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Old 15-01-2019, 07:24 PM
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mynameiscd (Andy)
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Polar alignment breakthrough

Hi all,
I finally got it!!!!!!!!!
For years ive been aligning my scope to the SCP normally with my spotter, then guide, and finally with my main scope. Ive always been that close I've never really worried about software etc.
I've been trying drift aligning but its always not the same as my visual alignment and then I just adjust back to visual and wonder what i did wrong.
Im pretty lucky here as there are no towns nearby so i can normally see the part of the" Chinese hat " by eye so I know the stars aroud the SCP pretty well by now.
I took an image (I put a red dot on the SCP) the other night after a visual alignment and i was a fraction out but a drift alignment i was a fair bit out.
And thats when the penny dropped! !!
My scope is not aligned with my my mount!!!
I have never checked if its spot on with the mount and taking it off all the time of course there will be variation but its the mount is what needs to be spot on and not the scope and thats why the drift alignment is always right.
After the drift (DARV) and a 3 star alignment the goto was spot on and I took a 4 minute unguided image and there was only a tiny bit of trailing.
So next thing is to adjust my scope to be square with my mount.
I cant believe I've been doing this for so long.
Cheers
Andy
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  #2  
Old 16-01-2019, 06:41 AM
kens (Ken)
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PHD2 has three polar alignment tools you could try. One of them, Static Polar Alignment or SPA addresses the problem of the misaligned scope by recording three points as you rotate the mount to determine where the mount is pointing. You can then align that point visually to your red dot.
PHD2 also has a classic drift alignment tool that you may find easier/quicker to use than DARV. The third, polar drift alignment or PDA is another drift method used near the pole. Of the three is it independent of field of view and camera orientation but not as accurate. Tutorials for SPA and PDA accessible at https://openphdguiding.org/polar-ali...deo-tutorials/
The video series includes how to use PDA and SPA to aim your scope to be pointing at the same place as your mount.
There are many tutorials on classic drift alignment with PHD2 but I have not reviewed them.
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Old 16-01-2019, 07:51 AM
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The_bluester (Paul)
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I have been using drift alignment with PHD2 though I have just bought a Polemaster from another IS user as without ready access to a good compass it is easy to find yourself degrees off the mark and then you chase yourself all over the sky drift aligning. I put in alignment notes in PHD2 to say which way to move the mount to correct which misalignment (E.G. "Raise axis to lower trend line when pointing east" and similar) but when you start a few degrees away it is easy to doubt yourself when adjustments don't seem to be working when the problem is that you are just a way off and cant readily see the trend line difference as it looks like a ski jump.

I am expecting once I am familiar with with the Polemaster to align closely to the pole fairly quickly with that and then drift align for a final check.

I would say a couple of things if you drift align, point up near the meridian and do the mount azimuth axis first and always point to the same side of the meridian as east or west will reverse the direction of drift for the same error in mount azimuth, same goes for the altitude.

It is easy to isolate the error in azimuth by pointing close to the meridian, where when you do the altitude adjustment you will probably be pointing 15 or 20 degrees above the horizon which means that any drift you see is a result of both altitude and azimuth errors. If you had to point 45 degrees off the horizon to do your altitude adjustment the drift will be equally impacted by both alt and az errors.

When you are happy with the azimuth, lock the center bolt down properly, I was fiddling on the weekend and even a tenth of a turn back from being properly tight makes a huge difference to the altitude of the RA axis, you are trying to be down in low arc second errors and a tweak of the lock bolt is more like degrees. I realised that as I was playing around and looked through the polar scope while I did the bolt up, I had Octans nicely in the reticle and when I did the bolt up it moved vertically by half the size of the asterism itself!
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Old 19-01-2019, 09:17 AM
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Icearcher (Chris)
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Another good polar alignment program that I have found easy to use is Sharpcap,

Im running 2.9 at present and on my first go using it with my star adventure, I was able to get very close to spot on in about 15 min. I recon on a more advanced mount and/or with a bit more practice, this could be reduced to around 5 mins.

