[Aavso-photometry] Re: Crud on filters

[Bruce Skelly]

I noticed in the article "UBVRI Filters for CCD Photometry" by Michael S. Bessell, CCD Astronomy; Fall 1995, that he has a prescription for 5mm thick filters, where the V filter uses 3mm of BG40 in place of the usual 2mm of BG39 in the 4mm prescription.  The B filter still uses 2mm of BG39, but at least the BG39 could be sandwiched between the 1mm of GG385 and the 2mm of BG1.  
 
I know this doesn't solve the crud problem, it just hopefully avoids it.
 
Bruce
Richard Miles <rmiles.btee at btinternet.com> wrote:
Bruce wrote:
>I haven't seen this crud, so I don't know if this is appropriate, but what about CO2. I know that they use CO2 to clean telescope optics, and I heard that Boeing uses it to remove the paint from airplanes. I believe that they use liquid CO2.

Hi Bruce.

Two things here - CO2 is sometimes used to clean optics by blowing off any dust by the rapid expansion of the CO2 gas from a small cylinder of the liquefied gas.

Liquid CO2 is also used as a special solvent for certain chemical species although I don't think it works on inorganic salts which is what this crud will be (a sulphate, carbonate or hydroxide or combination).

Of course liquid N2 is also used to remove chewing gum from pavements by freezing it and it cracks and comes away from the stone surface.

Shame I'm not working in the lab anymore as it would be a simple matter to get the residue analysed. I imagine Schott, the manufacturer, will know what it is - They do rank BG39 as one of the three worst resistant glasses (with UG5 and UG11) and it has very poor acid resistance and pretty poor alkali resistance. The following (all one web address) contains information in general:

http://www.schott.com/optics_devices/english/download/tie-30_chemical_properties_of_optical_glass.pdf

This web address has details about BG39:
http://www.besoptics.com/html/body_schott_bg39_filter_glass.html

Schott recommend that BG39 be laminated or coated. Personally, I prefer a 1mm silica laminate. I know of a source of V filters that have this (not a commercial operation) and shall be having discussions as to whether there is sufficient demand from AAVSO and BAA VSS members for a supply of these to be made available. They would normally be 3mm thick - the main question is what diameters are needed by people?

Cheers,
Richard
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