Why is my telescope drifting South while guiding with PHD2?

[Dorin-Lucian]

I have noticed that my telescope and the frames are drifting South constantly. Initially I thought it is the telescope flexure but the same drift happened imaging towards South (I.e Hamburger Galaxy) and North (Iris Nebula) despite the fact that gravitational forces should cause an opposite drift. My PA was reasonable (with 1 Star alignment the scope pointed to the target with 8 minutes error- still in the camera field of view). I use a 12 inch SW with a focal length of 1500mm and a 60mm guidescope with a 2.25X Barlow on it to achieve 540 mm focal length. I modified the guidescope so the most of the length it's not flexible and it is attached with  screws on the main scope. My dark library for guiding camera is good(green on PHD2 and I choose the stars), and as I go through the main camera subs they are always drifting South and a bit West no matter the scope orientation. The overall drift in a 6 hours is about 6-7 minutes with a good guiding graph and the stars within 3 arcsec on the bull's eye target. Can I turn off the dithering option in PHD2? The lowest setting is 0.1 and changing tk this from 1 didn't change much. I use SharpCap as imaging program, can it be the cause by requesting dithering?

[malc-c]

It might help if you stated the make and model of the mount you are using, and the scope involved.   Also if you can post you PHD2 guidelog that would be helpful for some of the PHD gurus to diagnose any possible causes

[michael8554]

16 hours ago, Dorin-Lucian said:

My PA was reasonable

What figure was that ?

The drift sounds like the cumulative effects over 6 hours of a small PA error, plus a little due to Dither.

16 hours ago, Dorin-Lucian said:

Can I turn off the dithering option in PHD2? The lowest setting is 0.1 and changing tk this from 1 didn't change much

The PHD2 setting is a multiplier of the setting in the software that is generating the Dither.

So if you have a 12 pixel Dither in SharpCap, then a 0.1x setting in PHD2 will change that to a 1.2 pixel Dither.

But why would you want to turn off Dither, it is usually beneficial to images ?

I guide a 1280mm scope with a 200mm guidescope, 540mm sounds excessive to me.

But depends on the image scales of the guidecam and image cam, let's see the GuideLog.

Barlowing the guidescope raises the f/number by 2.5X,  so now the guidecam will require much longer exposures.

16 hours ago, Dorin-Lucian said:

I choose the stars

You should let PHD2 choose the stars, another sign that the high f/number is giving PHD2 a poor choice of stars .

Michael

[Dorin-Lucian]

The mount that I use is SW AZ-EQ6 Pro and Sw 305p Flextube main telescope. I didn't save a guidelog but I will once the sky gets cleared. Anyway I didn't receive any message from PHD2 that it cannot keep up with the mount movement as you sometimes get when you have a bad alignment. I have a picture with the guiding from an imaging session at the end of which the guiding was suboptimal. I have also attached the main camera view, (red) guidescope view with Barlow (yellow) and without it(green)

22 hours ago, malc-c said:

It might help if you stated the make and model of the mount you are using, and the scope involved.   Also if you can post you PHD2 guidelog that would be helpful for some of the PHD gurus to diagnose any possible causes

Regarding the star selection I always play with exposure gain to maintain the guide camera in between 1 and 3 seconds exposure so I would not select a bad pixels for example. I know that dithering is good, but I want to track down the error. I saw after my post that the SharpCap setting was maximum dither step was 20 pixels with a minimum and maximum settle time of 5 and 60 seconds( I don't really understand this values)

 

20 hours ago, michael8554 said:

What figure was that ?

The drift sounds like the cumulative effects over 6 hours of a small PA error, plus a little due to Dither.

The PHD2 setting is a multiplier of the setting in the software that is generating the Dither.

So if you have a 12 pixel Dither in SharpCap, then a 0.1x setting in PHD2 will change that to a 1.2 pixel Dither.

But why would you want to turn off Dither, it is usually beneficial to images ?

I guide a 1280mm scope with a 200mm guidescope, 540mm sounds excessive to me.

But depends on the image scales of the guidecam and image cam, let's see the GuideLog.

Barlowing the guidescope raises the f/number by 2.5X,  so now the guidecam will require much longer exposures.

You should let PHD2 choose the stars, another sign that the high f/number is giving PHD2 a poor choice of stars .

Michael

I you need other information, gladly, but I don't have a guidelog unfortunately. I did order an off axis guider which I will try ASAP 

[michael8554]

Did you understand my opinion on your guidescope focal length ?

Did you understand my opinion on the cause of the drift over the 6 hours ?

How did you Polar Align ?

PHD2 saves a GuideLog for every session in the PHD folder.

Look for the correct date in the filename.

I've never had the exposure set to Auto.

46 minutes ago, Dorin-Lucian said:

dither step was 20 pixels with a minimum and maximum settle time of 5 and 60 seconds( I don't really understand this values)

These are explained in the SharpCap documentation:

Max Dither Step – this specifies the maximum distance that any dither movement can be, measured in guide camera pixels (i.e. pixels in PHD2, not in SharpCap). Note that MGEN does not take any notice of this setting, and that if Dither Only Guiding is selected then this setting is interpreted as the size of the dither movement step in arc-seconds.

Settle Threshold – when the movement between two guide camera frames drops below this number of pixels then the dither is considered to have ‘settled’ – i.e. the mount movement is complete and images taken should no longer be blurred by movement. Only applies to PHD2 guiding.

Minimum Settle Time – a dither will not be considered to be settled until this amount of time has elapsed after the end of the movement, even if the Settle Threshold is met before this time has elapsed. Only applies to PHD2 guiding.

Maximum Settle Time – a dither will always be considered to be settled after this amount of time has elapsed, even if the settle threshold has not been met. Only applies to PHD2 guiding.

Michael

[Dorin-Lucian]

59 minutes ago, michael8554 said:

Did you understand my opinion on your guidescope focal length ?

Did you understand my opinion on the cause of the drift over the 6 hours ?

How did you Polar Align ?

PHD2 saves a GuideLog for every session in the PHD folder.

Look for the correct date in the filename.

I've never had the exposure set to Auto.

These are explained in the SharpCap documentation:

Max Dither Step – this specifies the maximum distance that any dither movement can be, measured in guide camera pixels (i.e. pixels in PHD2, not in SharpCap). Note that MGEN does not take any notice of this setting, and that if Dither Only Guiding is selected then this setting is interpreted as the size of the dither movement step in arc-seconds.

Settle Threshold – when the movement between two guide camera frames drops below this number of pixels then the dither is considered to have ‘settled’ – i.e. the mount movement is complete and images taken should no longer be blurred by movement. Only applies to PHD2 guiding.

Minimum Settle Time – a dither will not be considered to be settled until this amount of time has elapsed after the end of the movement, even if the Settle Threshold is met before this time has elapsed. Only applies to PHD2 guiding.

Maximum Settle Time – a dither will always be considered to be settled after this amount of time has elapsed, even if the settle threshold has not been met. Only applies to PHD2 guiding.

Michael

Thank you, that is very instructive, I haven't had time to go through all the settings yet, throughly. I am not sure how much it helps me with my problem. The guiding graph shows movements in both directions on Dec axis so if it were a backlash issue you'd see PHD2 struggling to compensate with repetitive and increasing signals in opposite direction. I mean you know when something is wrong with the tracking, it must be flexure ultimately. 

[Dorin-Lucian]

Thoroughly ^^

[Dorin-Lucian]

I have installed the off axis guide,  I am more than happy with it, I am testing it, but with a 15 minutes frame I pictured some great stars with no trails.