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Starman73
17-01-2008, 10:19 AM
Hello all,

I have recently bought an Skywatcher EQ6 Pro mount to put with my Meade LX200 10" OTA. I have a couple of questions regarding Cone Error.

1. How much of a problem is cone error. Is it something that I really need to worry about?

2. With cone error correction that the EQ6 Pro mount factors into its startup when doing a 3 star alignment, is this enough to cater for long exposure astrophotography or is it only good enough for obervations?

3. When adjusting the dovetail bar to correct for the cone error, we don't have Polaris to look at, as in the method in the SynScan manual, so would it be sufficient to use a star close to the South Celestial Pole? Or alternatively could we do the adjustment during the day, focusing on a distant object similar to aligning a finderscope? Would you need to use an object at a great distance, being km's away?

3. I was told (by a telescope shop) that Cone Error can be corrected for by the way the telescope and dovetail bar is put onto the mount, can someone confirm or deny this, and if its true tell me how to do it.

Thanks in advance for your help.

Paul

sheeny
17-01-2008, 12:36 PM
G'Day Paul,

I don't know whether you need to worry about cone error. Only you can decide that unless you can provide some more info.

Doing your 3 star alignment to correct for cone error will improve the goto accuracy of your scope. It is not enough for long exposure astrophotography. Good accurate polar alignment is needed for long exposure astrophotography, and I doubt the cone error will matter much for that. (i.e. the centre of the FOV of a scope with cone error should not be any different to a point further out in the FOV of a scope with no cone error as long as polar alignment is accurate).

Cone error depends on a few things: how parallel your OTA is to your dovetail, how square your dovetail is to the dec axis and how square your dec axis is to the RA axis. There are manufacturing tolerances on the squareness of the mount axes, so they won't be perfect, but you might still be able to make some improvements if your OTA is not parallel to your dovetail. Even the accuracy that you align your dec axis on start up will affect cone error.

To me, polar alignment is probably more important than cone error. If I am imaging only I often only do a drift alignment and don't worry about the 3 star alignment to correct the cone error - unless I know I want to use the goto a lot or to be accurate enough to find my target. If I'm just observing, I usually do a rough polar alignment and then a 3 star align so the goto is pretty good when changing from object to object.

You can check and correct misalignment of your OTA and dovetail (only part of the cone error problem) during the day. Use a guiding EP and focus on a nice distinct feature as far away as possible with the RA axis of the scope pointing at it i.e. your dec axis is set to 90 degrees so the OTA is parallel to the RA axis. Rotate the scope 180 degrees on the RA axis while looking through the scope. Your target should remain aligned on the cross hairs as the scope is rotated. The biggest source of error when you do this is the accuracy of the alignment on the dec axis alignment position, so play with this first before you go trying to "fix" a problem with your dovetail. On my mount, the dec alignment mark is about 1mm off where it should be!

I hope this helps,

Al.

gary
17-01-2008, 12:42 PM
"Cone error" is Chinese telescope instruction manual speak for one class
of geometric errors with mounts/OTA's. In the world of professional observatories
this particular error is "Collimation Error in Hour Angle" or CH for short,
the naming of which can be attributed to Patrick Wallace. I will therefore
talk in terms of CH rather than "cone error".

Don't confuse Collimation Error in Hour Angle (CH) with what is popularly
known as optical collimation. Collimation is simply an engineering term
meaning putting everything in a straight line. Though optical collimation
of the scope affects CH, it is just one possible component of this source of
pointing error.

When the Dec axis of the scope is not at right angles to the "pointing axis"
of the scope, that is termed CH. This results in the telescope optically
pointing east or west of where the mount and its computer "thinks" it is
pointing. There is actually another north-south component as well but
you need not worry about that as it gets mopped up easily in the position
calculations a pointing system makes.

