South Celestial Pole

[leon]

Hi All

Now i am assuming that my G11 mount is doing what it is suppose to, by this i mean, when i finish using it for the night i send it to park position, and it is suppose to point directly south and shut down.

Again assuming the scope is in the right place, and every thing is as it should be, i decided to take a 20 minute exposure just to see where, and if it was pointing to the right place, the SCP.

This is the result, it looks pretty close to me, ;) what do you think.

Leon :thumbsup:

[Doug]

I'd say it proves nothing Leon. You could take the same phot with your scope mounted on a tripod. If you G11 is a 'goto' there is in my opinion only one valid test for polar alignment, and that is 'goto' accuracy. Guiding will cover up small errors in polar alignment, but a 1 star alignment followed by some goto operations will tell you how well polar aligned you are. A good polar align will repeatedly place a target on a webcam chip, or be close to central on a DSLR sensor. Drift aligning is good, infact mandatory, but after drift aligning, a sure test is goto accuracy. All your image shows is that the scope was ponting south. Where was your polar axis pointing? How parallel to the Polar axis is your OTA?

cheers,
Doug

[ballaratdragons]

I was about to say 'Looks spot on Leon" until I read Dougs post.

I know my mount is pretty close to SCP because after I do a 3 star alignment, everything I GoTo is in the FOV on my monitor. I have never drift aligned as I haven't needed to.

I suppose the true test IS the GoTo accuracy. :shrug:

[leon]

Hmmm, Doug, I see what you mean, that put a pin in my bubble :lol: :lol: didn't it, but after saying that, it's go-to operation is pretty good actually, and the drift align i performed a couple of weeks ago also appears quite good.

A faint Star at the meridian will stay in the 9mm recticle square for well over 15minutes so i expect that i am pretty close to the pole, maybe not spot on but close enough for what i'm doing.

Cheers leon :thumbsup:

[leon]

Hi Doug, I had a think about what you said, and it is true, one can take a image of the SCP, with a tripod, or for that matter with any scope pointed in the right direction, if one can find it and align to it.

I assumed my accuracy with the equipment that i was using, being a G11 mount and Takahashi FSQ 106, which is mounted with double Clam Shell Tube Holders, so i can assure you it is rigid, and well aligned to the polar shaft.

After a successful drift align, and some imaging i found it to be pretty well right and have produced some good results without guiding.

This led me to believe that i must have been fairly close to the SCP.

The G11, as you probably know, will, according to it's accuracy of the drift align park it's self in the true south position after each imaging or viewing session.

It was this position of the mount that had me wondering, how close to the pole is this thing pointing, hence the image above.

After slewing and moving across the sky during the evenings, it always lands exactly to that SCP position, and to the image i posted.

So i suppose it was that which led me to believe that the mount was pretty much pointed in the right direction, but i may be wrong and missed something here, and am eager to hear from other members and the opinions.

Anyway that is my story, and I'm sticking to it :P :lol:

Cheers Leon :thumbsup:

[erick]

To my ill-informed way of thinking, I believe that the question is, Leon, is the centre of all those concentric circles of stars located exactly in the centre of your image? Point the camera 10 deg away from the SCP and you'll still have an image of concentric circles, but the centre will not be in the centre of the image. Having said that, I suspect that the error in determining if the centre of those circles is in the centre of your image is greater that the errors you can tolerate in your imaging work.

Or am I just talking rubbish??

Eric :screwy:

[leon]

Yes Erick, The image is as it was taken, not cropped etc, I parked the scope, attached the camera and took a 20 minute image, that was the result.

I agree, if i move the scope or the mount to the west or east, those rings will be still there, but off center.

Leon

[OneOfOne]

I would guess, if you locate the "centre" of the arcs and see how close this is to the centre of the image, it would at least indicate that you can't be far wrong! Not a precision way of checking it, but a good start?

[JohnG]

Hi Leon

Technically speaking, your Gemini system has calculated where the scope should be pointing, whenever you do an Additional Align, the Gemini updates it's Pointing Model and therefore makes slight adjustments to where the SCP is.

By doing what you did, an accurate Drift Align, then building a Gemini Pointing Model, you are effectively removing any major errors, the fact you hit your target each times proves that.

