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DEC guiding problem using PHD

alcol620

By alcol620
in Discussions - Software

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alcol620

alcol620

Posted
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Hi folks

Out tonight and using PHD guiding for only the second time. I must have been lucky at the first attempt as everything went fine and following calibration the guidescope followed the star and I managed a 5 minute image of the Orion Nebula before cloud rolled in. I simply polar aligned with the Jason Day chart - no drift alignment carried out.

Tonight I followed the same routine with a new PHD calibration and the software began guiding. However, the RA results on the displayed PHD graph looked to settle down to a reasonably accurate guide, but DEC was off the scale and I kept being asked to increase the DEC Max duration to enable DEC guiding.

Can someone advise what the problem is. My only guess is that my polar alignment was way off.

Advice appreciated. thanks

Alec

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Thalestris24

Thalestris24

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Hi

Hard to say... Most likely cause is a balance issue or cables catching/dragging. I guess you don't have a screen capture of your session? Always useful... Failing that, a log file loaded into PHDLab tells you a lot about what might be going wrong: http://countingoldphotons.com/phdlab-intro/

Failing that also, then next time a screen capture and log analysis would help.

Louise

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alcol620

alcol620

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Hi Louise.

Thanks. I was hoping it was something obvious. There were a lot of cables about!  I must admit that PHA etc is all at the edge of my competence. I think I was recording a log file of what was happening. I haven't had time to look for these, but hopefully later today I will do so.

Regards

Alec

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alcol620

alcol620

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Hi folks

I have attached the PHD debug file for last evening. A few lines are recognisable, but it doesn't mean much to me. If anyone can shed light, excuse the pun, on the reason that the software kept asking for the DEC Maximum duration to be increased, I would be grateful.

Thanks

Alec

PHD2_DebugLog_2015-02-21_184751.txt

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Thalestris24

Thalestris24

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Hi Alec

Um rather than the debug log it's the guide log that contains the useful info. Next time if you can enable the logging it will save a guide log. You can leave it in this mode as guide logs don't take up much space in the whole scheme of things. You can disable debug logging though. Here's a handy guide http://www.astrophotography.id.au/Downloads/PHD/Tools.htm 
It would be useful if you could post a screen capture of your PHD settings (click on Brain icon). What guide scope are you using?

Louise

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Thalestris24

Thalestris24

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Hi Alec

First of all, it seems you're actually using PHD2 which is ok. Now, is that the 50mm mini-guider with a focal length of 162mm? If so, I think you might be up against it using that to guide a scope with a fl of over 2 metres... You might get away with it!

Here is the PHDLab screen:

post-33532-0-39369600-1424630245_thumb.p

Looking at the calibration sections with a text editor it looks like you didn't actually calibrate successfully.

PHDLab does suggest a very large PA error so that might be one problem but if you're not controlling the mount in the first place, you need to address that first. I take it you had 'enable guide output' set in the Brain/Mount tab?

Perhaps you could take us through exactly what you did. How is PHD2 connected to the mount? Are you using eqmod? Are you using pulse guiding?

I had a look at the first part of the log file - lots of your settings seem wrong!

Calibration Begins at 2015-02-21 19:09:43
Equipment Profile = My Equipment
Camera = ZWO ASI120MC
Exposure = 3000 ms
Pixel scale = 5.56 arc-sec/px, Focal length = 139 mm
Mount = On Camera, Calibration Step = 380 ms, Assume orthogonal axes = no

I believe the ZWO has 3.75 micron pixels = 3.8 for PHD2. As far as I can establish your guide scope has a focal length of 162mm. There seems to be a zero missed off your calibration step size! If you use the calculate option under the Mount tab, that should give you a value which is a good starting point.

So... first make sure all your settings are correct. Then make sure PHD2 is controlling your mount.

Hope that helps

Louise

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alcol620

alcol620

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Hi Louise

Many thanks for taking time out for this.

Yes focal length of the scope is 2032mm, I have this reduced with a f6.3 reducer to get it down to 1280mm.

It is the 50mm mini guider with a 162mm focal length, but this has had a spacer taken out reducing it to 139mm. Without this reduction it was not possible to bring the ZWO into focus.

Not sure if I turned on 'enable guide output'! But it did seem to start guiding after calibration completed, at least on 1 occasion.

The ZWO as you say has a pixel size of 3.75.  I didn't realise it was possible to calculate a calibration step size. presumably it does this based on the parameters given to the program?

PHD2 is connected to the mount via the ZWO which is the guidecamera.

The strange thing is, when I used this set up for the first time a few days ago, although it took a while to calibrate and I lost the star on a couple of occasions, the graph seemed to indicate that it was guiding within reasonably limits - not that I has any experience of what reasonable limits are. Indeed as a check I took a 300s image of the Orion Nebula with little or no star trails and I took this to mean that everything was fine. Now a few days later something is amiss. 

I have attached the guide log of what I thought was my successful guide. The 300s Orion was taken during the last guide on the camera before losing the star - not for the first time!

Many thanks for looking

Best regards

Alec

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Thalestris24

Thalestris24

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Hi again

From the log you provided PHD2 didn't calibrate. This may have been because the calibration step size was much to small. You can do the step size calculation whilst 'off line'. It asks you to enter the needed parameters. You want to aim for maybe around 15 steps. I can't say exactly but you probably need a step size of around 1200 - 1500. I use 1500 with my 170mm finder guider and qhy5l-II mono which has the same sensor as your ZWO. Your focal length is the distance between the objective and the ZWO sensor which won't be the same as the length of the tube.

