[Aavso-photometry] newbie filter question & [AVSON] filter question - which to buy ?

Sun May 20 13:54:03 EDT 2007

Hi Ian

You sound as you have already some scientic projects in mind that
require more than one photometric filter and a CFW.

But photometric filters and a CFW are actually not needed for all
VSOing applications.
E.g. with monitoring cataclysmic variables and searching for novae
and supernovae a filter is not a must. The first ones are in outburst
pretty colorless (B-V ~ 0) and don't change color much (except on
the rise to maximum). So using a filter with these will usually just
"cost" LM and SNR (see CBA policy). With the later ones its most
important to get deeper to detect as many and as faint ones as
possible.
Many VSOer that just want to derive the time of maximum or minimum
of short periodic pulsating variables and eclipsing binaries by CCD
also don't use a filter as it usually doesn't have much influence on the
derived time.

But when it comes to "real" photometry, its always better to use a
photometric filter. Most start with a single V filter (that doesn't  need
a CFW on its own) in the 1"1/4 nosepiece (if the chip is as small as
a KAF-402). This is already sufficient for most of the photometric
applications like creating for light curves that are comparable to the
V filtered CCD measurements and visual obs of others.

If one wants to reach higher accuracy and precision (by including
color depending corrections) one needs to observe in at least two
colors and a CFW is more than recommendable. Then it depends
on the object which filters are best. Some might have just sequences
or comparisons star that are calibrated in UBV or BVRcIc while
others might have just ones in BRc or VIc.
BVIc are the most widely used and most recommendable filters in
CCD photometry. Rc just a bit less (probally as the V-Rc color index
doesn't span as wide as V-Ic) but it might be a better choice if you
want to go faint and e.g. just USNO BR mags are available (these
are pretty bad, but better than nothing). U band does often give the
"most scientific" data as it contains light below the Balmer jump.
Although many now common CCDs with KAI and KAF-ME chips
have a nearly sufficient NUV QE, making U band measurements
won't match the required system response with most amateur
scopes. As soon as there is some glas or a visually optimized AR
coating in the optical train, one will catch usually just about the red
half of the U band. Low altitude sites and high humidity might also
shift the effective system response to the red. Bad with U band is
that it will take about the same total exposure time to reach the
same SNR as the BVRcIc filter exposures together.
Some variables have increasing amplitudes towards the NUV (UV
Ceti (, ZZ Cet, BY Dra) stars, EBs of subtype zeta Aur) that will be
significantly larger than in the B band even under the mentioned
less than ideal conditions. You might try to image clouds on Venus
with the U band filter too but else there are not many usefull appli-
cations for it.
Some other vars might be just well observable in Ic and maybe Rc
due to severe interstellar reddening (see e.g. the Mira stars with
just JHKLMN mags in the GCVS). With a photometric filter set and
a CFW you can cover all of them and are equipted for all kinds of
scientific projects

The Schüler filter (now sold by AstroDon):
http://www.astrodon.com/whyschuler/whyschuler.cfm?WhyID=2
http://www.astrodon.com/_img/Image/oldsitejpgs/SchulerUVBRI.jpg
are the least expensive ones and not specially polished nor AR
coated.

The other vendors usually sell especially polished and AR coated
photometric filters that are much more expensive:

SBIG = Custom Scientific:
http://www.sbig.com/sbwhtmls/filters.htm
http://www.sbig.com/sbwgifs/UBVRIfilters.gif
http://www.sbig.com/sbwhtmls/ubvri.htm
http://www.sbig.com/sbwgifs/ubvri_u_chart.gif
http://www.sbig.com/sbwgifs/ubvri_b_chart.gif
http://www.sbig.com/sbwgifs/ubvri_v_chart.gif
http://www.sbig.com/sbwgifs/uvbri_r_chart.gif
http://www.sbig.com/sbwgifs/ubvri_i_chart.gif
http://www.sbig.com/products/ubvri_filter_curves.htm
http://www.sbig.com/products/u_filter.gif
http://www.sbig.com/products/b_filter.gif
http://www.sbig.com/products/v_filter.gif
http://www.sbig.com/products/r_filter.gif
http://www.sbig.com/products/i_filter.gif
(the Ic one is like in most Bessell prescription filter sets without
the recommendable short-cut)

Custom Scientific:
http://www.customscientific.com/astrostand.html
http://www.customscientific.com/astroresearch.html
http://www.customscientific.com/ubvri.jpg
http://www.fli-cam.com/FLIproducts/cs-filters.htm
http://www.fli-cam.com/images/bessell.gif
(the Ic one is like in most filter sets without short-cut)

