Help understanding PHD2 log

[rnobleeddy]

I have an EQ5 with GOTO that's not been adjusted since it was bought. I got a couple of long-ish guide logs last night (one hour in total, shown below) and was hoping someone could help me make sense of them.

Few other points:

- I also had problems with slightly saturated stars in PHD2 last night (perhaps I chose 8-bit for the guide cam by mistake, or focus is a little out) but don't think that made a massive difference

- Using EQMOD for guiding, no synscan handset

-  Ignore the very large spikes at the end of log1 and the middle of log 2 - this was me touching equipment

- I noticed I had  camera USB cable trailing over the scope, but it was not tight

- The DEC backlash is large, which is visible in the logs below. The DEC axis feels too easy to move with the knob on the mount. I intend to try to adjust the mount to remove some of the DEC backlash but wanted to get a baseline first

- PA was with sharpcap, PHD2 shows this drifted to 1.9' but as far as I can tell, this is good enough?

 

According to the internet, the RA axis takes 638s to rotate once, and I can just about buy that accounts for the periodicity in RA. However, what I'm less sure about is why there appears to be such strong periodicity in the DEC axis?

I'd have guessed that there could be either drift in one direction and/or random movement - but would be interested to know what might cause what looks like a periodic error in DEC that is roughly out of phase with the RA error? 

I don't think it's simply DEC overshooting because I initially tried to guide in one direction only, but this cycling north/south error meant I had to switch to both directions.

 

I know the EQ5 is limited and will upgrade in due time - it's more about understanding what's going on that expecting the mount to perform well!

 

Log 1:

Log 2: 

Log 2:

 

[knobby]

Really no expert but my 2p worth ... looks like guide speed is too low ? takes a long time to correct errors.

[rnobleeddy]

5 hours ago, knobby said:

Really no expert but my 2p worth ... looks like guide speed is too low ? takes a long time to correct errors.

Yeah, even if the DEC taking a long time is driven by backlash,  I hadn't really spotted how poorly the RA was keeping up.

That rings  a bell about seeing another post about the EQMOD default guide rates being low, so I'll try that out on the next cloud free night.

[michael8554]

It not easy to diagnose from screen grabs rather than the whole PHD2 Guidelog.

But from the visible text your guide speeds are  RA = 10Arcsec/sec, Dec = 7.7arcsec/sec, from a previous Calibration, so not particularly low.

1 second exposure would probably be considered as too fast, "Chasing the Seeing".

So onto the Dec swings.

You say you have Dec Backlash, so one scenario has the Dec balance too good, so that Dec is teetering between the extremes of the backlash.

Then it takes ages to bring back, and a bit of stiction too means it eventually overshoots, and repeats the process.

So if you can't improve that by worm adjustment you could try slightly unbalancing in Dec, so that it is always at one end of the backlash.

Michael

[rnobleeddy]

 

7 hours ago, michael8554 said:

It not easy to diagnose from screen grabs rather than the whole PHD2 Guidelog.

But from the visible text your guide speeds are  RA = 10Arcsec/sec, Dec = 7.7arcsec/sec, from a previous Calibration, so not particularly low.

1 second exposure would probably be considered as too fast, "Chasing the Seeing".

So onto the Dec swings.

You say you have Dec Backlash, so one scenario has the Dec balance too good, so that Dec is teetering between the extremes of the backlash.

Then it takes ages to bring back, and a bit of stiction too means it eventually overshoots, and repeats the process.

So if you can't improve that by worm adjustment you could try slightly unbalancing in Dec, so that it is always at one end of the backlash.

Michael

Thanks - I'm happy to post the full logs, I just thought expecting someone to use them was a little much! 

That makes sense. I have had better DEC guiding on a previous session when I had a much larger PA error. I imagine other's can use PHD2 drift alignment better than I can, but I found sharpcap made it much easier to get v. close to PA (and is the best £10 I've spent since starting this hobby!).

I'll have at tuning out some of the backlash and see what happens on the next clear night.

 

 

 

[michael8554]

9 hours ago, rnobleeddy said:

I have had better DEC guiding on a previous session when I had a much larger PA error

Similar effect to unbalance, PHD2 will only be correcting in one direction.

Michael