AM5 Random RA/DEC Spikes

[Apennine]

Hi Everyone,

I have been having an ongoing issue with random RA/DEC spikes while guiding. My equipment is:

• ZWO AM5 Mount
• FMA 135 Guide Scope
• ASI 220MM Mini Guide Camera

For the last few months, I have had what I believe are random RA/DEC spikes which ruin a portion of my images even though my main scope is only 408mm FL. I am attaching two sessions which show this behavior. Things I have tried so far:

• Change feet on TC-40 tripod to ones with larger contact area (sitting on grass)
• Ensure cabling seems "ok", picture attached
• Checked all screws on guide scope, even when using two screws attaching finder foot to scope, there is no change
• Various guide setting changes, internet is full of advice for AM5 to guide fast with low aggression and small min moves, no change from stock settings proposed by guiding assistant (last segment from 12/22/24)
• Following ZWO guidance, checked belt tension, seemed good

Where I have thought to go from here:

• Reinstall my OAG, maybe that will be more successful?

Thank you for any advice, Happy Holidays!

-Chris

https://openphdguiding.org/logs/dl/PHD2_logs_kRdF.zip

[Bruce Waddington]

I think the most telling evidence is that you're getting these excursions in Dec.  Here's a typical example (Dec in green):

There are two important points here: 1) The Dec guiding excursions are not triggered by guide pulses and 2) the Dec motor wasn't even running at the time.  So it's not the mount and it's not guiding.  Typical problem sources include:

1) A small, unwanted movement of the guiding assembly.  Yours looks very vulnerable in this regard with a coarse image scale, lots of thumb-screws, and a stalk mount.

2) Wind gusts or movement of the entire scope assembly (vibration, flexure of whatever surface the scope is sitting on, etc)

3) Something in the mount payload that's creating movement, e.g. filter wheel, camera shutter, etc.

Because of the guiding assembly you're using, you have a very coarse guider image scale, over 6 arc-sec/pixel.  So a 4 arc-sec guiding excursion like these corresponds to a movement on the guide sensor of 2/3 of a pixel.  With your guide camera, that's 2.6 microns, keeping in mind that a human hair is about 50 microns thick.  How can you be sure that your guiding assembly, with all its thumb screws and stalk mount, can't move by 2.6 microns?

The first thing to try is to go back to your OAG and eliminate all this other stuff.  Be very careful that you get the guide camera well-focused on the OAG, use a technique that gives you quantitative feedback on star sizes.  The Star Profile tool, for example, can be used or you can focus using a different application - whatever suits you.

Regards,

Bruce

[Apennine]

Bruce,

Thank you for your detailed analysis and equating the movements to a human hair. I definitely can't ensure that the assembly isn't moving 5% of a hair's width! I will install the OAG and go from there as you suggested.

Thanks again,

Chris