That is why a permanent setup is so good.

Cheers

JohnG :thumbsup:

[leon]

John, you make me feel better now :lol: at least i am heading in the right direction this time.

Leon :thumbsup:

[Doug]

Leon, I do not have a G11, I use an EQ6. Having said that, and admitting thereby that I have no hands on with a G11, my point is that any mount (go-to) can only know where it is if you tell it. John G implies that the Gemini can learn from your successive star aligns (I think), but if I have understood him correctly, then it is only masking out polar errors.
Here is the way the EQ6 works. When I do a 1 star alignment and subsequently park, the mount goes to what should be south based on it having been pointed at a known target. But this assumes that it is in fact polar aligned! Question: If my mount was pointing East instead of South, and I did a 1 star alignment, would park set it pointing south? Answer: No. So park depends on me setting the thing up correctly in the first place, because it will drive the Dec axis the required amount to go-to 90deg from where I have said it was, and it will rotate the RA sufficient to bring it to zero hours, again based on where I have said it was.


When I drift aligned, rather than watch a star for just 15 minutes, I used K3ccd tools and a web cam. I turned off Dec drive corrections and tweaked till the star trace oscillated +/- over the center line. There will be a little drift due to atmospherics and various mechanical abnormalities, some slight imbalance etc. I found that chasing these was a waist of time, so I reasoned that they were red herrings and any real Dec drift due to mis alignment would never come back to the zero line without correction. After watching the trace vary +/- over an hour satisfied me that statistically speaking the trace would remain the same over 48 hours ( if possible), there is no cumulative drift even if there is some sporadic drift now and then. In other words my mount is Polar aligned...period. As a result, whenever I start an observing session, I initiate a 1 star alignment. The target star is always well within the fov of the imaging chip, DSI or Toucam to start off with. Once centered, The mount will place any go-to target as near as anything to center. The biggest error I get is if I need to cross over the meridian,(cone error) but even then the target remains on chip.
Now that I am satisfied that my mount is polar aligned, if I were to do the same test as you have done, (all I would see is a %$#@ pine tree) I would need to center the OTA to the mount, not touching the mount to get the star trails centered on the sensor. I venture to predict that any go-to mount that is truly polar aligned will faithfully bulls-eye any target after a 1 star alignment. 2 and 3 star alignments are for field setups where time prevents accurate Polar alignment.
cheers,
Doug

[JohnG]

Hi Doug

Yes, you are correct about the Gemini refining any errors associated with a polar alignment.

I have always done an accurate Polar Alignment using the Drift method, my aim is to have a star stay within the central X-hairs of a high powered guiding eyepiece for at least 20 minutes or so with only drift in RA.

On startup, the Gemini uses the Counterweight Down position with the OTA pointing at the Pole, the more accurate this is, the better, although saying that, it does not really need to be absolutely perfect. From that position, the Gemini will go to a selected bright star, you then center that star using a X-hair and synchronise the system. From there on in, you go to at least 4 to 5 stars on that side of the Meridian and do what is called an Additional Align, the Gemini is then building a Pointing Model, all the time it is giving you A and E numbers (Azimuth and Elevation), these numbers are in Arc Minutes and are the distance the telescope is from the Pole. This is for a Cold Start.

You only need to Synchronise on one star for a Warm Start, then GOTO whatever object you want.

You then select other stars on the other side of the Meridian and continue to Additional Align, all the time the A and E numbers should be falling. You can use the Hand Controller or the current ASCOM driver or Gemini Control Center to show you the detailed results.

So, in reality, if you get A and E numbers of 0 and 0, you are within 1 Arc Minute of the Pole, if need be, you can use the PAC (Polar Alignment Correction) feature to get closer.

I have A and E numbers of 0 and + 2 on my mount and find that my Autoguider has no trouble correcting these. :thumbsup:

Cheers

[leon]

Thank you John and Doug, you have both been very helpful in my quest to get a good polar alignment.

Leon

[h0ughy]

well this has been a very enlightening thread, thank you gentlemen for your frank discussions of a benign image I thought? Man I have some learning to do with my G11

[montewilson]

In the start I drifted and then moved to the STV which can act as a super sensitive guiding eyepiece but now I have found TPoint and Automapper.

