10 replies to this topic

[martinontheroad]

I have recently been unable to obtain a successful PHD2 calibration with my EQM-35 Pro, using the ZWO 30mm f/4 scope (120mm f.l.) and the ASI-120MM-mini camera.  Guiding has worked fine until the last several sessions. For some reason it has started failing calibration and giving me the error "Calibration failed.  Star did not move enough."  And it is not consistently failing, or consistently failing at the same place in the calibration process.  Tonight, I spent two hours trying to get a good calibration, to no avail.   I use the ASCOM drivers and EQMOD for a direct connection into the mount.  I have tried adjusting the EQMOD RA and DEC sliders but nothing seems to help.  My hope is that some of those wiser than I in the ways of PHD2 may be able to shed some light on this issue.  Thanks in advance!

 

Here is the latest PHD2 log file.  I finally had to give up due to clouds.  The resuts shown by PHD2 log viewer are not good.

 

 

Attached Files

•  PHD2_GuideLog_2023-09-18_192105.txt   55.12KB   4 downloads

[John Rogers]

Did you build a library of dark frames?  I've encountered failed calibrations due to the software locking onto a dead pixel thinking it was a star.  A dark library appears to have solved that problem.

[michael8554]

Hi Martin

 

From the GuideLog:

 

You seem to have a huge Dec Backlash.

 

Either you've been lucky up to now, or something has been changed.

 

1) In the Cal PHD2 moves about 12 steps west in RA.

 

Returns to the origin.

 

Moves about 12 steps north in Dec

 

Returns to the origin.

 

Starts Guiding.

 

In your case the mount completed the RA Cal in 13 steps - good.

 

But spent 20 steps going nowhere in Dec, due to Backlash.

 

2) The Log shows you changing the EQMOD "ASCOM PulseGuide setting".

 

From 3arcsec/sec, up to 13.5, back to 1.5, up to 7.5.

 

So how to proceed.

 

A) Set EQMOD to give a guide rate of at least 7.5arcsec/sec.

 

B) Adjust the Dec worm mesh to reduce the Dec Backlash, you'll never remove it completely

 

So if you've read the PHD2 Instructions you will know how to clear the remaining (small) Dec Backlash BEFORE Calibrating.

 

Which you don't appear to be doing ?

 

C) After a good Calibration run the PHD2 Guide Assistant.

 

Tick the Measure Backlash box.

 

The Assistant will advise best settings for the guiding.

 

And give a suggested start setting for the PHD2 Dec Backlash Compensation.

 

Or maybe it will advise it's still too much for good guiding.

[martinontheroad]

Thank you so much! I made the changes in the EQMOD RA and DEC settings trying to make it work.

I admit to not having read the PHD2 manual in a while, as I have it working fine with my Star Adventurer using the same guide scope and camera. Time to reread with your points in mind, and proceed accordingly.

Edited by martinontheroad, 19 September 2023 - 10:43 AM.

[martinontheroad]

Did you build a library of dark frames? I've encountered failed calibrations due to the software locking onto a dead pixel thinking it was a star. A dark library appears to have solved that problem.

I do have a dark library, but it may be time to build a new one. Thanks!

[BucketDave]

You could try bumping up the minimum star size, that would help prevent locking on to a bad pixel.

Also, check your signal to noise ratio. The default value (6) is really low. I get better guiding when I use 25-75 and it is also more resistant to thin cloud.

[AhBok]

When my mount had a lot of Dec backlash, I found I could get a good calibration by making a final short move north just before calibration. Of course the real cure was adjusting my worm spacing to remove as much backlash as possible without binding. BTW, if like me, you have had a bit of a temperature drop lately, you may find it necessary to tune up Dec backlash. Many of us with less expensive mounts (mine’s an EQ6R Pro) require a bit of tuning as autumn or spring approaches for best performance. I only tune up a couple of times a year. The good news is once I got the procedure down, I can do it on the fly with virtually no loss in imaging time.

[martinontheroad]

Thank you for the words of hope and encouragement! I had seen some postings where people had mentioned the short movement North. I had not considered temperature change, but it makes sense, even though San Diego changes are barely noticeable compared to most of the world. :-) I’ll have to give the movement a try, whilst I learn to adjust backlash without a mount tear down.

I found an excellent video on YouTube where Petter Åström did a complete rebuild of hs EQM-35 Pro, replacing the washers and fiber washers with proper needle thrust washers. I think I can do that, so I’ll likely be tearing into my mount soon to do so. I am sourcing and costing out the bearings and washers now.

We have a week of clouds coming in, but I’ll report back on any successes. Thanks again.

[martinontheroad]

In the course of researching how to correct my problem(s), I ran across this excellent "How-To" video by Petter Astrom.  I have the parts on order.

 

https://www.youtube....h?v=SKZT5tiFX_s

 

This seems to be a doable upgrade to the EQM-35 without investing sacks of cash.

[Spaceman 56]

Backlash is a mechanical problem and needs to be fixed at the level of mechanics.

 

software adjustments are not the right place to fix Backlash.  

[martinontheroad]

Spaceman56 - Right you are! New and improved mechanical components are inbound. Hope to have it better than factory new with some thrust bearings for RA anD DEC, and ball bearings for the worm gears.