Meade LX200 ACF - Polar Alignment and Go-To's

[Cimitar (Evan)]

Hi everyone

I own an 8" Meade LX200 ACF and operate it on a wedge and standard field tripod. I have two questions about polar alignment and GO-TO's that have been bugging me for quite a while now and I am unsure how to tackle them. The end result is I spend most of my time setting up the telescope instead of observing/astrophotography.

Normally I complete a standard 1 star polar alignment (Sigma Octantis) in the Southern Hemisphere, and then complete the relevant Go-TO. After doing this I am "reasonably aligned". That part is fine.

However, once I commence the drift alignment process, or iterative process to improve the polar alignment, and make manual alterations to the wedge, this subsequently throws my Go-To's out. Not a lot, but just enough that it makes finding things really difficult.

So I have two questions:

1) After I complete the drift alignment (or iterative process), should I re-do the Go-To alignment procedure?

If so, I'm limited to using the Meade one-star alignment procedure (apparently Meade has a bug in it's software that limits the use of the two-star alignment in polar mode) and I end up in a bit of a loop... (i.e. align with Sigma Octantis, sync with relevant star, perform drift align... wash, rinse and repeat)

2) Can I simply use the SCP for polar aligning (instead of Sigma Octantis) right from the start and when the hand-controller asks me if Sigma is centred, just say YES and move onto the relevant sync star? Would this save me the trouble of needing to perform the drift alignment procedure?

Thanks in advance for your time,
Sincerely
Evan

[redbeard (Damien)]

Hi Evan,
The first thing to do here is to get your mount polar aligned. Don't worry about goto at this stage because there is no point until your mount is aligned.

How you correctly polar align is up to you as there are many different methods. I have a Meade LX200GPS and the setup should be the same. The method I use is by using the software 'Alignmaster'. It is very easy to use and it helps to polar align by selecting a set of stars selected from its database that are visible to your sky at the time.

That's just how I do it because it is very easy to use as I have to setup my scope each night that I want to use it as I have no obs as yet.
Once polar aligned, the next step is to ensure that all you scope settings are correct eg time/date, location, ect. GPS does most of this on my scope.

At this point, I put the scope into the home position and simply navigate with the hand controller to a star in the sky I know the name of. Once centred, I select that exact object from the hand controller object menu and I hold the enter key on the hand controller for about a second and a message comes up asking to Sync the object. Press enter again to sync the object.

I then do this to 2 other stars and the scope will goto really well from there. You can always sync on other stars throughout the night if need be.

I don't use any of the built in star alignment methods in polar mode because they have never worked well for me.
OK for Alt/Az though.

Hope this helps and ask any other questions if you have them.
I'm sure others have other methods on setting up their scopes and I would like to know if there is an even easier method of setup, but for me and a friend of mine who has the same scope, this does work very well.

Cheers,
Damien.

[AndrewJ]

Gday Evan

If you do a rough polar one star align and centre Sig Oct using the wedge controls, it will get you close as you note.
You can certainly drift align after that without messing up the alignment, as in Polar one star, there is no alignment "model". The scope just tracks at sidereal and "assumes" its RA axle is pointed at the pole.
What you would do after the drift aligning is select a bright star near the celestial equator and do a goto ( which will be off ). Manually centre the star using the Hbx keys and do a synch. That will reset the internal datums and you are now properly re aligned.

Another approach that i use at times is to put my LX200 into polar home and spin the declutched RA axle until the stars make a circle in the EP, ie i am at perfect DEC = -90.
I then use my wedge controls to adjust the pointing until i can see the triangular asterism that marks the pole.
It just fits into the FOV of my 26mm EP, so centring the pole is pretty easy.
When that stays centred in the EP as i spin in RA, i know i am mechanically pointed at the pole.
After that, just do a polar one star align and just hit enter when it asks you to centre Sig Oct. Dont even bother to look.
Centre the second star correctly and you are done.

Andrew

[Cimitar (Evan)]

Hi Damien,

The first thing to do here is to get your mount polar aligned. Don't worry about goto at this stage because there is no point until your mount is aligned.

Excellent, thanks for that! In my head I've always linked Go-To and alignment together. After reading your comments and Andrew's I've realised that they are in fact 2 separate elements.

The method I use is by using the software 'Alignmaster'. It is very easy to use and it helps to polar align by selecting a set of stars selected from its database that are visible to your sky at the time.

