Cone Error and Guiding

[Jammy]

Happy new year everyone!

I've recently started drift aligning, and also tried using the PHD2 drift alignment tool.  I'm still new to this but I'm slowly getting better at it with practice.

After doing this I've noticed just how badly I have my polar scope aligned within the mount.  I decided to correct this at the weekend against Polaris by adjusting the screws, and this got me wondering about cone error.  Unfortunately the clouds rolled in before I could check how bad (or good) it is.

Does cone error affect how good your guiding will be?  I'm using PHD2 and an OAG for giuding through my C9.25 SCT.

I've read a few things now about cone error, and know my mount corrects this with a 3 star alignment (NEQ6 Pro).  However does this mean the DEC is constantly changing to compensate?  Would guiding be smoother if I managed to remove (or reduce) the cone error?

I also understand about SCT mirror flop and that this can give false cone error readings.  I think I'm quite lucky though as my SCT hardly has any mirror flop at all.

When I do get chance to go outside again and check, if I find any cone error, should I bother correcting this, or am I wasting my time?

I'm not sure how to shim the standard Celestron vixen style dovetail bar anyway if I need to.  Two screws one end, and one screw the other.

I'm going to check it anyway, just out of curiosity.  I've only had the scope 5 years and never throught to check it before

[kens]

Cone error does not affect guiding at all. It only has an impact on goto's (at least at the levels of cone error you can normally expect to see).

Your three star alignment takes cone error into account on its gotos. The mount will not take the three star alignment into account when guiding - only when slewing.

[MarsG76]

No, dont worry about cone error... guiding/tracking will not be effected by cone error at all.

[newbie alert]

As I understand it cone error is to do with star alignment..polar alignment is totally different..separate subject..

With regards to the sct and mirror flop,the oag is in the same image train so if the mirror moves so will the guide star with it..unlike if you're using a guidescope the mirror moves and the guide star doesn't.. 

[ollypenrice]

The way to adjust your polar scope is to set the mount up in the daytime such that you can look through the polar scope at a distant reference point (over a mile away). Centre the point in the crosshair by moving the mount. Use a steeple, the apex of a roof, etc. Then rotate the mount in RA. If the polar scope is on-axis its central cross will not move relative to the reference point. If it is off axis the central cross will describe a circle as RA rotates. Use the three adjusters to rectify it.

Regarding cone error, here's a thought experiment. I just dreamed it up so I'd welcome the opinion of others regarding its validity!  Imagine a perfectly polar aligned mount with zero cone error. It's carrying a very widefield set of optics. A beam coming from a star in the corner of the picture is very far from parallel with the axes of RA and Dec and yet it can be perfectly round in a long exposure image. It seems to me that this beam has 'cone error' since it is far from parallel with the perfectly aligned axes of the mount.

Olly

[ian_bird]

17 hours ago, Jammy said:

I'm not sure how to shim the standard Celestron vixen style dovetail bar anyway if I need to.

Replacing the Vixen dovetails with Losmandy ones would be a really good investment.

Cheers

Ian

[Jammy]

Thank you for the replies.

I wasn't sure how cone error affected guiding, but you've all put my mind at ease.

I'll keep an eye out and see if I can get a Losmandy upgrade for my scope - I'll try and find a used one.

Out of curiosity, I'm going to check if I have any cone error anyway!

Thanks again

[RayD]

22 hours ago, ollypenrice said:

Regarding cone error, here's a thought experiment. I just dreamed it up so I'd welcome the opinion of others regarding its validity!  Imagine a perfectly polar aligned mount with zero cone error. It's carrying a very widefield set of optics. A beam coming from a star in the corner of the picture is very far from parallel with the axes of RA and Dec and yet it can be perfectly round in a long exposure image. It seems to me that this beam has 'cone error' since it is far from parallel with the perfectly aligned axes of the mount.

Would this error not only be relative to a single flat point on the mount and not the optics though as the lens shape at the corners would be relative to the light path on the optics?

[ollypenrice]

1 hour ago, RayD said:

Would this error not only be relative to a single flat point on the mount and not the optics though as the lens shape at the corners would be relative to the light path on the optics?

Hmmm... what I was thinking was this: we know a camera lens widefield will work provided the optics are good enough. So now we replace the lens with 4 small scopes/cameras of longer focal length and mount them in a 4 scope array each diverging slightly so as to cover the same bit of sky in a 4-panel. I cannot see that, from a guiding point of view, there could be any difference (ignoring any change in resolution, but that doesn't concern us here.) The same beams are arriving from the same stars onto chips being carried by the same mount. Why would it not work? Now given that all 4 scopes, by definition, have cone error this says to me that cone error doesn't prevent good guiding. (As others have said already.)

Olly

[dph1nm]

If cone error was a problem for tracking then those camera sky-trackers wouldn't work! When I stick my DSLR on my Ioptron Skytracker I can point it (with the aid of a ball-head) in any direction I like  and it works just fine - it does not have to be parallel to the rotation axis.

NigelM

[RayD]

3 minutes ago, ollypenrice said:

Hmmm... what I was thinking was this: we know a camera lens widefield will work provided the optics are good enough. So now we replace the lens with 4 small scopes/cameras of longer focal length and mount them in a 4 scope array each diverging slightly so as to cover the same bit of sky in a 4-panel. I cannot see that, from a guiding point of view, there could be any difference (ignoring any change in resolution, but that doesn't concern us here.) The same beams are arriving from the same stars onto chips being carried by the same mount. Why would it not work? Now given that all 4 scopes, by definition, have cone error this says to me that cone error doesn't prevent good guiding. (As others have said already.)

