How do you drift align a Goto CG5 mount that self corrects pointing error?

[g__day (Matthew)]

Can anyone help - I've directed this query in the following e-mail to Celestron technical support. I'm not asking what is the process to do a drift alignment, I want to know how to instruct a CG-5 not to automatically correct for mis-alignment error of the axes so I can properly drift align in the first place!

* * *

Lance,

I've now built a permanent pier for my CG-5 goto mount and I'd like to drift align it to get it very accurately point to the SCP. Three questions:

1. If I go under utilities and run the polar alignment process it moves close to Sigma Octans then says do a manual adjust so its perfect and re-align. To familiarise exactly where these two dim stars should be can I easily first just say go to Sigma Octans so I can see where it should be pointing, then say polar align to sigma octans and see how much it moves so I have a better indication of the amount and direction of correction I need?

I can't find a menu item to get the mount to just normally slew to Sigma Octans - how can this be done?

Shame Celestron didn't choose any bright alignment star that is visible to execute! For instance imagine if it said fine I'll slew to where say Antares is pause for 30 seconds then move to show how far it believes the scope is away from being properly drifted aligned using bright Antares in this example as the relative checkpoint, and then you manually adjusted the base until you had it right and then re-aligned it would be much easier, given you have one very bright star to point too. Maybe consider this on your next major upgrade, as you are just trying to correct both East/West and Up/Down delta's. You need not choose one of the dimmest stars on the sky to do this with if the scope itself has already calculate how far away from the SCP its axes are!

2. under the Menu -> utilities - display alignment menu it gives me a read out of two numbers something like elevation 67 (which looks like 90 - Sydney's latitude of 33.5) and negative 1 degree 3 minutes which I am unsure means East or West of the SCP? Does the -ve mean I am too far East or West of Celestial South?

3. Can you actually drift align a CG5 once you have done a 3 star alignment, or does the self correction of the mount itself make this impossible to do after alignment?

The way I understand it say my mount is within +/- 3 degrees East/West and Up/Down of the South Celestial Pole and I do a 3 star alignment. Then the mount determines my error and automatically compensated for this in both RA and DEC when tracking any star. If it was perfectly aligned then my pointing and tracking performance is probably greatly improved as the mount only has to every slew the RA to keep any star in view. However because of this compensation if I align and try and then attempt to drift align my mount is working against me because it is automatically compensating for DEC error meaning such error is hidden from me?

So if I want to drift align do I either somehow let the mount know this, not align at all or should I just do a 1 star align so the mount can't adjust for DEC error and then do my drift align?

[JohnG (John)]

Hi g_day

Can't you just bypass the alignment protocol and allow the mount to track at siderial rate with no alignment.

When I do a DA that is all I do with the Gemini, it goes through the alignment procedure and when it receives no responses it defaults to Siderial Rate.

Does CG5 do that?

Cheers

JohnG

[g__day (Matthew)]

John,

I'm not sure, the documentation is poor to say the least. I think you can skip alignment but then you obivously can't goto (not a worry) - if you try it will insist on an alignment process being run.

I don't know if it defaults to siderial rate or does nothing - its not documented!

My best guess is a one star alignment where you don't do any correction at all by the hand controller - just say its perfect and manually tweak it all the way after that on your drift alignment is probably the best bet...

[JohnG (John)]

If you want to do a DA you really dont need GOTO, I must admit I know nothing about the CG5 but what if you do a 1 star alignment then when it is tracking at Siderial, loosen the clutches, do your Drift Align, the mount will not know you have loosened anything, after you have got your DA right, turn off the mount and restart and allow it to do it's thing.

Just a thought.

JohnG

[gbeal]

I'll take a stab here as well. Don't use the motors at all.
Do the drift align without the motor(s). Set it up, pointing as close as you can, and let it drift through. Lock the dec axis, and simply hand push the RA axis to sit the star in the crosshairs. I have a Lumicon crosshair eyepiece that has both hairs as parallel lines, and I can have the star fit between them, slightly out of focus, taking up the whole width of the "gap". I align the hairs so they are in line with the RA/Dec.
Make your adjustments with the alt and az settings, repeating as necessary to get it bang on.

[g__day (Matthew)]

gbeal - that was an option I also pondered, basically if I can turn off or disable automated corrections, Drift Alignment should be straight forward.

