I have AR coated photometric filters and noticed that my V filter is cloudy. Does anyone have any advice on how to clean filters?
What you describe sounds like what happens to the colored glass V filters, where the main Schott glass (BG39) is hygroscopic. If moisture reaches that glass, it can react and cloud, usually from surface crystallization. The clouding is a common problem in humid climates, but is less common with those filters that are AR coated. The V filter is a sandwich of two glasses (BG39 and GG495), so one surface is kept away from air; the AR coating generally keeps the other surface away from air. The other common effect are crystals growing inbetween the two glasses; that problem cannot be solved as usually the glasses are cemented together. If the clouding is on the outside surface of the BG39, you can usually polish the crystallization off. However, polishing will likely remove the AR coating.
Since you are AR coated, you might start with an alcohol wash to see if it is just junk that can be removed easily. That requires alcohol (or lens cleaning solution) and a cotton swab, with gentle cleaning motions to prevent scratching. If you want to proceed and polish, the best method I've found is to use cerium oxide (I have WAY more than I need, and can send you some), and a clean cotton swab, and gently clean the surface until the haze disappears. Washing, followed with an alcohol wash so as to dry without streaking, usually completes the process.
You can often solve the problem this way. The new interference filters aren't hygroscopic if you do have to replace the filter.
Unsurprisingly, I fought this same problem for several years, and you're closer to the ocean so it is probably even worse there. I had some success using the cerium oxide polish approach Arne mentioned but eventually even determined cleaning had no effect. After going through three V filters (getting about 2 years out of each one) I replaced my Schuler filters with Astrodon Photometric filters a year ago and so far they remain clean. These filters don't include the BG-39 glass which react with water that Arne mentioned so there shouldn't be an issue even here in Florida. An added bonus is that the Photometric series has a higher transmission, especially the B filter.
Thanks Arne and Shawn. Someone emailed me privately yesterday and recommended red rouge for polishing from Willmann-Bell so I ordered that. He had the same problem with his Schuler v filter. On Arne's advice I just took the filter out and tried to clean it with 91% isopropyl alcohol. It didn't make a difference so I will wait for the polish. Can I have instructions on how to polish it? All of the other filters look fine it is just the V filter.
I used optical rouge with good results on my hazy Schuler V filter. Eventually, I replaced the B, V, and Clear with Astro Don interference filters. The Schuler R and I filters are still in the wheel and look good (but are rarely used).
Question for Arne: I have a five position filter wheel. If I decided to replace R and I would you suggest Sloan filters; if so, which ones?
You might want to take Arne up on his offer of the cerium oxide. Red rouge is a slower polish than the cerium oxide. I would suggest starting your polishing with the cerium oxide diluted in mineral oil. Polish against a thick piece of flat plate glass. Do not press hard and wear latex gloves. Clean the filter off every so often with isopropyl alcohol and view your progress. When the filter looks good optically, switch to the red rouge and do a final polish to remove the micro scratches and improve the transmission of the filter. Store the filters in a desiccant when they will not be used for a long period of time. Please let us know this works for you.
If you look at the responses, Sloan g' is roughly equivalent to the combination of Johnson B and Johnson V. Therefore, you can look at it in the opposite manner: Johnson B and V give additional spectral information in comparison to g', and so by keeping B&V in your wheel, you not only get color information, but you match with the decades of existing photometry. On the other hand, r' and i' are pretty similar to Cousins Rc and Ic.
If you have a 5-position wheel and want to keep a slot for Clear, then I recommend staying with Johnson B,V and Cousins Rc,Ic. If you don't mind giving up Clear, then I recommend Johnson B,V and Sloan g',r',i' - basically the same filters used by APASS.
This method worked well for me.
Moisten the tip of your finger (I used distilled water), then pick up the smallest amount of rouge possible by just touching the moist finger to a small amount of rouge . Polish the hazy side gently with your fingertip. Remove the rouge with a cotton ball or Q-tip. I used medical cotton balls.
The extreme periphery of the filter near the cell didn't get completely clear because of the raised edge of the cell. A Q-tip might do a better job there, but I was worried about possibly dislodging some dirt hiding in the glass-cell junction that might cause a scratch.
Can I take you up on your offer of a little cerium oxide?
I'll send you a small quantity.
Hi Dr. Barbara!
The Schuller B filter was notorious from getting the "crystalline fog." After switching to the AstroDon's interference B & V I dont see this problem anymore. Arne is nice for sending you some cerium oxide rouge...he probably expects you to make another great "U Sco-like" recovery!
Los Angeles, Ca
James: I think if this doesn't work I will order the Astrodon filter. That is nice of Arne to send me the Cerium oxide. Can't wait to get my camera and filters back on the telescope. I have had beautiful skies the past week in central Florida.
Phil: Thanks for the instructions.
Arne: Thanks for the Cerium oxide. I guess you can get my address from the AAVSO membership database. I would be happy to pay you for the Cerium oxide and shipping or I can just add an extra donation when I renew my AAVSO dues!
Thanks for the instructions Donn. Why polish against a thick piece of plate glass?
Don't bother paying us for the cerium oxide - include an extra donation on the next go-around. Cerium oxide usually comes in pound weight increments, and you need about 1/100 ounce for the purpose. That is why I have so much here in the lab. I consider the polishing as a stop-gap measure. It will extend the life of your filter, sometimes for years, but if your environment is such that the haze occurred once, it will re-appear.
Barbara, I hope polishing would be fine for your filter. Anyway, if someday you order an astrodon you will do a great purchase, it's worth the cost.
My old Schüler filters are still intact, I cross my fingers . Dry weather at my location helps a lot.
If the filters are polished on a piece of thin glass that is flexing, it will turn the edge of the filter enough that it will cause the filter to act like a lens. This will cause vignetting on the corners of your image. A piece of garden variety, double strength window glass will work fine if it is used on a rigid surface. I mention using a thick piece of glass because I use a 1/2" thick sheet of glass in my lab for jobs like this. Again, use a very gentle pressure on the filter when polishing as increased pressure will also turn the edge slightly; similar to the edge turning problem when grinding a primary mirror.