[Aavso-photometry] Question about "R" and "I" filters

Sat Jun 21 10:04:48 EDT 2008

Hi Bob

For blue and white stars it probably doesn't make a significant difference.
For "normal" yellow and orange ones it probably can still be calibrated out.
http://en.wikipedia.org/wiki/Stellar_classification
http://www.peripatus.gen.nz/Astronomy/SteCla.html
But as cooler and redder the stars are or as more molecular absorbtion
bands they have in the far-red (normal G-K-M-L-T-Y vs. R,N stars or C,S
stars with TiO, ZrO, ...):
http://adsabs.harvard.edu/abs/1985JRASC..79..277M
http://adsabs.harvard.edu/abs/1996ApJS..105..419B
http://www.astrosurf.com/aras/papers/c_s_stars.htm
http://en.wikipedia.org/wiki/Carbon_star
as larger the differences will be and cann't be calibrated out anymore.
Its also important that the photometrc filters (actually the total system re-
sponse) match the quoted ones with stars that show very strong emis-
sion lines (at least in the range where these occure). This are especially
the Halpha, Hbeta, ... lines with novae and Type II SNe. But also e.g. the
very strong and very broad (up to ~ 15 nm) He, C, O and/or N lines with
WR stars (these sometimes show just a very weak continuum compared
to the emission lines):
http://en.wikipedia.org/wiki/Wolf-Rayet_stars
http://www.peripatus.gen.nz/Astronomy/WolRaySta.html
http://cfa-www.harvard.edu/~pberlind/atlas/htmls/wrstars.html
http://adsabs.harvard.edu/abs/1987ApJS...65..459T
http://adsabs.harvard.edu/abs/1987RMxAA..14..287T
http://www.roe.ac.uk/~pmw/Wr140int.htm
http://www.physics.usyd.edu.au/~gekko/wr104.html
http://www.stsci.de/wr140/index_e.htm
http://www.stsci.de/wr140/description.htm
http://www.stsci.de/wr140/images/fg5.h.jpg
http://www.stsci.de/wr140/shots.htm
http://www.stsci.de/wr140/results.htm
http://www.stsci.de/forum/viewforum.php?f=17

If one wants to know more about:
- the acually recommended/quoted total UBVRcIc band system responses
and
- designing and adapting the Bessell prescriptions to an actual system
I recommend reading the following two articles:

1990PASP..102.1181B - UBVRI passbands, Bessell, M. S.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990PASP..102.1181B
available for free online

1995CCDA....2...20B - UBVRI Filters for CCD Photometry, Bessell, M. S.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995CCDA....2....20B
available for $ in the S&T archive

The Ic band filter as it should be (with an additional ~ 880 nm short-cut)
must not be a pure interference filter. It could be also a normal Bessell
prescription colored glas filter with an additional ~ 880 nm short-cut
interference layer.

The only Ic band filters with a short-cut I know of are:

SBIG (thin photometric filter set just for the ST-402XME):
  <http://www.sbig.com/products/402_new.htm>
  <http://www.sbig.com/sbwhtmls/402bvi.htm>
  <http://www.sbig.com/sbwgifs/cfw_402_bvi_graph2.gif> (ST-402 filter set) vs.
  <http://www.sbig.com/sbwgifs/cfw_402_bvi_graph.gif> (pure Bessell filter set)
  <http://www.sbig.com/sbwgifs/cfw_402_I_filter_and_qe.gif> (Ic response comparison)

Idas UBVRIL filter set ("photometric" interference filters):
  <http://www.sciencecenter.net/hutech/idas/ubvril.htm>
  <http://www.sciencecenter.net/hutech/idas/pics/ubvril.jpg>
  <http://www.fli-cam.com/FLIproducts/cfw.htm>
  <http://www.fli-cam.com/FLIproducts/hutech.htm>
  <http://www.fli-cam.com/images/Product%20Images/Hutech_ubvril.jpg>
  <http://www.ne.jp/asahi/starfactory/home/products/Accessories/IDAS/index.html>
  <http://www.ne.jp/asahi/starfactory/home/products/Accessories/IDAS/ubvril_sp.jpg>
  <http://www.icas.to/e/lineup/ubvril.htm>
  <http://www.icas.to/e/lineup/ubvril.pdf>
  This one looks like a inteference-only filter set. The V and Ic band
  filter look about OK. The U band filter looks a bit shifted to the UV.
  The B band filter looks a bit wide. The BVR band filters don't have
  the typical red tail of Bessell prescription colored glas filters. But
  this influences the results probably just for the R band filter signifi-
  cantly (probably a contribution to the ppi; interference filters cann't
  have arbitrarily shallow transits anyway). At least the others should
  be OK when calibrating the measurments, maybe also without
  calibration. The Ic band filter has a short-cut as it should and as
  we want it.
  They don't say how thick these filters are, but they are probably
  2-3 mm thick as most non-Bessell prescription filters. You might
  ask for the actual thickness at hutech-web at sciencecenter.net
  and if they are still available.

