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Re: Settings for use of AutoGUIDE

 

I will add one note to this info - and thanks Gabriel - MUCH more than I was asking for but very much appreciated.?

That note is that using a standard refractor as a guide scope (i.e. a guide scope) if you position the camera such that the name on the back of the camera is upside down but horizontal, you should have proper arrangements that correlate to movement - i.e. east moves the image east, north moves it up.?

I found this to be helpful if for any reason I wanted to use the guide image to move around the sky. (that orientation I believe is universal - name upside down - but I can't say that for a fact, it just makes sense to me since on my SVBony guide camera, the 305 Pro, it is as noted.)?

When you do this the chip is horizontal and by making it upside down you basically counter the effective 'mirror image' that the lens produces.?

g.




------ Original Message ------
From "Gabriel Wiklund via groups.io" <gabriel.wiklundholeshot@...>
Date 2/17/2025 1:11:52 PM
Subject [ServoCAT] Settings for use of AutoGUIDE

[Edited Message Follows]

Here comes a description of the hardware and settings i have used to successfully Autoguide with my ServoCat driven Starstructure:
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1. Hardware:
- Generic 50mm/180mm Guidescope, i ordered this one:
it's very light and stiff, comes with mounting options so i even could use one of the threads for the Telrad base to mount it.?
It is almost a crime in imaging to guide with 180mm focal length on 2725mm FL that i am using, but it is enough to provide corrections for and verify guiding down to 0.8" arc seconds total RMS.
?
- 5 meter ST4 cable, if you can find the right length RJ12 6P6C in a local store it will be much cheaper, at least here 3 meters was in stock.
- ZWO ASI 120 MM Mini Guidecamera (comes with a 2 meter ST4 and USB cable). Touptek or a similar brand could have a cheaper alternative.
?
The above hardware was selected because it was the lowest cost alternative that i could find which did not require any modification to my telescope.
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1. Software?
- PHD2
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After mounting the Guidescope, connecting the ST4 cable from the Guidecamera to the back of the ServoCAT AUX/AG port and connecting the USB cable from the Guidecamera to your PC you are ready to go from a hardware standpoint except aligning the guidescope with the main scope.
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In ServoCAT sky i use 6 arc seconds per second for the GUIDE 2 value (0.40 x Sidereal).
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Download PHD2 and the camera driver, start PHD2 and follow the instructions. When choosing mount select "On camera" and for the Mount guide speed enter 0.30 x Sidereal since the measured guidespeed at least in my case has been lower.
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The instructions will end with creating a dark library where it asks you to cover the scope, after that the setup is done!
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Click on the brain icon and choose Algorithms at the top of the window. Change the declination Algorithm to "Hysteresis" like is already selected on RA and make sure dec mode is on auto. In our case RA is AZ and DEC is ALT. Now click on Global and uncheck "use multiple stars" and uncheck Star mass detection.
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I then pointed the main scope at the moon with a 28mm eyepiece and the guidecamera looping exposures connected to SharpCap. I then adjusted the Guidescope untill the moon was centered in the guidescope as well. You could align the guidescope off center with regards to the main scope as long as the main scope is pointing within the guidescope field of view, and as long as the guidestar you choose when imaging is as close to that point as possible since the field will appear to rotate around the guidestar. PHD2 has a crosshair overlay that can be used.
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You can now focus the guidecamera on the moon by slightly moving it in or out before locking the lock screws. Personally i reached better focus later on by doing the same on stars (the guide assistant will tell you if you need to focus), be aware that the movements required to reach focus are very small.
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Make sure that the "Guide" button on the ServoCAT handpad is selected or PHD2 will not be able to send pulse commands.
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Slew the scope to the South slightly east of the meridian about 35 degrees above the horizon, PHD2 recommends that the calibration should be done close to the Celestial Equator. Begin looping 1s exposures, now slew the scope UP in Alt untill it moves to resolve backlash. Make Sure you are on the east side of the Meridian since Alt will change directions and move down after the meridian.
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At the bottom of the window hit Guide, now the calibration will start. PHD2 will move the mount East, Weast, North, and South in order to understand how the mount behaves. When the calibration is done click on tools at the top of the window then "review" calibration data.? Verify that between 8-12 steps where used on either axis during the calibration.
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If much more than 12 steps where used the guidespeed entered when creating the PHD2 profile is faster than the actual guidespeed, that will result in PHD2 not moving the mount enough, the reason being PHD2 sending short guide pulses assuming a fast guidespeed.
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If less than 8 steps where used that means the entered guidespeed when creating the PHD2 profile is slower than the actual guidespeed, causing PHD2 to send long guidepulses moving the mount to much = same distance covered but with fewer steps.
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If the guidespeed is incorrect PHD2 advises not to change that manually, but to create a new profile. In the new profile enter the measured guidespeed you got from the calibration.
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The scope will already have begun guiding after the calibration, and don't worry if the guiding is bad, since the scope is only 35 degrees above the horizon the atmosphere will cause the guidestar to bounce around causing the scope to chase the seeing if it is bad with the 1s exposures from the calibration.
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Stop guiding and slew to the object, loop 2s exposures and begin guiding and wait a minute or so, then click tools on the top of the window then choose "Guiding assistant". PHD2 will stop guiding and measure the mounts unguided tracking performance/behaviour. Uncheck "Measure declination backlash" in the lower left corner.
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PHD2 recommends waiting 2 minutes, click on "STOP" and follow the recommendations, you don't have to go and change the settings, you simply press on "apply" for the suggestions PHD2 comes up with, with the exception of improving guidescope focus which it may suggest, or polar alignment which is not relevant for us.
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If you after all this see very large guidestar deflections especially after corrections, the PHD2 documentation recommends that you should not wildly begin changing settings, you should however take a closer look at how the guidescope is mounted.
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Just a few microns of flexure is capable of throwing of the guidestar by 6 arc seconds, several star diameters, a human hair is 50 microns thick.
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I followed that recommendation and tightened the adjustments screws as best i could on the Guidescope, it did change the pointing of the guidescope slighlty but my main scope is still pointing in the lower right corner of the Guidescope field of view so i just select guidestars there instead. It helped a lot taming the deflections.
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Besides that seeing plays a huge roll, if the seeing is really bad guiding will be bad since the stars will move even if the scope is not moving.
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If the seeing is good, guiding should work very well, If the guiding looks bad but the exposures look good, don't worry about the guidegraph, if the guidegraph looks bad and the exposures are bad then it's worth taking a look at.
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I tested this 2 days ago for 1 hour on M106 and it stayed locked with M106 exactly in the center for 1 hour with 1-1.3 arc seconds total RMS guiding with round stars.
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I did not mention the following above to not complicate things but it is possible to connect the Meade Generic or ServoCAT? driver to PHD2 in order to provide PHD2 with pointing information so that it can use dec compensation. If you use "On Camera" like i have suggested above you will have to enter the DEC (ALT) manually before calibration and before guiding on a new object.
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Clear skies!
Gabriel
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Re: Settings for use of AutoGUIDE

