Re: AP1100 Notification Suggestions

Roland Christen

Encoder mounts are no machined more sloppy than non-encoder mounts. That's a fallacy.

With an encoder mount the periodic error in RA is gone, so you get true sidereal tracking rate. Non-encoder mounts can get close by measuring the periodic error and compensating for it with PEM.

Encoder mounts improve guiding. That's the bottom line. This is especially true for the Dec axis where the encoder basically eliminates any backlash. When the guiding program sends a command to move 1 arc second, the axis will move by that amount, regardless if it's in the same direction or the opposite direction. In a non-encoder mount the motion may take several 1 arc sec commands during a reversal due to backlash and other subtle non-linearity around the zero point. You may or may not get the commanded motion in a non-encoder mount. This is true of all non-encoder mounts, regardless if they are belt driven, have servo motors or steppers.

All of our mounts can be modeled for unguided imaging, but the encoder mounts will follow the model more accurately. So it depends on what focal lengths you are working with and what your tolerance for error is.

Here is a typical guided run with the Mach2 encoder mount. The scope was a 160DEF refractor:

-----Original Message-----
From: Mark Lamb <lamb_mark@...>
Sent: Fri, Mar 5, 2021 1:45 pm
Subject: Re: [ap-gto] AP1100 Notification Suggestions

Thanks for the responses so far!

I think I will end up going for the AP1100 instead of waiting (how long??? Daleen said at least 2 more runs) for the Mach2, even though the AP1100's capacity is overkill.  (I have zero appetite for mounting/dismounting an unwieldy 70#+ load, as this will be uncovered on my back patio.)  I also expect the AP1100 is machined to tighter tolerances vs. the Mach2, as it designed to perform at a high level without the need of encoders.  My mind has an analogy of the sound of simpler Class-A audio amplifiers compared to much more complicated Class-AB with extensive feedback.  The feedback intensive Class-AB have much better specs, but the Class-A amplifiers sound MUCH better and realistic (I have actually built my own Class-A preamp and power-amps, and they sound very realistic).  The downside is the Class-A amplifier requires MUCH higher quality components, better power supplies, and more heat sinking.

My concern with reliability of the encoders is more with the read heads and not the encoder rings, as I expect the failure rate of the read heads would be an order of magnitude higher than the rings due to their electronic nature.  It also seems that the read heads are much more expensive than the rings.

I still do not have a good feel for the practical/realisable/tangible performance improvement to be expected w/ the AE version compared to the base AP1100.  I have read reports than base AP1100s guide as good as seeing allows, though I expect this assertion should be conditioned on the actual seeing conditions.  My seeing is better than most, w/ 1.5" typical and 0.7" on the rare good nights (Meteoblue est. for my location).  With my good seeing location, I expect that can actually realize resolution improvement down to ~0.4"/px, a level that I expect to image at for galaxies (EdgeHD925 & QHY268M).   At 0.4"/px scale and assuming guiding w/ and OAG, would the AE add a meaningful improvement?  Or is AE "gilding the lily"?

Daleen called me while I was in the middle of composing this reply, and I had her put me down for a new AP1100, as I do not want to wait 2 years for a Mach2.  She told me to expect September delivery.  I need to talk to George about deciding whether to go for encoders or not. 

I plan on keeping my G11, to allow running 2 imaging rigs to make the most of the precious clear skies.  I will put the G11 on my LW tripod, and put the AP1100 on my FHD tripod beast.  I need George to explain whether I need attach the AP1100 to my FHD; do I need both the 119FSA and 119FP?  Does the combo 119FSA-FP work for the FHD and does it drop into the FHD's normal MA collar?

I know the encoders eliminate DEC backlash.  But, is DEC backlash a practical problem for the base AP1100?  DEC backlash does give me problems with my G11 from time to time, and is my main frustration with my G11.
I know the encoders will reduce RA PE to ~0.25".  What is typical PEC adjusted PE for the base AP1100?  At what image scales does this give problems?

I think I will go for the DOVEDV10 saddle, as I feel the wider clamp spacing of the DOVELM162 could give problems with my Esprit's after-market 7" dovetail plate; I want at least 2 clamps engaged.  With my G11's 8" saddle, the end of my Esprit's 7" plate extends ~0.5" past the front edge.  With the DOVEDV10 extra 1" on both sides of center, it should clamp in both the middle and end.  Does this make sense?

Roland Christen

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