I used to drift align and found that I could get very close but it would take me up to an hour sometimes to get there.
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Old 20-01-2019, 05:13 PM
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The_bluester (Paul)
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Having had a play last night with the Polemaster, if you have to set up and pack up with any regularity and want a decent polar alignment I reckon they are just about essential to a frustration free night.

My first use of it having never used the software or camera before I was well aligned in about 15 minutes. The only drawback I can see is the need to be able to see Octans, if you are really stuck on that count I suppose you can always drift align.
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  #6  
Old 20-01-2019, 05:29 PM
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Andy01 (Andy)
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+1 ^ just do it!
I too used one for the first time last night.
<10 mins & done!
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  #7  
Old 20-01-2019, 05:36 PM
Startrek (Martin)
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I have no view of SCP at both my imaging sites so I rely on the Skywatcher Synscan “polar alignment routine” which usually gives me a PA error less than 1 arc minute.I use BYEOS drift align tool to align my stars which works a treat.I bought a 3m curly cord so I can sit at my laptop table and use my Synscan handcontroller to star align and polar align. It usually takes anywhere from 15 minutes to half an hour. I have tried drift alignment but you need a fairly bright star close the meridian and celestial equator ( which may not be available all the time ) plus it can take a bit of time ( half an hour to an hour ) to get close PA
Cheers
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  #8  
Old 20-01-2019, 05:38 PM
Karlzburg (Karl)
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+1 for it.

I have to set up all the time and strip down, aligned now with pole master in 10 mins max and 5 mins later I'm imaging. Once I worked out what stars were needed when I first used it, now so much quicker. I'm glad I got one, well worth the money.
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Old 20-01-2019, 11:38 PM
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darrellx (Darrell)
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Andy

I got quite a chuckle when I saw this. I did exactly the same thing. Took an image a long time ago and marked a red dot for the SCP. I was happy when I setup to have the centre of my field of view within the circle (now shown).

I never did the spotter then guide scope. I just went straight to the main scope for alignment.

Maybe we should start a “red dot” club.

Darrell
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  #10  
Old 21-01-2019, 10:36 AM
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The_bluester (Paul)
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I did not reckon I would go down the polemaster route, I bought mine almost on impulse as it came up secondhand here at a good price. No looking back I think.

The only thing I reckon they should do, though it might take longer than drift aligning is, I reckon a different routine should make it possible to determine the mount axis like normal then park to the home position with tracking off to produce some star trails and find the pole from that, without having to be able to see Octans. It is just a problem that with the field of view it might take so long to produce good trails that is is not really useful.
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  #11  
Old 21-01-2019, 11:10 AM
gary
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Quote:
Originally Posted by mynameiscd View Post
Hi all,
And thats when the penny dropped! !!
My scope is not aligned with my my mount!!!
I have never checked if its spot on with the mount and taking it off all the time of course there will be variation but its the mount is what needs to be spot on and not the scope and thats why the drift alignment is always right.
Hi Andy,

Specifically the optical axis of the optical tube assembly (OTA) needs
to be precisely at right angles to the mount's declination axis.

Since many OTA's on German Equatorial mounts are mounted via rings
or dovetail plates, if sufficient care is not taken then the OTA will not
be "square" with respect the mount.

An optical axis to declination axes non-perpendicularity will result
in the OTA pointing away from the pole when the mount is otherwise
oriented to be pointing at it. This is termed "collimation error".

The direction of the error will also reverse when the OTA undergoes
a median flip.

It is a phenomena that surveyors have known about and corrected for
when using theodolites for the past several hundred years. Traditionally
they would take both a "face left" and "face right" observation with
the instrument by doing the equivalent of a meridian flip where the
telescope would become inverted. With the two readings in hand, they
could correct for the collimation error of the instrument.
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  #12  
Old 21-01-2019, 02:01 PM
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The_bluester (Paul)
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That is one definite advantage of the Polemaster method, the first steps are to establish the mechanical axis of the mount. I have not tried any other ways than that apart from dead reckoning (Degrees off) and drift aligning via PHD2 and a guide cam.

