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Old 14-10-2019, 08:12 PM
poider (Peter)
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Celestial Pole

Just curious, If I was to get my Tracking mount perfectly lined up with the pole, and then bolted it to a wall or large post would I have to change it again, does the pole move around night to night or year to year or decade or is it stable?
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Old 14-10-2019, 08:28 PM
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Lognic04 (Logan)
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It should be fine to go, there isnt much variation, however having NO adjustment is probably not a good idea as things can shift mechanically.
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Old 14-10-2019, 08:30 PM
gary
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Hi Peter,

It will precess and nutate in about the order of 50 arc seconds a year.

See https://en.wikipedia.org/wiki/Precession
https://en.wikipedia.org/wiki/Astronomical_nutation

Also keep in mind that here is no such thing as a perfect polar alignment
and equatorial mounts are really just engineering compromises.

Even if one were to align an equatorial mount's polar axis perfectly
with the celestial pole, field rotation will still occur.

How much and to what extent is trigonometrically relatively complex
and is a function of what area of the sky you are imaging and the
time you are imaging for.

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.

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.

What comes as a surprise to some amateurs is that the problem
of field rotation due to refraction for long exposure times was first
extensively studied by professional astronomers, such as Arthur A.
Rambaut, as far back as 1893.

The photographic plates in those days were not very sensitive and long
exposure times were the norm.

Astronomers realized that not only field rotation was upsetting their
long exposures on equatorial mounts but that the first differential, that
is the tracking rates, would have to be dynamic as well.

In a theoretical sense, guiding cannot compensate for the field
rotation due to refraction either. One would still need to either
have the camera revolving on a third axis -a derotator - or would one
would have to dynamically change the mount's alt axis as well.

Conveniently the field rotation caused by refraction is in the
opposite sense to that caused by elevation.

A good compromise is to attempt to align the mount's polar
axis not with the true pole but the refracted pole.

The effect of refraction is to "lift" the apparent position of an object
so the refracted pole is always above the true pole.

For your latitude in Adelaide, an appropriate amount to raise the mount's
polar axis would be about 85 arc seconds above the true pole.

Last edited by gary; 14-10-2019 at 08:55 PM.
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Old 14-10-2019, 08:32 PM
Sunfish (Ray)
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No. If you have good alignment with the South Celestial pole that should be good enough. I have mine bolted to a steel and aluminium pier on concrete with scribed markings and it seems good enough to bolt down the mount each night.

The apparent pole does wander subtly due to the seeing and atmospheric refraction but it will not affect most amateur users.

If you really want to read about that and how to check with drift alignment see here:

http://canburytech.net/DriftAlign/DriftAlign_1.html
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Old 14-10-2019, 08:43 PM
Sunfish (Ray)
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What Gary said. I wish I was precise enough that 9.2 sec made a difference . I assume that software for digital setting circles takes this into account.
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Old 14-10-2019, 09:40 PM
Sunfish (Ray)
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Concise, informed and a practical outcome. What more could you ask of an explanation than Gary has given. 85 sec is adjustment well within our grasp in an illuminated reticle 12.5 mm eyepiece I think. Roughly 1/40 my field of view 102mm f8 unless my calculation is off.
https://astronomy.tools/calculators/field_of_view/

Last edited by Sunfish; 14-10-2019 at 09:47 PM. Reason: Error
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Old 15-10-2019, 05:45 AM
poider (Peter)
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Thank you both
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Old 15-10-2019, 09:32 AM
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OICURMT
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Quote:
Originally Posted by gary View Post
Hi Peter,
Even if one were to align an equatorial mount's polar axis perfectly
with the celestial pole, field rotation will still occur.

I suspect you meant cone error?
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Old 15-10-2019, 10:50 AM
gary
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Quote:
Originally Posted by OICURMT View Post
I suspect you meant cone error?
Hi.

No, that is a totally different.

"Cone error" is the name used by Chinese mount manufacturers in their
instruction manuals for optical axis to declination axis non-orthogonality.

Historically that particular phenomena is referred to professionally
as "collimation error".

The use of the term goes back at least to the early days of surveying
with optical theodolites.

When the optical axis of the theodolite's telescope is not at right angles
to the theodoite's horizontal axis it is referred to as "collimation error".

With transit theodolites surveyors would take two readings, one with the
telescope on the left side of the vertical circle and the other with the
telescope flipped to the right side of the vertical circle.

They would then average these "face left" and "face right" readings
to eliminate any "collimation error".

Collimation error is often a significant source of pointing error particularly
in German Equatorial Mounts (GEMs).

Since the end-user is usually the one responsible for ensuring the OTA
is "square" in the rings or with respect the dovetail plate and they are
in turn "square" to the mount, the simple act of mating the OTA with the
mount often brings it about.

To get back to what I am referring to is not collimation error but
field rotation due to the "stretching" brought about within the FOV
from atmospheric refraction.

Unless you are standing precisely at the north or south poles on Earth,
if you take a time-lapse photograph of the stars apparent rotation around
one of the celestial poles, the trails don't form a perfect circle.

That is because refraction "lifts" the position of the star in elevation
as a function of the stars non-refracted elevation.

Stars on the horizon appear about 1/2 degree higher than they actually
are if Earth had no atmosphere.

At 45 degrees elevation, the amount of lift is only about 2 arc minutes.

At the zenith, the lift caused by refraction is zero.

The ‘true’ polar axis is the one around which the Earth actually rotates.

To appreciate this, a question to ask oneself is if there were a star with
a catalog declination of -90 degrees, i.e. right on the ‘true’ south celestial
pole, where would it appear to an observer, say in Adelaide?

Because refraction causes the apparent place of an object to be ‘lifted’
above its ‘true’ position, a star exactly on the ‘true’ pole will, to an observer
at Adelaide, appear lifted to a position known as the ‘refracted pole’.

So then one might ask, how much higher is the apparent refracted pole
compared to the true pole?

It depends on many factors.

Latitude is one of the most important.

At Singapore, which is at +1° north latitude, the true north celestial pole
will be nearly on the northern horizon, so the refracted pole will be lifted
by refraction by about ½ a degree. That’s 1800 arc seconds!

At Adelaide, the latitude is -35° south and so the true SCP is at 35° above
the southern horizon and the apparent refracted pole is about 85 arc seconds above that.

At Scott Base at the South Pole, the true pole is directly overhead and the
refraction is zero at the zenith. So the refracted pole and the true pole
appear at the same point.

There are other factors that affect refraction and therefore how much
higher the refracted pole is compared to the true pole.

Amongst these are :-
* Ambient temperature
* Atmospheric pressure at the observing site
* relative humidity
* Temperature lapse rate through the troposphere
* The wavelength you are observing at (red, yellow, radio, etc.)


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.

The bottom line is that 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 your 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.

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.

One of the first to consider this problem was Arthur Alcock Rambaut who
was a Trinity College Gold medallist mathematician and Royal Astronomer
of Ireland in 1892.

Andrew Robert Hinks who was at Cambridge Observatory elucidated the
problem further in 1898.

These guys were amongst the pioneers of long exposure astrophotography.
The slow emulsions on their plates meant they would require long
exposure times and the problem of field rotation due to refraction became
a real practical problem for them which they endeavoured to analyze and
solve.

These days amateur astrophotographers enjoy the benefit of higher
virtual ISO numbers and can keep exposure times relatively modest
and the FOV small to avoid it becoming an artefact.

Many, I suspect, are not even aware of it despite it being well appreciated
by astronomers over 127 years ago.

Image, Rambaut, 1893 paper :-
Attached Thumbnails
Click for full-size image (rambaut_1893_2.jpg)
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