Andover:
http://www.andcorp.com/Web_store/UBVRI/Johnson.php
http://www.andcorp.com/Web_store/Images/Graphs/UBVRI_Johnson.gif
(here the R one is Rc and is what we need, the I one is like in most
filter sets without short-cut)
http://www.andcorp.com/Web_store/UBVRI/Kron.php
http://www.andcorp.com/Web_store/Images/Graphs/UBVRI_Kron.gif
(here the R filter looks weird: the Ic one is what we actually want
as it has a short-cut)

Omega Optical:
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
(Bessell 1990 5 mm filter prescription for NIR sensitive UV-
coated Thomson and GEC CCDs; Ic one is like in most filter
sets without short-cut; they offer also Fukugita 1996 SDSS
filters)
http://www.konkoly.hu/staff/racz/Schmidt_UBVRI.html
http://www.konkoly.hu/staff/racz/OmegaOptical/XBSSL_U.gif
http://www.konkoly.hu/staff/racz/OmegaOptical/XBSSL_B.gif
http://www.konkoly.hu/staff/racz/OmegaOptical/XBSSL_V.gif
http://www.konkoly.hu/staff/racz/OmegaOptical/XBSSL_R.gif
http://www.konkoly.hu/staff/racz/OmegaOptical/XBSSL_I.gif

Optec:
Johnson 7.0±0.2 mm UBVRIJH Filters (12.7mm unmounted for SSP)
http://www.optecinc.com/optec_014.htm
http://www.optecinc.com/17031_johnsonu.jpg
http://www.optecinc.com/17132_johnsonb.gif
http://www.optecinc.com/17133_johnsonv.gif
http://www.optecinc.com/17134_johnsonr.gif
http://www.optecinc.com/17135_johnsoni.gif
http://www.optecinc.com/17137_johnson_j.gif
http://www.optecinc.com/17138_johnson_h.gif
(they offer also Strömgren ubvyHbnw filters for SSP)
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
(Bessell 1990 5 mm filter prescription for NIR sensitive UV-
coated Thomson and GEC CCDs; Ic one is like in most filter
sets without short-cut)

Chroma Technology :
UBVRI Filters for Photometric Astronomy
5 mm UBVRcIc Filter Recipes (Bessel type)
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

Glen Spectra:
Astronomy Bessell Photometric Filters
http://www.glenspectra.co.uk/glen/filters/astronomyfilt.htm
http://www.glenspectra.co.uk/glen/filters/images/ubvri.gif

Fingerlakes Instrumentation FLI:
http://www.fli-cam.com/FLIproducts/cfw.htm
http://www.fli-cam.com/FLIproducts/fli_filters.htm
(no further details)

Astronomik:
http://www.gerd.neumann.net/v2/deutsch/astronomik_lrgb.html
http://www.gerd.neumann.net/v2/bilder/Astronomik-LRGB.gif
(the BAV recomended the LRGB Typ IIc G Filter as V filter some
years ago; inteference-only filter that doesn't have the typical red
tail of colored glas filters)

Hutech:
http://www.fli-cam.com/FLIproducts/hutech.htm
http://www.fli-cam.com/images/Product%20Images/Hutech_ubvril.jpg
(looks like a inteference-only filter set; V and Ic looks about OK;
the UBRc one look pretty far off from the standard; the BVR ones
don't have the typical red tail of standard colored glas filters)

It looks like as nearly all Bessell prescription filter sets use
the old prescription for UV-coated, NIR-sensitive CCDs as
published in:
- 1990PASP..102.1181B - Bessell, M. S.
  UBVRI passbands
  http://adsabs.harvard.edu/abs/1990PASP..102.1181B
and don't use the advantage of adapting the filters to the
nowadays much more blue sensitive CCDs:
- 1995CCDA....2...20B - Bessell, M. S.
  UBVRI Filters for CCD Photometry
  http://adsabs.harvard.edu/abs/1995CCDA....2...20B

Parfocality is IMO a pretty overstrechted word. Its usually just
used as synonym for "equally thick" that doesn't sound as good
in advertising.

