Serious Stray Light Issue - William Optics UltraCat 56 - Forums

I’m posting this to see if anyone else has experienced or can verify this problem in their UC 56 telescope.

I received the telescope February 2, 2026 from Agena Astro. The quality control certificate that was enclosed was dated December 5, 2025. The telescope serial number is 250130.

I want to use this telescope with an ASI6200 MM camera and LRGB SHO filter set at a remote site. I also want to use a rotator. I live in Phoenix so the initial testing was done in a Bortle 9 sky but I was interested mainly in what the stars looked like in the corners and edges.

I early test exposures I noticed stray light in the form of concentric arcs. I eventually identified the source of the stray light as bright stars positioned 3-4 degrees off the long end of the full frame sensor, approximately 7 to 8 degrees off the optical axis of the telescope. For really bright stars I could see the arcs in short exposure plate solving images.

I removed the filters and rotator and replaced them with M54 extension tubes to maintain 55 mm of back focus. The stray light was still there but there was only one ring. I tried two other cameras, a Nikon Z6 mirrorless and a Nikon D850 DSLR, and the stray light arc was exactly the same size and shape. My thought was the stray light was being generated in the telescope optics.

I put a flat panel in front to the telescope, and removed everything from the back of the telescope except the WO provided M54 adapter that screws into the back of the telescope. When I looked into the back of the telescope, I was shocked to see a bright ring well past the exit pupil of the optics - first attached photo.

On close examination of the last element of the telescope optics, I was able to confirm the bright ring was being created by a reflection from the inside wall of the aluminum lens mounting ring. This rear ring is anodized but it has not been blackened or flocked with black anti reflection material. Since the light is at a grazing angle, the surface is highly reflective. I hope this is a one-off quality control failure and not the intended design since it would be a major problem for anyone using a full frame sensor.

I informed WO early on about the stray light and they responded with an assessment that it could be a light leak. I followed up with the photos in this post and my experiments with 3 cameras but have not heard back.

Agena Astro has issued an RMA and I have shipped the telescope back to them for a refund. I very much like the optical quality of the images from this telescope but lack of stray light control is a deal breaker. I am reluctant to try another copy unless I can verify others are not seeing this problem. By the way the M48 adapter still passes the stray light so it is possible in some framings this would be a problem for APS-C sized sensors.

Has anyone with an UltraCat 56 (or any UltraCat) had this problem or can verify the stray light is properly controlled in their copy?

Bob Fugate

Attached images:

Stray light ring observed looking into the telescope - flat panel illumination.

Close up of stray light ring.

Photo showing the rear lens mounting ring.

Typical stray light effect on full frame camera - no filters installed.

📷 Z60_7446.jpg📷 Z60_7460.jpg📷 UC56 stray light annotated photos.005.jpeg

📷 ASI6200-2026-02-25_20-33-27.jpg

Good instincts with performing this test - I have had similar issues in the past with my Nitecrawler WR25. It’s a very frustrating design oversight that seems to make it’s way into all sorts of gear. I often find myself thinking “they should have known better”… but I digress. My solution ended up being pretty simple and I think may be applicable here if you have the space for it and would prefer not to RMA. I ended up 3D printing a couple knife edge baffles that were able to sit between the gap of two adapters just after the nitecrawler drawtube. They don’t need to cut very far into the light cone to be effective, just enough to cover the affected area so that reflections off it cannot reach the sensor. Here’s a diagram of a cross section demonstrating what that might look like:

Note that the baffle does not actually take up any backfocus because it is sandwiched between two existing adapters/spacers in the stack.

Here was the original problem:
And a before and after of the arc reflection (with and without the baffle)