Deformed star shapes ZWO OAG Guiding

SB (Chris) · #1 20-08-2022, 07:21 AM

Hi Everyone
I’m getting deformed star shapes recently with my ZWO OAG. It doesn’t affect the guiding too much but I would like to see nice round guide star shapes again.

I have checked the prism and it is at the correct angle and positioned well into the scopes image path. The camera is seated well and the glass protector is seated correctly over the ASI120mm-s sensor.

Does anyone have any thoughts on what else could be the problem?

Many thanks

SB

wayne anderson (Wayne) · #2 20-08-2022, 08:29 AM

Hi Chris,

I had the same happen with my OAG and guiding stars just after I made a few small adjustments to the systems focal path, it turned out that when the stars were focused on the main camera sensor the guide stars were out of focus and looked just like yours.
With a few small adjustments to the variable adapter i have to the guide camera it was fixed and round stars again, not much change to guiding performance though, hopefully this helps, others may also have thoughts on this.

Regards, Wayne

The_bluester (Paul) · #3 20-08-2022, 09:34 AM

Is that with a refractor using a flattener or reducer/flattener?

I have found with mine that if the spacing is not quite right you get the same sort of effect on the guide camera that you do on the imaging cam (Comets or fishbowl effect) As you generally have the prism out as far from the center of the image as practicable to make sure it does not cast a shadow on the main sensor, whatever effects there are will be really pronounced.

Try tweaking the guider focus in and out a little to see if the star shape changes. While it may put them out of focus a little, if it improves the shape that might indicate the spacing is not quite right.

SB (Chris) · #4 20-08-2022, 11:08 PM

Thanks Wayne and Paul,
Your feedback was very helpful. I rearranged the OAG to be positioned over the long side of the main camera sensor moving it further into the light path. While not getting round star shapes there was a noticeable improvement. I was using the OAG with an FMA180. As it was difficult to mount a guidescope on this scope.

SB