Frustrating Issues with PHD last night.....

[kirkster501]

Got set up ready to go early but wasted 2 hours faffing about with PHD. PHD set up is the last thing I normally do before setting the imaging run off. So everything was ready to go and FoV was on M81 and M82, mount aligned, camera connected and focused etc. Selected my target star (and it was a star not a dead pixel) in PHD and set it off. It tries to calibrate as it does and then....

RA: Scope did not move enough.

I had not touched my setup one iota from last weeks outing. I tried absolutely everything to get this to work with no luck. Rebooted PC (so had to realign), power cycle everything, No joy. In the end I just switched it all off - mount included - polar aligned again and tried again. Same issue. So I changed to another subject - Leo Triplet - to the south. This time it guided fine!

What gives???? I hate strange issues like this - is it going to happen again is the question I always think.

Anyone got any ideas please? I am on latest and greatest software in terms of EQmod/Ascom/PHD etc.

[russellhq]

I'm not sure I can help on the error, but I calibrate PHD before I frame a target in case anything goes wrong with the calibration and I lose the framing.

[Photosbykev]

Normally you need to adjust the Max RA duration value (found under the brain icon) to get the star to travel far enough during calibration. The value will depend on what guidecam and the guide scope magnification

[chrisshillito]

PHD only works in pixels. As you move around the sky the amount of angular movement in RA that produces a 1 pixel movement in image will vary and is inversely proportional to the cosine of the declination. So if you're guiding up near the pole you don't get much apparent pixel movement for a whole load of actual angular movement of the axis (that is also why objects up there are easy targets - pec has far less image impact!

So to get phd to calibrate you need to choose a calibration step value appropriate to you're target is. One size dies not fit all. Try increasing it.

Chris.

[Ant]

Last time I got that error message it was because the scope was so well balanced that I didn't realise that I'd not tightened the axis locks on the mount.

Didn't realise until I started packing away in frustration... Could it be something that simple?

Good luck.

cheers

ant

[russellhq]

PHD only works in pixels. As you move around the sky the amount of angular movement in RA that produces a 1 pixel movement in image will vary and is inversely proportional to the cosine of the declination. So if you're guiding up near the pole you don't get much apparent pixel movement for a whole load of actual angular movement of the axis (that is also why objects up there are easy targets - pec has far less image impact!

So to get phd to calibrate you need to choose a calibration step value appropriate to you're target is. One size dies not fit all. Try increasing it.

Chris.

Does that mean I should calibrated near my target or equally on a star on the same DEC as my target?

[kirkster501]

....one size dies not fit all. Try increasing it.

Chris.

Your explanation makes perfect sense Chris. What parameter do i need to increase though?

EDIT: calibration step ???

Steve

[chrisshillito]

So to get phd to calibrate you need to choose a calibration step value appropriate to you're target is.

Increasing the "Callibration Step" will increase the duration of each pulse issued during calibration. The longer the pulse the more the mount will move. I think phd tries something like a maximum of 60 pulses before it gives up.

Chris.

[chrisshillito]

Does that mean I should calibrated near my target or equally on a star on the same DEC as my target?

No, calibration is for the guide system only - so phd can determine how responsive the mount is and how the guide image aligns with the RA/DEC axis.

If you move to another target however you must re-calibrate as the system as a change in declination would mean that phd would no longer be issuing appropriately scaled corrections for the errors is sees.

Chris

[kirkster501]

i think phd tries something like a maximum of 60 pulses before it gives up.

Chris.

Correct, I spent two hours of precious clear sky last night watching it count up to 60 only to fail.....

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[Martin-Devon]

Chris, I'm interested in your comment about declination having an effect on PHD accuracy. I always thought that if you calibrated on one side of the meridian, you could use PHD with any target on that side - it was only when you crossed the meridian and the mount flips that you needed to recalibrate PHD again. I've used PHD for a while now like this, usually I'm fixed on 1 target the whole night but on some occasions I have switched to a new target on the same side of the meridian (but at different declination) and not reclaibrated PHD again - so far I've not had any issues, but maybe I've just been lucky.

[chrisshillito]

PHD is designed to be as simple to use as possible and to work identically whether guiding an ASCOM connected mount or a mount where the only connection is via an ST-4 interface. As such it does not know anyhing about the imaging system, target location or mount responsiveness.

During callibration PHD will issue a series of guide pulses of a known duration and watch to see how far, and in what direction the star appears to move. To avoid the uset having to enter, focal length, pixel dimensions and declination it measures the movement in pixels. It then calculates the guide pulse duration to pixel ratio so that when guiding it can measure a guide error and determine what lenth guide pulse is required to correct it.

If you move to another declination without recalibration then phd will be applying the wrong amount of correction for any given RA error. You could counter that by adjusting the RA agressiveness parameter, the mounts guide rate,or, if using EQMOD pulse guiding, the RA gain.

Changes in DEC affect the apparent RA pixel movement by a factor of 1/cos(dec) and so this effect is not linear and changes of target DEC at low declinations only have a very slight effect. When you get up into the higher declinations a give change in dec has a much greater effect.

Chris.

[kirkster501]

Thanks Chris. I have the tick box "force calibration" ticked. But if I slew to a new object (I am still in that "capture at least three objects per evening" AP phase) it does not do a recalibrate but starts guiding again straight away..... This does not appear to have given me any issues with guiding thus far.