CORRECT ORIENTATION OF CHROMA FILTERS

Their website isn't useful.

“CN will always give conflicting information.”

I completely agree with Bob on this point! There are some very knowledgeable people on CN but unfortunately the good information is often buried under so much garbage that it’s hard to find.

The question about which orientation is best for filters is a common one and a bit difficult to answer without some careful analysis.

First of all, the effectiveness of the thin film stack that makes up the filter is not affected by which way light passes through it. So that isn’t a driver for which way to orient the filter. The more important question is which orientation reflects the least amount of stray light back onto the sensor? If you sit down and carefully ray trace all of the potential single bounce strays, you will find that the amount of stray light will be the same no matter which way you orient the filter. Here are a couple of diagrams showing the situation for two assumptions:

This first diagram shows the simplest situation if the AR coatings are assumed to be perfect. I’ve used assumed values for the transmission and reflectivity so that we don’t end up with some long analytic expression. As you can see, the amount of light from a single bounce stray will be identical no matter how the filter is oriented. That should be intuitively obvious.

📷 image.png

In the second case shown below, I’ve shown the situation where the AR coatings are just 2%, which is a garden variety, low-end coating. Here, the result is not immediately obvious. Again, I only considered a single bounce stray and again, you can see that the stray level is identical no matter how the filter is oriented.

📷 image.png

Unfortunately, this sort of calculation doesn’t quite close out the argument because it doesn’t consider the SIZE of the strays, which will determine the irradiance of the stray light on the sensor. Here’s what just one case looks like (ignoring the effects of refraction):

📷 image.pngI haven’t had time to make a similar diagram for the flipped filter but you can see what happens. If there is a difference between the reflectivity of the AR coating and the reflectivity of the filter (at the transmitted wavelength), there will be a small difference in the brightness of the smallest stray from the surface closest to the sensor. In reality, these values shouldn’t be wildly different so I don’t believe that this should be a strong driver for how to orient the filters.

The ultimate conclusion is that the folks at Chroma are right! It doesn’t really matter how you load your filters. Any differences between the two orientations will be extremely small. And, I should add that this conclusion doesn’t just apply to Chroma filters. It should apply to all filters, so I don’t know why some companies mark the orientation, other than to make it easier to load all of the filters with the same orientation. Up or down…it doesn’t matter.

John

i followed chroma’s guidance, but installed all the filters in the same direction. i had some star reflections with the OIII filter, so i flipped just that one, and the problem went away. i’m not even sure which side has the AR coating, to be honest.

i dont’ doubt john’s analysis but in my case that small difference seemed to make a difference. this is on a GTX110 with the 110TCC and a 6200MM w/ 50mm round chromas.

rob

pfile · Jun 27, 2026 at 04:51 AM

i followed chroma’s guidance, but installed all the filters in the same direction. i had some star reflections with the OIII filter, so i flipped just that one, and the problem went away. i’m not even sure which side has the AR coating, to be honest.

i dont’ doubt john’s analysis but in my case that small difference seemed to make a difference. this is on a GTX110 with the 110TCC and a 6200MM w/ 50mm round chromas.

rob

Rob,

That is interesting. The only way that I can imagine that the orientation might matter would be due to a seriously faulty AR coating, but maybe there’s something else going on that I haven’t considered—and it wouldn’t be the first time. I would really like to see what the difference between the two orientations looks like if you can dig out a couple of subs and post them. Adam Block had a similar issue with his scope in Chile and he went to the trouble of flipping the filters the last time we were down there and as I recall, it didn’t make any difference whatsoever. We tore our hair out trying to figure out where some strays were coming from in both of our scopes and we did a lot of experimenting. In my scope, I removed the field flattener and swapped filters to show that neither of those components were causing a particular stray. I ultimately concluded that it had to be due to a bad coating on the camera chamber window. Unfortunately, I wasn’t able to swap the camera to confirm that idea. Dealing with stray reflections is tricky business!

John

John Hayes · Jun 27, 2026, 03:18 PM

pfile · Jun 27, 2026 at 04:51 AM

i followed chroma’s guidance, but installed all the filters in the same direction. i had some star reflections with the OIII filter, so i flipped just that one, and the problem went away. i’m not even sure which side has the AR coating, to be honest.

i dont’ doubt john’s analysis but in my case that small difference seemed to make a difference. this is on a GTX110 with the 110TCC and a 6200MM w/ 50mm round chromas.

rob

Rob,

That is interesting. The only way that I can imagine that the orientation might matter would be due to a seriously faulty AR coating, but maybe there’s something else going on that I haven’t considered—and it wouldn’t be the first time. I would really like to see what the difference between the two orientations looks like if you can dig out a couple of subs and post them. Adam Block had a similar issue with his scope in Chile and he went to the trouble of flipping the filters the last time we were down there and as I recall, it didn’t make any difference whatsoever. We tore our hair out trying to figure out where some strays were coming from in both of our scopes and we did a lot of experimenting. In my scope, I removed the field flattener and swapped filters to show that neither of those components were causing a particular stray. I ultimately concluded that it had to be due to a bad coating on the camera chamber window. Unfortunately, I wasn’t able to swap the camera to confirm that idea. Dealing with stray reflections is tricky business!