Olly

I totally agree Olly.

There seem to be 2 threads running currently, both actually saying different things.  Personally I believe that if your PA is right, then cone error makes no difference to guiding when imaging as once you've aligned and synched, that cone error is then accounted for.  However, one caveat here is that I am dim, so could be talking absolutely nonsense 

[newbie alert]

2 hours ago, Jammy said:

'll keep an eye out and see if I can get a Losmandy upgrade for my scope - I'll try and find a used one.

If you're using a 9.25 sct then yes definately get a losmandy plate..made a difference to my 8inch.

[ollypenrice]

3 hours ago, RayD said:

I totally agree Olly.

There seem to be 2 threads running currently, both actually saying different things.  Personally I believe that if your PA is right, then cone error makes no difference to guiding when imaging as once you've aligned and synched, that cone error is then accounted for.  However, one caveat here is that I am dim, so could be talking absolutely nonsense 

Me too, which is why I asked for a second opinion!!! I have no head for 3D spatial awareness. I have a friend who is a seventh dan Aikido master and Cambridge science graduate. On holiday, once, he blacked his eye by walking into a wardrobe. There are more mistakes than right answers out there!

Olly

[Davey-T]

Here's a second opinion.

Dave

[ollypenrice]

10 minutes ago, Davey-T said:

Here's a second opinion.

Dave

Yes, this is in regard to GoTo. I don't doubt for a moment that GoTo needs to have cone error accounted for. But for accurate tracking I'm struggling to see why it matters. Are we talking about two different things?

Olly

[steppenwolf]

Here's a third opinion for what it's worth but as I have a virus-induced headache right now, I too could be talking absolute nonsense! To answer the OP's question, I don't believe that cone error has any detrimental effect on guiding.

[tomato]

Here’s my take on it:

There is a whole bunch of stuff that will mess up your guiding before cone error even gets a mention.

[MattJenko]

If the light beams enter the scopes perfectly parallel to the pointing of the scopes, then I would have thought that it makes no difference if that system is one single scope, or multiple smaller scopes all perfectly parallel with one another. Light from stars arrives in parallel beams, so unless we are talking about extreme widefield, where projection of the curvature of the celestial sphere comes into play, I would have thought they are completely interchangeable.

[coatesg]

Cone error won't make any difference to guiding - cone error ("collimation" error as opposed to RA/Dec orthogonality error) is simply where the telescope's optical axis is not exactly orthogonal to the Dec axis. The only implication here is that the pointing on a goto will be off (and you might have a "hole" around the NCP/SCP that you can't point to). You don't need to guide in Dec to correct for cone error.

I suppose it *could* have an impact on the RA guide rates (in terms of apparent px/sec guide rates) as the autoguider usually compensates for the Dec of the object. I doubt any sensible mounting arrangement would show a large enough error to be noticeable: the amount of RA correction in px/sec goes as cos(dec) - if the dec varies enough to make the cosines of the actual and mount declination wildly different from each other, then you'd see an issue. I can't see this happening in real life....

[IanL]

Think of it like this:

- Assuming the RA axis is polar aligned...

- Mount a green laser on a dovetail bar using some guide rings and attach it to the mount in the usual way.

- Use the handset buttons to slew the mount in RA. The laser will describe a circle along one of the lines of declination depending on where the mount is pointing. So if you slew the mount to a declination near the pole it will describe a small circle, near the celestial equator a large circle, but always along a line of declination. That's basically the point of an equatorial mount, so no controversy there I hope.

- Now detach the laser and fix it to a 90 degree camera bracket attached to the dovetail, so that the laser points straight out along the declination axis, i.e. perpendicular to the RA axis.

- Now use the handset to slew in RA again. The laser will still track along a circle of declination, in this case the celestial equator.

- You can of course adjust the angle of the laser relative to the RA axis track along any other circle of declination you choose.

So you have proven that tracking (and by extension guiding using an OAG) do not depend on anything other than the polar alignment of the RA axis. It is a purely mechanical issue relating to the mount and polar alignment, not an optical one.

Of course extreme cone error will lead to a problem: If we mount the scope and camera perpendicular to the RA axis as we did with the laser, then we could only ever point at targets along the celestial equator. Rotating the Dec axis rotates the field of view around its centre, but that centre must always lie on the celestial equator.

By extension, if we mounted the scope at (say) 15 degrees off the Dec Axis (i.e. at an angle of 85 degrees with respect to the RA axis), we could never point at anything that was more than 15 degrees above or below the celestial equator. Going back to the laser example, rotating the Dec axis through 360 degrees would draw a cone with its tip at the laser and a base on the sky that is 30 degrees across, with the centre of the base on the celestial equator.

You can thus deduce that if you have cone error with an angle (with respect to the RA/Polar axis) that is more than half the camera's angular field of view, there will be a part of the sky adjacent to the celestial pole that you can never image. You are mechanically prevented from pointing the centre of the field of view sufficiently close to the pole to do so. (In theory there is also an area adjacent to the other celestial pole with the same problem, but in practice that would only matter if you lived on the terrestrial equator.)