Schuler filters

Hi Ed,

if those are the original Schuler colored glas filters, then with time thy might have degraded especially in a humid environment.

I know from the V filter that between the two glas plates the glue disassembles when getting humid and a greasy view will be left. Maybe yours is like this.

I have not heard about this to happen in the R, I and B filters. I have had Schuler V filters and had this issue.

I hope this helps,

Josch (HMB)

The BG39 glass used in the Schuler (and other) B and V glass filters tends to absorb water over time and get "crusty." I an not aware of any particular tendency toward degradattion in the R and I filters but I have experienced degredataion of those as well over time but to a much smaller degree.   If the most susceptible glass is sandwiched between layers of more durable glass as in the B filter, the moisture tends to work in from the edges. I have heard that the Schuler filters are perhaps more susceptible to degredatation over time than some others but I went through two sets of Custom Scientific filters over 6 years in the Texas humidity. The first set only lasted two years because I left the camera on the telescope. The last set lasted about 2x as long becuase I removed the camera from the telescope and stored it inside the house in an air conditioned environment when not in use. The V ilter was always in the worst shape followed by the B. I might have been able to salvage the R and I filters but since I was replacing two I thought it best to start with a completely fresh set.  

To avoid replacing filters on a frequent basis I changed to Astrodon Interference type Johnson/cousins filters when I bought a large format camera that required 2" filters which are significantly more expensive than the 1.25" I used with my old ST7. The interference type are not hydroscopic, have almost 2x as good transmission in U,  much better transmission in B, and when couple with a CCD, much more closely match the and Ic response curve  of Cousins Bessel  R and more importantly I with the RCA 31034-A PMT than glass filters without dialectric coatings. 

Two informative articles are attached and passband curves of the Astrodon Interference filters. I have no affiliation with Astrodon and only provide the information since I have been very satisfied with them so far in the Texas humidity. Baader filters appear to have similar passband shape for Ic but the lower transimission in U and B typical of traditional glass filters. However, I haven't seen them advertised in sizes larger than 1.25". The cusom scientific I filters has curve with a longer red tail similar to the Is curve in the Astrodon paper, which may make transformation of I magnitudes less accurate.

Sorry you ended up with some degraded filters, but I hope if you decide to replace them the attached info is helpful.

Brad Walter

Hi -

I learned this on this discussion group, so am merely repeating it.  However, I had good luck with this technique for cleaning crud from my Schuler V filter.  Also, my friend Lew Cook tried it with success:

Carefully avoid getting water near edges of the filters.  Dab your finger in a few drops of water, then dip in some optical rouge (obtainable from Wilmann-Bell (http://willbell.com/)). Gently polish with your finger tip.  Clean with damp lint-free cloth.  Not 100% effective, but helps a great deal.  The edges will probably remain clouded - don't try to polish there, since you might allow moisture into the sandwich.

- Jerry Hudson

HJA

Thanks for the replies, I think I will just buy new filters and be done with it.

Ed