Dithering with an Off Axis Guider


#1

I used my QSI 683wsg last night for the first time with my 120APO f7. The guider reported dithering but after going to process it today I noticed that the stars barely moved. I had the dither set to medium and settle at 1. I noticed it having trouble settling so I change that to 2.
I guess I am having trouble setting the dither up. I am use to BackyardEOS and a guide scope with the standard settings working well from the start but now that I have moved to an off axis guider I feel a little lost. Any help or insight is welcome, thanks
Chris


#2

Your guilder sampling has likely changed dramatically unless you were using a fairly long guide scope. The dither amount (Small, Medium, Large, etc) tells PHD to move a certain amount of pixels. So if you increase your guider focal length but do not increase the dither amount then you will effectively be dithering fewer pixels on your main scope.

For instance let’s say that the focal length of your main Scope is 1000mm and your guider was 500mm…and for simplicity let’s assume that their pixels are the same size. 1 pixel distance on your guider is 2 pixel distance on your imager. Now you tell PHD to dither at a Medium rate (and for argument sake we’ll assume this is 5 pixels, I can’t recall the actual amount…). That moves the guider 5 pixels and your imager 10 pixels.

Now let’s move your guider onto the same scope as your imager with an OAG. Focal length is the same and the guider pixel equals the imager pixel. So if you keep the Medium dither setting then you’ll be dithering 5 pixels on your guider and 5 pixels on your imager…so half the amount.

Now I don’t know the actual focal length of your guider…but if we assume it’s the ever popular ST80 (at 400mm focal length) then moving the guide cam to an OAG would effectively be cutting your sampling size in half.

So, long story short. To get the same amount of dither that you were seeing with the guide scope you’ll need to increase your dither amount. You can do this on the Auto Guide area of the Control Panel or by editing your Profile:


#3

Thanks for the info. I have been using guide scopes so long, yes the Orion 80 and recently a 60mm, that I never had to change the profile. Now with the off axis guider I have a little more learning to do.I updated a new profile for PHD2 to adjust for the different focal lengths for my different scopes and will try out the different dither options.
SGP has been great, I just didn’t understand the dither option as well as I do now but to many cloudy night have not allow me to play with everything.


#4

Set it to Very High or Extreme Dither and Settle at < 1 pixels for 8 seconds. My setup is similar to yours. I think dithering in SGP is a little too conservative and I did tried at Medium and the guide star hardly moved.

Peter


#5

The dither amplitudes – in guiding cam pixels - are:

Dither	    Pixels
Small	    0.5
Medium	    1.0
High	    2.0
Very High   3.0
Extreme	    5.0

These values are defined by PHD2 and are eventually multiplied by the Scale setting (Brain -> Global tab -> Dither Settings).

You can convert this in arcsec by multiplying the result with the image scale of the guiding cam. If you then divide by the pixel scale of the imaging cam, you get the dither amplitude in imaging pixels.

Regards,
Horia


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