If you have a goto mount, a CCD camera, TheSky CCDSoft TPount and Automapper (which is free) then it can determine your misalignment in arcseconds. In about half an hour,I can get to within a few arc seconds of the pole. Of course there are refracted pole issues too but for now lets just say that by taking a couple of dozen frames, letting the software compare the images with its own maps and determining the offset, I have beaten polar aliging for good!

This way always a challenge because a big frame like the STL/FSQ gives will show field rotation very quickly if less than well aligned.

[Doug]

Yes TPoint has had rave reviews alright. Must be a real blessing for those having mobile imaging situations, but of no cost effective value to a permanent setup me thinks.
cheers,
Doug

[leon]

Well guys this has certainly been a very informative thread, and i thank you for your imput.

Tonight i tested my theory, and all that fiddling and imaging the SCP did actually pay off.

After manourvering the mount to get the SCP centered in the scope, it was time to see if it was all worth the effort, and i can honestly say it worked a treat.

Started with a cold start with the Gemini and did a 4 star pointing model, and it landed smack in the middle of the field of view each time.

The A and E readings were (A was 0, and E was minus 0) so i dont expect it will get much better than that.

Then decided to take some un guided images and, at one minute the stars were perfectly round, any more and a slight trailing was noticed.

So, there you have it, we all work in mysterious ways to get the same results.

Leon :thumbsup:

[Doug]

Hey Leon that is good news! Does the pointing model created remain saved in memory or do you have to repeat the ritual at each power up?

Doug

[Doug]

Monte, when I purchased THE SKY 6 from software Bisque they shipped it via UPS and I finished up being charged Import, customs + local freight+ GST. It cost me a lot more than the quoted price (adjusted to AUD). Just wondering how/from where you obtained your copy of T-Point? The current AUD is about $299, and I gues by the time UPS declare it at customs it would cost around $380- $420 or so. That makes it a very expensive package. I wish they would take a leaf out of Diffraction LTD and make their products downloadable.

Doug

[leon]

Yes Doug, all information is saved and it is not necessary to repeat the procedure, i just do a Warm Start as it is called.

It will use the saved information for as long as i want, unless i wish to cold start it again.

The mount will also do a Warm Restart, where it will continue on from where it left off from a previous nights viewing or imaging.

Pretty clever that mount ;)

Leon :thumbsup:

[gary]

Hi Leon,

When discussing the topic of polar alignment, one needs to
differentiate between what is practical and what is the actual
reality.

Please allow me to explain.

If the ends justifies the means, what most amateur
astrophographers are really seeking is no discernible image
anomaly as a result of polar misalignment.

However, in reality, different parts of the sky have different
optimal polar axis points.

One practical reason for this is atmospheric refraction. The
effect of refraction is to cause objects to appear slightly
higher than they really are. At the zenith, there is no
displacement whereas at the equator, the displacement is about
half a degree. Even at 45 degrees elevation, the effect accounts
for about 1 arcminute.

To complicate the matter even further, the current barometric
pressure and temperature also impact upon refraction and the
optimal polar axis for imaging any one point in the sky and
strictly speaking have to be taken into account as well.

Refraction will also mean your circular trails are never quite
perfectly circular, not unless you image by pointing at the
zenith from the geographical north or south pole itself.

In practice, unless your exposure time is long, it is unlikely
to make a difference. However, the concept that there is some
magic point in the sky that one can align the polar axis of the
mount that results in optimal imaging in all parts of the sky is
a myth.

If one sought absolute perfection for the sake of it, the
reality would be that one would have to continually adjust the
mount in elevation whilst tracking.

And it gets worse.

Since tracking is the first differential of pointing, the
tracking rate would need to continually change as well.

And it gets worse still.

All mounts have geometric, gravitational flexure and more often
than not bearing eccentricity errors. Not only does this make
optimal polar alignment difficult, for the perfectionist, some
of these errors make it absolutely impossible. For example, if
the RA and Dec axes of the mount have not been constructed to be
at absolute right angles to each other, then there is a region
around the scope's polar axis to which it can never point. The
same is true if the mount's Dec axis and the nominal pointing
direction of the scope is not at right angles.

