Please help - RA axis is not guided well.

[JinYoung Heo]

Hi,

Recently I got a new NYX-101 and RA guiding looks very strange and bad. 

Context:

0. Focal length: 420mm. Using OAG. Guiding via ASCOM/USB, not ST4. The guiding speed is 0.5 of the sidereal.

1. While it is not critical for this mount, tried to place OTA and devices on a balanced point of the DEC axis

2. No cable dangling or dragging. Double/triple-checked. 

3. Polar alignment with Sharpcap. Within 1 arc-minutes.

4. Calibrate at RA: 06h54m46s DE:+04°54'23". Cannot do it around the junction of the meridian and celestial equator due to house structure. But tried to get close to DEC 0. The altitude was between 50 and 60 degrees. 

5. Calibration error as shown:

6. Calibration review:

7. In an ideal way, I should throw away the calibration and do it again. But this is not the first time (happened already multiple times while I tested at home - just did not have dedicated time for serious testing), I decided to go anyway. The below picture is guiding on the same position of the calibration.

8. Guiding, indeed, was not good. RA did not seem to be corrected by guiding pulse well. I'm not sure whether this is caused by a mechanical issue or I did set something wrong. So I'm asking for your help. 

Thanks,

Jinyoung 

[wave...@talktalk.net]

Hello Jinyoung,
I'm not one of the experts, but I've learned a lot about high periodic error from my old Meade LX200 Cllassic. So, what I see is a significant PE lasting around 400 seconds. I guess this is the cycle period of the stainwave drive gear (?). It's around 90 ercsec peak to peak, which seems rather high for a new precision mount. The following data are from the PHD2 analysis tool on one of your guiding sessions

Whether this PE can be reduced by a mechanical adjustment of the drive, I couldn't guess. The use of the  Predictive PEC (PPEC) algorithm for RA may also be a better choice. I'm sure the PHD2 experts will advise you better when they return.
Cheers,
- Jack

[Bruce Waddington]

Hi Jinyoung.  Unfortunately, I think this mount is in pretty bad shape, the RA tracking appears to have serious problems.  Here’s a typical section when you were pointing near the celestial equator:

 

 

Since it’s a strain-wave drive system, I don’t know how to interpret what’s wrong here.  It looks like there’s a resistance that impedes the ability of the mount to track west at the sidereal rate, then that resistance is finally overcome and the mount “snaps back” to its intended location with some over-shoot.  I recommend that you run a long session with the Guiding Assistant and capture the unguided tracking performance of the mount.  Then take that info along with the graph above and ask the mount manufacturer to repair or replace the mount.  There’s nothing that software can do to control this behavior.

 

Most of the strain-wave systems don’t have periodic error in the usual sense of the term because the point of gear engagement doesn’t transition from one tooth to an adjacent tooth in a predictable way.  So periodic error correction often isn’t very effective.  I think that’s why some of these mount manufacturers have introduced follow-on versions with absolute encoder mechanisms.  But those are just personal observations by me, what matters here is that this native RA tracking performance is unacceptable.

 

Hope you can get it fixed soon,

Bruce

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[JinYoung Heo]

Hi Bruce and Jack.

Thanks for confirming the mount's behavior. I'll contact PegausAstro for further discussion. 

Clear sky!