Cone Error in the soutthern Hemisphere

[rider]

Greetings and salutations,
This is probably not really a beginner question, but

Is there a way of adjusting for cone error in the southern hemisphere?

So far, every article Ive found needs Polaris as part of the process...

Rider

[gary]

Originally Posted by rider

Greetings and salutations,
This is probably not really a beginner question, but

Is there a way of adjusting for cone error in the southern hemisphere?

So far, every article Ive found needs Polaris as part of the process...

Rider

Hi Rider,

"Cone error" is what Chinese instruction manuals refer to when discussing
the area around the pole of the mount that the OTA is unable to point to
because of geometric errors within the mount/OTA.

Professionally, we prefer to talk about two separate and distinct geometric errors
that can give rise to the area of denial around the pole, specifically
"Collimation Error in Hour Angle" or CH for short, and "Non Perpendicular
Axis Error" or NP for short.

Whereas CH arises from a non-perpendicularity between the optical axis
and Dec axis of the mount and is a constant value for all values of Dec,
NP arises because of a non-perpendicularity between the RA and Dec axis
and reaches a maximum value at the pole of the mount.

In practical terms, many German Equatorials have measurable CH
error simply because the operation of mounting the OTA squarely on the head
of the mount is partly placed in the hands of the end user, whereas NP tends
to be "built-in" to the mount when it was fabricated at the factory.

Beyond these two geometric errors there are additional effects that can
be worth looking for depending upon your pointing demands. For example,
eccentricities in bearings and gravitational flexure effects are not uncommon
in all amateur mounts to some extent or another.

All these sources of errors are entangled in a complex way and are best
unraveled by performing a star pointing test in combination with software
analysis.

For example, we manufacture a telescope computer where the pointing
error analysis software is in-built.

For stand alone applications, the commercial TPOINT package, one port of
which is marketed by Software Bisque, is an excellent diagnostic tool and will tell
you more about your mount than could a simple pointing test using Polaris.
However, depending upon what your ultimate pointing goal is, TPOINT may tell you
even more than what you might need to know.

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

[rider]

Thanks Gary,

If my suspected error was on our other GEM mount, it would be a simple matter to check, cos its got Argo Navis!

I think that you are saying that there is no manually based way of finding and adjusting or "cone errors" in the southern hemisphere.

My problem is that our SW HEQ5 (with GPS) does almost perfect 2 star alignments, with stars in the correct position all over the sky, but it wont do a successful 3 star calibration, - the third alignment star is always a long way away from its predicted position, and the alignment fails.

The incredibly minimalist Skywatcher manual (especially when compared to the "Bible according to Saint Argo") says that the difference is that the 3 star alignment takes into consideration "cone error"

Now the obvious answer is "hey - just do 2 star alignments!" but I am interested in tracking down the reason for the failed 3 star alignment process.

Regards Rider

[Wavytone]

Several possible sources of errors - not just "cone error":

a) RA & dec axes are not perpendicular,
b) dec and the optical axis of the scope are not perpendicular,
c) the motors are stalling or the pulses are being miscounted (missing pulses) on either or both axes - these mounts use pulse counting of the drives to measure position, and they do miss,
d) backlas, especially on the dec axis,
e) flexure of the mount and tube assembly, in several places;
f) atmospheric refraction (I often wonder whether these mounts compensate for it) which can be half a degree near the horizon.

All mounts suffer from all of the above to some degree. If you have a scope larger than a 4" on a Vixen dovetail the first suspect I would guess is flexure right there.

[Robh (Rob)]

Originally Posted by Wavytone

f) atmospheric refraction (I often wonder whether these mounts compensate for it) which can be half a degree near the horizon.

This is something we quite often don't think of. Refraction at the horizon is about 34' of arc. The apparent diameter of the sun is about 32' of arc. As the sun begins to set and its disk appears to be sitting on top of the horizon, it is actually below the horizon. Refraction also accounts for the flattening of the sun's disk at the horizon.
Atmospheric refraction is about 1' at 45 degrees and 5' at 10 degrees above horizon. At the height of the South Celestial Pole it is about 2'.

Rob

[gary]

Originally Posted by Robh

This is something we quite often don't think of. Refraction at the horizon is about 34' of arc. The apparent diameter of the sun is about 32' of arc. As the sun begins to set and its disk appears to be sitting on top of the horizon, it is actually below the horizon. Refraction also accounts for the flattening of the sun's disk at the horizon.
Atmospheric refraction is about 1' at 45 degrees and 5' at 10 degrees above horizon. At the height of the South Celestial Pole it is about 2'.

Rob

Rob,

Not only that, refraction is why there is no such thing as a "perfect polar alignment",
a commonly misunderstood concept by many enthusiasts. Instead, for any given
point in the sky you want to image, the optimal point at which to align the polar
axis will differ. As a compromise, it is best to align with the refracted pole.
In reality, most imaging times are kept sufficiently short so that the effects of
not altering the scope's elevation dynamically doesn't become appreciable in the image.

As far as its impact on pointing analysis, refraction needs to be accounted for
and again it is this complex entanglement of various phenomena that makes
whole-sky star pointing tests combined with software analysis the best
prescription for improving a scope's pointing performance.

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