For this reason, the drift method is not the gold standard for
polar alignment that many mistakenly think it to be. It can't
take into account all the other errors within the mount.

A simple one, two or three star alignment is no better
as is simply perfoming a GOTO to a few stars.

Instead, to have a chance of getting closer, one needs to
perform a more sophisticated analysis, taking into account the
effects of refraction, mount errors and polar misalignment, all
at the same time. This is achieved by performing a star pointing
test on a reasonably large number of stars distributed across
the sky and then using sophisticated analytical methods to
separate the polar misalignment from the mount fabrication
errors.

All of the world's major professional observatories, including
the Kecks, Geminis', VLTs, do exactly this and almost all of
them employ a software package originally written within the
professional astronomical engineering community called TPOINT.

The Losmandy Gemini system also performs mount errors analysis
as does the Argo Navis Telescope Pointing Analysis System (TPAS)
that we happen to produce. However, that is not the point of
this response.

The real point is that you should use whatever means to polar
align your mount at a point that is optimal for the point in the
sky you plan on imaging. The best methods are to use a
sophisticated analysis technique such as TPOINT, TPAS, etc.
Any system that claims mount error compensation but only uses,
say, three stars, is lacking, to say the least. The modeling system
in your Gemini will undoubtedly give an excellent result.
However, keep in mind that a good package should always provide
statistical metrics with regards the analysis, otherwise the
results become more difficult to prove. TPOINT and TPAS both
provide such feedback.

For many amateurs, the practical reality is that aligning the
scope's polar axis to the refracted pole rather than the true
pole and combining it with guiding software that will keep
you on track will tend to provide excellent results.

Hope this is helpful and allows you to concentrate on enjoying
your imaging rather than chasing a pole that really is a
shifting target itself.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[leon]

Gary, thank you indeed for that very informative response to this interesting thread.

I think I understand what you are saying.

Please don't take this personally, but if i were to follow that procedure each time i wished to do some imaging, I would abandon Astronomy and give all my stuff away, and take up stamp collecting.

Although I do my work in a strange way, i am enjoying myself, and having fun, so to speak, and i feel that is what it is all about.

I do not wish to be the top astro photographer in this field, but simply do what i do, and hang the occasional image one my wall, although not perfect.

I would not be enjoying the experience if i were to do what you suggested each time i went out side.

But thank you Gary, at my age i doubt if i would ever get to that level of expertise before i die, but one never knows.

Cheers Leon :thumbsup:

[Doug]

I want!!
I got no free cash!!
Drat!

[JohnG]

Hi Gary, nice to hear from you again.

What you have written was an interesting and well written response to alignment and associated problems, unfortunately a lot of us don't have the funds or equipment to approach this in an advanced sort of way, me included.

Therefore I would still suggest that a good Drift Alignment, old fashioned as it is, is still a good way to get a reasonabley accurate pole alignment. I would love to have T-Point etc but in the real world some of us are limited to what we have.

The current Level 4 Gemini will show all the various flexures, pointing errors, cone angles etc if you wish, in fact any error you might think of. These can be accessed through the Hand Controller, or better still, if you are running an ASCOM driver, though the Pointing Model menu, or if you have the companion Gemini Control Center program.

This why I do an extremely accurate Drift Align and using the Gemini's Pointing Model, coupled with an old ST-4 autoguider, very few of my photographs will show any trailing, the autoguider handles the errors without any problem.

Cheers

JohnG

[JohnG]

I have attached the Modeling Parameters menu from the current ASCOM driver to show some of what is taken into account when a Pointing Model is produced.

Cheers

[gary]

Hi Leon,

Thanks for the post and I appreciate exactly where you are coming
from. As I mentioned, I hope my comments will help you concentrate
on just getting out and enjoying yourself.

However, I wanted to just make a quick technical qualification.

In my response, I recommended best practice would be to perform
a star pointing test and an analysis of the data and that is indeed true.
I also mentioned that in order to perform this analysis, one needs to
sample a reasonable number of stars scattered across the sky, which
is also true.

