Photometric Filters - Intereference type Chroma brand

American Association of Variable Star Observers (AAVSO)
Tue, 09/21/2021 - 17:08

I'm looking it to acquiring some photometric filters. I'm interested in the interference type since I live on the west coast of Canada where we have a fair bit of moisture most of the time. I have a Schuler V from a number of years ago and it has the characteristic hygroscopic crusting that happens with the colour glass filters. 

When examining the response curves of the Bessell V and B of the Chroma brand I can see that the peaks of the curves are relatively flat for their range as opposed to the more gaussian "pointy" look of the traditional coloured glass filters.

Does anyone know if this is a problem? or should a stick with coloured glass and deal with the inevitable maintenance involved.



American Association of Variable Star Observers (AAVSO)
Photometric filters

Hello David,

I think that your question about filters is better suited to the "Instrumentation and Equipment" forum these days.

You'll see several other threads having to do with filters already there and your question may already have been answered but if not, do ask it again.

Best regards,

American Association of Variable Star Observers (AAVSO)
Wrong forum


I didn't see your comment when I posted my reply to David's original post.  Please feel free to move my response to the appropriate forum if you think that is the best was to straighten this out.    If David's original comment is moved or re-posted somewhere else I will repost my comment there.


American Association of Variable Star Observers (AAVSO)
Chroma filters

"Does anyone know if this is a problem?"

I'm afraid the answer to your question is, "no".   There has been no guidance from the AAVSO on this yet.  Until that happens I'd suggest you try some alternatives.

1)  Is your Schuler V beyond hope?  If the contagion has not spread to the internal glass to glass surfaces it is not difficult to polish it out of the external surface.  It always returns, but you could get a few more months out of you Schulers while you wait for further developments in the J/C filter availability saga.  Perhaps you have already tried this.

2)  How does the Schuler B look?  If that is still okay, you could experiment with transforming a tricolor imaging Green filter with the Schuler B for transformed B-V observations.  I have never seen this done, but it just might work.  At the least it would be an interesting experiment.  I think it would work best if your Green filter had the same thickness as the Schulers.  I think the Schulers are 4mm thick. 

Again, this would just be temporary.  In my experience the Schuler B also eventually deteriorates.  On the other hand the Schuler R and I filters seem to last a long time if they are not abused.

3.)  If the Schuler B is also hopeless, you could transform tricolor Blue and Green to B and V, or just report TB and TG magnitudes  until there is more clarity in the BVRI market.

4.)  One final strategy:   If the Schuler B is still good, you could buy just the Chroma or the Custom Scientific interference V filter, then calculate your B-V transforms with this hybrid pair.   I have a friend who was lucky enough to find a used Astrodon B on the internet.  He is now using a Chroma V with the Astrodon B (and with a Schuler I) to make good transformed B-V and V-I measurements.





Variable Stars South (VSS)
Chroma red band and emission lines

Interestingly, both the Chroma red filter and my ZWO astrophotography red filter, both of which have rectangular profiles with sharp cutoffs, will easily pick up 656nm, the H alpha emission line, in novae.

In contrast, this emission line will not be captured well by the red channel of at least some DSLR cameras.