Quote:
Originally Posted by bobson
Hi Peter,
What are you expecting to achieve with 10k over 8k encoders?
bob
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He hopes to achieve a noticeable increase in pointing accuracy. While the raw numbers at face value (8k to 10K) may not look like they achieve much of a gain, they in fact do.
Going from 8k encoders to 10k encoders reduces the pointing error residual significantly. You are working in 2 axes therefore the pointing error residual based on the arc subtended by each encoder step of each encoder is squared. Each encoder step of an 8k encoder subtends 2.7' . Each encoder step of a 10k encoder subtends 2.16' . The pointing resolution of 8k encoders is therefore 7.29' and the pointing resolution of 10k encoders is 4.67'. This is a 36% gain, or reduction in the pointing error. Also note this does not allow for other errors which can further affect pointing accuracy like collimation errors, mount fabrication errors or non orthagonal mount axes. Putting the above aside, in practise, with smaller scopes which have a much larger true field of view than larger scopes, 8k encoders and in many cases 4k encoders are perfectly adequate for putting the target in the FOV each and every time. However, Peter has a 30"/F4.8 scope which has a focal length of 3660mm. Even with a 31mm Nagler this gives 118X and a TFOV of only 42' or .7 of a degree. With a small field of view like this going from 8k encoders to 10k encoders will greatly increase the chances of putting the target into the FOV each and every time. With a telescope costing way north of $30K the gain from a < $200 set of encoders is well worthwhile. Similarly, when astrophotographers are using Argo Navis to place the selected target onto a small CCD chip, the gain by going from 8k encoders to 10k encoders is significant.
Cheers,
John B