Guiding problem

[Thommy]

Hi,

I hope you have some suggestions for me. The problem is that my guiding in RA is quite poor. Not very bad, but there are some spikes in the graph (see the attached graph). The decl. is much better. Some information: I use PHD ver. 1.14 and the pixelscale is 1.9"/px (same as for my imaging setup). The main scope and the guide scope are mounted side by side. Everything is balanced carefully in decl, but there is a slight overweight to the east in RA.  

Thanks,

/Thommy

[Tinker1947]

You could try to lengthen the time#ing from 1.5 to around 3, that about the limit of my PHD Knowledge......

[Leveye]

Try starting from the beginning. Check everything over again and basically the mount it to your true polar position and not magnetic then carefully leveling it.  Then polar align. Then balance everything very carefully. It all ties together. If you do this a few times and still have issues it could be the gears and some backlash. Periodic Error perhaps.

[Adamar98]

I've stopped paying attention to the PHD graph. It tells fibs! My graph would be all over the place but my subs would be spot on. Don't know if anybody else has this?

[Owmuchonomy]

As Adam says, looking at the trace is one thing but what are your subs saying? It's also hard to comment on your RA trace without knowing what scale it is running at? The RA trace doesn't look any worse than mine on a normal night's imaging. Your OSC looks almost exactly where it should be 0.3 ish.

[Thommy]

Thanks for your replies! The scale is 1.9"/px which is the same, incidently, as for the imaging setup. I think you're right, it probably doesn't look that bad. I guess I was unhappy with the graph, because I have seen people presenting very smooth graphs. Anyway, my subs show slightly elongated stars quite often, but it seems that I have problems with flex as well, so that could be the explanation.

/Thommy

[jsmoraes]

As told, the graph is important, but you must know how to use those infomations.

See, your graph shows 0.1 pixels maximum of drift. What is the scale for your guiding camera ? And what is for your imaging camera ?

Let me explain: My set, the imaging camera has 0.65 arcsec/px. My guinding camera (ASI120MC) has a bit little: 0,45 arcsec/px. So my graph can shows 1 arcsec of drift and the imaging camera (Canon 1100D) will not see it.

The distortion of refraction will be more intense than this 1 arcsec.

If I use photos with the full resolution of Canon 1100D, I will perceive any drift only if my graph shows 2 arcsec or more. With it I will have 1 pixel of drift error. With 4272 x 2848 pixels ... it is possible that I see something.

If I reduce the image to publish it here, the stars will appear as more than round star. Even though I have graph with 2 arcsec of drift.

[Thommy]

Am I misunderstanding something here ... I thought the dotted horisontal lines indicated one pixel on the guide camera? If this is right, I would have a maximum drift of 1 pixel, right? As the scale for my guiding setup is 1,9"/pixel this would then be roughly 2" drift. The scale for my imaging setup is the same.

Last night my graph was much smoother - I didn't change anything, though, and the object imaged was the same as before. Something weird happened - a plane crossed the frame and one of the lights on one wing passed exactly over my guiding star making PHD go for the plane instead of the star

[johnrt]

What mount and what imaging / guiding scope are you using?

[Thommy]

The mount is a CGEM and the imaging/guiding telescopes are mounted side by side using an ADM bar. The imaging telescope is a 8" F4.5 Newton and the guide telescope is a 80mm F7 ED refractor. The guide camera is a QHY5 and the imaging camera is a modified MX7 with a Sony ICX 829 chip.

[jsmoraes]

I thought the dotted horisontal lines indicated one pixel on the guide camera? If this is right, I would have a maximum drift of 1 pixel, right?

No, taking account my experience. The horizontal lines indicate the min. motion set for RA.

By now I am using PHD2 that shows drift in arcsec. You can try it. And  In PHD2 you can set min. motion for DEC, too.

Actually, PHD is always registering the drift, but the pulse correction is only generated if it cross the min. motion set.

You were having only 0.1 pixel of drift. But attention: if your pixel in guinding camera correspond to 10 pixels in imagin camera ... you were having 1 pixel in imaging camera.

The most important is this relation: how amount in guiding camera is translated to imaging camera.

[Thommy]

 Ahh, I see. But to me it seems that corrections are made even within the first set of horisontal lines. Otherwise I agree with your arguments.

[jsmoraes]

yes, and they are made, but the values are so small that you haven't action of your mounting. What I am sure is that, at least with my set of equipments, up to the first horizontal line I never perceive any action. But if the drift take that value or cross it... you can hear the mount changing speed.

You can see it better with DEC, since RA has much movement. The pulse correction will happens always the drift is over the first line. I use EQMOD platform, and this is very visible in the graphic of EQMOD.

Maybe I'm not absolutly correct in my interpretation. But, even with your version of PHD, I always used the first line as limit for drift. I work with OAG, and my scale is very short: 0,45". The refraction cause much false drift. always under that first line.
 

An example of my PHD graph with your version. Note that to see something in EQMOD, I needed to move the slider of graphic to its top.

And when I am testing the OAG together PHD look the drift with 900 sec - ISO 100 in photo with full resolution of the Canon. Target: Eta Carina.