[Aavso-photometry] filter question - which to buy ?

[Greg Crawford]

I had a set of Custom Scientific filters. Unfortunately they deteriorated;
possibly due to my humid coastal environment, a mere 6 metres above sea level,
who knows? After reading discussions on this and other lists, it seemed to me
that *all* the different brand filters deteriorate. Therefore, the only thing to
do is to replace them more frequently. Consequently, I concluded that constantly
replacing cheaper filters was more economical than constantly replacing more
expensive ones, especially if the cheaper ones were of pretty good value. 

I chose Schuler filters sold by Astrodon. Although they lack the anti-reflective
coating, (which is probably why they are cheaper), this only seems to affect
very bright stars. It seems to me that if you have sufficient SNR in the first
place, any tiny light loss through non-antireflective coatings will be
insignificant anyway.

For not much more than the cost of replacing my deteriorated Custom Scientific B
filter, I could purchase a BVR set of Astrodon filters. I know Arne is now
recommending the use of Ic filters for wider colour separation, but my CS Ic
filter was still operational, (though not now parfocal with the Astrodons), and
in any case, I have doubts about Ic measurements at 6 metres above SL. 

Greg

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Wolfgang Renz
Sent: Saturday, 12 May 2007 4:12 PM
To: AVSON at yahoogroups.com; AAVSO-PHOTOMETRY
Subject: Re: [Aavso-photometry] filter question - which to buy ?

> Problem 1:
> no antireflective coating, which reduces transmission
> significantly. You've gotta use every photon you can!

Uncoated glas surfaces usually reflect ~ 4.3 % if the light.
So an uncoated filter should have a loss due to reflections
of at most ~ 8.4 % when compared to a perfectly coated
one with 0 % reflectivity. But this should make a difference
of not more than ~ 0.1 mag in LM. So it doesn't make a
too big difference. But uncoated filter are prone to internal
reflections and ghost images that might get visible with
bright stars or long exposure times.

Actually the reflectivity of AR coated surfaces (no matter if
filters, FRs, FFs or other lens surfaces) is just lower than
the reflectivity of uncoated ones over a limited range in
wavelength (the range the AR coating is optimized for).
Outside this range the coated ones behave even worse
than uncoated ones and often show increased reflection
issues (e.g. in U and Ic band with visually optimizes FRs
hat might spoil flats). But in the case of photometric filters
this should not be an issue as the AR coating can be
choosen and optimized to cover the limited wavelength
range of the passband of the photometric filters well.

> Problem 2:
> mine developed a fungal or bacterial invasion between the
> glass layers, eating the glue. A discussion of this issue on
> one of the email lists a year or so ago (AAVSO-disc?)
> revealed that several others had suffered from this. I moved
> to Custom Scientific for my SBIG CFW8, and my present
> filters are as supplied by Optec for their filter wheels.

Are you sure it was not just humidity that entered the cement ?
Fungus is widely used with camera lenses to describe a class
of defects in the coatings and cement. But it doesn't necessarily
mean that there are actually fungi or bacteria. Sealing the edge
of the filters might prevent such issues (this might be given for
coated filters (depending on the coating process) but not
necessarily).

When I read the thread on the crude on uncoated photometric
filters with certain blue filter glasses I thought his issue is
caused by evironmental influences on the not so well resistant
glass sorts.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Tom Richards" <tom at woodridgeobsy.org>
To: <AVSON at yahoogroups.com>
Sent: Wednesday, May 09, 2007 4:55 PM
Subject: RE: [AVSON] Re: filter question - which to buy ?


> I had a Schuler set.
> Problem 1:
> no antireflective coating, which reduces transmission
> significantly. You've gotta use every photon you can!
> Problem 2:
> mine developed a fungal or bacterial invasion between the
> glass layers, eating the glue. A discussion of this issue on
> one of the email lists a year or so ago (AAVSO-disc?)
> revealed that several others had suffered from this. I moved
> to Custom Scientific for my SBIG CFW8, and my present
> filters are as supplied by Optec for their filter wheels.
> 
> Regards
> Tom Richards
> 
>  
> 
> From: AVSON at yahoogroups.com [mailto:AVSON at yahoogroups.com]
> On Behalf Of Bernard Heathcote
> Sent: Wednesday, 9 May 2007 22:03
> To: AVSON at yahoogroups.com
> Subject: [AVSON] Re: filter question - which to buy ?
> 
> Ian,
> I very much doubt you will get a more authoritive, or 'blue chip', 
> reply to your question than that already given to you by the AAVSO 
> Director (copy of Arne's very valuable reply copied below). As for
> the choice between CS/SBIG and Schuler (now owned by Astrodon) 
> I see it much like the choice between a Lexus and Toyota ... the 
> significantly cheaper car will give you around 90% of the utility 
> value of the more expensive model. I, and many others, have used 
> Schuler filters for a number of years and I don't believe my VS 
> photometry (at both f/6 and f/12) has suffered. If you are 
> contemplating the detection of exoplanets, where 0.003 mag 
> differences are of the order, then maybe at least your V filter 
> should be a CS/SBIG!
> 
> Bernard


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