PHD Issues with CEM120EC2

[MountainAir]

Hello,

I am a new owner of a CEM120EC2 (relative encoders on both RA and DEC), and a new user of PHD2.  Last night I tried to test my mount's sub-division error by using a long focal length scope with tiny pixels (.2 arcsec/pixel) unguided, but logged.  Unfortunately I could not get a normal calibration:

I  just read the CEM120EC posts in this group. I will happily try those recommendations when the clouds clear, but for now, any ideas why this wouldn't calibrate?  I suspect the calibration pulses are just too small.

My logs are here (sorry I didn't upload them through your normal interface):

https://drive.google.com/drive/folders/1iTSBRk48GhNINe8xlnlVXfXHAmmSeHwQ?usp=sharing

I hope someone can help!  Thanks in advance, and clear skies!

[Brian Valente]

Hi Brett

it looks to me like your pulses are fine - you got about 12 steps in each axis

there are a couple problems i see:

1. your image scale is way too small. PHD really wants 0.5" or larger, so if your image scale is really 0.2 you should bin2 or bin3

2. your orthagonality is the main issue. the RA and DEC should be at 90 degrees, but yours are consistently at much smaller degree. 

Common causes of orthagonality errors are
bad polar alignment,
large declination backlash, or
large periodic error in RA.

I'm not sure which if any relate to your setup, but they are worth investigating

3. i think your exposure time is a little long - try 3 seconds to start

I think you are right to focus on calibration right now, without a good calibration nothing is going to matter here

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/0ff247d7-c258-4a06-b4ba-03e5f2e30dfdn%40googlegroups.com.

--

Brian 

Brian Valente

portfolio brianvalentephotography.com 

[MountainAir]

WOW, that was fast Brian!

1.  I know, the image scale is really small.  I will bin it and try again; I was following advice to use the smallest image scale possible in order to detect SDE, but no one actually suggested a target scale.  Maybe "as small as possible" was meant for refractor owners, not almost 8 feet of focal length from my SCT.

2. I wonder if this was the result of the image scale.  Every other time I have calibrated PHD2 with this mount and OTA combination, it came out almost orthagonal -- never perfect, but within a few degrees by eyeballing the chart.  I did have a very good polar alignment (< 20").  PHD2 does say that I have a large DEC backlash (1.4 seconds after I tightened gear mesh), however I can't reproduce this with the hand controller (at speed 1x it feels like .5 seconds to me).  I spent a lot of time re-centering my collimation star and backlash would have stood out like kitchen fire.  Lastly, iOptron claims the PE in RA is < .15 arcsec, and I haven't had any issues in this axis.  I'll try again with binning to see if it helps.

3.  This one is a long story, but I am still digesting guidance you gave to other EC2 owners here.  In short, it seems this mount with its relative encoders really doesn't like short exposure times.  For guiding, I actually plan 5 seconds with 5 seconds of time lapse, but as I'm still "testing" the mount I don't know what I'll settle on.  I also need to spend more time choosing algorithms, because  I initially chose LowPass2 since I have encoders on both axes, but they are not absolute and probably not super-high-resolution.  Someone recommended going back to ResistSwitch, but I'm still curious about LowPass2 and ZFilter for this particular mount.

I will try again on Friday night when this week of clouds will hopefully end.  Saturday I need to send the mount back if I can't work this out.

Thank you!