PHD not enough star movement??

[Astroblagger]

Hello was out trying to bag bodes galaxy tonight was using a finderguider and qhy5, when trying to set up guiding i got not enough star movement error message, Any hints as to why it was happeneing?

[Beyond_Vision]

Either you have selected a hot pixel instead of a star or the connection to the ST4 port of the mount was not in properly. If both of these are not the case you may need to increase calibration step time to 2000ms in the advanced settings

[swag72]

Here are my settings for PHD that I use with my QHY5 and finder guider, may give you some pointers. You definately need a high calibration steps number.

[chrisshillito]

PHD calibrates by issuing guide pulses and the looking for the movement of the star. Beacuse it works only in Pixels the amount of movement in the RA direction you get for any fixed period pulse is entirely dependent on the targets declination. The greater the declination the less the movement. Because you were targeting an object up near the pole, the movement in RA per pulse is therefore very small. By the time PHD has issued its maximum number of calibration pulse the star will not have appeared to have move enough for PHD to make accurate calculations and so it issues the calibration failed message..

The solution is to increase the PHD calibration step size. By doing this each calibration pulse will persist of a longer period - thereby generating a greater apparent star movement.

Chris.

[Astroblagger]

Ok thanks for the suggestions, that does make sense as i then went to try and image the pliades ie (further from the pole) using the same settings, and it calibrated no probs. i did try and up the calibration step up to 1000 but it still failed. Still on a learning curve with PHD!

Also another question- if iv set up guiding on one part of the sky and then move the mount to another part of the sky to image another object do i need to calibrate again or can i just start guiding on another star in the fov without going through the calibration again?

[swag72]

If you move to a different part of the sky altogether then generally it's recommended to do a recalibration according to what I have read. If it's just a small nudge for framing for example, then there's no reason to recalibrate for that.

[FrankieValley]

Assuming you are using the ST4 port on the camera and you have 'On Mount' selected in PHD and your ST4 connectivity is not the problem (trying to use a telephone cable instead of ST4 cable, poor connections - wiggling plugs helps), then I would say calibration step size will help as suggested above. Another thing that helps me bring down calibration time is to increase the settings in the screenshot below. I change mine between x.75 & x1.0 for different areas of the sky as Chris elaborated on (assuming you're using the excellent EQMod).

[Astroblagger]

Iv not been using eqmod, wouldnt know where to start! will try swag72 settings first then see how that goes. I was having a real hard time framing last night, taking short exposures then moving the mount then move again and again till it was positioned right, took some time! is there an easier way? or is there a more effiecient way of doing it?

[swag72]

Framing can take an absolute age!! I don't think there's a way round it to be honest. It is easier with a CCD for framing, but with a DSLR you just have to take the subs look and move accordingly.

[FrankieValley]

Iv not been using eqmod, wouldnt know where to start! will try swag72 settings first then see how that goes. I was having a real hard time framing last night, taking short exposures then moving the mount then move again and again till it was positioned right, took some time! is there an easier way? or is there a more effiecient way of doing it?

I think there are the same autoguide settings that can be made via the mount hand controller. Something to bear in mind, but good idea to start with swag72 settings.

When framing, put up iso, gain, use no/luminance filter if mono .....anything that helps you see something with min exp duration. I get an idea of where the subject is and then memorise the position of bright stars and reduce exp duration so I see just the stars and I can then gauge where I want to move things to by watching the stars move. I'm all ears for anything better. Accurate star alignment will also help get things close to centre.

[Beyond_Vision]

Iv not been using eqmod, wouldnt know where to start! will try swag72 settings first then see how that goes. I was having a real hard time framing last night, taking short exposures then moving the mount then move again and again till it was positioned right, took some time! is there an easier way? or is there a more effiecient way of doing it?

You can use 'plate solving' to work out a more accurate position of the mount then you can command the mount to centre on your object. Maxim in conjunction with a program called pinpoint or Astro Art 5 has this feature. There is also a freeware program that works with nebulosity and Astro Art 4.

If you can it's worth leaving the camera fixed to the scope so that you can maintain the same orientation when imaging the same object over a series of nights. This also helps with Flats as you can use one set of flats to calibrate the sub frames.

[chrisshillito]

Another thing that helps me bring down calibration time is to increase the settings in the screenshot below. I change mine between x.75 & x1.0 for different areas of the sky as Chris elaborated on (assuming you're using the excellent EQMod).

Please note that the speed of calibration, or number of steps (pulses) used to complete calibration is not in way an indicator of the suitability of the the guiding parameters for optimal tracking whilst guiding. PHD doesn't abort the calibration process on elapsed time - it only aborts when it has issued a set number of pulses and the star hasn't moved a set percentage of the image size. For objects at high declinations you just have to accept that PHD's calibration process will take longer and will may require a longer calibration step to be used - its really not a bad thing.

