Regular Spikes in Dec

[Billy Liang]

Hello fellow PHD guiding experts,

Here is my guide log:

https://openphdguiding.org/logs/dl/PHD2_logs_XE6i.zip

I would like to pick your brains on my persistent issue of recurring DEC spikes in 4-7minute intervals.  Here is my equipment:

AP 900 with CP4 Controller

AG Optical 12.5 inch CDK @ 2080mm focal length

Guider: QHY 5IIL

Main Imager: QHY600m

Focuser/Rotator: Nitecrawler 3.5inch

House in a 8ft Explora dome

I have performed the following but none improved the DEC guiding error:

1. Clean and regrease DEC  gears;

2. Re-mesh the DEC gears and feel no play when wiggling the scope;

2. Rebalance mount

3. Clear any potential cable snag

The issue happens when the mount is pointed at different declinations. Even with dec guiding output turned off, I still see the error as a "cliff" instead of spike in the guide graph, so I can pretty much rule out mount issues.

I am really lost. Any ideas and tips to trouble shoot this are extremely appreciated :)

Billy

[Billy Liang]

A sample guide graph for the issue

[Brian Valente]

Hi BIlly

yeah wow that is some interesting DEC behavior

I was tempted to say stiction, but i think you have other issues presenting themselves with your DEC axis, possibly mechanical, cable snag, etc.

As you pointed out, even with no guide pulses, you can see some significant jumps in DEC. Assuming you aren't running any sort of pointing model i.e., APCC/APPM and DEC motor is not running at all, you have about 2" jumps as shown here:

you can also see it clearly here where there is a large a fairly regular DEC jumps when you are tracking/guiding:

i was tempted to think it was stiction, but you also have runs at the same declination that look okay:

So i can see signs of stiction, possible mechanical problems, and possibly cable snag, or other non-guiding issues

The one that really throws me is your jumps in DEC with no guiding and presumably no motor movement (again assuming you aren't using a pointing model or other tracking accuracy type )

Another possibility is you have a bad PEC? I've seen this kind of thing before when PEC is invalid. Don't know if you're using that  

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Brian Valente

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[Brian Valente]

PS the mean and median for those jumps is somewhere around 250-260 seconds

On Sun, Jul 4, 2021 at 6:37 PM Billy Liang <leung...@gmail.com> wrote:

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[Billy Liang]

Yes. the PEC is turned on. Let me try to turn off PEC, point correction and rate correction tonight and give it another try.

[Bruce Waddington]

Well, this is different – a 4^th of July head-scratcher. J   As you would expect, I don’t know what’s going on here but I can offer a few thoughts:

 

1.       This is probably not the mount drive system because the Dec motor and gear train is idle most of the time. 

2.      It’s not a guiding problem because the big excursions aren’t a reaction to guide commands – they are being triggered by something external

 

So now for some speculation about what might be happening:

1.       Are you sure the Dec axis clutch is sufficiently tight?

2.      Are you using some kind of automated focus adjustment such as temperature compensation?  Any movement of the focuser can affect the guide star position.

3.      Is there something like a fan assembly on the main camera or on the OTA that is cycling on and off at these intervals?  With your image scale of 0.7 arc-sec/px, it doesn’t take much movement or vibration to produce a guiding spike

4.      Are you using any kind of cable wrap or cable ties that have ridges or small protrusions that can rub across part of the mount?  Any kind of non-smooth surface on cable wraps can briefly catch then release when it tries to slide across part of the mount.

5.      Do these roughly 4-minute events correlate with exposure times on your main camera?

6.      What other software is connected to the mount?  Are you running anything that can do periodic re-centering?  For testing purposes, it would be good to disable *everything* except guiding to see if the spikes remain.

 

That’s all I can think of at the moment, but I’ll still keep thinking about it.

 

Bruce

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[mark T]

Just a thought - I determined that regular DEC spikes I was experiencing was due to filter changes.  I loop continuously through LRGB with 1 or 2 subs each, and the spikes occurred right on time.

Mark

[Brian Valente]

Bruce i'd just add invalid PEC as a possibility

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[Billy Liang]

Thank you all for the tips. I performed more testing last night.  I tried to point the scope at different declinations at 45 degree altitude with scope on the west side of the pier

https://openphdguiding.org/logs/dl/PHD2_logs_ynq6.zip

One of my previous observation was not true. The guide curve looked OK if I guide near 0 degree declination (guide curve 4 and 6 in the log). When I point east and north, the spike appears. This indicates the problem is within the imaging train, not from external force. It looks like mount was causing this since other components were tested and can be eliminated (see the list below). I am just not sure why and how it would cause this issue.

I have eliminated these causes:

1. PEC / rate correction / pointing correction - spike still appears with the settings off;

2. cable snag - I observed through the dome camera and saw no cable snag when the spike happened;

3. vibration from dome rotation - No dome rotation when the spike happened. I also manually trigger the dome rotation during guiding and see no impact. the pier is nicely isolated from the dome;

4. filter wheel - I powered off the main camera and filter wheel when guiding and still saw the spikes;

5. Loose OAG or loose pick off prism - I tried to guide with the main camera and saw spikes;

6. Scope fan / Main camera cooling fan - still had spikes with fan off;

7. Guiding software - I tried to guide with Maxim DL and PHD2 - same results;

8. Focuser - no temperature compensation, still have spikes with focuser powered off.

 
Billy

[Gin ger]

Commenting on this as I have a very similar issue and also as lost! :)