 

Thank you Gary, much appreciated!
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/Gabriel
Re: Settings for use of AutoGUIDE

 

Gabriel,

HATS OFF TO YOU SIR!?

Super exercise ... and definitive results. Great job.?

Gary Myers
StellarCAT



------ Original Message ------
From "Gabriel Wiklund via groups.io" <gabriel.wiklundholeshot@...>
Date 2/17/2025 5:53:55 PM
Subject Re: [ServoCAT] Settings for use of AutoGUIDE

Just for fun this evening i tried using Easytrack on it's own without any DSC just to see if autoguiding is able to compensate for such basic tracking. I have marks on my scope so i manually can point it at roughly 0 az and 63 alt, so with SPEED 2 i pressed the CCW button on the handpad, switched on the ServoCAT and let go of the button when the yellow SPM light turned on = scope is tracking.
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Here is a 60 second sub (crop of the very center), i just slewed the scope randomly to the north east at about 50 degrees in altitude. In actual capture i would of course never use that kind of exposure time since my scope is sky fog limited after only 20 seconds.
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Here is the graph in PHD2:
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Pherhaps hard to see but total RMS is 1.28" arc seconds, not bad for only using Easytrack if you ask me.
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So in other words, the 2-star alignment is less critical when using autoguiding.
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/ Gabriel
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Re: Settings for use of AutoGUIDE

 

Just for fun this evening i tried using Easytrack on it's own without any DSC just to see if autoguiding is able to compensate for such basic tracking. I have marks on my scope so i manually can point it at roughly 0 az and 63 alt, so with SPEED 2 i pressed the CCW button on the handpad, switched on the ServoCAT and let go of the button when the yellow SPM light turned on = scope is tracking.
?
Here is a 60 second sub (crop of the very center), i just slewed the scope randomly to the north east at about 50 degrees in altitude. In actual capture i would of course never use that kind of exposure time since my scope is sky fog limited after only 20 seconds.
?
?
Here is the graph in PHD2:
?
?
Pherhaps hard to see but total RMS is 1.28" arc seconds, not bad for only using Easytrack if you ask me.
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So in other words, the 2-star alignment is less critical when using autoguiding.
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/ Gabriel
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Re: Settings for use of AutoGUIDE

 

Hi Raphael,
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Your welcome
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Thank you and clear skies!
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/Gabriel
Re: Settings for use of AutoGUIDE

 

Hi Gabriel,
Thank you so much to have described in detail how to autoguide !
It will be very usefull if I decided to autoguide in future !
Thanks
Rapha?l
Settings for use of AutoGUIDE

