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

Sat May 12 11:55:13 EDT 2007

Hi Greg

How did your CS/SBIG photometric filters deteriorate ?
On the surface although the CS filters are AR coated ?
Or in the cement layer(s), due to a missing edge sealing ?

Just the CS B filter ?
Or the UBV filters with Schott BG glasses ?
U filters use usually BG39 or S8612.
B filters use usually (BG1, BG4, BG12 or BG37)+BG39.
V filters use usually BG18+BG38, BG39 or BG40.

There are some issues due to the telluric O2 and H2O lines
(minor: 592-599 nm H20, 627-633 nm O2, 646-653 nm H2O,
693-709 nm H2O, 789-797 nm O2; medium: 686-692 nm O2,
810-839 nm H2O, 895-920 nm H2O; major: 716-741 nm H2O,
759-771 nm O2, 925-970 nm H2O). See e.g.:
http://www.astrosurf.org/buil/us/spe2/hresol4.htm
http://www.astrosurf.org/buil/us/spe2/hresol6.htm
http://www.astrosurf.org/buil/us/spe2/radio2.gif
http://www.astrosurf.org/buil/us/spectro11/specalib.htm
http://www.astrosurf.org/buil/us/vatlas/vatlas.htm
http://www.astrosurf.org/buil/lhires_ir/obs_us.htm
But the missing recommended short cut of the Ic filter should
have a higher influence on red star photometry even at .

The influence of the telluric O2 and H2O lines will increase with
airmass (lower altitude, larger zenith angle) and the telluric H2O
lines with higher humidity too.

With Rc one might get some minor issues with stars with very
strong Halpha emissions (like novae) or absorbtions.

Clear skies
 Wolfgang

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

----- Original Message ----- 
From: "Greg Crawford" <gc at nelsonbay.com>
To: "'AAVSO-PHOTOMETRY'" <aavso-photometry at mira.aavso.org>
Sent: Saturday, May 12, 2007 8:38 AM
Subject: Re: [Aavso-photometry] filter question - which to buy ?

> 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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