Hope that helps

Louise

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alcol620

alcol620

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Thanks again Louise

It did go into guiding mode as the graph illustrated the movements and these seemed to be held reasonably close to the x axis. Must have been lucky to have got the 300s sub with no star trailing.

Just put the data into the calculator and it came up with a calibration setting of 650, it seems that I had the guide speed rate set at 1. if I reduce this to 0.5, the calibration figure will double to 1300. I will check out the focal length of the guider plus camera. It is possible that this will increase slightly as the sensor is set back slightly in the camera housing.

Thanks again for your help, much appreciated

Alec

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Thalestris24

Thalestris24

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Ah yes, I have my guide speed set to 0.3 in eqmod! :)

Yes, your PHD2 went into guiding mode but the calibration didn't go as one would expect. There were 21 backlash corrections! You can see for yourself with a text editor. When guiding, you lost the guide star on a number of occasions - must have been cloud?:

67,249.831,"Mount",9.580,4.852,-0.776,-10.735,-0.472,-10.735,339,E,800,N,,,13805,4.21,0
68,253.062,"DROP",,,,,,,,,,,,,3162,8.69,5,"Star lost - mass changed"
69,256.552,"DROP",,,,,,,,,,,,,792,9.86,5,"Star lost - mass changed"
70,260.009,"DROP",,,,,,,,,,,,,652,9.65,5,"Star lost - mass changed"
71,263.465,"DROP",,,,,,,,,,,,,828,10.96,5,"Star lost - mass changed"
72,266.938,"DROP",,,,,,,,,,,,,808,10.43,5,"Star lost - mass changed"
73,270.430,"DROP",,,,,,,,,,,,,565,7.48,5,"Star lost - mass changed"
74,275.461,"Mount",-2.197,17.534,16.160,-6.364,10.148,-6.364,680,W,800,N,,,630,10.32,0
75,278.633,"DROP",,,,,,,,,,,,,14591,5.16,5,"Star lost - mass changed"
76,282.097,"DROP",,,,,,,,,,,,,14408,5.14,5,"Star lost - mass changed"
77,285.682,"DROP",,,,,,,,,,,,,15433,5.04,5,"Star lost - mass changed"
78,289.233,"DROP",,,,,,,,,,,,,14583,5.06,5,"Star lost - mass changed"
79,292.690,"DROP",,,,,,,,,,,,,14667,5.39,5,"Star lost - mass changed"
80,297.669,"Mount",-6.617,26.860,26.431,-6.885,17.362,-6.885,680,W,800,N,,,14792,5.46,0
81,302.419,"Mount",-6.392,26.595,26.087,-6.958,17.650,-6.958,680,W,800,N,,,12460,4.30,0
82,305.591,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
83,309.041,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
84,312.520,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
85,315.998,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
86,319.475,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
87,322.896,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
88,326.442,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
89,329.974,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
90,333.465,"DROP",,,,,,,,,,,,,52,3.09,5,"Star lost - mass changed"
91,337.066,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
92,340.600,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
93,344.063,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
94,347.578,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
95,351.078,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
96,354.575,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"
97,358.034,"DROP",,,,,,,,,,,,,0,0.00,2,"Star lost - low SNR"


Do you have the star profile displayed? It's very useful to have this so you can keep an eye on the guide star. Keep an eye on the snr - it should be in double figures.

Louise

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alcol620

alcol620

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Hi Louise

I seem to have made a fundamental error in thinking that by taking out the spacer on the mini guider that I was reducing the focal length of the guider. What I was doing was moving the focal point of the guider. The focal length staying the same at 162mm. Can you confirm that this is indeed correct??

next time I run the software I will change the calibration setting based on a focal length of 162mm and a guidespeed of 0.5

One further question do you have the tracking on sidereal or PEC = sidereal?

Alec

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Thalestris24

Thalestris24

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Hi Alec

Yeah, as I said above, the focal length is the distance between the objective and the plane of the sensor (when in focus, of course!). It's determined by the objective lens but varies slightly according to how far away an object is but normally that will be at infinity. So the value given by the manufacturer is probably correct :)

Louise

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alcol620

alcol620

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Hi Louise

Just came in from the garden, defeated by the wind and cloud. Not a lot of images for processing but a lot of experience gained. Slow start, as it took me a while to work out why I was getting such noisy images from the guider and couldn't get PHD to run. It turned out that I hadn't achieved focus with the guider. Once this was solved I was able, after a few false starts, to get PHD to carry out a calibration. It did then go on to guide. The wind was gusting quite strongly at the end and it was almost impossible to get images without star trails, almost certainly due to the movement of the gear.

However, taking the suggestion you made, and now I have got the hang of it, I am pretty certain with better conditions, PHD will work fine. 

One last question. If after a calibration and imaging the scope is moved to another part of the sky, does another calibration have to be carried out or can PHD simply be put into guide mode?

Thanks again for all your help.

Best regards

Alec

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Thalestris24

Thalestris24

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Hi Alec

Sounds like you're making progress! :) PHD2 lets you image different parts of the sky without recalibrating. However, there are settings for when you might do a meridian flip (Brain/mount tab) - see Meridian flip in the PHD2 Help.

Sorry you had a problem with the wind. The bad weather means there's little chance of me attempting anything for at least the rest of this week :( Sigh.

Good luck next time!

Louise

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