 
Edited

Here comes a description of the hardware and settings i have used to successfully Autoguide with my ServoCat driven Starstructure:
?
?
1. Hardware:
- I use a 50mm /180mm focal length Guidescope (Omegon Guidescope Modul-Finder) and it has worked just fine, of course a larger guidescope? ? ? ?would be better as long as it does not flex.
- ST4 cable, if you can find the right length RJ12 6P6C in a local store it will be much cheaper, at least here 3 meters was in stock.
- Guidecamera like the ZWO ASI 120MM Mini (comes with a 2 meter ST4 and USB cable). Touptek or a similar brand could have a cheaper alternative.
?
The above hardware was selected because it was the lowest cost alternative that i could find which did not require any modification to my telescope.
?
1. Software?
- PHD2
- Camera driver
?
After mounting the Guidescope, connecting the ST4 cable from the Guidecamera to the back of the ServoCAT to the AUX/AG port and connecting the USB cable from the Guidecamera to your PC you are ready to go from a hardware standpoint except aligning the guidescope with the main scope.
?
In ServoCAT sky i use 6 arc seconds per second for the GUIDE 2 value (0.40 x Sidereal) and in the PHD2 Profile Wizard i use 0.20x Sidereal. The reason is beacuse the measured guidespeed for my scope is lower than the speed entered in ServoCAT Sky due to friction.
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When you first start PHD2 and follow the Profile Wizard, choose "On camera" when selecting mount.
The instructions will end with creating a dark library where it asks you to cover the scope, after that the setup is done!
?
Click on the brain icon and choose Algorithms at the top of the window. Change the declination Algorithm to "Hysteresis" like is already selected on RA and make sure dec mode is on auto. In our case RA is AZ and DEC is ALT.
?
When aligning the Guidescope to the main scope you could align the guidescope off center with regards to the main scope as long as you know where in the guidescope field of view the main scope is pointing, and as long as the guidestar you choose when imaging is as close to that point as possible since the field will appear to rotate around the guidestar.?
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Make sure that the "Guide" button on the ServoCAT handpad is selected or PHD2 will not be able to send pulse commands.
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Slew the scope to 30 -40 degrees above the horizon in altitude and in azimuth around 90 degrees has worked well for me, as long as you don't point directly north or south. Take out the backlash in Altitude before performing the PHD2 calibration.
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At the bottom of the window hit Guide, now the calibration will start. PHD2 will move the mount West, East, North, and South in order to understand how the mount behaves. When the calibration is done the scope will begin guiding, this is when i stop guiding and slew to my target to begin capture.
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You don't have to recalibrate if you change target. PHD2 will prompt you to enter declination when starting to guide (ALT for us). If using a mount driver like the Meade Generic PHD2 will automatically know where the scope is pointing.
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If you check "Auto restore calibration" in PHD2 settings you don't have to recalibrate the next imaging session, and as long as the scope behaviour has not changed like if the bearings have been lubricated or become dirty it should work.
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In my experience Multi-star guiding gives much better results than guiding on 1 star, it averages the star movement on up to 12 stars (if i remember correctly). There is a box in PHD2 settings to check "Use multiple stars". However you don't get to choose the main star so if it is very far from where your scope is pointing the object will move out of the main field of view faster, however it has not been a major issue for me.
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I recommend running the Guide assistant when you have begun guiding on the nights target. It will suggest improvements and you only have to click on "apply". The standard settings should work well though! I uncheck "measure declination backlash" since i have not found a reason to measure it.
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It's important that the guidescope is firmly mounted since just a few microns of flexure is capable of throwing of the guidestar by 6 arc seconds, several star diameters, a human hair is 50 microns thick.
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The best i have achieved so far with my Starstructure is 0.69" arc seconds total RMS during a 2.5 minute period, something i never even thought was possible with a dob.
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Seeing plays a huge roll, so even if everything is "perfect" you could get 2 arc second RMS guiding just beacuse of rough seeing.
I have found that Multi-star guiding does a lot to mitigate this. I think my seeing is below avarege compared to global standards.
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1.5 arc seconds and below is what i find acceptable. It's easy to get hung up on the numbers, if the results are good forget the numbers!
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Clear skies!
Gabriel
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Re: ST4 Guide Port action

 

Raphael,

Yes indeed! Since there was only the one GUIDE switch position on Gen1/2 hand pads I had to come up with a system that would support the dual buttons. It is as the labels suggest, though one need not have to know the particulars.?

So if Speed 1/2 is set to 1 which is what probably 90%+ of users use (and most never touch the switch - to my chagrin as it is handy when going to really high/narrow fields) then GUIDE fast is faster than GUIDE slow (by a factor of 2:1, although again one need not know the particulars)!?

Likewise if in Speed 2 once again?GUIDE fast is faster than GUIDE slow (by a factor of 2:1)!?

The programmed GUIDE 1 speed is the reference (base if you will) for slow vs fast
The?programmed GUIDE 2 speed is the reference (base if you will) for fast vs slow(er)

g.


