Another PHD problem...

[fwm891]

I'm having problems getting PHD to guide evenly, some nights better than others and it responds differently depending on the altitude I'm guiding at. It doesn't necessarily guide better at more northerly than equatorial regions which I would have expected.

It doesn't seem to matter if I change the settings.

Some nights I can guide with the RA Aggressiveness at 10, another it need 100?

Sometimes it will guide well in RA but the dec will wander wildly or vis versa...

Happens whether I capture with the Artemesis or Nebulosity+PHD link

Does anyone else have these difficulties with PHD?

If you solved them can you tell me your secret?

I'm using an OAG through a SW MN190 with a QHY5 as the guide camera.

Using an NEQ6 Pro mount on a pier with EQMOD and CdC as control software.

Comments, remedies welcome:icon_scratch:

[FrankieValley]

Have you polar aligned as that will make it (too) hard work for PHD if you haven't.? Is the mount level?

I use PHD and an OAG with a 250PDS on NEQ6 and it works great. The graph isn't the flattest in the world, but the stars are round. I also use a QHY5 on a finderguider for a refractor with a smooth graph.

RA ag is between 95 & 105 and hys is 13 - the rest are defaults. Things do change with PHD depending on what part of the sky you are and that may also depend on the angle/orientation of the camera, but not sure on that point, but it may explain different behaviour across the sky from one night to another.

I always calibrate on a new subject or if the object has moved significantly since I was last there.

I use ST4 on both guide cameras. In EQMod there are ST4 autoguide settings. Most times I set these both to x0.75, but if I'm guiding near the pole or zenith then I get slow calibration and I increase them to x1.0

Calibration step size in the brain I have at 1500 for the short FL finderguider and 1000 for the OAG for the longer focal length.

I set exposure to 1sec if I can to speed up calibration and then set to 2s once guiding so not chasing the seeing.

Hope this helps.

[fwm891]

David, Thanks. My mount is pier based in an obsy and polar alignment is checked periodically with drift alignment and rarely needs any adjustment its certainly OK at the moment.

I can rotate my QHY5 on the OAG so that its always guiding with the star motions (NSEW) along rows/columns of the camera.

I too recalibrate when moving any distance across the sky although not for subjects that are close (i.e. M51 & M101 would use same calibration).

My calibration steps are smaller than yours - typically 300 with the OAG and 800 with a small frac.

My RA Ag is about the same but I've found (tonight) using a bigger Hyst (50) it calms things down but needs to run for a few minutes before starting a set of exposures for Hys to kick-in properly.

I'm using 1.5 or 2 sec for guide exposures with dark frame to remove the noise.

Francis

[FrankieValley]

Hi Francis,

All sounds good to me.

ST4 connectivity problem? I get them all the time and have to wiggle the ST4 plug in the mount. Had to do it twice tonight. The symptom is that PHD is trying hard to move the mount and will tell you along the bottom bar i.e. you keep seeing N 150, N 150 but dec still drifts off, because the pulses aren't getting through. Something to look for i.e. phd is trying but mount not reacting.

[fwm891]

I don't know about the ST4 but it sounds like me at the moment! (not reacting that is)

I'll check that and clean the contacts ready for tomorrow if it stays clear

Thanks

[Psychobilly]

You can use the handbox option in thd tools menu to see if the mount is responding to guiding signals from phd

[kookoo_gr]

Also you could try to guide via ascom pulse guiding since you can fine adjust the pulses that reach the mount. with st4 the pulses are perdefined and sometimes that can cause an issue. Also check the balance of your equipment, if it is off it will be difficult for phd to calibrate and also guide. If you turn the oag camera you also need to recalibrate. Finally you need to check for flexure at your system.

[kookoo_gr]

i also forgot to mention that you need to have the guide camera at perfect focus with the main camera or you will have guide issues

[MG1]

i also forgot to mention that you need to have the guide camera at perfect focus with the main camera or you will have guide issues

Hi there...could you elaborate a little as to why this is the case? I only ask because I'd picked up from numerous threads that a slightly de-focused star was better for guiding....but I can't recall exactly why. ?!?

[robbieince]

A slightly defocussed star is less prone to seeing perturbations and so will throw fewer guiding commands.

Regards

Rob

[MG1]

A slightly defocussed star is less prone to seeing perturbations and so will throw fewer guiding commands.

Regards

Rob

And I'm guessing this is good because then you reduce over corrections due to fighting the seeing?

But would a sharp focused star with a loner exposure do the same (say 5sec)?

[fwm891]

Thanks All, Points noted.

Re rotation, focus and calibration of the guiding camera. I have a purpose built OAG (http://stargazerslounge.com/diy-astronomer/164051-diy-oag.html) which allows me to rotate the camera to match the 'sky', focus independently of the main scope and my system is calibrated each time I move a significant amount between objects. My guiding exposures in PHD are usually 1.5 or 2 seconds after applying dark frames to remove background noise effects.

At present I have not set-up PEC, but now have enough data to do so, so that will take out the majority of the cyclic guiding errors.

What really bugs me though is the number of times PHD suddenly decides to go 'walkies' in one or other of the axes for no apparent reason. They are never gradual motions, can be in either direction. Its always a 'kick' throwing the graph way off scale which usually results in quite a time lag for it to restore to normal guiding or I need to intervene, stop and reset guiding so it starts again from scratch, hopefully without ruining the exposure if caught early.

On flexure that is taken out because of it using the main optics to guide by rather than a second guider scope.

Thanks Francis

[chrisshillito]

And I'm guessing this is good because then you reduce over corrections due to fighting the seeing?

But would a sharp focused star with a loner exposure do the same (say 5sec)?

Yes it would - but introducing a log into a control loop is a bad thing to do. A five second exposure means 5 seconds the mount can move subject to its PE uncorrected for 5 seconds (unless you have PEC active). Sure eventually guide corrections are made eventually but your image may already compromised. You really want to be using the shortest exposures you can without chasing seeing and defocusing is one way to achieve this - it also gives a larger area for the guiding centroid calculations to work on which again is a good thing.

Of course if guiding through an OAG you may be dealing with pretty dim guide images anyway and defocusing may not be practical at all - there's no point having to use really long exposure just to use a defocused image. So if you get a nice bright star with a guide exposure or 2s you might consider defocusing - but if it is dim don't increase the exposure length just for the sake of defocusing.

Chris.

[robbieince]

And I'm guessing this is good because then you reduce over corrections due to fighting the seeing?

But would a sharp focused star with a loner exposure do the same (say 5sec)?

Yep - defocussing is also good for really bright stars that would saturate and clip at max level.

As you increase exposure length the star will effectively average out the seeing variations anyway.

Regards

Rob