[Aavso-photometry] Where do the professionals source

[Richard Miles]

Stan / Arne / Michael,

Firstly, I have just taken some nice close-up images of B and V filters 
which I have extracted from a filter wheel that has sat in my attic the last 
few years.  I've produced 5 images showing (a) edge of the B filter 
comprising 3 elements showing the white residue on just the one component 
(BG39), (b) 45 deg view of BG39 side of the B filter, which in this case is 
cemented on one side, (c) front view of B filter, (d) front view of V filter 
showing degraded surface (Arne - I had previously tried cleaning this, which 
explains why there are no longer any spots of white residue showing), and 
(e) view of B and V filters on a white background.

Of course manufacturers in the case of the B filter should sandwich the 
sensitive BG39 in between the other two components (BG12 and GG385) for 
protection: this would go a long way to maintain its stability.

The BG39 never had to be used for B and V in the case of the 1P21 PMT and 
similar pm tubes.  It's the inclusion of the BG39 in both the V and B 
filters which is the only change in going from PMT to CCD work.

In my view, manufacturers should not vacuum coat filters as water molecules 
can usually diffuse through these layers which often have pinholes (see more 
positive suggestion below).

As I said before, the transmission curves of different batches of BG39 can 
vary quite a bit especially at the red end of its range where there is a 
small shoulder, the height of which varies with batch.  This could mean that 
different batches also have differing chemical mix, etc. sufficient to 
affect the hydrolytic stability of the glass.  However I believe that all 
BG39 filters will succumb if stored in a sufficiently high relative 
humidity, it's just that some are more resilient than others.

As to where we go from here.  An old friend, E. Norman Walker (ex-Royal 
Greenwich Obs., Herstmonceux) produces CCD filters which are sandwiched in 
1mm silica plates and polished optically flat.  So far I have been using the 
V and the I filters for the last 2 years and I am ecstatic as to their 
quality: the images show no evidence of degradation or optical faults even 
when imaging close to the full Moon when the sky is heavily illuminated.

I'll send the 5 photos to you 3 guys separately - maybe Arne could put them 
on-line somewhere convenient for people to browse.

Happy observing,
Richard

----- Original Message ----- 
From: "Stan Walker" <astroman at xtra.co.nz>
To: "arne" <arne at aavso.org>; "Michael Koppelman" <lolife at bitstream.net>
Cc: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, September 27, 2005 9:21 PM
Subject: Re: [Aavso-photometry] Where do the professionals source


> Hi Arne & Michael,
>
> There's obviously a major change from UBV filters with pm tubes to the 
> more complex CCD filters. We used to buy filter glass directly from 
> Germany and had no trouble with any sort of 'crud'. The original set made 
> in 1965 was still going strong in 1990 and may still be in the unused 
> photometer.
>
> But some U glass bought in the 1980s caused some trouble as it appeared to 
> be coated. This makes me wonder whether all of these commercial filters 
> have some type of coating - anti-reflective or something - which is 
> probably unnecessary. Or is it needed with CCD photometry where the 
> filters are much closer to the focal plane and you're not just looking at 
> one defocussed image?
>
> Auckland is very humid but we had no trouble with filters except for the 
> adhesive on some sets of filters that had come from Mt Stromlo. They 
> replaced these very promptly and helpfully.
>
> Maybe if this continues to be a problem the AAVSO could get into the 
> filter making game and produce sets that don't cause problems. This was 
> the way things were done in NZ and Australia in the 1970s with UBV 
> photometry. But perhaps the manufacturers could be approached to see what 
> 'improved' treatment of filters isn't working? The idea of throwing a set 
> away each year is alarming in that the degradation must affect all 
> measures during that year.
>
> Regards,
> Stan
>
> ----- Original Message ----- 
> From: "arne" <arne at aavso.org>
> To: "Michael Koppelman" <lolife at bitstream.net>
> Cc: <aavso-photometry at mira.aavso.org>
> Sent: Saturday, September 24, 2005 7:17 AM
> Subject: Re: [Aavso-photometry] Where do the professionals source
>
>
>> Michael Koppelman wrote:
>>
>>> I can do that, although I tried to clean it so it looks worse than it 
>>> did. I can just send you the dang thing, too, if you'd like. It's of  no 
>>> use to me anymore.
>>>
>>> Michael
>>>
>>> On Sep 23, 2005, at 12:21 PM, arne wrote:
>>>
>>>> Thinking about the actual paper, I was wondering if someone has one  of 
>>>> the
>>>> bad filters and would be willing to take a picture of it for me so 
>>>> that I
>>>> can include a jpeg image in my presentation.
>>>
>>>
>> I'd rather have an "unadultrated" filter picture if possible, if anyone
>> can get me one.  Michael's cleaning results can be another useful
>> picture, though.
>> Arne
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