------ Original Message ------
From "Raphael Guinamard via groups.io" <rguinamard@...>
Date 2/16/2025 4:34:47 PM
Subject Re: [ServoCAT] ST4 Guide Port action

Dear both,
Thanks to your message, I realise I made a confusion :
I though the guide 1 was used for guide fast value and guide 2 for guide slow value. But this is completely wrong as you explained and as written in the manual :
"Guide speeds:
With SPEED 1/2 set to 1: GUIDEslow = GUIDE1 GUIDEfast = 2X GUIDE1
With SPEED 1/2 set to 2: GUIDEfast = GUIDE2 GUIDEslow = 1/2 x GUIDE2"
?
Sorry for my mistake and thanks for explanations !
Regards
Rapha?l
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Re: ST4 Guide Port action

 

Dear both,
Thanks to your message, I realise I made a confusion :
I though the guide 1 was used for guide fast value and guide 2 for guide slow value. But this is completely wrong as you explained and as written in the manual :
"Guide speeds:
With SPEED 1/2 set to 1: GUIDEslow = GUIDE1 GUIDEfast = 2X GUIDE1
With SPEED 1/2 set to 2: GUIDEfast = GUIDE2 GUIDEslow = 1/2 x GUIDE2"
?
Sorry for my mistake and thanks for explanations !
Regards
Rapha?l
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Re: ST4 Guide Port action

 

Indeed! The tracking can be extremely good. I had nights where I could measure it using a 240mm guide scope and a camera... and know what it was ... and in AZ it stayed UNDER 1 arcsec for a long period of time.?

We have to remember that this is ALT/AZ. Field rotation is GOING TO BE dominate. MY goal in playing with ST4 was only - only to reduce severe drift in either axis. If the alignment was not great and/or if there are mount errors causing the model of the sky in the DSC to differ from reality - that WILL cause tracking to be off ... and for me it was purely a matter of keeping the view roughly centered to maximize field size with field rotation nibbling away at the edges. This would reduce the necessary cropping that inevitably has to be done. So the intent and hope was to keep 'the center' within maybe 1/2' of total movement over the full time of the desired image capture (15 - 60 minutes).?

I've unfortunately only been able to use it a few times due to mostly weather and the fact that I'm now in a 55+ community with no backyard to set up in... thus limited as to what I can test.?

Gary?


------ Original Message ------
From "Gabriel Wiklund via groups.io" <gabriel.wiklundholeshot@...>
Date 2/16/2025 4:29:02 AM
Subject Re: [ServoCAT] ST4 Guide Port action

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My theory is that the unguided tracking already is so good that the corrections need to be very subtle to make things better instead of worse.
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/Gabriel
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Re: ST4 Guide Port action

 

Raphael,?

Good question.?

First the main intention of GUIDE speeds is for visual use. At the eyepiece. It is there to allow you - looking into an eyepiece, to move around a field at a comfortable rate. That field size might typically be say 25 arcmins. The GUIDE1 speed might be 250 arcsecs/sec. So it would take just over 3 seconds to move from the edge of the field to the center. You can very conveniently select GUIDE fast or GUIDE slow to customize this depending on the actual power and field size... I designed the system to be extremely flexible and conveinent for this action which is probably near 100% of what one does - uses the hand controller for. AND if you go to high to really high power on say a night of good seeing you can flip the SPEED switch from 1 to 2 and have slower speeds available. So as long as the user has taken the time to understand how GUIDE works and programmed the speeds according to their particular telescope (FL) ... they should be quite comfortable using it.?

When you use ST4 AG you now dedicate Speed2 to AutoGUIDING. This, too, isn't an issue as the vast majority of users never flip the Speed switch from 1 to 2 and thus it goes unused for the most part. You can leave the Speed 1/2 switch set to 1 ... have the comfortable guide speeds as discussed above when looking through the eyepiece (or centering on a camera frame) and yet still have the programmed speed2 values for AG. No flipping of switches needed! Maximum convenience.?

Now that all stated - I personally would not set the AG speed to anything less than 10 or so. That is because most of the time the AZ track value is going to be around sidereal (just over 15 arcsecs/sec). But as mentioned in the last note - it is very dynamic and having a "right" speed depends heavily on where you are in the sky.?

Gary?


------ Original Message ------
From "Raphael Guinamard via groups.io" <rguinamard@...>
Date 2/16/2025 3:52:29 AM
Subject Re: [ServoCAT] ST4 Guide Port action

Dear all,
Thanks Gary to have explained the way the guiding correction works.
You say "The way ST4 applies guiding is to add a fixed amount (programmed in GUIDE 2 speed value....".?
OK
Then in his message /g/ServoCAT/message/7395 I see Gabriel has lowered the guidespeed in ServoCAT Sky to 6 arc seconds per second to achieve a good auto guiding
But I see at same time the classical guide2 values are around 160 arc sec/sec for obsessions scopes, whatever their diameter, which is very far away from the 6 arc sec/sec that found Gabriel.
There is a conflict here : if the guide2 value is 6 arc sec/sec for autoguiding, it will be way too slow for visual observing.
Gabriel, how do you manage this if you want to observe visually ?
Changing the rate settings in servocat sky each time you change from autoguiding to visual would be boring...
With your guide 2 being 6 arc seconds/sec, did you change your guide 1 speed -which is classicaly in the 250-280 arc sec/sec - to 160 arc sec/sec to use it for visual observation ? Then you also need to reajust the slew and jog rates also to have smooth ratio change...
In this case the guide 2 will be only allocated to autoguiding and guide 1, slew, jog for visual observing.... Right ???
Unless I miss something ???
Regards?
Rapha?l
Gabriel : I send you a private email to discuss your scope settings and avoid polluting this group. I hope you have received it.
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Re: ST4 Guide Port action

