Serious Stray Light Issue - William Optics UltraCat 56

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)

I have an Ultra-Cat 108, and I have no light leaks at all. It is the best telescope I have ever owned, other than my Stellarvue SV70T. I am sure it is a one-off issue, and if they exchange it for you, my guess is that the next one will be perfect. It sucks that this happened to you.

rqfugate · Mar 2, 2026 at 01:48 AM

I really do like the optical quality of the images it produced. Thanks for the info on your 108. Do you know if that last lens mount is blackened on your telescope? It is something one wouldn’t pay attention to if there isn’t a problem, so no worries if you don’t know.

I have no idea if it is blackened or not. I will get back with you tomorrow and let you know.

Hi. I did not see where your system came into focus in your initial description. I own both Cat 91 and Ultra Cat 56 scopes. I have not yet seen this issue with either using APS-C or Full Frame sensors. I did notice both of mine came into focus at the nearly all in position. I thought they should be more toward center of focus travel. I found a support article discussing this while researching the issue. It specifically references light leaks and where the focuser should be operated at on their Petzval WIFD designs. I wonder if this is to prevent the issue you are having. Article is at: https://support.williamoptics.com/guides/back-spacing-range-on-petzval-systems

Here is an image to show where WO wants the scopes to focus.

CS,

Jeff

rqfugate · Mar 2, 2026, 03:46 PM

I just received this response from William Optics:

—————

Dear Bob,

Thank you very much for your thorough testing and for sharing the additional photos.
We truly appreciate the time and care you’ve taken to investigate this issue.
The lens mount finish on WO telescopes is designed to be glossy (not matte), and this does not affect imaging quality under normal conditions.

However, the light ring you described — caused by reflection from the inner wall of the end lens retainer — may be an isolated case.

We recommend that you contact Agena Astro to request an exchange.

Best regards,

WO

Karen

————————

So, the design includes a glossy lens retainer ring. I don’t think my telescope was an isolated case - the laws of optical physics are in effect at all times and all places. And I guess ‘under normal conditions’ means don’t look at any part of the sky near a bright star.

This telescope may still be useful for sensors smaller than full frame - seems what most people are using. But for full frame use, the built-in stray light is a deal breaker for me. I have requested a refund.

And a note to Astro Jeep: you will probably find an anodized retainer ring. But one thing that may be different for the 108 is the distance from the last lens element to the focal plane. If the lens assembly is further up the tube it may not be a problem for full frame.

Hope this is useful to others considering this telescope.

I would tend to agree with you, especially in light of Charles’ post where he clearly shows the ring before and after his baffle addition. (Great fix!)

As an avid DIY’r I’d probably seen if I could black it out myself. But for a high-end model Petzval I’d expect it to be correct out of the box.

Shame on Karen! But then, she is a Karen…. Now that WO is informed, they should be doing quality control on the remaining stock. They have a reputation to defend.

Bob,

The sort of thing you found by looking into the back of your scope with the objective illuminated by a flat panel is not uncommon. I have been on the warpath to figure out why I get odd colored arcs in my calibrated frames on several image trains. I started looking at my image trains in this manner and found they all have issues to a varying degree.

One of the more interesting cases was my Edge HD1100 with dedicated 0.7X focal reducer. There was a rainbow-colored reflection off of the back end of the focal reducer. Inspection showed this to be a reflection on the very shiny anodized inner surface of the focal reducer just behind the backmost optical element. The colors change wildly as you move around looking at the reflection. I put a quarter inch strip of flocking around the shiny inside edge of the focal reducer and it was gone. I have attached the before photo. In addition, in this shot you can see that the inside of the baffle tube is also not black, but a dull gray. The inside of the baffle tube is machined with threads, but that was not totally effective. I was able to put some flocking inside the rear part of the baffle tube up to the corrective optics. It helped, but it is not fully effective. I have attached an after photo as well.

📷 EdgeHD 1100 with 0.7x FR.jpg📷 EdgeHD 1100 with 0.7x FR and Some Flocking.jpgI had similar issues with my WO Megrez 90. The tube has knife edge baffles, but the inside of the very front of the optical tube behind the objective and before the first baffle, while painted flat black, reflected a dull gray that can be seen by the sensor. I flocked that and the reflection was dramatically reduced. Unfortunately, I did not take any photos.

Many telescope and accessory manufacturers either paint the inside of optics flat black or machine threads the length of the optics, all thinking that this will eliminate light reflection down the light path. My examination of many items, from extension tubes to focal reducer/field flatteners to focusers to telescopes themselves, reveals that this is far from the truth. Sometimes threads are especially reflective at the glancing angle involved. Knife edge baffles work well if placed well, be even they can have a reflection on the inner flat circumference that is however thick that the baffle is if it is at all reflective.

I think every astro imager should take a look at their optics in this manner to see what can be addressed. Flock accordingly if possible. Unfortunately, Protostar, which was a source for some excellent flocking products, is no longer in that business as of December. I have not researched what other products are out there that can replace theirs. I know that there are some paints out there that are significantly black and non-reflective, but I have not researched that.

Unfortunately, the weather has not been my friend this winter, so I have no results to see how these improvements have affected my image calibration quality, but I have developed a method to evaluate flat performance from the flat itself. Here is what I do with an OSC camera using PixInsight:

1. Generate a calibrated master flat made up of flat subs calibrated using an uncalibrated master flat dark (takes out bias and hot pixels).