However, what I did not take the time to point out is that once you
have performed such an analysis you can then save any persistent
mount error terms into a pointing model for use on a subsequent
observing session. The good news is to re-use that model for a mount
that is only roughly polar aligned requires you to sample perhaps as few
as four stars on a subsequent observing session.

Initially, the various error terms within the mount are entangled
in a complex way. A long sampling run allows you to unravel this knot.
Once the errors are know, this then allows you to hold 'fixed' those
error terms that are likely to be persistent from session to session, for
example, any non-perpendicularity between the RA and Dec axes.
One can then re-synchronize any non-peresient terms, such as polar
misalignment on a portable mount, using a much shorter sampling run.

The good news is that this takes only minutes to perform and is subtstantially
faster than a drift test with all the benefits of accuracy as discussed earlier.

Anyway, I hope this additional qualification is helpful as I would not
like to leave anyone going away with the impression that they need
to spend most of the night performing a sampling run in order to
polar align their mounts from night to night.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[gary]

Hi John,

Thanks for the post and great to hear from you again. I did note a few
error terms that are missing from the mix, namely those associated with
eccentricities in the RA and Dec axes. We've seen examples on the G-11
and other mounts where these can contribute several arcminutes to the
Root-Mean-Square pointing performance of the mount.

There is an case study on our web site here which might be of interest -
http://www.wildcard-innovations.com.au/group_post_5573/
Be sure to grab the free Adobe SVG plugin to view the graphics -
http://www.adobe.com/svg/viewer/install/main.html

A good autoguider, as you point out, can do a great job of 'mopping-up'
some of the residual errors.

Thanks again for the post.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[ballaratdragons]

Geez, I think I'll join Leon in the stamp collecting club! :eyepop:

After I do a 3 star alignment (EQ6) I can GoTo anywhere in the sky and the object is in the FOV on my monitor, and that is with the Toucam which is equal to a 6mm EP!!! That sounds pretty accurate to me. And for long exposures, my autoguiding set-up works fine. :thumbsup:

[Doug]

I think most modern Goto systems have atmospheric refraction factored into their calculations; a natural enough spin off from having longitude inputted.
Tpoint, at around $249USD is an expensive toy for an occasional use item, or something for the idle rich.

Doug

[ballaratdragons]

I know for a fact that Vixens Skysensor2000PC has asmospheric refraction compensation.

Dunno about our EQ6's though :shrug:

[gary]

Hi Ken,

Good to hear.

What this says is that your mount/OTA probably has at best only small fabrication
errors, which is exactly what you want. If your mount/OTA were entirely free
of geometric, flexure and bearing eccentricity errors, then you would only
require a two star alignment, not three.

However, there are some users who are less fortunate than you.
It's a bit like having impaired vision. If one has twenty-twenty vision,
then who cares about eye glasses or laser surgery. However, if
everything always appeared totally blurred, the prospect of corrective
lenses or surgery would be heaven-sent. It's the same with telescope
pointing. if you don't have the problem, then you probably would
not be interested in the cure. Move on, nothing to see here. However,
if, like some users, you have invested subtantially in a mount to find it
points like a bad shopping trolley steers, then you will want to know
how to fix it.

The problem is that when the mount has more than one type of pointing
error and you add polar misalignment into the mix, then something as
simple as a one, two or even three star alignment just doesn't
cut the mustard. For that, more sophisticated analysis is required.
However, the good news is that as long as the errors are systematic
(no system can account for random errors such as a sudden mirror flop),
then there is a prescription available.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[gary]

Hi Ken,

Our own Argo Navis performs atmospheric refraction compensation,
and takes into accounts effects such as refraction and nutation.

Refraction is one of those transformations that unless taken into account
can degrade the whole-sky RMS pointing performance of the telescope.

Refraction can be as much as about half-a-degree down on the horizon
and about an arcminute at 45 degrees elevation. Which brings us back
to the original line of this thread with regards the pursuit of some
"holy grail" exact polar alignment position. There is no such point and
perfectionists will be disappointed. A good compromise for such
people is to aim for the refracted pole.

Best regards

Gary Kopff
Wildcard Innovations Pty. Ltd.