The idea of letting looses the clutches - not sure, a whoopsie could always happen!

[rogerg (Roger)]

Hmm, so your problem really is understanding what tracking mode the mount is in.

You can do a drift alignment with a GoTo no problem providing it's tracking at Siderial, I used to do it all the time with my LX200 until I learned better. Now I do it by hopping between 2 stars using the GoTo and adjusting the mount appropriately:

http://www.rogergroom.com/rogergroom...m.jsp?Item=343

Using that method I can currently take a 30 minute exposure with no declination drift at all (2160mm focal length). That's better than I ever got with drift alignment.

But that would still require that your mount isn't correcting for known polar alignment error, so it still comes down to knowing what tracking mode it's in.

Hopefully someone with CG5 knowledge will be able to help. With my LX200 it doesn't have anything other than Siderial and with my GM-8 I make sure it's on Siderial not Adaptive King or anything else. Presumably the CG5 would have a menu option somewhere for choosing, but I haven't any idea.

Roger.

[gbeal]

Heck, I wouldn't trust the clutches to save a whoopsie. The whole lot should be that well balanced that the whoopsie factor isn't an issue.

Originally Posted by g__day

gbeal - that was an option I also pondered, basically if I can turn off or disable automated corrections, Drift Alignment should be straight forward.

The idea of letting looses the clutches - not sure, a whoopsie could always happen!

[JohnG (John)]

I probably should have explained it better, I meant loosen the clutch, move to the required star retighten the clutch and make your adjustment then do the same with the second star or as already been said, leave the motors off.

JohnG

[sheeny (Al)]

G'Day g_day.

Here's the go mate! It's not too difficult, really (I had some trouble getting my head around it to start with too!)

1. When you fire up your mount, choose "Quick Align" not "Auto Align".

Quick align just models the sky in the controller, it makes no adjustments for cone error, etc.

2. Do your drift alignment.

3. After doing your drift alignment, run it through the full "Auto align" procedure. You may have to power it down to do this, I think? Try it straight from the menu first .

Once accurately drift aligned and the cone error has been calculated by the auto align, from then on you should be able to use the "Last Alignment" option on start up.

That should get you going!

Al.

[g__day (Matthew)]

Many thanks guys, Celestron really have great gear but lousy documentation and how to guides!

Will get it sorted as soon as the weather clears, and post a few piccys of my new permanent pier I built myself last week (45cm square by 8mm steel base plate mig welded to 1.3 metre high 140 mm diameter steel cylinder 5mm thick with machined aluminium scope holder bolted to a stainless steel top plate and onto the peir. All this is permanently bolted to a 160kg block of cement hidden beneath pavers with just the galvanised bolts passing through the pavers so if I ever want to sell and move I unbolt the pier, lift four pavers, cut the blots, put four fresh pavers in and you'd never know the scope was ever there!

Alot of thought went into the construction and fab, but once done it looks beautiful and stable as you could wish for. Then the rain came down since I finished!

[sheeny (Al)]

Originally Posted by g__day

with machined aluminium scope holder

I'd love to see a photo / details of this. Any chance?

When I move to a darker site I'll be looking to do the same sort of thing!

Al.

[g__day (Matthew)]

Absolutely, as soon as the weather clears I'll get some pictures of it. I wanted it to be cheap, practical, simple and look stunning. I think I got it right this time.

Looking at piers from overseas they are very expensive and even trying to mode a EQ6 pier would cost upwards of $600. For me the bill was

Materials

1. 45cm square base + 1.3 metre cylinder - $190 from Jacobson Silverwater
2. Aluminium holder $65 + stainless steel top $15 from Edcon Brookvale
3. Professional milling of aluminium + stainless steel $100
4. 4 * 30 cm long 15mm galvinised bolts and nuts $20
5. 160kgs of pre-mix cement $40
6. 8 * Stainless steel bolts to hold the aluminium sleeve to the steel cylinder $24

The welding was free from a friend, and I had left over black satin paint and cold gal to protect the base of the pier.

For tools I needed a 16mm drill bit $30 a 10mm tapper $12 and a crowbar to lift the pavers $9.

So about $350 for materials, $100 for milling services (worth every cent) and $50 for tools. The result was worth every cent IMHO and as far as I am concerned look absolutely professional. Will post a how to guide with pictures as soon as the weather clears and I get a few good shots!

