Have I got cone error?

[OzDave]

Hi,

I've been having problems getting accurate slews with my EQ3-2 for a while now. So tonight I abandoned the SynScan controller and hooked up EQMOD and then went on an alignment-slew-fest to see how it would perform.

Before I started, I did a fairly accurate polar alignment using the polarscope. Checking this at various times during the night, I saw that Polaris was tracking anti-clockwise around the circle in the polarscope view.

As I started the alignment slews with EQMOD, I was picking the brightest stars I could see. I started with Vega, then went to Arcturis, then one from Hercules, then over to Regulus, Mars, and a couple of others nearby in Leo. As I was doing this, I found EQMOD was getting pretty accurate.

But then I decided to align with one of the stars in Ursa Major and found that the slew was well off. Like, at least two moon diameters away. So I thought that was a bit odd and corrected things and did a SYNC. Then I tried to slew to a different nearby star in UMa and again found the slew way off. Not as bad as the first star (Dubhe I think it was), but still pretty awful. Thinking something was wrong, as it really should have got closer than that, I tried to slew to Kochab, and again found it quite a way off.

Then I tried Polaris. This is when I discovered that I actually couldn't view Polaris at all through the scope. I disconnected the GOTO system so I could do it manually with the clutches and found that no matter what, there was no position of RA/Dec that would put Polaris in the field of view (25mm eyepiece of FL 1500mm, aperture 127mm).

At the time I discovered this, I checked the polarscope and found polaris at the 12 o'clock position roughly, which means it was really at 6 o'clock with respect to the NCP as you look north at the NCP. So I figured that I should be able to get Polaris centred with the RA axis rotated so the scope and weight bar are horizontal to ground and then use DEC adjustment to find Polaris. Using a red dot finder I put the scope where I should have been able to see Polaris, but couldn't.

I then found that if I flexed the scope in the mount a bit, that Polaris came into view. When I let go, Polaris slips out of view again and cannot be found no matter where I put the RA/Dec.

I think this is happening because the scope is totally out of alignment with the polar axis of the mount. And judging from the flexing I did, it probably needs one end of the dovetail bar jacked up a bit in the mount to get it to align properly.

Is this what cone error is?

Would this explain why slews to stars far away from the NCP were reasonably accurate, but slewing to anything fairly close to NCP seemed to exhibit weird inaccuracy, becoming more inaccurate the closer the target was to NCP?

Assuming I can correct the cone error by adjusting the dovetail w.r.t. the scope, should I expect slews to get a lot more accurate in general?

Thanks for any advice,

David

[kniclander]

i thought that was when your ice cream fell on the floor...

sorry - couldnt resist -i am sure someone more knowleegabe and less daft will give you a proper answer, and hey, i bumped your post:)

[Merlin66]

Cone error is where the OTA is not parallel to the RA Axis. You'll note on all the Skywatcher dovetails, where the OTA rings are attached that there's an additional two small screws....

Guess what these are for??

It must be a pretty common problem for SW to add this feature to their dovetails.

In the Skywatcher manuals they use three pages of Appendix ( Appendix A - Cone Error Calibration) to walk you through the process.

Ken

(BTW they also say using a 3 star alignment that it "corrects" for cone error.......)

[roundycat]

Is this type of error also known as orthogonality, or lack of it. If you line up on a star near the meridian and then pier flip and go to it again where does it end up? Maybe those two little screws provide the answer.

On a slightly less helpful note why worry? There is a saying in fairly common use in England, 'don't lose sight of the ball for the sake of playing the game'. If you can find your target to look at or image it does it really matter if a target over your shoulder might not end up in the 'X' ring after a goto?

Dennis

[martin_h]

I have exactly the same error - last night did a quick allignment on Vega and Arcturus, then went over to Ursa Major and was miles out, but I was going to image Bodes, so did an allignment on 3 stars in UM (1st was way out 2nd pair were on the chip,and it was good enough to get bodes on centre on the chip.

So I'v found that its best to just do a quick allign then zone in on the area you want to image/view.

[OzDave]

I think my problem would be ok normally, and correctable by the GOTO alignment, if it weren't that the cone error was so big. In my case, the error is so large that there is actually a region of the sky that my scope just can't point at. That area is a circle around the NCP with a radius equal to my cone error and it seems to include Polaris and almost includes Kochab.

So because the error is so big, the GOTO alignment kind of tries to account for it and does alright away from the NCP, but slewing to stuff like Bode's and Ursa Major, the GOTO seems to compute the ideal place to slew to, but gets it wildly wrong because the cone error is so massive.

Today I had a play with the screws on the dovetail bar and tried to get the scope's optical axis vaguely in line with the polarscope axis. It seemed to need to be raised a few millimetres, which judging by how much the FOV moved, would totally account for the problems. However, it seems to be very fiddly and difficult to get things exactly aligned and I have to keep taking the scope out of the mount to make the adjustments. But at least now the scope's FOV kind of matches the polarscope FOV and so I should finally be able to see Polaris thru the scope next time I go out.

But since my scope attaches directly to the dovetail, not with tube rings, I started thinking that it could be misaligned in 2 dimensions, not just up and down, if the dovetail or tube mounting plate were not drilled precisely in a straight line along the tube.

I'm hoping I've corrected things enough so that the GOTO alignment can now cope better. I'll have to see next time I go out, and maybe do some fine tuning of the cone adjustment screws as the Skywatcher manual suggests with Polaris. At least I'll be able to see it now to carry out the procedure.

David

[OzDave]

I should also say that I think the times that I was using SynScan and having my 3-star alignment fail, I think that is very likely because the alignment stars included one in Ursa Major. So when it would get the 3 slews done, it would have what looked like 2 small corrections and one massive one for the UMa star.

It would then throw a wobbly fit and reject the alignment data, presumably because it isn't able to correct for the errors it "sees".

So, if you're using SynScan and finding that a lot of the time the 3-star alignment fails, it might be worth checking your scope's optical axis alignment with respect to the polarscope (if you have one). Chances are the misalignment may be enough to cause SynScan alignment problems. Not sure what you do without a polarscope to check this though

David