Why do I get a Bright Centre when I use a Newt?

[PeterC65]

I've been trying out my Player One Uranus-C camera with my APO and Newt and noticed last night with the Newt that there is a bright central glow around fainter targets that I don't see with the APO. It almost looks like noise. You can see it in the image below of M33.

Is this something peculiar to Newts, and if so, what is it? Or am I just seeing glow from the target? Any ideas about how to reduce it (I much prefer the APO images)?

 

[Paul M]

That's Vignetting. The fully illuminated image circle formed at the scope's focal plane is smaller than the camera sensor. The image brightness tapers off from the center. Your Apo will have a larger image circle that illuminates more of or most of the sensor.

It's common and easily remedied by taking "flat frames" that your stacking software will use correct your final image.

 

[ONIKKINEN]

Indeed, it is vignetting and flats are the cure.

[PeterC65]

Thanks!

[markse68]

If you wanted you could fit a larger secondary to get a larger image circle too