors that point due west, with the reverse pattern being evident on the west side of the pier. I interpret that as either, my pier could be rocking? I can check that - or the mount is not properly attached to forks of the mount base.

Any thoughts?

As mentioned in my previous post, some classical error terms such as CH
reverse their vector direction when the mount is flipped, so the pattern
you saw is not strange but in fact normal.

It is typical to achieve a small RMS when you begin sampling just on
one side of the meridian, but as soon as you do a flip see the RMS values
go out the door until you sample some more.

But it is key that whatever automation you have in place that is
conveying the mount's position to TPoint is also able to denote
which side of the meridian the data was sampled on.

If not, you can be chasing your tail.

[Barrykgerdes]

Polar alignment sounds like the trouble. 10 arc mins in elevation is much too much. This will give quite large goto errors in some parts of the sky. particularly at high declinations.

Elevation is far more important for accuracy than most people think and the error will not show up on a simple drift alignment at the zenith. You need to do a drift at the horizons as well.

Barry

[gary]

Originally Posted by Barrykgerdes

Polar alignment sounds like the trouble. 10 arc mins in elevation is much too much. This will give quite large goto errors in some parts of the sky. particularly at high declinations.

Elevation is far more important for accuracy than most people think and the error will not show up on a simple drift alignment at the zenith. You need to do a drift at the horizons as well.

Barry

The high fitted RMS won't be due to polar misalignment as
the MA and ME terms will mop that up very nicely.

The polar misalignment terms have very distinctive mathematical signatures
and so the analysis typically has no problem characterizing them.

[gary]

Hi Lars,

Within TPoint, select File->Export Text File and type in a suitable name.
That will produce a .dat file which you can then attach here so we can have a quick
look for you.

[Merlin66 (Ken)]

Just a comment:
I can get good GOTO results with the NEQ6pro.
The weight limit for "serious" work is more like 18Kg.
Have you double checked that cone error isn't one of the problems - re your earlier post.
Meridian flip would certainly aggrevate the accuracy...