John

i’m looking back at the images. there’s a further complication here in that i originally ordered 3nm NB filters for this system but pretty quickly came to realize that there was some problem with the transmission of the OIII filter.

on a field containing M42 and the running man, what i see is a flipped and reversed image of the brightest stars in the field, with a significant amount of coma, only thru the OIII filter. so near the running man i see the reflection of the bright stars at the bottom of M42, and on the other side of the image near those stars below M42 i see a fainter reflection of the stars in the running man. or at least i think that’s what they are, it’s hard to tell exactly. it’s easiest to see this using blink so i might make some movies.

i also see these same reflections with the images taken right after i got a replacement set of filters, again only thru the OIII filter. however i then flipped that filter and the problem went away. the replacement set is 5nm.

looking back thru my emails, they told me that if the writing on the edge of the filter is right-side up, then the AR coating is on the bottom. this is how i installed all the filters at first. now after flipping the OIII filter, the AR coating is facing up. that means on the OIII filter only, the AR coating is facing the 110TCC reducer.

there was definitely something wrong with the initial 3nm OIII filter - it also exhibited halos and other strange, more diffuse reflection artifacts in addition to poor transmission. but since the replacement 5nm filter has the same subset of star reflections, it’s hard to say if the problems the 3nm filter had were the cause of the “pinpoint” star reflections. i never tried the 3nm filter in a “flipped” orientation.

side note, i’m a little disappointed in chroma given the cost of the filters. when i reached out to ask about the transmission issue i got no response for more than a week, at which time i tried again and finally got a reply. at that point because of the possibility of having received a bandshifted 3nm filter (which this system should not need at f/5.5), rather than asking for troubleshooting help i asked to exchange for 5nm filters and they agreed. subsequently i had the reflection issue with the 5nm and asked them about flipping the filter; when that resolved the issue i just dropped it since everything seems to be fine (and continues to be.)

John Hayes · Jun 27, 2026, 06:41 PM

Mark McComiskey had similar problems with a 3 nm OIII filter and Chroma replaced it for him.

sounds like they must have had a bad run then.

John Hayes · Jun 27, 2026, 06:41 PM

I can’t imaging why you were getting a flipped stray image without involving more of the components in the system. Flipping the filter really shouldn’t make a difference; but, I believe you that it did. I just don’t understand how that could have made a difference.

i feel like i must be getting a reflection off of the back surface of the 110TCC reducer… otherwise i’m not sure where the flipped image would come from.

i tried to make a little movie with blink of the various artifacts but failed, maybe there are too few frames and ffmpeg only builds a movie with 2 out of the 10 or so frames i fed it.

John Hayes · Jul 1, 2026, 04:34 PM

. I suggest that you pull your reducer to see what happens. That will at least show definitively if that’s where the stray is coming from.

this is probably easier said than done for me due to spacing requirements. i own the flattener equivalent of the 110TCC, so i could try that instead, though if the reflections persist i didn’t learn anything.

also of course this requires me to open the filter wheel and flip the filter, then find a clear night with the right stars (so far i only observed this on M42 and so i can’t repeat that experiment at this time of year.)

to be honest i probably do need to open up the filter wheel anyway because i’m getting different artifacts, maybe off of the filter masks (which are 3d printed and thus a little shiny.) on fields where there is a bright star pretty far out of the FOV i get spikes pointing to those stars. i hesitate to raise this with AP because the initial reaction always seems to be incredulity when it comes to issues like this. so i have to do a bunch of debugging first.

Bob Lockwood · Jul 1, 2026, 09:55 PM

pfile · Jun 27, 2026, 04:36 PM

flipped and reversed image of the brightest stars in the field,

The only time I ever see this is after a pier-flip, but this is beside the point.

maybe i was not clear about what i was seeing; it’s not that the image is flipped, it’s that the reflected stars i see (faintly) in the images are flipped/reversed images of the bright stars in the image.

As for the reflections from the AP 110TCC, I’m pretty sure this came up some time ago on the AP user group. They were getting reflections at the rear of the 110TCC and I believe it had to do with the shine of the anodizing and the angle of the taper of the housing. The fix at the time was to make a mask that covered the taper, my 110TCC threads into my OAG and I made a mask that threads into the OAG between the TCC. I use the 110GTX with the 110TCC with chroma 3nm filters with a Moravian C3-61000 and see no reflections of any kind.

I believe AP has since fixed that issue.

Edit: forgot to mention that I’ve had the filters in both orientation and see no difference.

This is the ring I made installed in my OAG with the 110TCC attached.

📷 IMG_1811[1].png

that is interesting. the other day i was trying to determine if i could fit a rotator in my image train, and i happened to look back at the AP website for mechanical drawings. i noticed that the various adapters that are used to step down from the TCC to the camera are now different than the ones i have. it’s possible they eliminated the various tapers in favor of flat surfaces.

anyway i’m not sure if something like this could be the cause of the reflected stars (which were cured by flipping the filter anyway) but these kind of shiny surfaces could be responsible for the rays of light coming from stars far out of the FOV of the sensor. i should attempt to flock or mask these surfaces the way you have done.