First attempt at PhD guiding - what is this showing me?

[AlistairHowie]

As the clouds were in yet again tonight, I thought I'd try my hand at the dark art of guiding using Star Guiding Emulator. I set my scope up in the kitchem and had SGE running on a separate laptop positioned 5m away down the hallway!

I pointed the North leg of my AZ-EQ6-GT straight at the screen and rotated the RA(?) axis so the counterweight bar was parallel to the ground and switched on sidereal tracking. Using SGE I had a "star" move horizontally across my laptop at the end of the hall from the left side of the screen to the right.

I used PhD guiding, successfully got it to calibrate my mount, and then apparently track the moving pixel all the way across the screen.

I got the below graph .... and haven't really got a clue what it's telling me!

The red line looks good (I have no clue what the y axis scale is), but the blue lines looks to be a concern. But then again, I don't know if this was a decent test of the system. The pixel "star" was moving horizontally across the screen, not in an arc as a star would, so I'm guessing that might have messed up the results somewhat. I didn't change any of the default settings in PhD and as I said, my mount was doing sidereal tracking, so I don't know if PhD was battling against that sidereal tracking. You can ignore the end of the trace where the blue line goes vertical - that is where it lost the "guide star" as it went off my laptop screen.

All thoughts gratefully received. I'm hoping that PhD guiding really is as easy as it seems ... although nothing else in my AP journey has been so far!

[Jannis]

I'm far away from experienced in PHD, but as far as the graph goes:

Ideally you shuold see two stright lines similar to the red line. That means you have so good tracking and alignment to begin with that PHD don't need to correct nearly anything.

Your red line is DEC axis correction, and blue is the RX axis correction. What your graph is showing you here seems to be that your mount is tracking the target more or less correctly in DEC axis, but the RX axis is needing a Lot of correction to keep target in position.

Now, basically, this doesn't mean anything else then that your system is working as intended, and you shuold ignore how the graph looks like for now untill you get outside and actually try it on a star after polar aligning.

[AlistairHowie]

I think I need to look again at the SGE instructions again. I thought it was meant to be a bit more helpful than this. Hmmm ....

[AlistairHowie]

OK ... I think I may have got my set-up a little wrong. I looked again at the instructions and among other things, they say .... "The movement of the star being exempt of any PE you can test then correct your PEC in RA .... Place your telescope at a few meters from the PC screen and adjust the mount direction in order that its North axis is precisely towards the computer screen. - [in set-up] DEC movement in pixel/min (positive value moves upwards, negative value moves downwards), default is 0."

So I just pointed the north leg of my mount at the PC screen then rotated the (RA?) axis so my counterweight bar was horizontal and my guide scope was pointing parallel to the ground and straight at the PC. I'm now wondering whether those instructions mean I should basically be "polar aligning" with my laptop screen, which I'd need to raise up high near the ceiling?

If I did that, would it work then?

Still confused and since it's snowing outside just now, I'm really keen to try as much of this out in the warmth of my house!

[Merlin66]

I think what you're doing is OK

Just point the polar axis direction towards the screen.

SGE is just to allow to verify that the guide camera is seeing the target star, and that the connections between the camera/ software and mount all work as intended.

I don't think I even worried about the shape of the PHD graph!

I was more interested in being able to get all the bits to "talk" correctly and work properly.

Sounds like you're ready to set-up and use your guider under real stars.....

Hope this helps.

[AlistairHowie]

Many thanks. I've obviously been reading too much into the capabilities of SGE! Next stop real skies ... once the snow clears!

[cp255]

It looks like your scope got knocked off course somehow, and is trying to recover. The only problem is backlash, and the amount PHD will step for each guiding iteration. Try increasing the max RA to 2000 and increase the frequency PHD takes pictures. This will help it recover faster. I have this problem with my DEC axis some times. What sometimes fixes the problem is making one side of the axis heavier than the other. Since your problem is with RA you can try moving your counter weights slightly further out on the rod.

[Jannis]

It looks like your scope got knocked off course somehow, and is trying to recover. The only problem is backlash, and the amount PHD will step for each guiding iteration. Try increasing the max RA to 2000 and increase the frequency PHD takes pictures. This will help it recover faster. I have this problem with my DEC axis some times. What sometimes fixes the problem is making one side of the axis heavier than the other. Since your problem is with RA you can try moving your counter weights slightly further out on the rod.

This is a good tip, but you Did read that this wasn't an actual star test, but a fake "star" moving accross the computer screen only several meters away from the scope that he set up only to see if PHD was working properly...?

[Freddie]

Good tip by CP255 re balance but you should move your weights to make the balance bar slightly heavy to the east. This will obviously only be out towards the end of the bar when the scope is orientated in that way. Obviously to go east heavy, you will sometimes need to move the weights UP the bar.