 

I am not using my scope for visual anymore, but if you leave the guide 1 values as they are and the switch on the ServoCAT is flipped to "1" visually you will not be using the guide 2 values when using the handpad, if you decide to image the Autoguide port uses the guide 2 value regardless if you have the switch to 1 or 2 (as i have come to understand it, Gary correct me if i am wrong).
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So it's only if you flip the ServoCAT switch to "2" your handpad will have the very slow value.
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When i got my scope the Guide 1 and 2 values where 110 arc sec/sec.
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I have not seen anybody recommend a guide 2 speed lower than 25 arc sec/sec, but the guide deflections where so massive even when i had 20 arc sec/sec it looked like windgusts where hitting my scope, so i had no choice but to try other values even if no one had recommended them, and autoguiding works very well now! Yesterday i did tests all over the sky and also let it guide for 1 hour on M106 with 10 second subs, no problems so far! I did not even have any problems with backlash, and to be precise.. the measured guidespeed in PHD2 is not even 6 arc seconds per second, but 2.5 arc seconds per second, and for some strange reason it even worked when i did test exposures at about 74 degrees in altitude.
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My theory is that the unguided tracking already is so good that the corrections need to be very subtle to make things better instead of worse.
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Rapha?l i found your email now, it was in the junk mail, i will read it and respond soon!
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/Gabriel
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Re: ST4 Guide Port action

 

Dear all,
Thanks Gary to have explained the way the guiding correction works.
You say "The way ST4 applies guiding is to add a fixed amount (programmed in GUIDE 2 speed value....".?
OK
Then in his message /g/ServoCAT/message/7395 I see Gabriel has lowered the guidespeed in ServoCAT Sky to 6 arc seconds per second to achieve a good auto guiding
But I see at same time the classical guide2 values are around 160 arc sec/sec for obsessions scopes, whatever their diameter, which is very far away from the 6 arc sec/sec that found Gabriel.
There is a conflict here : if the guide2 value is 6 arc sec/sec for autoguiding, it will be way too slow for visual observing.
Gabriel, how do you manage this if you want to observe visually ?
Changing the rate settings in servocat sky each time you change from autoguiding to visual would be boring...
With your guide 2 being 6 arc seconds/sec, did you change your guide 1 speed -which is classicaly in the 250-280 arc sec/sec - to 160 arc sec/sec to use it for visual observation ? Then you also need to reajust the slew and jog rates also to have smooth ratio change...
In this case the guide 2 will be only allocated to autoguiding and guide 1, slew, jog for visual observing.... Right ???
Unless I miss something ???
Regards?
Rapha?l
Gabriel : I send you a private email to discuss your scope settings and avoid polluting this group. I hope you have received it.
?
Autoguiding is now working as it should

 
Edited

At last guiding works as intended and without changing the settings wildly or going against PHD2 recommendations.
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From the last time out I have tightened the screws on my guidescope which caused it to shift slightly, so the main scope is pointing to the lower right corner of the guidescope fov, which is not much of a compromise since i just have to pick the closest guidestar in that corner instead of the center.
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In ServoCAT Sky? i have set backlash comp to 0 on alt and az, lowered the guidespeed in ServoCAT Sky and the Meade Generic driver to 6 arc seconds per second (0.40 x Sidereal).
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I also created a new profile in PHD2 where it detected the 0.40 x Sidereal guidespeed from the Meade driver.
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It was windy outside, the type of wind that makes me very skeptical to even give imaging a try but i gave it a shot anyway!
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PHD2 calibration was completed without any warnings, and after running the guide assistant it recommended min mo of 0.12 in RA/AZ and 0.19 in DEC/ALT. So i followed that.
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I tried guiding first to the north at 38 degrees altitude, then to the east at 50 degrees altitude, in both cases, despite the windy conditions i got 1.0-1.3 RMS guiding in arc seconds. Similar error in both axis and round stars in the test exposures.
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The guiding graph looked similar to my 200PDS on the Sky-Watcher EQ mount without any large deflections apart from a windgust here and there,?
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Then the clouds rolled in! Can't wait for the next clear night hopefully without any wind!
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Moral of the story: Read the PHD2 documentation:
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From the trouble shooting document:

