PHD Guiding - "star did not move enough"

[Alveprinsen]

Whats the deal with the "star did not move enough" error when using PHD guiding.

Is my polar alignment too spot on, or what?

right now my rig is down at the observatory imaging NGC7023 through a 200mm Sigma DSLR lens, guiding with a 500mm Skywatcher refractor.

The mount is a NEQ6 Pro, the guide cam is an Orion Starshoot Autoguider.

I have set the camera to run:

15x 150 sec

15x 300 sec

18x 600 sec

exposures, so I am worried the stars are gonna start to get elongated when it starts running the 300 and 600 sec pics. On 150sec they look good though...

NGC 7023 is pretty close to Polaris, so I guess the rate of rotation at that location is quite a bit slower than what you would expect closer to the horizon. But still - the star must certainly move a LITTLE at 600mm during 15 minutes.....

What I can gather from other posts on the subject is that I need to increase the calibration step? Where do I find that option?

Sincerely, Alveprinsen.

[Tinker1947]

Have you set the guiding to a dead pixel and not a star....the software works just a case of getting the right numbers in the right boxes.....

[wuthton]

Click stop > click brain symbol > click mount.

Start at about 1500 and adjust until you calibrate in approximately 15 steps.

Matt.

[Alveprinsen]

Click stop > click brain symbol > click mount.

Start at about 1500 and adjust until you calibrate in approximately 15 steps.

Matt.

Done, and done...

Set it to 2000... its guiding nicely now. It had already started on the 300 sec frames though. Looked good, at least all zoomed out.

[Pete Presland]

Click stop > click brain symbol > click mount.

Start at about 1500 and adjust until you calibrate in approximately 15 steps.

Matt.

i find 15 steps just about right!

[wuthton]

i find 15 steps just about right!

I've been using AstroArt lately. One of its best features is the guiding calibration which is essentially one step for each axis, it's a little disconcerting when you're used to the ten minute PHD calibration. It calibrates in less than a minute and the plate solving isn't far behind.

[IanL]

NGC 7023 is pretty close to Polaris, so I guess the rate of rotation at that location is quite a bit slower than what you would expect closer to the horizon. But still - the star must certainly move a LITTLE at 600mm during 15 minutes.....

Glad you sorted the problem out, but just to explain the cause and why you need to increase the calibration step:

- When calibrating, PHD will rotate one axis at a time, first in one direction and then back in the other.

- The mount will move at whatever guiding rate you have set, which will be some fraction of sidereal rate, i.e. if you have the guiding rate set at 0.1x the mount will first guide the RA axis at 1.1x sidereal (i.e. slightly faster than one rotation every 24 hours), then it will guide the RA axis at 0.9x sidereal (i.e. slightly less than one rotation per 24 hours).  It will then do the same for the Dec Axis.

- Worth noting that for the RA axis, the mount is driven West constantly, but for the East direction it is driven at less than sidereal rate the sky "overtakes" the mount and thus the stars appear to move East.  For the Dec axis the mount is actually driven North or South so backlash in the gears is a concern whenever guiding reverses direction in Dec (i.e. there will be a delay whilst the gears change direction due to mechanical tolerances).

- For calibrating in Dec, the calibration step shouldn't matter too much, but it does matter in RA depending on the declination of your target.  At low declinations (near the celestial equator) moving the mount at 0.1x sidereal rate will cause stars to move across the camera's field of view very quickly.  At high declinations (near the celestial poles) the same guiding rate will only move stars very slowly across the field of view.  (The upside is that a tracking error that produces a visible trail in an image near the equator will produce a much smaller trail near the pole).

- The reason for this is simply the distance the scope travels over time - circle made by the celestial equator across the sky is physically much longer than the circle made by (say) line of declination at 80 degrees North or South.  Since a star will take 24 hours to travel the full length of each circle, the distance covered in (say) one minute is far less for the star near the pole.  An  equatorial mount inherently deals with that requirement when tracking at sidereal rate.