2. Debayer the calibrated master flat.

3. Apply GraXpert to the debayered calibrated master flat. It can be done with Correction set to Subtraction or Division to get two different GraXpert extractions. This removes the smooth contour of the master flat and leaves any more detailed variations from what should be nice, smooth contours.

4. Apply the ScreenTransferFunction (STF) with unlinked channels on each of the resulting GraXpert extractions.

If the flat were perfect, the result would be a perfectly flat result, except for dust motes, and possibly with a color cast of a single hue. I see something very different on all my scopes. I am attaching JPEGs with the results of this process on flat panel flats of my Megrez 90 with Televue TFR-2008 before and after flocking. I have included the subtraction version. As you can see there are more than just dust motes left in the results. The image with flocking is stretched much harder by the STF, so It can be seen that the flocking has greatly reduced the bright ring in the center. It’s not perfect, but then again, viewing up the tube with a flat panel shows it to be still not perfect, but vastly improved.

📷 Panel Flat Subtraction.jpg📷 Flat with Flocking Subtraction.jpgI would be interested to see what others find out about their imaging trains using these methods.

Greg

GregsAstrobin · Mar 3, 2026, 12:29 AM

Bob,

The sort of thing you found by looking into the back of your scope with the objective illuminated by a flat panel is not uncommon. I have been on the warpath to figure out why I get odd colored arcs in my calibrated frames on several image trains. I started looking at my image trains in this manner and found they all have issues to a varying degree.

One of the more interesting cases was my Edge HD1100 with dedicated 0.7X focal reducer. There was a rainbow-colored reflection off of the back end of the focal reducer. Inspection showed this to be a reflection on the very shiny anodized inner surface of the focal reducer just behind the backmost optical element. The colors change wildly as you move around looking at the reflection. I put a quarter inch strip of flocking around the shiny inside edge of the focal reducer and it was gone. I have attached the before photo. In addition, in this shot you can see that the inside of the baffle tube is also not black, but a dull gray. The inside of the baffle tube is machined with threads, but that was not totally effective. I was able to put some flocking inside the rear part of the baffle tube up to the corrective optics. It helped, but it is not fully effective. I have attached an after photo as well.

📷 EdgeHD 1100 with 0.7x FR.jpg📷 EdgeHD 1100 with 0.7x FR and Some Flocking.jpgI had similar issues with my WO Megrez 90. The tube has knife edge baffles, but the inside of the very front of the optical tube behind the objective and before the first baffle, while painted flat black, reflected a dull gray that can be seen by the sensor. I flocked that and the reflection was dramatically reduced. Unfortunately, I did not take any photos.

Many telescope and accessory manufacturers either paint the inside of optics flat black or machine threads the length of the optics, all thinking that this will eliminate light reflection down the light path. My examination of many items, from extension tubes to focal reducer/field flatteners to focusers to telescopes themselves, reveals that this is far from the truth. Sometimes threads are especially reflective at the glancing angle involved. Knife edge baffles work well if placed well, be even they can have a reflection on the inner flat circumference that is however thick that the baffle is if it is at all reflective.

I think every astro imager should take a look at their optics in this manner to see what can be addressed. Flock accordingly if possible. Unfortunately, Protostar, which was a source for some excellent flocking products, is no longer in that business as of December. I have not researched what other products are out there that can replace theirs. I know that there are some paints out there that are significantly black and non-reflective, but I have not researched that.

Unfortunately, the weather has not been my friend this winter, so I have no results to see how these improvements have affected my image calibration quality, but I have developed a method to evaluate flat performance from the flat itself. Here is what I do with an OSC camera using PixInsight:

1. Generate a calibrated master flat made up of flat subs calibrated using an uncalibrated master flat dark (takes out bias and hot pixels).

2. Debayer the calibrated master flat.

3. Apply GraXpert to the debayered calibrated master flat. It can be done with Correction set to Subtraction or Division to get two different GraXpert extractions. This removes the smooth contour of the master flat and leaves any more detailed variations from what should be nice, smooth contours.

4. Apply the ScreenTransferFunction (STF) with unlinked channels on each of the resulting GraXpert extractions.

If the flat were perfect, the result would be a perfectly flat result, except for dust motes, and possibly with a color cast of a single hue. I see something very different on all my scopes. I am attaching JPEGs with the results of this process on flat panel flats of my Megrez 90 with Televue TFR-2008 before and after flocking. I have included the subtraction version. As you can see there are more than just dust motes left in the results. The image with flocking is stretched much harder by the STF, so It can be seen that the flocking has greatly reduced the bright ring in the center. It’s not perfect, but then again, viewing up the tube with a flat panel shows it to be still not perfect, but vastly improved.

📷 Panel Flat Subtraction.jpg📷 Flat with Flocking Subtraction.jpgI would be interested to see what others find out about their imaging trains using these methods.

Greg

Greg,

Thanks for the detailed information - quite an informative treatise! I’ve been lucky as the UltraCat 56 is the first telescope I’ve had that shows any stray light problem. I will use the technique you describe to look at my master flats. I did look into the back of my Takahashi FSQ 106 but didn’t see any bright stray light visually. I have used it since 2020 and have been lucky so far.

Thanks for your input, much appreciated.

Bob