Purchase of Dobsonion 10k encoders?

[Robbos30]

Hi Everybody,
I'm contimplating on replacing my dobs 8k encoders with 10k encoders.
Does anybody know what an average realistic price would be for new ones and where does one purchase these from?:help:

[Poita]

The Argo Navis guys sell 10K encoders I think.

[astroron]

Yes Peter, contact Gary at Wildcard Innovations or contact him by pm on here .
Cheers:thumbsup:

[Robbos30]

Thanks Guys will do!
Cheers Pete.

[bobson]

Hi Peter,

What are you expecting to achieve with 10k over 8k encoders?

bob

[alistairsam]

Hi,

Motiontech have encoders from 5 different manufacturers.
have asked them for a quote for 10k and 20k encoders.

http://www.motiontech.com.au/
http://www.motiontech.com.au/electrical.cfm?pc=45&link_catg1=45::53::83&parentcategory=83
http://www.motiontech.com.au/editable/contact_us.htm

the S1 from usdigital in lots of 100 is $39 each as per their pdf.
http://www.motiontech.com.au/assets/pdf/A%20Product%20Overview%20of%20US%20 Digital.pdf.pdf

[ausastronomer]

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

[gary]

Hi John,

Thanks for the post.

Actually, when going from 8192 on each axis to 10,000 on each axis, the pointing
resolution potentially increases by around 49%.

(10,000 * 10,000)/(8192 * 8192).



Peter's 30" is fitted with an Argo Navis.

By supplying it with pointing data that is of a higher resolution, the Argo Navis
Telescope Pointing Analysis System (TPAS) can also provide a more refined
analysis of these types of systematic pointing errors.

[naskies]

Hmm... I'm not sure about the maths provided so far on pointing accuracy improvements going from 8k -> 10k. Pointing error/resolution is measured by the straight line distance between where you wanted to point versus where the scope actually pointed to. The previous calculations measure something slightly different (the reduction in area or amount of sky that any given coordinate represents).

Sorry for the slight tangent... I had a few minutes to spare on a slow afternoon.

As engineers, we usually specify accuracy in the same units as the quantity being measured - like the length of a machined part might be 200 mm +/- 0.01 mm.

In this case, we can say that the 8k encoder is being used to measure 360° of sky, so the resolution of pointing would be 360°/8000 = 0.045° per step. If we ignore all other errors for now except for those due to encoder resolution, then the accuracy of pointing at any given coordinate is +/- 0.0225°. This is the worst case scenario for one axis, since you can't be more than half a step away from the target (as we're ignoring all other errors).

To extend the +/- 0.0225° pointing error to two axes (RA/DEC or Alt/Az), the worst-case-scenario error is the hypotenuse of the isosceles triangle with sides 0.0225°, i.e. +/- SQRT(2 * 0.0225° ^ 2) = +/- 0.0318°. To see why, draw a square grid on a piece of paper, pick any coordinate point, draw lines representing +/- 0.0225° error away from that point for both axes, and you'll see where the maximum error occurs. It's less complicated than it sounds - very clear once you draw it.

Similarly, we can do the above calculations for the 10k encoder, which gives us a single axis error of +/- 0.0180° and two-axis error of +/- 0.0255°.

To calculate the error of the 10k encoder relative to the 8k encoder, simply divide the 10k error by the 8k error. For one axis, it's 0.0180° / 0.0225° * 100% = 80%. For two axes, it's 0.0255° / 0.0318° * 100% = 80%.

Therefore... despite increasing the encoder resolution by 25% going from 8k to 10k encoder resolution, the overall pointing error is only reduced by 20%.

Intuitively, it makes sense that the overall reduction in pointing error has to be less than or equal to the improvement in encoder resolution. Imagine that you upgrade the encoders on both the altitude and azimuth axes, but then you leave the azimuth axis alone but re-point the altitude axis somewhere else. Your altitude axis can't suddenly become more accurate than before you upgraded the azimuth axis - they're obviously unrelated. Would be nice if it did though :lol:

[bobson]

Interesting how some posts with "correct" resolution calculations disappeared ;)

cheers

bob

[naskies]

Just to clarify... I believe that John and Gary's calculations are correct for the increase in spatial resolution of 10k vs 8k encoders. That is, if the scope is already pointing at an object, how much more 'accurate' is the coordinate reading with 10k vs 8k encoders? This is where you get the squaring effect because you're measuring the difference in the area of the "sky pixels".

My post was referring to the increase in pointing accuracy - i.e. you tell a GOTO system to slew to an object, by how much does it miss its target? Intuitively, you don't get the squaring effect in this case because each additional axis gives you new ways of missing the target: with 1 axis -> you can miss the target to the left or right; but with 2 axes -> you can miss it left / right / up / down... which cancels out the squaring effect.

Anyway, Peter, sorry for taking your thread off topic... I'll shut up now :)

[Robbos30]

Thanks John, Gary and Dave, Wonderful feed back!! My thoughts exactly on pointing accuracy.
I have put the 30 on the market to possibly fund a 40 f(5) with 10k encoders.
Cheers Peter.