Derek et al
If you live in anywhere but space, your viewing with air turbulence is about 1" at best. I'd say of all the parameters, the air turbulence limits is far more than mount accuracy. What say you?
Chuck
On Sunday, June 13, 2021, 12:30:09 PM PDT, Paul Kanevsky <yh@...> wrote:
Hi Derek,
You raise some good points. It is important to remember that RMS guiding error while important, is just one measurement that tells only part of the story. The ultimate goal is the best possible resolution of the image, and that depends on much more than just guiding.
Regards,
? ? -Paul
On Sun, Jun 13, 2021 at 01:52 PM, Derek C Breit wrote:
The REAL question is WHY??
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PHD2 guides to a fraction of a pixel *and* most everyone has seeing that is not good enough to realize such an improvement in encoder resolution.. Add in the size of the imaging chip, Nyquist /proper sampling)and..
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WHY?? It¡¯s a semi rhetorical question, but there is also ¡°Why should I not be happy with *my* 0.7¡± rms guiding?¡± i.e what am I missing (aka teach me) that makes people want more.. What imaging theory points to needing 0.2¡± rms guiding?? I mean I understand inventing, tinkering, etc..
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I see people ¡°struggling¡± with the adjustment of spring loaded mounts, changing encoder resolution, or swapping out 25:1 for 50:1 gearboxes.. Frankly I am *GLAD* I have my 2 piece worms.. My rms values are exceedingly stable, my imaging scale is at 0.5¡±.. Even if I switch out my perfect 130mm refractor for my less than perfect 12¡± SCT to increase resolution, why would my 0.7¡± guiding not be ¡°pretty darn good¡±??
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I don¡¯t mean to disparage anyone, I want to know what I am missing.. Whether we are talking guiding or pointing, why isn¡¯t a 1/2 arcsecond sufficient, especially considering seeing (environmental, as well as local)..
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Derek
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Page 31 of the Gemini-1 users manual has a table. It says the G11 and? GM8 have 6400 steps per worm revolution (25 gearbox x 256 motor encoder revolution).? There are 360 degrees per revolution and the G11 ring gear has 360 teeth, so that's one degree per worm revolution.? So each degree having 3600 arcsec per degree is divided by the steps per worm revolution that is 6400.? ? So that's 3600/6400 arcsec per step = 0.56 arcsec per step.??
As for reasons not to go with a higher motor encoder: the Gemini-1 clock is 1.5 MHz (per the G-1 manual).? I don't know the clock speed of a G-2.? You might find the encoder detector chip unable to keep up with the codes at a high slew rate....but that's only a factor of 2...so it might be just fine.? It's worth a test.? Getting the motor encoder out is a major soldering challenge.? Replacing the fragile encoder disk is easier but really...these motors are hard to get apart and repairs of them are rare.? (Brendan Smith and David Partridge are expert in this subject.? I tried and had no success.)??
To double the step accuracy, you can also replace the 25:1 gearbox with a 50:1 gearbox.? [I'm only familiar with the McLennan versions.? The McLennan company didn't recommend using their 50:1 gearbox with a HiTorque motor because it's shaft is ~3.2 mm and the small 50:1 gearbox pinion gear is 3.0 mm...they thought enlarging the pinion ID would get too close to the teeth.? Alternatively, Stuart Hutchins recommended using the motor rotation and a file to reduce the shaft OD to 3.0 mm to fit the pinion ID.? Stuart thinks this is the best way to keep everything I have not tried that idea...the question is what is best to modify. ].?
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Here is my math:?(360*3600)/(256*25*360) = 0.5625 arc sec per step.
This seems rather coarse. We have 2.3 million steps per rotation of the Ra shaft. From talking with Sideral Technologies high end mounts have a greater number of steps. If it is possible to change the encoder used in the motor this could be increased to 4.6 or even 9.2 million steps. So 0.28 or 0.14 arc sec per step. This I think should have a significant impact on improving guiding performance, especially in the Dec axis, as right now the minimum correction is 0.56 arc sec. My imaging system is 0.76 arc sec per pixel. I would welcome any thoughts. Perhaps Losmandy can comment on wether the controller will work with a higher resolution encoder. They cost around $65 per axis to do the upgrade, so well worth running a test I feel.
