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Photometric Filter Selections

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bln3614
bln3614's picture
Photometric Filter Selections

What are the options, criteria, and designs for selecting photometric filter set purchases? 

My primary interest at this point is in ExoPlanet observing (also interested in eclipsing binaries, CVs, cluster variables, pulsators, etc.).  Lots of questions;  ideas on best options and suggestions??

For general use, are the U,B,V,R,I choices adequate for general variable star observations using CCD cameras?  I see several manufacturers that make various filter designs like Bessell, Sloans, Cousins, etc.  Is any one type adequate for variable stars?  Some manufacturers seem to have discontinued their filter offerings and some offer regular vs. premium filter sets -- what are the differences?  Who offers what filter sets within the amateur price range?

I also see some kind of specialized filter intended for ExoPlanet observing.  What are they designed for that makes them preferable for exoplanets -- (instead of a V filter?)?

It seems some filters are dyed glass and some are interference coated glass.  Which are better and for what useage?  Are some filters better for faster f/ratio scopes, like f/6?  There are some suggestions that thinner glass filters work better at faster f/ratios.  If they are, how are they better?

NBL      Bob N.

Richard Berry
Richard Berry's picture
Photometric Filter Selections

Hello Bob--

I expected to find a single location with an answer to your multiple questions. If you use the search function, you'll come to this page:
https://www.aavso.org/search-results?query=filters+for+photometry&op=Search

For your immediate concern about an exoplanet filter, I understand that the special filters are broad-band blue-blocking filters that result in maximum throughput and lower extinction. Perhaps those with more experience in this art will jump in.

Richard   

spp
spp's picture
Photometric Filter Selections

"For general use, are the U,B,V,R,I choices adequate for general variable star observations using CCD cameras?"

These are the Johnson-Cousins filters.  They are still the most commonly used filters by amateurs.  If you plan to report observations to the AAVSO data base these would still be the best choice. 

You don't need them all.  You may never use the U filter.  If you plan to collect filters one-by-one start start with V, then B, then I.  Later you could get an R.  Once you start collecting filters it's probably best to stick with the same manufacturer.  The thickness of the filters varies from different manufacturers.  If you use filters from different manufacturers you will likely need to refocus when you change to a different filter during an observing session.  (An electronic focuser which supports "offsets" makes this easy.)

Nomenclature:  In the J/C filter system U, B, and V are the Johnson filters.  R and I are the Cousins filters.  Sloan filters are a different system, used more by professionals.

Bessell filters are J/C filters made from colored glass. Baader and Custom Scientific (and several other manufacturers) make Bessell filters.  Shuler filters, probably the first off-the-shelf J/C filters available to amateur astronomers, were Bessell filters.  N.B., Choma makes J/C interference filters, but calls them Bessell filters.    

Interference J/C filters are now made by Chroma, Optolong, and a few other companies.  Astrodon filters are also interference filter, but they are no longer being produced.   Astrondon was bought up by Optical Structures several years ago.  I spoke with one of the partners of O.S.  He said they intend to produce  photometric (and other) filters again, but there is now no plan when this will happen.

Advantages of colored glass filters: less expensive, usable with very fast optics, e.g. the  Rowe-Ackerman Schmidt at F2. 

Disadvantages of colored glass filters:  Less transmission of passed wavelengths.  The Bessell presciption for V and B fiters used a glass type which hazed up, expecially in damp climates.  The Schuler V and B filters were known for this.

Baader claims their Bessell filters are sealed and coated in a manner which resists this hazing up.  I would imagine the Custom Scientific probably does something similiar.  If you buy colored glass (Bessell) J/C filters, be sure to ask about this before buying.

Advantages of interference J/C filters:  better transmision of passed wave lengths.  No hazing up.  Disadvantages: more expensive.  Not good for very fast optics.  I think Arne Henden wrote, somewhere, that he thought interference filters were okay down to about F/3.

Phil 

 

 

 

pbealo@comcast.net
pbealo@comcast.net's picture
I am looking for BVRI filters

I am looking for BVRI filters in 2" round mounted format. Are these available??

 

CrossoverManiac
CrossoverManiac's picture
Sorry, but everyone I know

Sorry, but everyone I know that sells photometric filters only sells the unmounted type for that size. You may have to use the 50-mm mounting cells.

 

https://www.edmundoptics.com/p/50mm-mounting-cell/11757/

spp
spp's picture
Mounted 2 inch Johnson-Cousins filters

I believe that both Baader and Optolong produce these filters.   

