Re: Very oval stars with excellent guiding - how?


Hi Eric

I can offer a few observations:

On your model, you may have a few errant points (max error on east is 174 which seems high to me)
I suggest you enable these two options

Second, enabling/disabling your guiding should not be that dramatic, especially for an encoder mount.  your comment "although checking later showed azimuth off by about 15 minutes and altitude about 1 minute." sounds to me like something may have slipped or settled? maybe time to check things over and do another polar alignment

Finally, anytime I hear "great guiding, but oblong stars" my first reaction is differential flexure. Not sure what is your guiding setup, but if you are guiding on a separate scope, that is definitely a suspect


On Sun, Apr 24, 2022 at 3:58 AM Eric Claeys <AstroEric@...> wrote:
I have a TEC 140 refractor on a 1100GTO with encoders on a permanent pier.  I ran a 378 point model, and am getting excellent guiding with SGP and PHD2; mostly +/-  0.25 pixels, but getting very oval stars.  I'm not sure how that can happen, and don't know how to fix it.

Full-size pictures and additional information, including full setup, is at

Here's the model results.  I have no idea if this is "good" or not.

Here's guiding with tracking corrections On, then Off.  Even when Off I'm getting pretty good guiding.  Note the scale is +/- 1px.

The right side of this is with guiding OFF.  I believe it shows polar alignment isn't perfect, but it's pretty close.  Note the scale is +/- 4px.

And hear are the oval stars:

How can I be getting very oval stars with extremely good guiding and pretty good polar alignment?

This just started happening recently.


Brian Valente

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