"YouTube videos, unless done by someone close to the PHD2 project, have wildly variable quality, often reflecting misunderstandings and sometimes producing a witches' brew of good and bad advice - so be careful what you watch.? One of the most common mistakes is to wildly change the PHD2 guiding parameters in a futile attempt to correct for mechanical problems that exist in your setup. ?This only makes things worse. The default guiding parameters calculated by PHD2 reflect the parameters you entered in the new-profile-wizard, so they are already adjusted for your setup. You should be able to get reasonable results out-of-the-box if the equipment behaves and if you don't, you are probably dealing with operational mistakes or mechanical problems"
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Edit: I did however follow the below alt/az PHD2 guide (unchecked multi-star guiding and changed the ALT/DEC guiding algorithm to Hysteresis etc)
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Re: ST4 Guide Port action

 
Edited

Hello Gary,
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Thank you for the info, interesting and helpful! I have read some about guiding on Cloudy nights and it seems other systems do the same, they add to the current track rate, so when they mention "0.5 x sidereal" they have stated that the current rate increases or decreases by 7.52 arc seconds, as i am writing this i realize that EQ mounts always are very close to the sidereal rate which would mean that the guidespeed would have to be greater than 1x sidereal for the RA to change directions, that explains why you can't enter more than 1.01x sidereal in the PHD2 guide section, and why backlash only was a problem in DEC when i used my EQ mount.
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By the way, i saw some posts about the Mesu 200 mount with ServoCat Argo with insane tracking accuracy, so it seems, like i think you have written somewhere already - the scope and installation is the limiting factor not the ServoCAT when it comes to Astrophotography.
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/Gabriel
ST4 Guide Port action

 

Gabriel,?

Just a note on how the ST4 guide port works as implemented in the ServoCAT.?

First off both the AZ and the ALT tracking rates are changing continuously on an ALT/AZ mount. They will go from zero as, for example, AZ crosses over directly south in which case the ALT will go from UP through 0 speed and then back to down... it will of course be decrementing (on the east side of the meridian) and slowly incrementing on the west side. I believe the equivalent location in the sky for the same action only in the AZ axis is at a position of 90 - LAT in both AZ and ALT. So if you are at 40¡ã LAT it would be at a position of AZ=50, ALT = 50. Just below that in ALT and the AZ will be slowing down, just above that it will be speeding up.?

Second of course if you reverse direction you will due to the nature of mechanics in gearing, introduce backlash and have to unwind the gears going in the opposite direction.?

The way ST4 applies guiding is to add a fixed amount (programmed in GUIDE 2 speed value and used only when the HC is in GUIDE and the command is coming from/through the ST4 port). It does not replace the actual track rate. It simply modifies it. So if at a particular location in the sky the AZ track rate is 12.52123 arcsecs/second and the ST4 AZ GUIDE 2 speed is 10 then an ST4 command to go west (in the direction of sidereal movement) the track speed will increase to 22.52123. If the east command came in it would go to 2.52123. So it speeds up - or it slows down. NOTE however that if the ST4 rate were, for example, 15, then in one direction it would go to 25.52123 and in the other direction it would go negative to -3.47877 AND in the process lash will be introduced.?

Hindsight says I should have designed it not to go below 0 speed... but it is what it is currently (and there are more than likely additional complications if it were done that way).?

So you can see that ideally the GUIDE speed (ST4) values should, as well, be fluid as the AZ changes drastically going through infinity as one approaches dead-center to the zenith and passes through it. In 0 time it has to go from pointing directly east to directly west. Because of this for any and all factors related to tracking with a dob - avoid the pole! It works extremely well visually even as high as say 86¡ã Alt... but you're getting in to some very high numbers and rapid changes so from an imaging standpoint I'd stay a good distance from the pole - probably a good 10 - 15 degrees away.?

If I were to do it again, now having actually used it and experimented with it... it would be different than what it is. But when i did it very few were using it (I had a handful of people that would talk about it over nearly 20 years or so). But now that CMOS cameras have made very short exposures possible (down to sub 1 second on very large apertures) it becomes something that ideally would be optimized.?

I don't say this as any pressure on Bill.... he is doing fantastic work both in keeping up production as well as working on new and exciting products... but maybe a "what could I have done... " statement on my own part and accordingly: an explanation of what I did probably way way back in about 2004 or so.?

Gary Myers


------ Original Message ------
From "Gabriel Wiklund via groups.io" <gabriel.wiklundholeshot@...>
Date 2/14/2025 9:47:26 AM
Subject Re: [ServoCAT] Promising Autoguiding results in PHD2

[Edited Message Follows]

I would also like to take the opportunity to correct myself. I have now read the PHD2 documentation (mostly the troubleshooting section) and according to what is written there, if you get large/abrupt guidestar deflections like i was getting, wildly changing the settings in PHD2 is not likley to solve the problem, at least not in the long run, it is almost always due to hardware issues which should always be adressed first.?
?
This is what the PHD2 documentation states about the symptoms i had:
?

