How’s my guiding looking

[dannybgoode]

In new to guiding and don’t quite understand what the PHD graph is telling me. 
 

Does this look ok. I get the odd quite large ‘lump’ every so often but then it settles back down again. 
 

At the moment I’m only doing a quick visual PA through the PA scope. I could use the SharpCap PA routine of it would improve things. 

[teoria_del_big_bang]

Following your thread with with interest as although I have been guiding for some while now I have sort of jut set PHD going and like you not really known what to expect.

Also there is a lot you can do to tune things in with PHD and again I am still getting to grips with it.

It may help to say what your setup is regarding the actual mount and guidescope/camera ets as I am guessing you should be able to get better results with different equipment.

With my limited knowledge I think generally looks very good and certainly a little better than I got with my HEQ5. As you say that lump on the Dec axis looks like a tight spot in the gearing that maybe just binds very slightly and so takes a fair few correction pulses then goes all of a sudden. Hopefully somebody with better knowledge will confirm this or suggest another cause but if you can improve that then to me looks good (please take with a pinch of salt though).

Steve 

[vlaiv]

That does not look particularly good - that large excursion in DEC. It looks odd - it could be due to wind (large scope?) or cable tug/snag - or something else.

It shows that you have backlash in DEC or calibration was not good enough. In fact cal is yellow in status bar - meaning that it would be best to re run calibration.  Fact that DEC is not responding after numerous guide commands means that it could be backlash, or it could be coupled with backlash (either guide pulses are too small or they are right but are clearing backlash instead of making corrections).

What mount are you using? If you want to get smoother looking graph - try using longer guide exposure.

[teoria_del_big_bang]

2 minutes ago, vlaiv said:

That does not look particularly good - that large excursion in DEC. It looks odd - it could be due to wind (large scope?) or cable tug/snag - or something else.

As I said I am also trying to understand these graphs, my thoughts were that the fact the error continues to climb for quite a time then goes all of a sudden would suggest some sort of binding in the gears, or never thought of it myself but cable drag also, do you think wind could also cause this?

I guess if it is binding or something with the gearing it will be cyclic and will repeat.

Steve

[dannybgoode]

@vlaiv it seems to have calmed down now so don’t know the cause. It has been windy but the scope is pretty sheltered. Again don’t think it’s cable snag because bar a long USB cable back to my laptop the only cables run from the guide cam and FW into the USB hub built into the main camera so it all moves as one unit. 
 

Mount is an SW AZ EQ6GT and graph has looked like this for around 45 mins now. Next session I’ll double check the cable routing and balance of the scope and give it another go. 
 

it doesn’t appear to be affecting the subs so I’m just letting it run tonight. 

[vlaiv]

4 minutes ago, teoria_del_big_bang said:

As I said I am also trying to understand these graphs, my thoughts were that the fact the error continues to climb for quite a time then goes all of a sudden would suggest some sort of binding in the gears, or never thought of it myself but cable drag also, do you think wind could also cause this?

I guess if it is binding or something with the gearing it will be cyclic and will repeat.

Steve

Binding is unlikely in DEC to cause such large offset. This is because DEC does not move - it stays still and only correction it needs are ones that are related to polar alignment error. If it were binding - it would have very small change and would remain in that position for some time while guiding tries to move it back - but here error is building up over some period of time and corrections don't help even slow it down, let alone reverse it.

Strong wind can act as pressure on OTA and if OTA is such that surface is not balanced - it will tend to turn in wind. This can happen with large scopes but also scopes that are quite heavy on one end and therefore are not "centered" over mount (like front or back of scope sticking out). Newtonians are prime candidate, and to some extent longer refractors. Compact scopes suffer less from this because arm momentum is small (they are compact after all).

Cable snag is also more likely with RA - as it happens when cable is tight and prevents further motion. In that case deviation in RA is also very likely - above graph is not showing that. Cable tug is sort of obvious - something or someone needs to pull on cables for that to happen.

There is one more thing that can happen to produce graph like above, but ota needs to be in very specific position for that to happen. Local thermals can cause guide star to bounce that much - like scope sight crossing above chimney or similar and OTA needs to be at certain angle so that main motion is in DEC and almost non in RA (graph does not show great deviations in RA).

[vlaiv]

2 minutes ago, dannybgoode said:

Mount is an SW AZ EQ6GT and graph has looked like this for around 45 mins now.

Hm, that is interesting - mount seems to respond really well to DEC commands now - in both directions like there is no backlash.

Have look at my previous post - one thing can cause such wild behavior and that is "heat jet" from local heat source, but it lasted really short time - it means it must have been far away and due south?

[dannybgoode]

6 minutes ago, vlaiv said:

Hm, that is interesting - mount seems to respond really well to DEC commands now - in both directions like there is no backlash.

