PHD2 LOCK POSITION after dither calculation

[jkc...@yahoo.com]

Below are two lines from a PHD2 GuideLog file(PHD2 Ver 2.6.10dev3).

 INFO: SET LOCK POSITION, new lock pos = 64.374, 526.817

INFO: DITHER by -0.032, -1.746, new lock pos = 65.973, 526.114

 

I am assuming: 

the LOCK POSITION in the first line is the initial lock position with x =64.374 and y = 526.817

The DITHER is x = -0.032 and y = -1.746

The new dithered LOCK POSITION is x = 65.973 and y = 526.114

I don't see how the new dithered lock position(65.973, 526.14) is calculated from the DITHER parameters.

Could you please explain.

 

Thanks 

Jack

[Bruce Waddington]

There are two coordinate systems involved here.  The lock point is expressed in the X/Y coordinates of the camera sensor.  The dither amount is expressed in the RA/Dec sky coordinates, so there is a transformation done based on the current calibration data.  That’s the way it needs to be, dithering wants to move the mount in RA and Dec, regardless of how the guide camera is oriented.

 

Hope this is clear,

Bruce

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[Brian Valente]

btw you can view the RA/Dec coordinates overlaid with X/Y coordinates from the View menu

here's an example

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Brian Valente

portfolio brianvalentephotography.com 

[Florian Brill]

Hei everyone!

Is it normal that PHD2 is setting the new position after a dither command to the new reference position and for the next dither command, PHD2 will apply the random movement to the new lock position?! I always thought that the lock position would stay the same throughout the whole session and dither movements would always refer to this initial lock position. I checked my latest Guiding Log which was showing that after each dither movement, the new position was set as the new lock position. During this session, PHD2 dithered 7 times and when plotting the path in Excel, it looked like it would slightly "drift" away like a drunken sailor.

It would be nice to be able to stick to the first lock position for the entire session. I know that there is a "stick to lock position" option, but I think it does something different.

Hope you can help me.

Thanks a lot!

Florian

[Bruce Waddington]

Dithering is accomplished by changing the lock position – that is the definition of dithering.  The goal is usually to prevent many exposures from having exactly the same positioning on the camera sensor, something that is especially important with CMOS sensors and their fixed pattern noise.  If the original lock position was given some special preference as a reference point, that goal would be compromised.  If you want more predictability about the dithering pattern, you can try using the ‘spiral’ dithering pattern.  If none of this is satisfactory, you can implement your own dithering application using the PHD2 server interface.

 

Bruce

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