Cone error correction

[jambouk]

A friend has a 10" newtonian and has a fair amount of cone error (Polaris out of the fov of a 22mm EP when flipping 180 degrees around the RA), and at the edge of the fov of the finder scope, not corrected by adjusting the dec.

The cone error means his goto isn't great even after a three star alignment and he's keen to reduce it.

We shimmed it yesterday day time using a distant object about half a mile away and got the cone error nearly vanished. But he tested it again last night and polaris was still way off, and put of the fov.

What distance does something need to be away on the horizon to get as close as possible to mimicking polaris? It's far easier to shim it in day light than at night.

Thanks for any replies.

James

[rwg]

Half a mile - let's say 800m.

How far is the center of the tube from the center of the mount? Maybe 30cm? So if you flip sides the center line of the tube moves 60cm.

0.3m / 800m = 0.000375 radians = 0.021 degrees = (approx) 1.3 minutes of arc.

That should be small compared to the field of view (maybe 1 degree for a 20mm eyepiece on a 250mm Newt). So half a mile really ought to be far enough.

If you can connect the mount up to EQMOD (even temporarily) you could use ConeSharp to at least measure your cone error accurately at night. http://www.sharpcap.co.uk/conesharp

cheers,

Robin

PS. After a 3 star align, your mount gives you some polar alignment error values (MAZ and MEL). Do these come out the same each time you align - or at least close. I found that these would change quite a lot between alignments, even when I was really careful. This always worried me as if the mount calculates different values for these each time it is probably calculating a different value for what it thinks your cone error is too....

[orion61]

Are you sure his Dec Setting circle is calibrated accuratly?

I had the same issue. I set the EQ head level,leveled  the dec shaft, then leveled the tube. (like an Az/Alt would be set up ).

Thats how I found my circle was off!

It was only by under one degree, but once I got it accurate my computer was dead on.

Also are you sure the EQ Pole is straight? ot if he has a bubble on the cap of the tripod, If there isn't one he will need to put one there!

Also make sure there is no slop in the encoder bracket! Those little things will drive you crazy!!!

Fix these and unless there is slop in the mount Bearings you will be all right.

Cheers

Larry