Large/Abrupt Guide Star Deflections

Most users eventually encounter situations where the guide star appears to make a large, abrupt excursion away from the lock-point. ?The great majority of these problems?arise from neither the mount nor?PHD2's guide commands. ?Instead, they usually come from?unwanted mechanical movement in the gear that is riding on top of the mount, especially the guide camera/guide scope assembly. ?This is especially true if the large deflections occur in declination because the Dec motor is normally idle except for executing the very short, relatively infrequent guide commands it receives. ?The unwanted mechanical movement usually comes from several sources:
  1. Tiny movements of the various components in the guiding assembly as a result of the changing gravitational forces while the mount tracks the target object
  2. Dragging, binding, or snagging of cables, especially those that are connected to the guide camera
  3. Wind gusts or less commonly, effects from camera filter changes, auto-focusing, or mirror movement
  4. Use of mount features for backlash compensation - these should not be used with PHD2 guiding
Before rejecting these things as likely sources of? problems, think again about the tiny measurement scales and tolerances described in the previous section. ?With many guiding set-ups, a movement of only 5 microns can create an apparent tracking error (guide star deflection) of over 6 arc-sec, the equivalent of many star diameters. Every mechanical interface, every set-screw, every movable element has the potential to shift or move on its own by these tiny amounts. ?Even when cables have been routed in a purposeful way, they may bind or pull in certain sky positions or after a meridian flip. ?Cable ties or ribbed plastic cable guides hare small protrusions that can briefly catch on stationary parts of the mount. ?For large Dec deflections, it's easy to determine if these things are coming into play. ?Just use the PHDLogViewer tool to zoom in on the time of interest and see if the deflection was immediately preceded by a correspondingly large guide command in the direction of movement. ?In most cases, you will find this didn't happen. ?It can sometimes happen at the beginning of a guide session if you're using PHD2 Dec backlash compensation, but those events should disappear quickly. ?If the abrupt deflections occur in RA, the analysis is less straightforward because the RA motor runs continuously. ?But even then, unusually large, randomly space deflections are more likely to arise from the sorts of mechanical problems described here than from errors in the RA drive system.
?
?
So i took a closer look at how my guidescope is mounted and it could be a problem, i have tightened the screws and we will se if it makes a difference.
?
Guidespeed was probably also the problem in the beginning and just to see what will happen next time i will try 10 arc seconds /per second in ServoCat Sky, which is actually what the link with the Panther mount recommends and also the defualt value in the Meade Generic driver.
?
I also read in the documentation that you should not change the guidespeed in PHD2 "manually" you should reset and create a new profile where that is entered in the first steps, i noticed that if i enter 10 arc seconds per second in the Meade Driver and in the beginning in PHD2 when creating the profile, PHD2 does not change it to 1.01 arc seconds like i wrote earlier so now both the Meade driver, PHD2 and ServoCAT sky has 10 arc seconds /per second, i hope it clears up tonight so that i can give it a try! (i have also set backlash comp to 0 on both axis in ServoCat Sky this time)
?
In the first calibration, If the measured guidespeed in PHD2 turnes out to be say 5 arc seconds per second instead of 10, then i will re-do the profile and enter the new speed (0.33 x Sidereal in that case) so that the correct guide-pulse length is calculated for calibration and guiding.
?
Here is the PHD2 user guide: https://openphdguiding.org/PHD2_User_Guide.pdf
?
/Gabriel
?
?
Re: Promising Autoguiding results in PHD2

 

Thanks Gabriel,
clear !!!
Re: Promising Autoguiding results in PHD2

 

I have seen very few images taken with dobs that have been autoguided, i can only remember one and that was on an EQ platform from Tom O, so i have only read about 2 that have succeded in doing this in alt/az (Dan Price, and another Obsession owner).
?
So you don't need autoguiding, and you don't need to take even 10 second exposures, with a modern low read noise camera you can get away with 3 seconds even.
?
The fact that your guidescope has a long focal length is a lot better than the 50mm guidescope i am using which only has 180mm focal length, even if they are not perfectly aligned i would still give it a try and see the results, then you know for sure (if you want to try autoguiding).
?
The only reason i am trying to autoguide is because i am having great fun problemsolving and i am very fascinated bu the subject of tracking accuracy, it may even be difficult to tell the difference on my future images even if i get autoguiding to work well.
?
Even if the guidescope and mainscope are not pointing at the same spot in the sky, as you are indicating i suspect you can still keep the object reasonably centered probably for even more than 30 minutes, then you would just re-center and pick a new guidestar.
?
/Gabriel
?
?
Re: Promising Autoguiding results in PHD2

 