Andover (5 mm thick Bessell prescription):
  <http://www.andcorp.com/Web_store/UBVRI/ubvri.php>
  Here the naming and a few of the filter transmissions are a bit weird.
  Both U band filters don't extend far enough into the far UVA. But
  that range will be extincted away at low altitude sites or at high
  airmasses anyway.
  Both B band filters look about OK, maybe a bit too far into the NUV.
  Both V band filters reach their max transmission already at 500 nm
  which is considerably shorter than with other Bessell prescription sets
  (that have it usually a bit too far in the red) but just a bit shorter than it
  actually should be.
  Kron/Cousins R and I filters:
  <http://www.andcorp.com/Web_store/UBVRI/Kron.php>
  <http://www.andcorp.com/Web_store/Images/Graphs/UBVRI_Kron.gif>
  Here the R filter transmission looks like a pretty weird (flat top) Johnson
  Rj filter (shifted by ~ 60 nm to the far-red in comparison to the Cousins).
  They probably assumed some kind of major QE fading towards 900 nm
  to come up with such a R band filter design.
  But this Ic band filter is exactly what we actually want and like all true Ic
  band filters should look like.
  Johnson/Bessell R and I filters:
  <http://www.andcorp.com/Web_store/UBVRI/Johnson.php>
  <http://www.andcorp.com/Web_store/Images/Graphs/UBVRI_Johnson.gif>
  Here the R filter transmission looks like a common Bessell prescription
  Rc band filter, but has a bit weak max transmission value.
  This Ic band filter is a common Bessell prescription Ic band filter with-
  out a short-cut.
If you want to use this true Ic filter with other Bessel prescription filters
you might make sure that they are also 5 mm thick to avoid to have
to raw refocus every time. Most available Bessell prescription filters
are "just" 4 mm thick. If you combine them, you'll have to compensate
the 1 mm thickness difference by changing the focus by ~ 1/3 mm
when changing filters.
Omega Optical offers such a 5 mm Bessell prescription filter set:
<https://www.omegafilters.com/index.php?page=prod_astro_index>
<https://www.omegafilters.com/images/ubvri.gif>
<https://www.omegafilters.com/pdfs/Filters_for_Astronomy_Flyer.pdf>
<https://www.omegafilters.com/index.php?page=shop/browse&product_line_id=9>
Optec does too:
Bessell 5±0.2mm UBVRI Filters (25.4mm diameter mounted for SBIG CFW)
<http://www.optecinc.com/optec_065.htm>
<http://www.optecinc.com/optec065001.jpg>
<http://www.optecinc.com/bessell_25,4mm_transmission.pdf>
Bessell 5 mm UBVRI Filters (50mm diameter unmounted for IFW)
<http://www.optecinc.com/optec_021.htm>
<http://www.optecinc.com/17250_bessel_u.gif>
<http://www.optecinc.com/17446_bessel_b.gif>
<http://www.optecinc.com/17447_bessel_v.gif>
<http://www.optecinc.com/17448_bessel_r.gif>
<http://www.optecinc.com/17449_bessel_i.gif>
And Chroma Technology (5 mm UBVRcIc Filter Recipes (Bessel type)) too:
<http://www.chroma.com/index.php?option=com_content&task=view&id=34&Itemid=47>
<http://www.chroma.com/images/stories/filters/astronomy_graph2.jpg>
<http://www.chroma.com/images/stories/filters/astronomy_graph3.jpg>

BTW
AstroDon also offers a Schüler Iz band far-red filter. Here the z stands
for the SDSS z band.

Clear skies
 Wolfgang

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

----- Original Message ----- 
From: <robertjmodic at ...>
To: <aavso-photometry at mira.aavso.org>
Sent: Friday, June 20, 2008 7:08 PM
Subject: Re: [Aavso-photometry] Question about "R" and "I" filters

> Arne,
> 
> Regarding the Schuler/Bessell prescription for the Ic filter, you've mentioned
> before that this has a red tail that can cause very red stars (like Mira's) to
> appear too bright compared to true Cousins I.  You suggested that ideally the
> Ic filter should be an interference type to better control the bandpass.  Do you
> know of a supplier that stocks these or is this something that has to be custom
> ordered ($$$$$)?  I know that SBIG has one as part of the BVI set for their
> ST-402 filter wheel but these filters are rather small (~21mm diameter).
> 
> Bob
> 
> 
> 
>> Date: Fri, 20 Jun 2008 01:15:47 -0400
>> From: "Arne Henden" <arne@,,,>
>> Subject: Re: [Aavso-photometry] Question about "R" and "I" filters
>> To: "George Sjoberg" <george_sjoberg at ...>
>> Cc: aavso-photometry at mira.aavso.org
>>
>> Different names for the same thing, for the most part.  The big difference
>> is between Johnson R,I and Cousins R,I; the Cousins filters are recom-
>> mended. Cousins filter bandpasses are detailed carefully in various
>> papers, and then there have been a couple of papers that give filter
>> glass prescriptions to closely match (with CCDs) the Cousins filter
>> bandpasses.
>> Most vendors follow the prescription given by Mike Bessell (the so-called
>> "Bessell" prescription), including Astrodon.  The "s" subscript on Astrodon/
>> Schuler filters just refers to "standard".
>> Arne
>>
>>
>>
>> On Thu, Jun 19, 2008 at 9:23 PM, George Sjoberg
>> <george_sjoberg at ...> wrote:
>>> I'm just begining to try some photometry and have a question about I
>>> and R photometry filters. I've noticd that many AAVSO Field Photo-
>>> metry Tables  refer to Ic and Rc (Cousins) whereas Astrodon refers
>>> to their filters as Is and Rs (Schuler). Is there a significant difference
>>> in these filters or are they different names for the same things?
>>>
>>> Thanks
>>> ..george