PHD Guiding - How good is good enough??

[RayD]

Says to me your PA is out.  How are you doing your PA?

[Peje]

It was done using drift align about 6 months ago in PHD, perhaps it's worth redoing. Having said that, 1.5 arcmin doesn't sound like very much.

My DEC seemed impressively flat, I had expected it to drift like the RA

[RayD]

Worth checking again, it doesn't take long, and 6 months is a while without checking.

It looks fundamentally that your issues have been/are balancing.  You need to make sure you have the OTA balanced slightly East heavy (of course if you image past the Meridian you will need to touch the weight down a bit so it's then West heavy).  Also it has identified that you have a bit of backlash on the dec axis, and is suggesting you try guiding in dec in one direction only.  That means setting your OTA front heavy, then selecting guide on North only on the dec axis.  As with the RA it should prevent the oscillation caused by chasing.

I think you're not far away to be fair, and you need to also consider that seeing really does make a fair bit of difference, so I suspect it's only going to get better.

Once you get the full assistant to run you can just accept the settings it suggests, and it will automatically populate the relevant fields.

[Peje]

7 minutes ago, RayD said:

Worth checking again, it doesn't take long, and 6 months is a while without checking.

It looks fundamentally that your issues have been/are balancing.  You need to make sure you have the OTA balanced slightly East heavy (of course if you image past the Meridian you will need to touch the weight down a bit so it's then West heavy).  Also it has identified that you have a bit of backlash on the dec axis, and is suggesting you try guiding in dec in one direction only.  That means setting your OTA front heavy, then selecting guide on North only on the dec axis.  As with the RA it should prevent the oscillation caused by chasing.

I think you're not far away to be fair, and you need to also consider that seeing really does make a fair bit of difference, so I suspect it's only going to get better.

Once you get the full assistant to run you can just accept the settings it suggests, and it will automatically populate the relevant fields.

Sounds good, redoing PA will be handy enough but I'll wait for a night with good seeing.

In terms of unbalancing, how unbalnced should it be? If is disengage the clutch should it start to move? Currently it wouldn't move if it was sitting stationary but the unbalance can be see when you push it in both directions as it travels more in one.

[RayD]

Yes that should be fine. It doesn't need to be much, just enough to keep the OTA end pushing down as the mount is trying to push up.

i reckon with that and PA you'll be about there. With 0.6" RMS you're not far off already to be fair.

[Peje]

Thanks for all the advice. I was listening to my rig moving about earlier, my OTA certainly does make some noise...faint creaking and clunking and it moves around, not a good sign LOL

[SonnyE]

On 10/10/2016 at 14:54, Peje said:

Thanks for all the advice. I was listening to my rig moving about earlier, my OTA certainly does make some noise...faint creaking and clunking and it moves around, not a good sign LOL

Sounds like my back and knees...

Your graph looks to be a lot like what I get much of the time. And I have run some hour, and hour and a half one-shot images. (I hunt Nebulae as a rule, and have yet to be able to stack images. A Friend told me I just stack with time...)

I think the atmosphere is the trickier part to imaging. It is variable and can play hobs with your pictures.

[Peje]

Thanks Sonny. I've been watching nice tutorial on how to use the various settings so I'll have something to fiddle with, improving the PA will be first on the list followed by trying to guide in one RA direction only.

 

 

[SonnyE]

I tried something new to me last night. Instead of Arc Seconds, I switched to Pixels. (Left side under "history", Settings button)

I got the enviable very low flat line I've seen posted so often. Very little wondering or jagging. I felt like I was cheating.

So I tried changing it back to Arc-Seconds and there was the good old jags back.

My jury is still out as to if Pixels is any better than just old arc-seconds. Zooming in on Stellarium to see how well the sight was holding seemed to show it holding steadier. But I can't say it was because I had a rather unpleasant experience with Polar Alignment (PA) the night before. So was it Pixels? Or finally good PA?

If I find another hole tonight, I'll be back at it. As an aside, I use 0.5 seconds as my guide camera's exposure duration. I've tried long times, and I do use shorter times when trying to align at dusk. But for me, with my Orion 50mm SSAG guide scope, 0.5s seems to be a golden number for guiding with PHD2. I'm always interested in others trials and tribulations with PHD2. I look for any tid-bits I might absorb from the threads.

Best wishes for clear skies for you!

[Thalestris24]

Hi

I believe arc secs or pixels is just a display thing and won't have any bearing on your guiding performance or star shapes. Also, unless you have an oag, PHD2 is only reflecting what your guide scope sees. This isn't normally a problem since setups are capable of sub-pixel guiding. This is reflected in the PHD2 displayed rms errors. Ideally you want your imaging pixel scale to be compatible with the guide pixel scale. So normally you want bigger pixels on the imaging sensor and smaller ones on the guide sensor. 

Displaying guide data as arc secs gives more information than pixels or, at least, makes that data more accessible and visible.

Louise

[Thalestris24]

ps Craig Stark gives a good explanation as to how 'sub-pixel' guiding is theoretically possible. Of course, in practice, lots of other things come into play...

[SonnyE]

57 minutes ago, Thalestris24 said:

Hi

I believe arc secs or pixels is just a display thing and won't have any bearing on your guiding performance or star shapes. Also, unless you have an oag, PHD2 is only reflecting what your guide scope sees. This isn't normally a problem since setups are capable of sub-pixel guiding. This is reflected in the PHD2 displayed rms errors. Ideally you want your imaging pixel scale to be compatible with the guide pixel scale. So normally you want bigger pixels on the imaging sensor and smaller ones on the guide sensor. 

Displaying guide data as arc secs gives more information than pixels or, at least, makes that data more accessible and visible.

Louise

Hi Louise, Thanks!

I agree. I just hadn't ever tried the Pixel display as I had seen or read elsewhere that Arc-Seconds was more sensitive, so I had just stuck with that. I don't have an OAG as I use a smallish refractor (80mm). When starting out, I did try my very best to hang an OAG on my refractor, two in fact. Neither worked quite out. So I relented and took the obvious route for a refractor, a guide scope and camera. Lesson learned here. Don't try and reinvent the wheel.

Anyway, I wanted to try the Pixel setting so I could say I had. And the RA and Dec graphs truly ran sub-pixel in PHD2's graph. (Very sub pixel)

All I've been after is to be able to get decent images and after a year and a half now I think I might be getting somewhere. (My mount died twice, and I blew a focuser. But that's another story. )

I'm very interested in others experiences with PHD2, and often learn a little more with every post and thread I explore. As I have here. It's been an interesting learning curve to climb. And there seems to always be more for me to learn. And I try to share what I have found to work with my meager attempts and baseline equipment.

My night is approaching here, and I've reconfigured my telescope for a new look-see at the night skies on my side and spot of our spinning glob of dirt. So I beg to take leave now and yet again attempt to catch an image of something in our Universe.

Thank You for your help Louise! (Not just here, but everywhere.)

[Peje]

Finally got some free time last night so had another go at adjusting the PA to get it as close as possible. No azimuth ajustment required, fiddled with altitude and managed for get it from -3 to under -1. Altitude was a [removed word] to do, very sight adjustments make a huge difference so I think this is as good as I'll get. Going by some PHD tutorials +/-1 arcmin is as good as needed. Next on the hit list is PEC & figuring out how to guide the DEC (I think) in one direction only.