A) Filter thickness
But equaly thick is just equaly thick within the specs of the used
glas. With a cemented 4 or 5 mm UBVRcIc filter sandwich
(containing 2 or 3 layers of 1 to 3 mm colored glas) this might
result in a physical thickness difference of worse than ± 0.3 mm
that will result (in the convergent beam ) in an effective shift of
focal point of worse than ~ ± 0.1 mm between different filters
(see e.g. "Forms of supply" of standard Schott colord glas for
"polished" filters that say: thickness 1.0 ± 0.1; 2.0 ± 0.2 and
3.0 ± 0.2 mm; polished used here in opposite to thick float
glas).
With higher priced specially polished photometric filter sets
and especially some pretty picture RGBCL filter sets (these
usually consist just of a single ~ 2 mm base glas with an
interference coating and the specs the base glas might be
much smaller) this issue is usually less severe.
As the depth of focus depends on the f ratio (and FL) of the
scope, fast systems will be affected much more than slow
ones.

B) Chromatic focus shift
But as soon as an optical train contains a refractive element
(like the SCT Schmidt corrector plate, a field flattener or a focal
reducer), it will also show some chromatic aberration that will
result in a shift of the chromatic focal point.
As just a few common amateur scopes don't contain refractive
elements (like e.g. plain RCCs and Newtons without field
flattener and focal reducer) and should be therfore free from
chromatic aberrations, most amateur setups are are not.
This means that the filters of a true parfocal filter set would have
to have different thicknesses to not shift the focal point when
the filters are exchanged. And that this parfocal filter set will
be not parafocal anymore when changing the setup slightly by
adding or removing a field flattener or a focal reducer.

With chromatic (corrected for 1 wavelength) and apochromatic
(corrected for 3) lenses one will have the NUV and NIR focii at
different and with achromatic (corrected for 2) and superchro-
matic (when corrected for 4) one will have them on the same
side (this is a simle test to confirm if an apo is really an apo).
The rise of the focus change outside the range the lens was
corrected for will be usually steeper in the NUV than in the NIR.
E.g. with my 300mm FL f/5,6 mirror tele lens, BVRc focii are
pretty close (but not identical) and the UIc focii a pretty far off
on the same side (but not identical too). So this one behaves
like an achromatic although or better because it contains
some additional lens elements.

C) Thermal expnasion
Most scopes (especially the ones with metal tubes) change
focus throughout the night anyway if the temperature drops
signifivcantly (thermal expansion of the OTA, minor changes
in the optics, ...).

So parfocality and keeping the focus is pretty relative and
severely depends on the actual setup. Everybody has to be
prepared to at least to have to fine refocus after a filter
change (which is no problem if one has a computer
controlable focuser anyway).
With photometry its less severe than with high-end pretty
picture imaging as one can still get pretty good photometric
results with slightly defocused stars (even with severely
defocused ones if the star is very bright).

Clear skies
 Wolfgang

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

----- Original Message ----- 
From: "Ian Bacon" <ibacon at graduate.uwa.edu.au>
To: <aavso-photometry at mira.aavso.org>
Sent: Monday, May 07, 2007 6:32 AM
Subject: [Aavso-photometry] newbie filter question

> Hello,
> 
> I have been pestering people on the SBIG discussion group with a few  
> questions, and was directed here for follow up questions.
> 
> By way of background, I have been carrying out visual variable  
> observing for years (BIX), but have now decided to go ccd. My scope  
> is a 10" LX200GPS. I plan on buying a SBIG ST-7, a 10 position filter  
> wheel, a F5 focal reducer (NextGen), and Optec focuser.
> 
> My specific question is this, what are the best filters to buy for  
> variable use? I would like to get a complete set (UBVR, maybe even  
> I). I realise that in the beginning I will not use this full set, but  
> I plan to expand my observing as my experience grows.
> 
> So, any thoughts on filters (or any other part of my setup)?
> 
> Thanks for any assistance.
> 
> Ian Bacon

----- Original Message ----- 
From: "Ian Bacon" <ibacon at graduate.uwa.edu.au>
To: <AVSON at yahoogroups.com>
Sent: Wednesday, May 09, 2007 12:46 PM
Subject: [AVSON] filter question — which to buy ?

> Hi,
> 
> I have been kicking this question around two other discussion groups,  
> and, while I have received a number of answers, I am still not sure  
> just what I should do, so I thought I would my local Aust compatriots  
> for their views.
> 
> Here is the situation. I want to move into ccd imaging of variables,  
> and also do a little general imaging. I have a 10" LX200GPS, I will  
> be buying a ST7, an Optec focuser, a SBIG 10 position filter wheel,  
> and plan to work at F5. Now, the question, which brand of filters are  
> best for variables? The easy option is to go with the default SBIG  
> set, some people have suggested Astrodon.
> 
> All I am sure of is that I want parfocal system.
> 
> Does anyone have advice for me?
> 
> Any advice appreciated.
> 
> Ian Bacon