Thanks for mentioning this, I've had a look at the website and I'll download the trial version. I usually have my laptop with me in the field as I use it with PHD guiding. I will need to buy a USB/serial adapter for my laptop. I take it I still use the handset (via HBX port), and plug the computer cable into the RS232 port on the LX200 mount?

Cheers
Evan

[Cimitar (Evan)]

Hi Andrew,

What you would do after the drift aligning is select a bright star near the celestial equator and do a goto ( which will be off ). Manually centre the star using the Hbx keys and do a synch. That will reset the internal datums and you are now properly re aligned.

Thanks for this, makes perfect sense now. I think what may have contributed to my confusion is where the manual talks about high precision mode and manual star syncing. It says that it works well for that particular region of the sky, but if you move to a different part, another relevant local star sync may be required.

Another approach that i use at times is to put my LX200 into polar home and spin the declutched RA axle until the stars make a circle in the EP, ie i am at perfect DEC = -90.
I then use my wedge controls to adjust the pointing until i can see the triangular asterism that marks the pole.
It just fits into the FOV of my 26mm EP, so centring the pole is pretty easy.
When that stays centred in the EP as i spin in RA, i know i am mechanically pointed at the pole.
After that, just do a polar one star align and just hit enter when it asks you to centre Sig Oct. Dont even bother to look.
Centre the second star correctly and you are done.

Perfect! I shall give this a try first up. I recently stumbled upon the 90 degrees OTA alignment issue 2 weeks ago. I conducted an RA field rotation and the SCP disappeared right out of the FOV and then slowly back in again. (the circle was quite wide). After some minor adjustments I was able to keep the SCP asterism stationary whilst everything else spun around it.

Cheers
Evan

[AndrewJ]

Gday Evan

I think what may have contributed to my confusion is where the manual talks about high precision mode and manual star syncing. It says that it works well for that particular region of the sky, but if you move to a different part, another relevant local star sync may be required.

That is correct if you have a bad alignment model or arent properly polar aligned but did a one star align ( and hence dont have a model ), or you have orthogonality errors.
In its simplest form, the scope has an encoder on each axis and when you do a one star align, it reverse calculates what encoder value = 0 for each axis, based on the theoretical values for the target star. Every time you do a future synch, it simply resets this zero datum value to suit the currrent star, so it works well in the local region, but may not be correct elsewhere.
If you have got yr RA axle very well polar aligned, doing a synch anywhere will still reset the datums in the same way, hence why you can just do a synch after drifting, vs do a full realign.

After some minor adjustments I was able to keep the SCP asterism stationary whilst everything else spun around it.

Thats good, but one thing i find at times is it is difficult to get near at first, esp if the asterism at the pole isnt visible yet. In that case, you can sometimes do the polar one star first, and use the much brighter sig oct to get the wedge close "mechanically".
Then instead of drifting, just use the spin test to get the OTA parallel to the RA axle and again use the wedge controls to find/centre the asterism.
After that, again goto a bright star near the equator and synch.
Lots of ways to skin the cat.

Andrew

[redbeard (Damien)]

Originally Posted by Cimitar

I take it I still use the handset (via HBX port), and plug the computer cable into the RS232 port on the LX200 mount?

Cheers
Evan

Yep, that's correct. Be careful if you are making a cable to go between the Meade and the USB adapter to check pin-outs carefully.

Cheers,

Damien.

[Cimitar (Evan)]

That is correct if you have a bad alignment model or arent properly polar aligned but did a one star align ( and hence dont have a model ), or you have orthogonality errors.
...
If you have got yr RA axle very well polar aligned, doing a synch anywhere will still reset the datums in the same way, hence why you can just do a synch after drifting, vs do a full realign.

Thanks Andrew. I think I understand now. Kind of sounds like error creep... If the telescope is not very well polar aligned, then it's enough to throw the Go-To out when you move to a different part of the sky.

Thats good, but one thing i find at times is it is difficult to get near at first, esp if the asterism at the pole isnt visible yet. In that case, you can sometimes do the polar one star first, and use the much brighter sig oct to get the wedge close "mechanically".
Then instead of drifting, just use the spin test to get the OTA parallel to the RA axle and again use the wedge controls to find/centre the asterism.
After that, again goto a bright star near the equator and synch.
Lots of ways to skin the cat.

Thanks for reminding me of this! I was out last night around 6.15pm (my usual time when I begin to setup) and I could only just make out where Sigma might be located. I then remembered that I'd usually end up working on my alignment until 9pm, only then could I start taking pictures. Very timely advice

Cheers
Evan