Phil

jewan
jewan's picture
The two's
msheald
Photometric Filter Selections

Hello! I spoke with the Astrodon folks about interference filters and fast optics. The concern with the light cone was with narrow band filters such as sulfur and H-alpha. In that case, one may need to go to 5nm band width instead of 3nm band width in order to make sure the desired light is obtained since there is a 0.8 nm blue shift when going below f2.8. So, anything below about f3, Astrodon recommended its 5nm bandpass filters

    For purposes of photometry, it did not appear that a roughly 1 nm shift in the bandwidth towards the blue end of the spectrum would be significant, especially since red and I would have even less bandwidth shift.

    The problem of star image halos could not be completely overcome since that was dependent on the the thickness of the filters and the f-ratio of the scope.

    I will likely be purchasing these filters in the future for a fast scope, so I would appreciate guidance from anyone with real world experience. Best regards.

Mike

spp
spp's picture
Photometric filters for "fast" optics

Mike,

Look at Arne's comment #8 in this discussion: https://www.aavso.org/photometric-ubvri-filters.  There, he recommends F/3 as an approximate limit for use of interference filters for photometry. 

Baader, and perhaps other manufacturers, are making J/C filters that use both colored glass and interference coatings.  Perhaps these filters may do better with fast optics and still retain some of the advantages of interference filters.  I would love to see some tests.

Phil 

 

 

 

TRE
TRE's picture
Just freshening Phil's link

Just freshening Phil's link wihout the period so it will work:

https://www.aavso.org/photometric-ubvri-filters

I have used narrow band optical coatings on the laser table where the coatings are chosen for either 90 or 45 degrees angle of incidence. The angle versus transmissivity or reflection graphs were common on manufacture's web pages in past years. Apparently, our community has seen the effect with short focal length astrographs. It might be good to quantify that effect in the equipment forum. 

The shift is small and I am not sure that it moves the passband of a 5 nm FWHM filter by more than 2 nm. The transmissivity should remain relatively high for angles in the range of +/- 5 degrees and the small wavelength shift may not make much difference to CCD or CMOS sensors. You could do the ray-tracing for a RASA and see what the ray angles are that come off the last lenses and on to the focal plane.

I have no experience with this supplier, but they provide one calculation:

https://www.alluxa.com/optical-filter-specs/angle-of-incidence-aoi-and-p...

An AOI vs shift plot from Alluxa white papers is included here. These plots typically illustrate the simplified colimated beam at an angle. A long time ago, all the manufacturers included these plots in their spec sheet.

The cone half angle might be another way to look at the problem:

https://www.alluxa.com/optical-filter-specs/cone-half-angle-cha/

RayTRE

msheald
Photometric filters for "fast" optics

Thanks!

Mike

CrossoverManiac
CrossoverManiac's picture
So you couldn't use

So you couldn't use photometric filters with a Takahasi Epsilon 180?

TRE
TRE's picture
Tak filters

It is done all the time.

Ray

CrossoverManiac
CrossoverManiac's picture
What do you mean by fast

What do you mean by fast optics?  Some really low f-ratio like f/2 like the RASA?  An f/4 Newtonian astrograph?

pbenni
Exoplanet filters

As a exoplanet observer, the choice of filter for me evolved with experience.  For starting out and/or following known exoplanets with the goal of submitting data to refine the ephemeris such as the NASA project https://exoplanets.nasa.gov/exoplanet-watch/about-exoplanet-watch/ or   http://var2.astro.cz/ETD/index.php  a high SNR filter like the exoplanet filter (blue blocking clear) or a red filter (Rc, r', etc) worked best for me.  For more advanced projects such as follow-up of TESS targets or first step validation of exoplanet candidates, the Sloan series g' r' and i' are nice to determine transit depth chromaticity differences because of the sharp cutoff between filters.  Besides filters, minimizing star shift on images is important, so off-axis guiding is better than independent telescope guiding.  If you have not done so, check out the AAVSO Exoplanet Observing Section for more guidance.

-Paul

Richard Berry
Richard Berry's picture
Photometric Filter Summary

Hi all--

One of our goals in setting up the Instrumentation & Equipment section was to create guides to best practices. Drawing on past and current posts, created the document at this link, and I'm asking for input corrections, and missing details:

https://www.dropbox.com/s/4vfv1vpad763awi/AAVSO_Instr%26Equip_Photometri...

(I hope the link to an outside page will work.)

