Calibration - RA and DEC rates differ

[Pawan Singh]

Hi

I am new to PHD. I am using PHD 2.6.2 with ZWO ASI120 camera, guide scope fl of 162mm and aperture of 50mm on an Atlas Pro mount. After I Polar Align using the polar scope, I point the mount close to equator/meridian and use PHD to calibrate.

On eqmod, my RA rate is set to 0.5, DEC rate is set to 0.2. I think the pulse length is set to 20ms.

After calibration, PHD says that my RA and DEC rates vary by suspicious amounts. Looking at the guide, DEC rate should be:3.0 a-s/sec. Actual rate comes out around 2.3 a-s/sec. According to docs, RA rate should be cosine(declination angle)*DEC rate. The calibration star declination was 15.3 degrees. The expected RA rate should be: 7.3 a-s/sec - but the measured rate is: 8.864 a-s/sec.

Does this mean that mount is faulty? How does one interpret this information?

Attached are the log files and calibration screenshot.

-Pawan

[bw_msgboard]

Hi Pawan.  I think it’s pretty unlikely there’s anything wrong with the mount.  The alert message is really an advisory, not an error condition, and I suspect your guiding results would be pretty good.  That said, I think you will get better results with a higher Dec guide speed setting in the mount.  I can’t think of any reason why you’d run it with a 0.2x speed.  Is there a specific problem you’re trying to work around?  With such a low guide speed, it could take a very long time to clear any backlash you might have in Dec and it means the PHD2 calibration takes an overly long time.  I suggest setting your Dec guide speed to at least 0.5x and see what the calibration looks like in that case.  

 

Good luck,

Bruce

 

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[Pawan Singh]

Thanks Bruce.

What about the the "DEC" calibration image? It does not look as nice and clean as the RA. Is that because my DEC guide rate is set to 0.2x?

-Pawan

[bw_msgboard]

Hi Pawan.  The super-low Dec guide speed is probably complicating things.  It’s taking over 40 steps to do the Dec calibration, so there’s lots of opportunity for minor hiccups and statistical variations.  It can also be affected by uncorrected PE in RA and polar alignment error.  You should definitely boost that Dec guide speed and see if things improve – I expect they will.

[Andy Galasso]

Pawan,

You mentioned your pulse length is set to 20ms.  If you were referring to Pulse Width Override, you should definitely disable that. If referring to Minimum Pulse Width then yeah, you want that at the lowest value, 20ms.

Andy

[Pawan Singh]

Thanks guys for all the tips.

What is the ideal RA and DEC rate for PHD guiding? Does it depend on telescope focal length, camera, mount, etc... and one has to tinker with settings to find out?

-Pawan

[bw_msgboard]

Hi Pawan.  Most mounts will perform better with guide speeds in the range of 0.5x to 1x sidereal.  That doesn’t have much to do with PHD2 but it’s generally true unless there is some very specific reason to use a value below that range.  As I said, I don’t know why you ever ended up with a guide speed of 0.2x in Dec.  Once you have the speeds set in the mount, there should be no need to tinker with them.  Note, we’re talking about the *speed* at which the motors in the mount are running, not a calibrated rate of guiding.  The guide rates are computed by PHD2 automatically.  And as Andy said, you *never* want to use any of these funky EQASCOM tuning factors for guiding, they will just cause problems.

 

Good luck,

Bruce

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Pawan Singh
Sent: Sunday, December 11, 2016 9:56 PM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Calibration - RA and DEC rates differ

 

Thanks guys for all the tips.

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