Is This Good Guiding

[Terry]

Can anyone please help?  I am fairly new to astrophotography and PHD2.  My equipment seems to being working quiet well and my guiding seems okay, as far as I can tell.  But my question is, how can I really tell?  I look at my guide logs and I don’t know if the results are good or not.  I have attached last night’s guide log and Guide Assistant results and I wondered if someone, who understands these things, could give me some feedback.  I have also attached a single 7 minute sub of the Horsehead Nebula.  When I zoom in the stars start to get a bit oval.  Is this normal?  The seeing was not particularly good with some high wispy cloud.  Any advice would be greatly appreciated.

 

Many thanks, Terry.

[Bruce Waddington]

With a fairly brief look at the guide log, I'd say there's room for improvement.  You should use the PHDLogViewer tool to examine the guide logs and get the corresponding statistics.  Just as a crude benchmark, you should perhaps strive for 1 arc-sec total RMS in guiding unless you are dealing with bad seeing or have a very inexpensive mount.  That's probably a reasonable target if you are new to the hobby and are trying to figure things out.  You got 1.7 arc-sec RMS in the longer guiding session.  Since you didn't attach to the mount with an ASCOM driver, I don't know what mount you're using.  If you look at the guide log with PHDLogViewer (see http://openphdguiding.org/) you'll probably see a couple of things that are limiting your performance: 1) substantial, spontaneous deflections in RA of several arc-secs and 2) extended periods where the star has shifted in the Dec direction and the mount is not responding to the corresponding Dec corrections.  These are most likely due to limitations in your mechanical set-up, not settings in PHD2.  That's never what people want to hear, but it is usually the case.  If you are mechanically inclined at all or can get some local help, you may be able to improve the mechanical performance of the mount.  Otherwise, you'll have to work around the problems you can and live with the rest.  If you upgrade to the latest development build, you'll be able to measure the declination backlash in your mount with the updated Guiding Assistant.  That's a common source of problems with commodity mounts and may relate to the sluggish behavior you're getting with Dec corrections.

 

In the end, the guiding performance only needs to be good enough that you're satisfied with your imaging results.  For most of us, imaging is a continuous process of improvement (and frustration) as we identify and eliminate one limiting factor only to move on to the next one.  It is, without a doubt, an incredibly difficult hobby.  I say that as I'm sitting in my warming room with the dome closed up in the face of 25 mph wind gusts. :-( 

 

Good luck.

Bruce

 

 

 

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[Andy Galasso]

Terry,

Welcome to PHD2, you are off to a great start.

In addition to Bruce's excellent advice, I would also recommend using PHD2 Log Viewer to look at a guide log section collected during a run of the Guiding Assistant.  The reason I bring that up is that there appears to be some RA oscillation occurring causing your RA elongation. Here's a plot from PHD2 Log Viewer of a small section of the log:

The blue lines are RA offsets, and the red lines are Dec. You can see RA swinging back and forth. You may be able to improve this by decreasing your RA Aggressiveness setting. If you reduce it too much though, PHD2 will not be able to keep up with your RA periodic error. If you look at the guide log for a run of the Guiding Assistant you will be able to see how much RA periodic error you have, and in particular how bad the steepest sections of the PE curve are. Seeing that can help you gauge how aggressive your RA guiding needs to be to keep up with RA PE.  If your RA curve is very smooth, you can really back off the aggressiveness to get rid of the oscillation.  Seeing the RA curve from a run of the Guiding Assistant may also suggest whether your mount needs a tune-up to smooth its RA tracking.

Andy

[Terry]

 Hi Bruce and Andy,

 

Thanks for the help.  You are correct about me not using ASCOM.  I use the On Camera option in PHD2.  Is this okay?  Or would it be better to use ASCOM?  The mount I use is a Takahashi EM200.  Do you know where I can get some information about how to adjust the backlash on this type of mount?

 

Thanks again for you help, Terry.

[Bruce Waddington]

Hi Terry.  There's nothing wrong with using the on-camera guiding option in terms of guiding performance - assuming the cable is functioning (which it is in your case).  The shortcoming is that it gives us much less information in the log files to help you.  We don't know guide speeds or where the scope is pointing, stuff like that.  And it means you will have to keep doing calibrations every time you point to a different target.  If you want to continue using the guide camera interface, you should at least establish an 'Aux' mount connection in PHD2 - see the help docs for how to do that.  Now that you've said you're using a Takahashi mount, I would say you can definitely expect to get better performance - those are generally excellent mounts.  But we still don't know much about what you're trying to do in terms of what scopes are on the mount and how heavily loaded it is.  As for making any adjustments, you'll have to rely on advice from others who have the same mount, I don't think Andy or I have any familiarity with it.  You may find other web forums that can help you more.  

 

Cheers,

Bruce

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Terry

Sent: Saturday, November 07, 2015 2:34 AM
To: Open PHD Guiding

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[Terry]

Hi Bruce and Andy,

Thanks for the advice.  I have just had an idea.  The cable from the RS-232 port on the mount to the USB port on the computer is rather long.  It is a bit over four meters which I thought might be a little too long for a serial cable.  Is it possible that this could be affecting the RA guiding?

Many thanks, Terry.

[bw_msgboard]

I wouldn’t think so if it is really a serial cable.  According to the standard, such cables should be ok up to 15 meters.  I know I’ve used some very long ones in other contexts. Of course, they can be damaged – I’m famous for running chair wheels over them, which is not a good plan. <g>  Since they’re inexpensive, you could certainly replace the cable just to rule it out as a source of the problem.

 

Could you give us more info about what scopes and cameras you’re using and how much total weight you’ve got on the mount?  Have you programmed a periodic error correction in the mount?

[Terry]

Hi Bruce,

 

Thanks for taking an interest.

 

My equipment is as follows:

 

Takahashi EM-200 Temma 2 Jr mount

Explore Scientific 127mm f/7.5 ED APO Refractor

SBIG STF-8300 Mono camera with SBIG CFW8 filter wheel

Orion 80 Guidescope and Starshoot camera

 

Weight about 12.5 kilograms in total

 

As for the “periodic error correction”, that’s new to me.  There is no mention of this in the Takahashi manual.  Is this important?  How would I programme this?

 

Thanks for your help, it’s most appreciated.

 

Terry.

[Terry]

Hi Andy,

 

I am about to install the latest PHD2 Snapshot v2.5.0dev7.  Can you please tell me if I need to uninstall the earlier version of PHD2 first.

 

Many thanks, Terry.

On Saturday, 7 November 2015 19:06:59 UTC+13, Andy Galasso wrote:

[Andy Galasso]

Terry,

Un-installing the prior version is not necessary.  If you do un-install PHD2 it will remove all your PHD2 settings and profiles, so do not uninstall phd2 if you want to avoid that.

Andy