[AlistairHowie]

Well at long last I had a clear night tonight so I actually managed to try out PhD on a real star, and the result is attached. No real idea how to interpret it, but I think the fact that the deviations are small is probably good news!

This was the only thing that went right tonight. Artemis Capture and PhD kept crashing, my Titan and Atik 4000 were recognised, then not recognised, recognised again, then the Titan disappeared and my options were to connect to Artemis HSC instead - no idea what that is - which didn't work. I was raking draw tubes in and out all night trying to come to focus, and playing about with binning, sub frames, long and short exposures to try and get some stars on the sensor. Finally after several reboots and the stars disappearing behind trees and buildings I decided to give up. Oh well, no-one ever said this imaging lark would be easy.

If someone can confirm that my guiding looks ok (can you tell anything about the accuracy of my polar alignment with the guide trace?), that will at least let me salvage something from the evening's debacle and I can play about trying to connect things up during daylight hours in the warmth of my house.

[Brent]

That's a really good graph! Many of us only dream of a graph like that! I really wouldn't worry about it now if you can reproduce it reliably

Oh, and your PA looks pretty darn good as well.

[AlistairHowie]

Oh well, good to know I've salvaged something from the evening! Let's see if it's replicable ... another month to wait for that!

Can someone explain, if my PA is 100% perfect, would the guiding corrections only be in RA or DEC? Is that how you can see how good PA is, do you just look at RA or DEC alone?

[Brent]

Your DEC is quite flat but has slight corrections which means your PA is a little bit off. If it was perfectly flat then, yes, you could switch DEC guiding off and just guide in RA.

However, it's quite unlikely that you'd be able to get it that perfect, especially if you don't have a permanent setup.

[AlistairHowie]

Thanks.

[swag72]

The guide graph looks good Alistair - But don't become a slave to it!! I've wasted many hours in the past, looking at the guide graph bouncing about and trying to get it nice and flat prior starting gathering data. If only I'd looked at the subs first, I would have noticed that they were fine with the graph bouncing.

Next time out, just get a sub under your belt with a guide graph like this and see what the stars look like - I'm sure they'll be fine, especially with the focal length of the Tak and then you've saved yourself PA worries!!

[giorgio_ne]

I think this is the first PHD graph I've seen from an AZEQ6-GT. If quality control is consistent, this mount will be a serious player in its price/capacity bracket!

[chrisshillito]

Can someone explain, if my PA is 100% perfect, would the guiding corrections only be in RA or DEC? Is that how you can see how good PA is, do you just look at RA or DEC alone?

If your PA were perfect, and your mount perfectly level, and your guidscope perfcectly orthogonal to the mount, and your guide camera sensor is pefectly square to the guidescope, and there is no flexure, and you mount had zero PE,and there are no atmospheric or optical distortions, then no guiding corrections would be required. For any other case you could get RA and DEC corrections.

Poor PA will result in drift in either axis. Which is more significant will vary depending on where you are pointing.

Chris.

[AlistairHowie]

The guide graph looks good Alistair - But don't become a slave to it!! I've wasted many hours in the past, looking at the guide graph bouncing about and trying to get it nice and flat prior starting gathering data. If only I'd looked at the subs first, I would have noticed that they were fine with the graph bouncing.

Next time out, just get a sub under your belt with a guide graph like this and see what the stars look like - I'm sure they'll be fine, especially with the focal length of the Tak and then you've saved yourself PA worries!!

Thanks, Sara. Trust me ... if I ever get my software and camera's talking to each other properly without having to reboot, restart, unplug, etc, all the time I will be sure to try and get a sub!!

I think this is the first PHD graph I've seen from an AZEQ6-GT. If quality control is consistent, this mount will be a serious player in its price/capacity bracket!

I probably wouldn't read too much into the results I got last night. My mount is hardly stressed - it's got less than 8kg weight on it when set-up for imaging, and the specs suggest its capacity is 18kg for imaging and 25kg for visual. While it was balanced in RA, it wasn't in DEC since with my Titan and guide scope off to one side, and with my rings and plate set-up, I can't balance it without changing things around a fair bit. See here ... http://stargazerslou...asymmetric-ota/ ... but if last night is typical then it seems that I don't need to worry so much about DEC balance, as long as it's East heavy. Which makes me happy!

If your PA were perfect, and your mount perfectly level, and your guidscope perfcectly orthogonal to the mount, and your guide camera sensor is pefectly square to the guidescope, and there is no flexure, and you mount had zero PE,and there are no atmospheric or optical distortions, then no guiding corrections would be required.

Thanks, Chris. Something to aspire to next time I'm out