Hi Robin,
I was wondering about the accuracy of the polar alignment in Sharpcap as the readout shows the error in degrees minutes and seconds and so at the second level it is 3600th of a degree if the mount left right adjustment post was at say 115mm radius from the rotation point of the mount this would give roughly 1mm of movement per degree and if this is being moved via a m6 thumbscrew this also has a pitch of 1mm. When manually aligning if I go back and forth I can get to within a couple of seconds of alignment but find it hard to believe that I am accurately moving the thumbscrew by a few 1/3600ths of a millimetre roughly 1/3 of micron movements so is there some vagueness / tolerance in the values that Sharpcap is reporting.
I am only asking because as you know I am mechanising the left right and up down movement on the mount and am nearly finished so have been doing some calcs as to the current accuracy / precision of the movements I have and horizontally I have about 1.5 seconds per step of movement and vertically due to the 50:1 harmonic drive for torque purposes as it is lifting the mount / scopes I have 15.5 steps per second.
So was wondering whether to add a second harmonic to the horizontal to increase the step count hence querying how precise the readout is.
Cheers
Nick
Polar alignment accuracy
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Re: Polar alignment accuracy
Hi Nick,
there is certainly going to be a limit to accuracy based on the pixel scale of the camera you are using - with a 'standard' setup of a 200mm focal length guide scope and a typical guide camera with 3.75 micron pixels, you are looking at roughly 4 arc seconds per pixel. Sharpcap does calculate star positions at the sub-pixel level (based on the center of brightness of the star), but I think that picking the picel scale as the limit wouldn't be a bad guess in terms of how accurately stars are positioned.
Otherwise though, analysing the inaccuracy of the calculation is tricky - I think I would probably try to work out the error experimentally (repeat the first two stages several times and see what polar align error is reported each time) rather than trying to do a mathematical analysis on it.
In any case, aligning to nearer than 1 arc minute is typically not worth it unless you need to image close to the pole (I gave a talk on this a couple of years back - you can watch it here https://www.youtube.com/watch?v=bH4F5wjN_i8 - about 14 minutes in for details on alignment accuracy and the impact on maximum sub exposure length).
The other thing I'd note is that when aligning my NEQ6, I do the fine adjustment by tightening the two bolts against each other, so by tightening or loosening one side a bit you are flexing the whole alignment mechanism slightly rather than moving the mount at 1 degree per revolution - that tends to give fairly fine control of the adjustment, but might not help for a mechanized system.
cheers,
Robin
there is certainly going to be a limit to accuracy based on the pixel scale of the camera you are using - with a 'standard' setup of a 200mm focal length guide scope and a typical guide camera with 3.75 micron pixels, you are looking at roughly 4 arc seconds per pixel. Sharpcap does calculate star positions at the sub-pixel level (based on the center of brightness of the star), but I think that picking the picel scale as the limit wouldn't be a bad guess in terms of how accurately stars are positioned.
Otherwise though, analysing the inaccuracy of the calculation is tricky - I think I would probably try to work out the error experimentally (repeat the first two stages several times and see what polar align error is reported each time) rather than trying to do a mathematical analysis on it.
In any case, aligning to nearer than 1 arc minute is typically not worth it unless you need to image close to the pole (I gave a talk on this a couple of years back - you can watch it here https://www.youtube.com/watch?v=bH4F5wjN_i8 - about 14 minutes in for details on alignment accuracy and the impact on maximum sub exposure length).
The other thing I'd note is that when aligning my NEQ6, I do the fine adjustment by tightening the two bolts against each other, so by tightening or loosening one side a bit you are flexing the whole alignment mechanism slightly rather than moving the mount at 1 degree per revolution - that tends to give fairly fine control of the adjustment, but might not help for a mechanized system.
cheers,
Robin
Re: Polar alignment accuracy
Hi Robin,
Thanks for that will watch the video, I polar align with my main scope and camera and a 3 to 4 second exposure so my pixel scale is 0.99 pixels to per arc second. Have just finished machining everything and fitted it to the mount and pier just need a clear night to try it all out in anger. Will post some images of the vertical and MkII horizontal bits as an update with some notes on the previous thread relating to the pier / mount.
Cheers
Nick
Thanks for that will watch the video, I polar align with my main scope and camera and a 3 to 4 second exposure so my pixel scale is 0.99 pixels to per arc second. Have just finished machining everything and fitted it to the mount and pier just need a clear night to try it all out in anger. Will post some images of the vertical and MkII horizontal bits as an update with some notes on the previous thread relating to the pier / mount.
Cheers
Nick