CH can come about due to a variety of reasons. For example, the Dec axis may
not be at right angles to the OTA. This is particularly common if the dovetail
has not been mounted "squarely" to the mount, or the OTA not mounted
"squarely" to the dovetail. Scopes that affix with tube rings can have similar
problems. Another example is if the optical axis of the scope is not parallel
to its mechanical pointing axis. In other words, one can have all optical
components perfectly optically collimated but the whole train of optical
components may not be parallel with the tube itself. A further example
is a focuser not being mounted "squarely" or the CCD camera tilting when
it is mounted in the focuser.

All these sources of mechanical error (and there are some others) conspire to
make a difference between where the telescope "looks" and where it "points".

CH results in a constant error shift for all declinations. Note when you flip the
tube across the meridian on a GEM and observe the same object from this
flipped orientation that the apparent CH direction also reverses.

Whether it is something you need to worry about depends upon we call
your "pointing goal". For example, your pointing goal might be to blind
point the scope so as to land the object onto your CCD. A visual observer
on the other hand might articulate their pointing goal as having the
object fall within the FOV of their eyepiece of choice.

Therefore if the pointing error residual is such that the system does not
achieve your pointing goal, then it is a problem. Since the origins of this
particular source of error are not restricted to the mount itself, then your
mileage will differ from others and therefore nobody else will be able
to answer your question with any authority. One of course could determine
anecdotally whether you are achieving your pointing goal by simply
attempting to locate objects. To do it more rigorously, one would perform
what is called a star pointing test and measure and tabulate the error
residuals and apply a statistical metric to them. I won't go into that here.



First of all, there are several other sources of geometric, flexure and
eccentric bearing errors within the mount/OTA that CH alone does not account for.
For example, a non-perpendicularity of the RA and Dec axes is what is known
as Non Perpendicular axes error, or NP for short.

One of the side-effects of these errors is that there is a region around the
pole of the scope to which it can now never point. So imaging at the very pole
of the scope would be impossible. These errors also affect one's ability to
accurately polar align the scope, which can also affect your photography
during long exposures. However, keep in mind that errors such as CH and NP
primarily affect the scope's pointing performance rather than its tracking
performance. The reason is is that tracking is the first differential of pointing
and in most parts of the sky the magnitude of errors such as NP only
changes slowly.

Whereas NP tends to be "built-into" the mount, CH can often be corrected
by making mechanical adjustments to the OTA and how it is mounted.

Keep in mind that the various errors within a mount/OTA are all intertwined
in a complex way and that three stars are insufficient to account for them all.
On a mount such as this, one would realistically need to start with a pointing
model of probably 50 to 100 stars to account for the major errors, including
polar-misalignment and eccentric bearing terms. One could then re-synchronize
a pointing model with as few as three or four stars.



To eliminate one source of error, one could use a distant terrestrial object.
Back in the days of optical theodolites, surveyor's would in fact take
what was called a "left-face" and "right-face" sighting to help null out
collimation error.



Absolutely true. To visualize it best, I recommend you stand next to the
scope whilst reading this. Identify the scope's Dec, axis, that is the
imaginary line which it rotates around in Declination. Now imagine
the scopes optical axis, the imaginary line that runs up the middle of
the tube from the center of the mirror and out into space. Note how these
two lines (axes) are at right angles? Now note if one rotates the dovetail
or OTA on the dovetail, one can make these two axes be no longer at right angles
to each other.

As I say to people, when you consider that many mounts and OTAs
are manufactured to a fairly modest level of precision, it is often
the squidgy piece of biological machinery armed with a spanner
that finally mounts the OTA onto the mount that is the real culprit. :)

This is something obviously you can attempt to correct yourself mechanically
by adjusting the OTA.

Don't let any of the above get in the way or delay your imaging. If after
aligning the scope you find that you can land the object reliably toward
the center of the FOV across the whole sky, including at a range of Declinations,
then your whole-sky pointing performance is good.

Keep in mind that phenomena such as atmospheric refraction also affect
your tracking rate which may not be an issue unless you are imaging
close to the horizon with long imaging times.

As a disclaimer, I spent several years of my professional life working on
telescope error pointing analysis. Last May I gave an introductory talk on
this topic at the Texas Star Party and am scheduled to give a presentation
that might cover some of this at the forthcoming South Pacific Star Party.