[g__day (Matthew)]

Back on subject - Lance from Celestron replies, and what and undocumented insight / bombshell he drops. Although the mount perfectly models the sky and understands after alignment your pointing error, it only correct for this during Goto - not tracking!

What a 24 carat whoopsie! I don't see why this isn't programmed in, it surely wouldn't be that difficult! Failing this why not document that critical feature Celestron? I had pondered that drift alignment was a nice but unnecessary task on a clever Goto GE mount - boy was I wrong!

Also Lance still hasn't told me how can I go to Sigma Octans? It doesn't appear in the Named Stars section of the menu - is it a SAO or IC item, or is it normally un- goto - able?

Lance's kind reply to my three questions in my first post!

• I believe that you do not quite understand how this polar alignment routine works. The mount simply moves to an object that is close to the Celestial pole. For northern hemisphere observers, it goes to Polaris which is actually 0.8 degrees off of the NCP. For observers in the southern hemisphere, the mount chooses Sigma Octanis, which is unfortunately, the closest star to the SCP. The mount points to where Sigma Octanis should be with respect to the mount’s starting position at that exact date and time. The scope does not show you the amount of “offset”. You have to mechanically adjust the scope using the altitude and azimuth screws to bring Sigma Ocatnis to the center of the field of view. A star closer to the celestial equator will not work well for polar alignment as this introduces cone/hub errors into the equation. You need to use a star that is as close to the celestial pole as possible.
• This is similar to a compass coordinate. 0 degrees is at north. 90 degrees is east. 180 (or -180) degrees is south. 270 (or -90) degrees is west.
• Any time you manually adjust the mount after a computer alignment, the computer’s alignment is no longer valid. You will need to realign on three stars. Once aligned the telescope will compensate for poor polar alignment and will successfully point to any object, but may not track properly. Being a German equatorial mount, only the R.A. motor is operating during tracking. If you have a poor polar alignment, the scope will be able to hit the target, but will show declination drift when tracking. An accurate polar alignment is a necessity if you want excellent tracking.

[OneOfOne (Trevor)]

I have the same GoTo mount and I am not sure, but I seem to recall seeing "Polaris Australis" on the list at some time.

I looked on the net but couldn't find a SAO number for it, otherwise it is also known as HR7228 or HD177482 but I couldn't find anything that would link any of these to a SAO.

At the moment, I only do visual alignment and what the reply from Celestron seems to backup what I have found, haven't tried drift alignment yet. You can go to something and it is pretty spot on. Leave it for a few minutes while I go inside to get something I forgot and it has moved significantly, ie "drifted". But select something else and it is spot on, actually it would be interesting to go to something, let it drift for several minutes and then go to it again. I expect it would actually put it in the middle again. Essentially from what Celestron says, the mount uses both motors to go to an object but once it has found it, only one motor is actually moving. I imagine if it is a fork mount, it would track in both directions, but of course if you are shooting near the pole you will get a lot of "rotation" of the image.

[sheeny (Al)]

This is good, g_day.

If you were only interested in visual work, I can see why you might like a mount that tracks in both RA and dec. But if your polar alignment is poor, you will get field rotation while it tracks. This is the problem fork mounts have with long exposure images unless they are used with a wedge and also accurately polar aligned. If you want to do imaging of any length of time per image, accurate polar alignment is essential.

Interesting reply re: polar alignment. I have ignored the polar alignment function as I figured it was a N hemisphere procedure only. If I have read the reply correctly, when you choose polar alignment, the scope goes to where sigma octans should be, you then just adjust latitude and azimuth to do your rough align (I would still drift align if doing imaging) and Bob's your mother's brother in law. So you don't need to be able to select Sigma Octant from the list (though I'm sure it would have an SAO number).

SO I would revise my procedure for accurate alignment of a pier mounted CG-5 to:

1. Power on and do a "Polar Align".
2. Drift align.
3. Do a full 3 star "auto align" (to calculate cone error)

For all subsequent usages, select "Last alignment" on power up.

It is a pity the documentation that comes with the scope/mount isn't clearer isn't it!

Al.

[sheeny (Al)]

From Wikipedia:

Polaris Australis, HR 7228, HD 177482, SAO 258857, FK5 923, HIP 104382.

Al.