I still think that drift alignment has the potential to be more accurate than the Polemaster, but if it can reliably get you to a "close enough for government work" situation in ten minutes then what is the point?

While it would also render imaging more or less impossible, the wind I suffered under on Saturday night when testing would make drift alignment with the guider pretty hit and miss. You would need to drift for a long time on each iteration to be sure of the trend line reflecting the PA error, not wind buffeting the gear around.
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  #13  
Old 21-01-2019, 02:10 PM
raymo
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The DARV drift method can be as accurate as you want it; the longer you make the star trails, the more precise the result will be. If you get a perfect single star trail across the full width of your LCD screen you are as near to perfection as you can get. The mount's limitations will become your imaging
limitation, not the accuracy of your polar alignment.
raymo
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Old 21-01-2019, 02:27 PM
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The_bluester (Paul)
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I should have a go at that one night so I could use it in case of not being able to see Octans.
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  #15  
Old 21-01-2019, 04:27 PM
gary
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Quote:
Originally Posted by raymo View Post
The DARV drift method can be as accurate as you want it; the longer you make the star trails, the more precise the result will be. If you get a perfect single star trail across the full width of your LCD screen you are as near to perfection as you can get.
raymo
Hi Raymo,

Not quite.

It is important for practitioners to appreciate that there is no such
thing as a 'perfect' polar alignment.

It does not exist and cannot be achieved.

At best, one can find an optimal compromise depending on where you
are imaging in the sky.

One problem is that for any given elevation in the sky, the amount of
“lifting” to a star caused by refraction is different when compared to a
star at some different elevation.

What’s more, as the star advances across the sky in elevation, the
amount of “lift” is continually varying.

Plus within the FOV, points in the sky that are at lower elevations are
“lifted” more than those at higher elevations. The wider the FOV,
the more the “compression” within the image.

That also means there will still be some field rotation within
the FOV on an equatorial telescope.

So for any given point in the sky you wish to image, the optimal polar
axis will be slightly different and unfortunately continually changes with time.

So the fact that an equatorial mount moves around its polar axis in a
perfect circle can, in some ways, be considered a mechanical
compromise, as the stars don’t circle the sky in perfect circles.

What's more, since the amount of “lifting” to a star caused by refraction
is different when compared to a star at some different elevation,
that also means the tracking rate will continually vary.

So in the case of a long drift test, one is simply averaging the misalignment
error in the part of the sky the test was performed.

The result will differ at some different part of the sky.

Best Regards

Gary Kopff
Mt Kuring-Gai NSW
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Old 21-01-2019, 05:33 PM
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Wussell (Russell)
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Being a newbie I have been using Sharpcap. I have found it very easy and quick (most of the time) to use. Having a street light in the side road next to me makes it difficult to see Octans through the polar scope.

I have now opened another can of worms and started to use PHD2, I have got it guiding just need to tune it up a bit, but at the end of the day its another box ticked for me
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Old 21-01-2019, 05:52 PM
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Stonius (Markus)
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Gary - isn't the refractive index in the first instance mitigated by using the king rate of tracking?

Doesn't help with the second 'atmosepheric refraction in-frame issue', but has anyone calculated how much that is an issue in exposures of up to, say, 10 minutes?

Best

Markus
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Old 21-01-2019, 06:11 PM
raymo
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All of that is fine Gary,I didn't say that you can get perfect PA, I said as near perfection as you can get; but I stand by what I said about with the best possible DARV alignment the mount's periodic error will limit the roundness of your stars, not PA misalignment.[This does not include very expensive top end mounts, which often have very small periodic errors indeed.]
raymo
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Old 21-01-2019, 07:49 PM
gary
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Quote:
Originally Posted by Stonius View Post
Gary - isn't the refractive index in the first instance mitigated by using the king rate of tracking?
Hi Marcus,

Only partially in the time domain and by a fortuitous set of circumstances
also partially in the space domain if the elevation axis of the mount
is elevated to the refracted pole if your observatory is at mid to high
latitudes away from the Earth's equator.