- When you are driving at a different rate (to calibrate PHD) you need to increase the calibration step the closer to the pole you get.  That increases the time the mount is moving in order to ensure the guide star moves across enough pixels for PHD to measure the motion.  (You could of course increase the guiding rate and keep the calibration step the same instead, but you might well have problems when guiding the Dec axis, since the software and/or electronics might not be able to act reliably on the much shorter guiding commands that result from high guide rates).

- It is worth bearing in mind that this slight complication and the length of time needed to calibrate PHD in general are due to the fact that it is designed to be much simpler to set up than other guiding software.  If you already know your guide rate, the pixel scale of your guiding rig (a function of the guidescope's focal length and the size of your camera pixels) and you have the camera set up so that the pixels are precisely aligned with the RA and Dec axes of the mount, which side of the meridian you are pointing and you know the amount of backlash in the Dec gears  then you could in theory do away with calibration altogether and just plug the numbers directly in to software.  No calibration (or short calibration) is entirely possible and there is software out there that works like this.

- PHD does away with all those concerns and just figures it out from scratch each time, making it easier to get started without having to measure/test all sorts of things first.  In PHD 2 things are more intelligent and you can actually save calibrations for re-use, assuming you don't change your setup.

There is more here if you run in to further problems down the line:

http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

[Astrosurf]

This is a brilliant thread, as I get this sometimes.

IanL, your explanation has made things so clear. Thank you!!!

Alexxx

[IanL]

- The mount will move at whatever guiding rate you have set, which will be some fraction of sidereal rate, i.e. if you have the guiding rate set at 0.1x the mount will first guide the RA axis at 1.1x sidereal (i.e. slightly faster than one rotation every 24 hours), then it will guide the RA axis at 0.9x sidereal (i.e. slightly less than one rotation per 24 hours).  It will then do the same for the Dec Axis.

Just for clarity when I said "It will then do the same for the Dec Axis." I should have actually said 'It will then rotate the Dec Axis at 1.1x sidereal rate in one direction, and then 1.1x sidereal in the opposite direction".

[Owmuchonomy]

- The mount will move at whatever guiding rate you have set, which will be some fraction of sidereal rate, i.e. if you have the guiding rate set at 0.1x the mount will first guide the RA axis at 1.1x sidereal (i.e. slightly faster than one rotation every 24 hours), then it will guide the RA axis at 0.9x sidereal (i.e. slightly less than one rotation per 24 hours).  It will then do the same for the Dec Axis.

Ok, please excuse my ignorance. Why would I wish to choose a rate other than sidereal. I didn't even realise in PHD that this was an option. Could you explain. Thanks.

[D4N]

This is just to calibrate the guiding.

Unless you meant about other rates, they will be down to the mount, for example if you goto the moon it should track at lunar rate.

Sent from my iPad using Tapatalk

[IanL]

  

Ok, please excuse my ignorance. Why would I wish to choose a rate other than sidereal. I didn't even realise in PHD that this was an option. Could you explain. Thanks.

This is just to calibrate the guiding.

Unless you meant about other rates, they will be down to the mount, for example if you goto the moon it should track at lunar rate.

Sent from my iPad using Tapatalk

Exactly.  The tracking rate will be sidereal (for DSO/planetary work, obviously solar or lunar for those targets), or it might be King rate if you really want to be clever about it.  The guiding rate has to be something other than sidereal rate for RA otherwise the scope won't move in response to guide commands but just continue tracking at sidereal rate.  For simplicity systems usually just guide in Dec at whatever rate you have set for RA.

Put it another way:

Guide West = Sidereal Rate + 0.1 X Sidereal Rate = 1.1 X Sidereal Rate

Guide East = Sidereal Rate - 0.1 X Sidereal Rate = 0.9 X Sidereal Rate

Guide North = No Tracking + 0.1 X Sidereal Rate = 0.1 X Sidereal Rate

Guide South = No Tracking - 0.1 X Sidereal Rate = -0.1 X Sidereal Rate

(The + / - for North/South in Dec is arbitrary, just depends on which direction the mount considers to be 'positive').

I just used 0.1x sidereal as an example as it tends to be the default setting, but there is no reason why you can't use a higher rate if that woks better for your setup bearing in mind what I said about really short guiding commands not being reliable so don't go too high - for example EQMOD filters out really short guide commands as the motors can only be driven to a certain precision.