ConeSharp is a small program designed to help you correct cone error on a telescope mounted on a German Equatorial mount.

What is Cone Error?

Cone error is a misalignment of the telescope axis away from or towards the plane of the mount puck. This means that the front of the telescope is either pointing slightly inwards (towards the puck) or outwards (away from the puck). For a more complete description of cone error (and a manual correction procedure), check out this tutorial.

Why is it a problem?

If you have cone error your telescope isn’t pointing in the direction that the mount is aiming for. What’s worse, the error changes as you point in different directions (ConeSharp takes advantage of this to calculate the cone error). In theory, a 3 star align on a SynScan handset will correct for cone error, but my experience is that it doesn’t always do a great job. Much better not to have the error in the first place.

Once you have corrected the cone error you will most likely find that GOTOs become more accurate, also Polar alignment routines (Alignmaster or the handset ones) should work better.

How do you correct Cone Error?

Your dovetail will most likely have 3 bolts at each end. One is generally a locking bolt holding it to the telescope or tube rings, the other two are adjusting bolts allowing you to move that end of the telescope away from or towards the dovetail. Just loosen the locking bolt a bit and tweak the adjusting bolts then re-tighten the locking bolt. Do remember that the locking bolt is what is holding your telescope on the mount! Don’t loosen it too much or you may have an expensive accident.

How do I know which way and how much to adjust?

Run ConeSharp of course, and follow the step-by-step instructions :).

New version – download here.

You can download ConeSharp here.(New Version – should work with more ASCOM mount drivers)

Source code for ConeSharp can be found on GitHub.

Requirements for Running ConeSharp

  • A Windows PC with ASCOM 6 installed
  • .NET framework 4 Installed
  • A German Equatorial mount that performs a meridian flip (ie NEQ6, HEQ5, etc)
  • Control of the mount using an ASCOM mount driver (ie EQMOD)
  • Precise alignment (a reticule eyepiece or camera)
  • Rough polar alignment (within a degree or so)
  • Planetarium software that can drive the mount (ie Carte Du Ciel)
  • A decent view of the southern sky (due south), able to see stars with a declination of ~0

Using ConeSharp

Download and run the executable. There is currently no installer (or any need for one).ConeSharp will guide you through 7 steps to correct cone error – only press Next once you have completed the task for each step.  

1.   Select ASCOM Telescope/Mount driver. Press the ‘Select’ button to choose the device (eg EQMOD ASCOM HEQ5/6).  

2.   Set Mount Properties. Adjust any settings needed to properly control your mount via the ASCOM driver.

It is VITALLY important to turn off any pointing enhancements in the ASCOM driver (for instance multi-point alignment/sync) before proceeding!

If these features are left enabled they may be correcting or partially correcting for cone error already, meaning that ConeSharp will be unable to measure the error correctly.

3.   Goto a star to the west of the meridian. In your planetarium program find a star just west of due south, with a declination near zero. Pick a bright star in this area and goto the star. Let the GOTO complete and then press Next. Note that the star will almost certainly not appear centered in the field of view after the GOTO is complete. Do not try to center it yet – that has to wait until Step 4.

4.   Align the star centrally in the reticule eyepiece or camera field of view. Just get the star perfectly central, then press Next.

5.   Goto a star to the east of the meridian. Back to the planetarium again and find a nice bright star just east of the meridian with a declination near zero. Don’t pick one too close to the meridian as you need enough time to complete the procedure before it crosses the meridian. 5-10 degrees east of the meridian (Azimuth between 170 and 175) should be fine. Your telescope should GOTO this star the long way round and when it arrives it should be on the opposite side of the pier to where it was in step 3-4. Once the goto has completed (do not align yet!) press Next.

6.   Align the star centrally. You’ve got the idea of what is expected here – press Next when it’s bang in the middle.

7.   View the results. This step will show you the amount of cone error that you have (along with the raw measurements which have been filling in at the bottom of the screen). Anything above a minute or two of arc is definitely worth correcting.

8.    Correct the Cone Error. On arriving at step 8, ConeSharp will move the telescope slightly so that the last star is no longer central. ConeSharp calculates the new position based on the measured cone error. All you have to do to correct the cone error is get that star back in the middle (or as close as possible).

Remember that you are only allowed to make corrections using the alignment adjusting bolts at the front/back of the dovetail.


That’s it – all done! If you want you can run the correction procedure again to make sure that the cone error has been fixed and/or make even finer corrections to get alignment spot on.


How does ConeSharp measure the cone error?

Becaus the mount does a meridian flip when going from just west of south to just east of south, the cone error will be in opposite directions for the two stars visited. Other errors (such as errors in polar alignment) will be in the same direction for both stars. This allows ConeSharp to separate out the cone error component from other errors. Basically the cone error is the average of the RA error on the west side and minus the RA error on the east side of the meridian.

Will ConeSharp work in the southern hemisphere?

Yes, it should – I have tested this in the EQMOD simulator. It may not work in the tropics, if the celestial equator is polewards of you.

ConeSharp will give the best results when the stars you goto/align are below about 45 degrees of elevation.

Should I polar align before correcting cone error?

You need a rough polar align for ConeSharp to work, but you will probably find that you get better results from the polar align routines in the handset or programs like AlignMaster if you run them after correcting any cone error.

ConeSharp keeps complaining about my mount being on the wrong side of the pier – what should I do?

It turns out that many ASCOM mount drivers aren’t very good at reporting the side of pier correctly. I based the original version on what EQMOD (1.27L) reported, which now appears to have been wrong (EQMOD 1.27q changes the reported side-of-pier to be the opposite of older versions). To take account of this I have made a new version of ConeSharp that is much less fussy about which side of pier you are on. Re-download and use the new version ( and you should be fine. The new version will still report an error if the mount changes side while aligning on either star and will warn you if the mount *hasn’t* changed sides between the first and second stars.