Thanks Gabriel for clarification.
You indicate your unguided precision is "0.08 arc seconds per axis per second".
In the last session I did to improve my ratio, I measured initially a 0.05 arc seconds per second in az and 0.32 arc seconds per second in alt.?
I have then calculated at home the correction factor and the new ratio in alt and az have been put in servocat but bad weather then.
I will check next time I have good weather at observatory what are my new az and alt precision. But I will certainly be globally not that far from you on precision.
I then have some questions :
?
* I see wonderfull pictures taken with alt/az scopes here. Are these pictures taken using the autoguiding method ? Or with unguided scopes ? In the latter case what is your precision of these unguided scope in arc sec/ sec ??
Because I wonder if, as a newbie in astrophoto, autoguiding is necessary or not...
If I read correctly your initial message Gabriel, I understand that you have improved precision of guiding by a factor 2 when using the autoguiding method (from 2-2.5 arc" to 1.3 arc " for your exposure time of <13"). It this improvment necessary for a newbie ?
?
* I have a 10" (f=1150 mm) scope in // of the 36" (f=3150 mm). This could be used for autoguiding, no ?
The only potential problem is that the parallellism of the 2 scope may vary a little bit when I change location in the sky because the 10" is fixed by its back on a plate (with adjustment screw at back of the plate to parallelize the 10"), and I believe a little bit of flexion occurs certainly. Not a big deal in visual, but in astrophoto I don't know... Maybe on a given zone of the sky it will not change its flexion for a total 30 min posing? I don't know...
?
Thnaks for your feedback
Rapha?l
?
Re: Promising Autoguiding results in PHD2

 
Edited

I would also like to take the opportunity to correct myself. I have now read the PHD2 documentation (mostly the troubleshooting section) and according to what is written there, if you get large/abrupt guidestar deflections like i was getting, wildly changing the settings in PHD2 is not likley to solve the problem, at least not in the long run, it is almost always due to hardware issues which should always be adressed first.?
?
This is what the PHD2 documentation states about the symptoms i had:
?

Large/Abrupt Guide Star Deflections

Most users eventually encounter situations where the guide star appears to make a large, abrupt excursion away from the lock-point. ?The great majority of these problems?arise from neither the mount nor?PHD2's guide commands. ?Instead, they usually come from?unwanted mechanical movement in the gear that is riding on top of the mount, especially the guide camera/guide scope assembly. ?This is especially true if the large deflections occur in declination because the Dec motor is normally idle except for executing the very short, relatively infrequent guide commands it receives. ?The unwanted mechanical movement usually comes from several sources:
  1. Tiny movements of the various components in the guiding assembly as a result of the changing gravitational forces while the mount tracks the target object
  2. Dragging, binding, or snagging of cables, especially those that are connected to the guide camera
  3. Wind gusts or less commonly, effects from camera filter changes, auto-focusing, or mirror movement
  4. Use of mount features for backlash compensation - these should not be used with PHD2 guiding
Before rejecting these things as likely sources of? problems, think again about the tiny measurement scales and tolerances described in the previous section. ?With many guiding set-ups, a movement of only 5 microns can create an apparent tracking error (guide star deflection) of over 6 arc-sec, the equivalent of many star diameters. Every mechanical interface, every set-screw, every movable element has the potential to shift or move on its own by these tiny amounts. ?Even when cables have been routed in a purposeful way, they may bind or pull in certain sky positions or after a meridian flip. ?Cable ties or ribbed plastic cable guides hare small protrusions that can briefly catch on stationary parts of the mount. ?For large Dec deflections, it's easy to determine if these things are coming into play. ?Just use the PHDLogViewer tool to zoom in on the time of interest and see if the deflection was immediately preceded by a correspondingly large guide command in the direction of movement. ?In most cases, you will find this didn't happen. ?It can sometimes happen at the beginning of a guide session if you're using PHD2 Dec backlash compensation, but those events should disappear quickly. ?If the abrupt deflections occur in RA, the analysis is less straightforward because the RA motor runs continuously. ?But even then, unusually large, randomly space deflections are more likely to arise from the sorts of mechanical problems described here than from errors in the RA drive system.
?
?
So i took a closer look at how my guidescope is mounted and it could be a problem, i have tightened the screws and we will se if it makes a difference.
?
Guidespeed was probably also the problem in the beginning and just to see what will happen next time i will try 10 arc seconds /per second in ServoCat Sky, which is actually what the link with the Panther mount recommends and also the defualt value in the Meade Generic driver.
?
I also read in the documentation that you should not change the guidespeed in PHD2 "manually" you should reset and create a new profile where that is entered in the first steps, i noticed that if i enter 10 arc seconds per second in the Meade Driver and in the beginning in PHD2 when creating the profile, PHD2 does not change it to 1.01 arc seconds like i wrote earlier so now both the Meade driver, PHD2 and ServoCAT sky has 10 arc seconds /per second, i hope it clears up tonight so that i can give it a try! (i have also set backlash comp to 0 on both axis in ServoCat Sky this time)
?
In the first calibration, If the measured guidespeed in PHD2 turnes out to be say 5 arc seconds per second instead of 10, then i will re-do the profile and enter the new speed (0.33 x Sidereal in that case) so that the correct guide-pulse length is calculated for calibration and guiding.
?
Here is the PHD2 user guide: https://openphdguiding.org/PHD2_User_Guide.pdf
?
/Gabriel
?
?
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