CDK14 telescope having color splitting issues on stars

Tilt and UV or IR wavelengths in the mix can cause this, I currently have this issue due to my 2400 having an AR window instead of a UV IR cut, but with my tilt issues solved, the IR remains the issue. I noticed that the fringing is roughly inline with the RA axis, and as such, I suspect it’s caused by atmospheric density changes causing chromatic shift; which with an ultra-wide bandwidth from running my system unfiltered, causes some pretty gnarly issues.

Other nuisance is the presence of UV and IR microlensing halos around my stars, and the washing out of my colors from having pretty fast equalizing of CFA transparency on the 2400 past 700nm. I’d really like to hope this isn’t the issue since you’re using a Chroma L filter, but with it’s bandwidth there is that extremely slim chance that it could be a symptom, but not a cause.

A bit more tilt adjustment, and maybe a tighter UV IR cut filter will fix the issue. My only concern is that in your images, it appears to be radial to center versus mine which is all coplanar, which to me screams mirror spacing or backfocus issues.

📷 Star Chromatism.png📷 Star Rings.png

andrea tasselli · Nov 29, 2025, 08:04 AM

Anthony Grillo:
More issues with cdk14 stars! First it was wonky stop sign stars and blob artifacts due to a faulty primary and secondary mirror that planewave had to take the scope back and regrind and coat both mirrors to correct, then it was flaring and weird diffraction spikes due to holes in the upper truss ring that I needed to cover to sort of fix (still have weird flaring) now its color splitting on stars!!

📷 mehstars.pngAt first, I thought it was the reducer but after I went back and looked at other data, I realized I had separation on older data without the reducer, so now I’m stuck between the corrector and atmospheric dispersion if I imaged too low sometimes.

I do have some tilt with my 2600mc pro that I have been using to collect color data but its only at 3% according to astap and not that bad at all compared to both my 2600mm setups that show a whopping 13% (thanks zwo filter wheels!!) I haven’t fixed the tilt yet because like clockwork all my issues seem to pop up when its winter lol

I didn’t think tilt would cause the splitting on all the stars that’s why I wasn’t convinced it was causing it.

So, I came here to ask the pros what you think the issue could be!

If you look at my latest images nearly all of them have the splitting except my IC10 image where it isn’t as bad as it has been.

Lateral CA is rather common feature for some optical systems employing correctors and has nothing to do with sensor tilt, if this is what you're showing (the variation needs to be a function of the field angle). At a glance I'd put the onus on the alignment of corrector but obviously you can't run without it.

Thanks for the insight!! I went back to my IC10 data and WR134, the ic10 color was captured with the 2600mc pro and the wr134 with chroma filters. Neither show the splitting like my 7217 data (color captured with filters) and my ngc672 data (taken with 2600mc pro)

Only difference so far is exposure time even though ic10 and ngc 672 were with the reducer and wr134 and 7217 were without it.

I just used those examples to show that I got the splitting with and without the reducer.

I don’t see what else is different except for exposure time unfortunately, so at the very least that rules out the corrector right??

I never saw lateral color with my CDK20. Take the reducer off and confirm that you are seeing the problem with the base telescope. Is the color separation showing in a radial direction about the center of the field? The only thing that could be causing this problem in the base telescope is the field flattener/corrector located in the baffle tube and it would be something that you would have to discuss with Planewave. Sure BXT can fix it, but it is generally bad practice to fix fundamental optical problems with software.

If the base telescope is fine and you only see the problem with the reducer, then it’s the reducer. In either case, my recommendation is to always ditch any reducer. If you want a bigger field, get a bigger sensor. Reducers can work but most of them cause all sorts of difficult to fix problems and they aren’t worth the trouble they cause. Keep your optical system simple and you’ll be happier in the long term.

John

Anthony Grillo · Nov 30, 2025, 02:34 AM

John Hayes · Nov 30, 2025, 12:13 AM

I never saw lateral color with my CDK20. Take the reducer off and confirm that you are seeing the problem with the base telescope. Is the color separation showing in a radial direction about the center of the field? The only thing that could be causing this problem in the base telescope is the field flattener/corrector located in the baffle tube and it would be something that you would have to discuss with Planewave. Sure BXT can fix it, but it is generally bad practice to fix fundamental optical problems with software.

If the base telescope is fine and you only see the problem with the reducer, then it’s the reducer. In either case, my recommendation is to always ditch any reducer. If you want a bigger field, get a bigger sensor. Reducers can work but most of them cause all sorts of difficult to fix problems and they aren’t worth the trouble they cause. Keep your optical system simple and you’ll be happier in the long term.

John

Thanks john, always helping me out when I’m second guessing myself ha-ha. I think I can say with some certainty that it is the reducer; any color separation I had before that I noticed was only with the weird flaring I had around some of the brighter stars as you can see in my image of ngc 7217, that image was base configuration at full focal length with chroma filters. whereas my ic10 image was reduced and color taken with a 2600mc pro.

The separation is pretty much the same direction throughout the field, doesn't change about the center of the image at all.

I really wanted the reducer set up to work for the speed, at f4.8 the cdk essentially becomes a pretty neat astrograph but it also brought with it a host of other issues because of the tight back focus of the cdk reducer.

Watch my “Quest for Aperture” talk on TAIC. If you are sampling for resolution, the F/# doesn't matter. That reducer is just for field size. If all you want is field and you don't care about resolution, you don't need that 14” scope!

John