Have look at my previous post - one thing can cause such wild behavior and that is "heat jet" from local heat source, but it lasted really short time - it means it must have been far away and due south?

 could be a periodic error that’s going to take a while to come back round perhaps? 
 

will keep an eye on it over the next few sessions and see if I can nail down the cause. 
 

thanks for your responses - as always very informative   

[vlaiv]

6 minutes ago, dannybgoode said:

 could be a periodic error that’s going to take a while to come back round perhaps? 
 

will keep an eye on it over the next few sessions and see if I can nail down the cause. 
 

thanks for your responses - as always very informative   

It was error in DEC. Dec should simply stay still - it should not move, so no binding, no periodic error, nothing like that. It needs some sort of external thing to throw it off correct position - either move scope / mount or move guide star (poor seeing can make star appear to be displaced).

Other than that - DEC should change very slowly due to PA error - something like one correction every half a minute or minute. If you look at your recent graph - there are 5 corrections in one direction and 8 corrections in opposite direction - let's say that 5 of those "cancel out" (just seeing stuff) - only 3 are "true corrections" and display shows something like couple of minutes worth of data. PA error simply can't be cause of such large excursion in position.

[teoria_del_big_bang]

10 minutes ago, vlaiv said:

It was error in DEC. Dec should simply stay still - it should not move, so no binding, no periodic error, nothing like that. It needs some sort of external thing to throw it off correct position - either move scope / mount or move guide star (poor seeing can make star appear to be displaced).

Other than that - DEC should change very slowly due to PA error - something like one correction every half a minute or minute. If you look at your recent graph - there are 5 corrections in one direction and 8 corrections in opposite direction - let's say that 5 of those "cancel out" (just seeing stuff) - only 3 are "true corrections" and display shows something like couple of minutes worth of data. PA error simply can't be cause of such large excursion in position.

Yes I see this now, and now you have pointed this out it is pretty obvious. Also really interesting about how the heat sources can affect it as well. I guess these could be something as simple as a flue from a central heating boiler, although I guess the condensing vapour from those will show up on the subs as well so will be more obvious, but there are plenty of other heat sources.

Steve

[vlaiv]

4 minutes ago, teoria_del_big_bang said:

Yes I see this now, and now you have pointed this out it is pretty obvious. Also really interesting about how the heat sources can affect it as well. I guess these could be something as simple as a flue from a central heating boiler, although I guess the condensing vapour from those will show up on the subs as well so will be more obvious, but there are plenty of other heat sources.

Steve

Effect is very dramatic. I was observing Jupiter couple of years ago in winter time from my back yard with tracking mount (it was simple EQ2 with DC motor, but it did the job) and as Jupiter was climbing in the sky from the east in one instant it came right above chimney exhaust from a house 50-60 meters away - image instantly turned into something similar to seeing thru the jet engine exhaust plumes - like in this video:

https://youtu.be/XEpXh3vqdxI?t=75

At 1:10 - look at background scenery and how it is impacted by "poor seeing" caused by jet exhaust.

[dannybgoode]

@vlaiv quick thought on this. Could it be that might guide scope had not fully cooled? I set everything up and started imaging quite quickly and it seemed to happen earlier on in the session. Would tube air thermal’s be enough?  
 

Hopefully the weather will stay clear today and I’ll be out again tonight and see if I can improve matters

[wimvb]

On 15/01/2020 at 22:47, dannybgoode said:

@vlaiv it seems to have calmed down now so don’t know the cause. It has been windy but the scope is pretty sheltered. Again don’t think it’s cable snag because bar a long USB cable back to my laptop the only cables run from the guide cam and FW into the USB hub built into the main camera so it all moves as one unit. 
 

Mount is an SW AZ EQ6GT and graph has looked like this for around 45 mins now. Next session I’ll double check the cable routing and balance of the scope and give it another go. 
 

it doesn’t appear to be affecting the subs so I’m just letting it run tonight. 

That looks very good. 

With guiding problems, start here

https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

3 hours ago, dannybgoode said:

@vlaiv quick thought on this. Could it be that might guide scope had not fully cooled? I set everything up and started imaging quite quickly and it seemed to happen earlier on in the session. Would tube air thermal’s be enough?  
 

Hopefully the weather will stay clear today and I’ll be out again tonight and see if I can improve matters

Guide scopes are generally smaller than imaging scopes, so they should cool down fast enough. Unless you use a 6" apo as guidescope, of course. 😀

[vlaiv]

3 hours ago, dannybgoode said:

Could it be that might guide scope had not fully cooled?

Like @wimvb said, unlikely - small finder will cool down fast, and I believe that tube currents will cause effects similar to astigmatism - which will cause soft image for planetary, rather than causing guide star to jump around.

[dannybgoode]

Thanks both. Just ruling stuff out. Beautiful day here so going to get set up when I get home.
 

Usually get set up in the dark so will be able to spend a bit of time getting everything just so and then get some flats done before hopefully another imaging session tonight