A guiding rate of x1.0 is very fast - nothing in the mounts mechanics should be generating an error signal that changes at that rate so in terms of guiding that is rather aggressive.

Chris.

[swag72]

Chris I am interested in your post above and have a question if I may.

Prior to EQMOD I was always calibrating within 10 steps in PHD. So this I guess is a native kind of rate for my mount and setup. When I went to EQMOD, my steps changed to about 20-25, so I upped the guide rate to 0.50 and got back to 10 steps.

So, is it beneficial or otherwise for my guiding rate WITH EQMOD to be so very different as without? I guess I am asking whether it is better to keep the guide rate lower at 0.25 and have more calibration steps or up it to 0.50 and get back to where I was?

@OP - Hope you don't mind me asking this question!

[bamus]

!

Also another question- if iv set up guiding on one part of the sky and then move the mount to another part of the sky to image another object do i need to calibrate again or can i just start guiding on another star in the fov without going through the calibration again?

Let me come back to your question. I think that declination plays important role here. You can change orientation of your telescope to completely different part of sky. Providing that declination is only few degrees off the declinations used while calibrating, you don’t have to recalibrate.

[chrisshillito]

Hi Sara,

What you have to remember is calibration is a totally artificial process and performance during calibration does not reflect what happens during real tracking where overall deviations and pulse widths are much, much smaller. Its purpose is simply measure the guiding characteristics of your system. It should not be used to set those characteristics.

Upping the guide rate to 0.5 might get calibration steps back to where they were guiding performance itself will have been fundamentally changed! I'm sorry but I really can't advise you on whether that change would be good or bad for you. If you've made things better though you've done it through accident rather than design.

Guiding is a black art - even with PHD there are many parameters that affect how it works and when coupled with variables such as guide resolution, image resolution, guide exposure, image exposure it means there is unlikely to be a one size fits all solution to choosing the optimum guide parameters. All you can do is experiment and find which combination of equipment and settings works best for you. Once you've found that happy place you don't change parameters unless you see a problem in the image.

Chris.

[chrisshillito]

You can change orientation of your telescope to completely different part of sky. Providing that declination is only few degrees off the declinations used while calibrating, you don’t have to recalibrate.

There's a bit more to it than that. As part of the calibration process PHD measures the angle of the guide image with respect to RA and DEC and so knows which direction to issue corrections in when guiding. If your camera angle changes then clearly you need to recalibrate - however even for a fixed camera angle the direction of correction pulses required will change when you flip over the meridian. So If you move, recalibrate!

Chris.

[bamus]

There's a bit more to it than that. As part of the calibration process PHD measures the angle of the guide image with respect to RA and DEC and so knows which direction to issue corrections in when guiding. If your camera angle changes then clearly you need to recalibrate - however even for a fixed camera angle the direction of correction pulses required will change when you flip over the meridian. So If you move, recalibrate!

Chris.

hmm, not so sure about that. I have fixed guide wide angle lens, so I am never changing angle as I always can find guide star in FOV. Whenever I do drift align using PHD i calibrate only on Southern star close to declination 0 and when I move scope to eastern star close to dec 0, I never was recalibrating. I don’t remember which side of meridian southern star was though.

[FrankieValley]

Please note that the speed of calibration, or number of steps (pulses) used to complete calibration is not in way an indicator of the suitability of the the guiding parameters for optimal tracking whilst guiding. PHD doesn't abort the calibration process on elapsed time - it only aborts when it has issued a set number of pulses and the star hasn't moved a set percentage of the image size. For objects at high declinations you just have to accept that PHD's calibration process will take longer and will may require a longer calibration step to be used - its really not a bad thing.

A guiding rate of x1.0 is very fast - nothing in the mounts mechanics should be generating an error signal that changes at that rate so in terms of guiding that is rather aggressive.

Chris.

Hi Chris,

I agree with what you are saying. I probably do calibrate quite aggressively with cal steps and guide port rates, but that was after listening to/reading Mr Starks presentations. I set cal steps and guide pulse to achieve 10 or 11 steps in each direction. I could do 7 with my setups and when low in the sky, but I figure that is not enough time to spruce up my G&T... oh, and it probably wouldn't guide well As I say, it works for my combinations of imaging scope and guide scope focal lengths. I get good graphs and most importantly, I get round stars to 20mins (I haven't tried beyond that because I haven't needed to, but I'm quietly confident).

