Help understanding tilt

[Ferrante Enriques]

hi all,
using the ASTAP stack->inspector process I was able to measure the tilt of the sensor of my camera.
The results in focuser steps were then converted to microns (see attachment).

Are these numbers showing a tilt that needs to be corrected? or, in other words, when the tilt is 'bad'?
I understood from another software that when tilt equals CFZ it needs to be corrected. Does this rule make sense?

Thanks
Ferrante

[han.k]

Hello Ferrante,

The looks like a tilt which requires a correction. Microns or focus steps need some scaling to HFD. What does ASTAP show when you load a focused image and hit F5 (or F4 directly) for the image inspection. Any difference between the corners of more the 10% requires adjustment/correction. 5% or less would be good.

Regards, Han

[han.k]

Expressing the nine values is delta HFD is probably the best solution. From the focusing curve a HFD/step factor can be determinated. I will look into that.

Han

[Ferrante Enriques]

hi Han,
thanks for looking into that. I agree that the best focus plane is a better solution for tilt measurement.

Anyway, I took a short exposure (1s) at best focus* and used the Tilt (F4) tool on ASTAP.
The reported tilt is 8%, see attachment. The direction is consistent with the delta HFD method.

Regards,
Ferrante

• Ekos measures focus on multiple stars over the FOV so this 'best' focus is an average over the sensor.

[han.k]

That is indicating there is something to gain by adjusting the tilt.

For the ASTAP tab inspector, the idea is to copy the HFD's value of the ASTAP table into a spreadsheet. That will indicate the tilt or curvature in an other way as show in this image from cloudynights:

https://www.cloudynights.com/uploads/monthly_02_2022/post-241090-0-15056100-1646001078_thumb.png

https://www.cloudynights.com/topic/809050-tilt-discussion-astap/page-13#entry11741905

Any horizontal shift will indicate a tilt. Any vertical shift of the curves will indicate aberration.

As indicated here:
https://www.cloudynights.com/topic/809050-tilt-discussion-astap/page-18#entry11765253

C indicates tilt/curvature, E-B indicatates aberration.

Regards, Han

[Ferrante Enriques]

hi Han,
plotting HFD values makes clear the tilt and its direction and confirms the what the curve fitting shows.
I only moved the focuser by 1000 steps that should translate to 1mm. I will increase the travel on a next test.
But as the delta HFD between center and outer ring should be constant, it seems it's only 0.5 HFD.
(left chart).
The aberration is small.

Thanks
Ferrante

[han.k]

Yes it is all minor. You system is good to go imaging :)

Han

[Ferrante Enriques]

thanks Han. I will try to tweak these minor issues. I saw that astap has also a live monitoring session that will guide me through the process.