Refraction distorts things in two ways.

1) It lifts objects in elevation as a function of elevation.

2) The first differential of that results in the tracking rate constantly
varying as the objects rises and falls in elevation.

One of the first astronomers to raise concern over the refraction problem
was Arthur Alcock Rambaut in the 1890's.

Rambaut was the Royal Astronomer of Ireland when astrophotography
was in its infancy.

Back in those days the clocks which produced the sidereal rates for
telescopes weren't the most accurate and that in itself caused trails
in images.

Then the refraction problem exacerbated the issue further and in
1896 Rambaut wrote a paper for the Royal Society Monthly Notices
which attempted to derive optimal tracking rates for various parts of the
sky. The idea was to introduce variations into the driving rate of the clock.

Arthur Hinks, who was at Cambridge Observatory, was one of the first
to tackle the problem of trying to determine elevation axis offsets for
various parts of the sky to try and compensate for refraction in the
spatial domain.

Quote:
Originally Posted by Stonius
Doesn't help with the second 'atmosepheric refraction in-frame issue', but has anyone calculated how much that is an issue in exposures of up to, say, 10 minutes?
Hinks addressed this very problem in a paper to the Royal Society in
1898.

The answer is complex and depends on what the observer's latitude is
where in the sky you are imaging.

Edward S. King was an astronomer at Harvard between 1893 to 1931.
When his eyesight began to fail he was put in charge of photographic work.

In 1902 he wrote a paper which was published in the Annals of Harvard
College Observatory entitled "Forms of Images in Stellar Photography".

It is in this paper that King comments that if the observer is at a mid or
high latitude away from the equator, then it is advisable to align the
elevation axis of the mount with the refracted pole and to adopt the
'King tracking rate'.

In Australia, observers in the far north of the country are given
a different prescription where they should be adjusting the elevation
to the true pole rather the refracted pole.

With computer control and auto-guiding, the "King tracking rate" is
essentially a thing of the past.

But most of us can readily remember when it was offered as a selection
on amateur mounts from company's such as Celestron even up to the
mid-90's.

Now and then I have given a brief presentation entitled "The Myth of the Perfect Polar Alignment"
at some star parties. It discusses the refraction problem and touches on some historical background.

It is fascinating how astronomers first attempted to tackle the refraction problem back as far as the 1890's
to image photographic plates.

Despite that, even to this day, there are amateurs out there who will "chase their tail" in pursuit of finding
some magic alignment of their mount they hope will be perfect for all areas of the sky.

Alas, even changing atmospheric pressure changes the refractive index.

Ideally the elevation axis of the mount would be motor driven to continually adjust it as well.
The UK Schmidt had such a mechanism which was required to avoid field rotation in its exposures which
had a very wide FOV.

Last edited by gary; 21-01-2019 at 08:03 PM.
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Old 21-01-2019, 08:24 PM
gary
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Originally Posted by raymo View Post
All of that is fine Gary,I didn't say that you can get perfect PA, I said as near perfection as you can get; but I stand by what I said about with the best possible DARV alignment the mount's periodic error will limit the roundness of your stars, not PA misalignment.[This does not include very expensive top end mounts, which often have very small periodic errors indeed.]
raymo
Hi Raymo,

Thanks for the clarification.

The number of stunning images in the IceInSpace forums is also testimony
to the results people achieve in the face of the physical reality.

As Andy touched upon in the very first post, where, as i understand it,
he tweaked the OTA to mount alignment (collimation error), it touches
on what a multivariable problem alignment and tracking is.

You mention periodic error as another example.

Then there are flexures in the OTA and mount, mirror shifts, RA
to Dec non-perpendicularity and a range of other geometric and
gravitational flexure errors that are in play as well.

A more detailed analysis, beyond what can be achieved by a drift test,
is required to ascertain the magnitudes of these types of errors if it
transpires they are playing a significant role.
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