The intent is to answer broad recurring questions about photometric filters and filter sets, and it does not cover special cases such as Stromgren filters, narrowband filters, or filters for specific projects. It also lists but does not evaluate the cliams or quality of different suppliers, as this may be a matter of opinion and may change with time.

I will incorporate feedback from the experts into the document and post it as our first "Best Practices Guide" in the Instrumentation & Equipment Section webpage.

Richard

spp
spp's picture
Filters summary

Richard,

Thank you for this very clear overview of photometric filters.

...just a couple of comments on filter manufacturers.

Schuler filters are no longer being produced.  (I misspelled the name in my original post.)  I mentioned them in my forum post because some of them are still around, at least the R and I filters are still around.  I know at least one person who still uses Schulers.  I think he must live in a very dry climate.

Astrodon filters:  These are widely used, good quality interference filters.  The problem is that they also are no longer (summer of 2020) being produced.  Based on my (winter of 2019) discussion with one of the principal partners of Optical Structures, the current owner of Astrondon, there was then the intention but no clear plan to resume production.  I hope Astrodon will eventually return to the market.

Phil

TRE
TRE's picture
back in business

I talked to Astrodon a month ago. They had plans to continue. I ordered B and V last week. They predicted delivery no later than September.

 

Ray

Richard Berry
Richard Berry's picture
We should probably say to

We should probably say to "check with these suppliers for current availability" and list all those for which we can find a link. I can insert the links into the document. 

--Richard

spp
spp's picture
Astrodon

Ray,

Please post to the forum when (if) you get your new Astrodons.  I'll be very interested to know when Astrodon comes back.

Phil

TRE
TRE's picture
OK

OK

September is my latest news. Will Let you know.

Ray

TRE
TRE's picture
UPS has my filter order on

Astrodon. B27R, V27R filter arrived today.  

 

Ray

spp
spp's picture
Astrodons shipped

Ray,

Thanks for the update. 

I just went to the Farpoint site and checked the availability of the Astrodon photometric filters.

Today (July 28, '20)  all the 1.25 inch J/C filter are listed as "avaiable on backorder".  The same is true for the 49.7mm unmounted filters, all except the Ic which is "in stock".

I looked at a few colors and sizes of the Sloan filters.  All that I saw were back ordered.

It seems that Astrodon is back in production and filling the backorders of people who have been waiting the longest.   

Phil 

CrossoverManiac
CrossoverManiac's picture
Good because I'm thinking

Good because I'm thinking about selling my Ic filter for an R since my camera is more sensitive to the light from a R filter than the light from the Ic.

Tonisee
Sensitivity of the imaging

Sensitivity of the imaging system and observing efficiency are definitely important points of consideration. E.g. for exoplanets, Rc filter is a good choice because of suppression of atmospheric effects (second order extinction, scincillation) and typically (still) high quantum efficiency of sensor. Similar considerations apply for differential non-transformed photometry of stars or measuring positions and lightcurves of asteroids etc.

However, while the wavelength region covered by a Rc filter is astrophysically extremely interesting (e.g. Halpha line, but definitely not only), broadband photometry more or less fails to retrieve/reflect that information. Johnson-Cousins filters measure mainly the shape of stellar spectral energy distribution continuum. Because V and Rc filters are rather close to each other in effective wavelength, the measure of slope of continuum what V-Rc colour index represents is sensitive to measurement uncertainties. Definitely more than e.g. V-Ic colour index. IMHO V-Ic would be a much more suitable filter pair for high-quality transformed observations after B-V.

Best wishes,
Tõnis

CrossoverManiac
CrossoverManiac's picture
I ordered an Exoplanet filter

I ordered an Exoplanet filter and was planning on order a Johnson-Cousins Red filter to replace the Ic filter, but it looks like that's not going to happen.  Should have gotten the UBVRI filters from Baader instead.

spp
spp's picture
Ray's Astrodon. B27R, V27R filters arrived

In view of the long months during which Astrodon filters were unobtainium, I propose that the AAVSO make a substantial order of Astrodon 1.25 inch and 2 inch J/C filters in order to produce

1)  a strategic reserve for the AAVSO network of current and future telescopes,

2)  a financial stimulus for Astrodon to allow inceased production, and

3)  a backup source of these filters for AAVSO members should the retail supply again be cut off.

Phil

TRE
TRE's picture
check them filters

I wonder if it would be a bunch cheaper for AAVSO or any group doing photometry to purchase the equipment to check passbands and leakage out of band.  Then that group could certify filters from manufacturers.  Are there other groups that have the equip[ment, that  would do some runs for AAVSO? Can the equipment be rented once per year?

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