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Old 17-08-2019, 11:25 PM
HeavyT (Todd)
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PHD2 mirrored graph

Hi all

I'm very new to astrophotography and extremely new to guiding.

Finally got it working last night, but could only achieve acceptable 60 second exposures.

QHY5L-ii-mono through a ZWO mini guidescope, riding an ED100 riding a CG5. Guiding connection is "on camera".

I know my polar alignment wasn't super because of drift in the image over a couple of hours plus both ra and dec graphs consistently hovered around +1 arc second.

But the real thing seemed to be the intermittent clusters of correction spikes. (Photos attached).

I might think it's a periodic error caused by old gears, but why would the spikes be mirror images of each other? It wasn't all that windy, and I have read that dec might only receive correction for polar alignment errors (which would be consistent, not intermittent peaks?)

PHD2 calibration settings need a look perhaps?

Thanks for reading, and clear skies.
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  #2  
Old 18-08-2019, 06:46 AM
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Slawomir (Suavi)
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Hi Todd and welcome to IIS.

Your graphs show corrections going in one direction with every single exposure and the guide star is never on axis so perhaps polar alignment is too inaccurate for the mount to track and guide properly. There could also be significant backlash but my guess is that polar alignment needs to be more precise.

Good luck with your investigation and please keep us posted
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Old 18-08-2019, 07:14 AM
Startrek (Martin)
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Next night your out start your guiding and run the guiding assistant in Tools
for about 2 minutes , it will hopefully pinpoint your issue
Also you can send your guide logs to the PHD2 forum for some assistance
Other than that try to get your PA as tight as possible , get a good calibration from a location just above the celestial equator and the meridian ( at near zero Declination ) and check all your settings in the Brain especially camera settings
Good Luck !
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Old 18-08-2019, 09:03 AM
HeavyT (Todd)
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Thanks both, I have heard sharpcap makes polar alignment super easy so I am going to try that for polar alignment.

I can get acceptable 30-40 second subs using the mount's PA routine with an illuminated eyepiece (the reticle insert is loose, so still having to use judgement I'm afraid).

But a pa problem would cause constant correction (as you've pointed out) rather than spikes? I have a steep learning curve.

Thanks again and clear skies.
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Old 18-08-2019, 11:26 AM
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Slawomir (Suavi)
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It's hard to pinpoint the issue just from a graph. A glimpse of stars in on the images suggest these are highly elongated so maybe the centroid is not being calculated correctly? Also, 1.5s guiding exposures will be more strongly affected by seeing, so trying to go a bit longer (2-3s) might help in pinpointing the issue. There could be a few factors contributing to those spikes. But certainly improving the accuracy of the PA is a starting point.

Recently I was getting sudden spikes with my Mach1 and it took me a while to realise it was my guide camera not the mount or anything mechanical - every now and then a random subtle noise pattern would appear in the guide camera 's exposure and sometimes this noise would overlap with the guide star distorting it and consequently causing phd2 to miscalculate the star's centroid. Lowering star mass tolerance change fixed that, at least until I will get a new guide camera. This experience taught me that a good low noise guide camera is essential in getting the best out of the mount.
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Old 18-08-2019, 12:41 PM
Startrek (Martin)
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Todd
I just noticed your graph shows a saturated guide star with SNR of 86 which is not ideal for Guiding
Try an exposure setting of 2 sec
Once looping , go to tools and let PHD2 “auto select a star” for you which will be unsaturated and have a SNR value between say 15 and 40 depending on your star field and seeing conditions. Don’t be concerned the guide star that is auto selected is not in the centre of your FOV , as long as it’s not close to the edges
This may help ?
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Old 18-08-2019, 01:55 PM
HeavyT (Todd)
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Thanks again both, your feedback is helping me immensely, I'll be implementing all these suggestions next clear night for sure.
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Old 18-08-2019, 02:39 PM
Startrek (Martin)
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Todd
Below are some guiding tips I set up for myself 2 years ago when I first started using PHD2
They may be of some help to you ?

PHD2 Set Up and Guiding tips

1/ Try to Polar align to within 2 arc minute error on Alt and Az

2/ Auto select guide star (recommended by PHD2) or manually select star that’s not too bright or saturated to avoid lost stars

3/ If guiding is poor ( above 2 arc seconds ) utilise the Guiding Assistant

4/ Check and adjust focus of your guide scope and camera to minimise lost stars

5/ Set your guide camera exposure time between 2 and 4 seconds ( Looping ) 2 seconds is a good starting point

6/ The SNR value of the guide star should be between 15.0 and 40.0 to avoid losing the guide star

7/ Don’t over correct settings , under correcting will let you manage your guiding performance better

8/ If Guiding is poor try and keep your min mo above 0.25 in RA and DEC . Don’t go above 0.6. A good starting point for min mo is 0.20 or 0.25

9/ Build a Dark Frame library and a Bad pixel map to provide good calibration and stable guide star

10/ The RA Osc value should be between 0.3 to 0.5
If it drops to 0.1 or lower then increase the RA aggressiveness or decrease the hysteresis
If it gets quite high say 0.8 then reduce the RA aggressiveness or increase the hysteresis

11/ Use Ascom Guiding - Laptop direct connect to mount ( preferred) in lieu of ST4 connected guiding via camera to mount

12/ USB 2 cable to camera is not to exceed 5 metres

13/ Do not cable through a hub even a Powered hub, direct connect to the computer , you will have less issues

14/ Ensure you have the latest driver installed for your guide camera

15/ If guide camera is USB2 make sure you connect into a USB2 port with USB2 cable
And likewise with USB3

16/ Ensure you enter all the correct data for your guide camera into PHD2 brain tabs

17/ Make sure you get good Calibration. Calibrate within 10deg of celestial equator and within an hour of meridian
Once Calibrated save it and reuse it ( go to Brain in guiding tab and tick box “Auto restore Calibration”)

18/ if you lose guide star often , go to Brain in guiding tab and increase “Search region” and increase “ Star mass detection”

PHD2 Extra Guiding Tips

1/Try and keep your min mo in RA and Dec just above your arc sec error in both axis or leave as default (0.20) and see how the guiding performs over a few minutes
2/ Only make small adjustments in RA and Dec Aggressiveness. Wait for guiding to run for a few minutes to see any change or improvements
3/ If guiding is saw toothing in RA ( blue ) badly you are over correcting , reduce RA Aggressiveness and increase Min mo in RA only
4/ If DEC ( red ) is staying for the most part above the centre line you are under correcting , so increase DEC Aggressiveness a bit and reduce Min mo
5/ If guiding is still bad , run the Guiding Assistant to see what can be done
6/ Only use the Backlash Compensation setting If Guiding Assistant advises to do so.
Go to Brain then Algorithms
Use only if your backlash value from the Guiding Assistant is 3 seconds or less or if the Guiding Assistance says to use this function

Cheers
Martin
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Old 19-08-2019, 06:43 AM
HeavyT (Todd)
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Thanks Martin, those instructions are going to prove to be invaluable as I get to grips with guiding.

Clear skies mate.
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