[chrisshillito]

Hi David,

I'm sure there is a case for always looking to achieve a set minimum number of calibration steps particularly if there are variable lags present in the system. A minimum of 10 sounds reasonable as it much better for PHD to calculate a response resulting from the average to 10 individual pulses than use only 3 or 4 where one erroneous response could unduly skew the calculations.

My only point is that folks shouldn't change the mounts guiding rate to achieve calibration at their preferred step count - they should change the PHD "calibration step" parameter to do this.

Chris.

[FrankieValley]

Hi David,

.......

My only point is that folks shouldn't change the mounts guiding rate to achieve calibration at their preferred step count - they should change the PHD "calibration step" parameter to do this.

Chris.

Hi Chris,

Can we compromise, please Change cal step parameter first and if that doesn't work (and only if it doesn't) 'try' changing the mounts guiding rate. I love EQMod to bits and you are my absolute hero (stop me if you will), but when I increased the guide port rate via EQMod I got reasonably quick calibration 60-90secs (time enough to refresh a G&T - just) and great stars as well. I also calibrate with exposure duration of 1sec (decreases calibration time) and then increase to 2secs for guiding so as not to chase the seeing. I guess I'm too impatient and should make a longer G&T

In support of why I changed the guide port settings, here is a passage from another hero of mine, Mr Craig Stark - "If your using the Autoguide port.. Make sure to check the autoguide settings for BOTH RA and Dec. 50% to 99%."

That is taken from this page - tutorials:phd:faq [stark Labs Astrophotography Software]

Again, you are absolutely correct, change calibration step size first, but I would suggest that if that doesn't work, try what Mr Stark suggests if your guide port settings are at x0.25 or x0.5 and it ain't workin'

[chrisshillito]

Of course we can compromise how about this:

1. If PHD calibration completes in least than 10 calibration steps shorten the calibration step until at least 10 steps are taken.
   
   

   
   

2. If PHD calibration doesn't complete within 10 -20 steps the first thing to try is lengthening the calibration step or waiting for more steps - there is no harm in this other than the effects of over consumption of G&Ts.

   
   

3. If, on increasing the calibration step to its maximum, calibration still fails to complete within 60 steps then your guidescope/guidecam combination is seriously lacking in resolution and you should consider inserting a barlow.

   
   

4. If calibration still doesn't complete the only option left (other than using another guiding program) is to change your guiding rate for a faster one.I

   
   

5. If calibration still doesn't complete then either: Unpark the Scope! or Stop trying to image the Pole!
   
   

   
   

Note that progressing to step 4 implies calibration didn't complete within at least 10 minutes (60 steps of 10000mS) !

Guiding is complex and there are many variables some rather interdependent. What works for one person may not work for another. That said you only need to achieve a level of guiding performance such that your final image isn't compromised by mount movement. There may well be different paths you can take to achieve this and "perfect guiding" may not be required at all.

Chris.

[FrankieValley]

Of course we can compromise how about this:

1. If PHD calibration completes in least than 10 calibration steps shorten the calibration step until at least 10 steps are taken.
   
   

   
   

2. If PHD calibration doesn't complete within 10 -20 steps the first thing to try is lengthening the calibration step or waiting for more steps - there is no harm in this other than the effects of over consumption of G&Ts.

   
   

3. If, on increasing the calibration step to its maximum, calibration still fails to complete within 60 steps then your guidescope/guidecam combination is seriously lacking in resolution and you should consider inserting a barlow.

   
   

4. If calibration still doesn't complete the only option left (other than using another guiding program) is to change your guiding rate for a faster one.I

   
   

5. If calibration still doesn't complete then either: Unpark the Scope! or Stop trying to image the Pole!
   
   

   
   

Note that progressing to step 4 implies calibration didn't complete within at least 10 minutes (60 steps of 10000mS) !

Guiding is complex and there are many variables some rather interdependent. What works for one person may not work for another. That said you only need to achieve a level of guiding performance such that your final image isn't compromised by mount movement. There may well be different paths you can take to achieve this and "perfect guiding" may not be required at all.

Chris.

Hi Chris,

Deal

No.3 isn't something I ever considered on a guide scope. I went down rat holes with a x.6 reducer (to get focus) on a short focal length guide scope trying to guide a 1500mm newt..... and then a finderguider on the same newt. I didn't fully understand the effects of f/l's at the time. You live and learn and I realise now that focal length considerations are important and No.3 would have saved me so much struggle, time and money (if the QHY5 could pick up a star). Now you tell me

Is there a 'rule of thumb' for guide scope focal lengths to use with imaging scope focal lengths? My limited experience says that the guide scope should be a minimum of 1/3 of the f/l of the imaging scope for >10min exposures.

For No.5 you should add 'go get a longer G&T glass'

Cheers