Anyway, hope some of the above is helpful to you.

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

Suzy_A
18-01-2008, 02:06 PM
I thought that there were four types of 'cone error':

1) you don't pack the stuff in well enough in the cone and it falls out before you light it up.

2) you don't close your eyes when you light it up and ruin your night-vision.

3) after the cone, you trip over the 'scope in the dark and lie there giggling for a few hours.

4) you get the munchies, then fall asleep and miss the -25 fireball.

DeanoNZL
18-01-2008, 02:31 PM
Brilliant Suzy!:lol:

Starman73
18-01-2008, 11:29 PM
Hello all,

Thanks to Gary and Sheeny for your very informative answers. To tell you the truth I haven't even been able to use my scope and mount once yet I have had troubles with getting tube rings to fit and then discovered I had problems with the counterweights, ie not enough. So I hope i have solved all that but now Sydney is in the midst of a mini rainny season so I have to wait for that too.

It all seems to come down to the squareness of everything. Like I said before I am using tube rings to mount my scope to the dovetail bar, then to the mount. Is there any way that I can attach the tube rings and get all of that squared away or at least look at it to see the squareness. I am asking about that because it is something that I can fix easily.

I am thinking that i might just use the scope for a bit, have a little fun, then get into the seriousness of alligning everything properly and do some astrophotography.

Thanks again to all
Paul

g__day
19-01-2008, 10:29 AM
Try downloading a 30 day trial of Maxpoint - run a 30 star model and read its diagnostics of all your points pointing, flex, cone errors etc.

Product info here http://www.cyanogen.com/products/maxpt_main.htm

Download here:

http://www.cyanogen.com/downloads/maxpt_main.htm

skogpingvin
07-02-2022, 03:51 PM
This is something like 14 years old and still the funniest thing I've read on IIS!

Peter Ward
07-02-2022, 07:56 PM
After Gary's comprehensive and erudite response it is hard to add much more of consequence
though I would echo: many mount and optical systems rarely sit with an optical axis that is precisely parallel to the polar axis (i.e. RA shaft )

An easy test is to place a wide field eyepiece and 90 degree star diagonal
into the optics, rotate the optics to as close to DEC -90 (i.e. at the pole)
then, while looking through the eyepiece, slowly rotate the mount in RA and
observe the path of the field stars.

If things are bang-on, then you should see the stars follow nice concentric circles that care centered on the middle of the eyepiece field of view.
(picture 1)

What you are more likely to see however, is the stars moving through arcs that are centered somewhere outside the field of view. (picture 2)

To remedy this, first make a fine adjustment to the Dec axis and ensure you are really at a geometric Dec -90.
A small tweak may be all that is required to get to nice concentric motion. A little trial and error may be required to get as close as possible to this.
Once you are there, then a small shim may need to be added (to either the objective or eyepiece end of the telescope)
to converge on Picture 1.

After shim adjustment, your alignment Nirvana however may be further thwarted by the RA and Dec shafts on your mount being less than orthogonal
...resulting in something like picture 1, but simply not centered in the field of view. However the good news is small errors of this nature are easily corrected by packages such as T-Point and Max Point.

The_bluester
08-02-2022, 02:37 PM
To agree and slightly expand on how important the error is. Only you can decide if it is an issue. The only time I have found the cone error to be an issue on my Orion AZEQ6 clone was doing automated meridian flips while imaging and centering using plate solves.

I don't know about for any other software, or if it is baked in to the mount electronics, but as Gary mentioned, the direction of the error will "reverse" when you do a meridian flip. in the case of my Orion, the error led when the setup was west of the pier, pointed east for the scope to be pointed further west optically than the mount was. When the mount flipped the error was great enough that reversing it's direction would leave the scope optically pointing east of the meridian, and either in the driver or the mount firmware that leads to the plate solved sync being rejected and centering would fail. The easy solution was to image further past the meridian before flipping to ensure that the first plate solve was reliable on the west side.

If I was determined enough I could probably remove the scope mounting rings and have one machined or put a small spacer under the other to flip the error, so the scope would end up pointed further west than intended on the flip.