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.
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:
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.
"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. Schuler 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.
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.
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.
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.
Just freshening Phil's link wihout the period so it will work:
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:
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:
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.
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:
(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.
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.
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.
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.
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.
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.
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.
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.
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?
I had someone reach out to me about variables and filters. He mentioned ordering the Astrodon but I mentioned that the quickly becoming goto filter maker for pretty picture makers is Chroma. He reach out to them and found that they have scientific filters. They might be an option. The bandpass report they sent him looked as good or better than the one for the Astrodon (V) filter he ended up canceling the order for.
That is a great news, thank you for sharing the info! Their selection of any kind of filters is very interesting and promising, too.
I had a quick look over their offering and I found it a bit confusing. On webpage their Bessell filters look more like SDSS ones (rectangular ones with almost 100% transmission) but in an astronomy-related brochure, Bessell bandpasses look more conventional. However, there maxima of all the filters are well below 100% transmission. That is pretty different from Astrodon ones. I haven't tried with their data.
Compare to e.g. my measurements (http://jupiter.to.ee/~tonis/vaatlus/astrodon_LBVRI_filters.png) made in the radiometry lab of our observatory.
looks like they all leak in the NIR. Is that IR scattering or an artifact of the measurement set-up? Is your detector super sensitive at 1100 nm? The longer waves will scatter off rough surfaces.
Should we all be using an IR blocker on the camera after the UBVRI filter wheel? I suppose it depends on the spectral response of the camera and the atmosphere.
Very good questions! While those graphs are very typical to interference filters, there are indeed some points of concern as you pointed out.
I used a successor of Hamamatsu diode S12915-1010R (https://www.hamamatsu.com/eu/en/product/type/S12915-1010R/index.html). While Hamamatsu specifies that this diode is sensitive up to 1100 nm, it still works till theoretical Si diode limit at 1200 nm as it should be - so in that region detector is not exactly super sensitive. Each datapoint on my graphs is an average of 10 individual readings (i.e. readings are definitely not high-frequency temporal noise) and diode setup was fed by an alkaline battery. I had to scan one of the filters more than once and it's NIR behavior did not change over several hours, that gives me the confidence to say those peaks are real. Of course, it is stretching the limits past 1000 nm, but I was pretty confident about the quality and stability of equipment. In addition, measurements were not in photon-starving regime around 1 µm!
The setup was:
1 kW halogen bulb -> condenser lens -> double monochromator -> aperture -> projection lens -> filters in the filter wheel -> silicon photodiode catching all the light from monochromator, everything collimated by a laser.
All curves were collected by scanning with monochromator 3 nm step by step from 350 nm to 1200 nm. Every scan was forked by a reference scans (without filter) with exactly the same parameters. Of course, everything starting from the exit opening of the monochromator (included) was in a completely light-tight box. I followed standard procedures in our radiometry lab.
I suspect that I can't properly answer your question about IR scattering. Probably changing slightly the tilt of filter and re-measuring eveything would give slightly different results when there is some kind of scattering happening. And it seems, that I really have to repeat measurements with an InGaAs detector, too!
I haven't been too worried about those very far NIR leaks, because typical CCD QE there is very low and my typical targets are not very red ones. So the that leak really could be detected when the object is an extremely red one (V838 Mon?). If I recall correctly, Arne has done such test before and he did not found anything problematic. Regarding atmospheric effects past 1 micrometer - unfortunately that area (1-1.2 µm) is just moderately attenuated by atmospheric absorbtion (see e.g.: http://www.gemini.edu/sciops/ObsProcess/obsConstraints/transnir1.gif).
Still, maybe there is some kind of lab setup that could be used as well (ordinary incandescent light with and without NIR blocking filter) having both filters and the camera in test. I will discuss my colleagues about that.
By the way, technically it is possible to extend that NIR blocking range further into the red. Few years ago I iterated (with Schott) a design of a custom NIR blocking filter and every attempt to ensure wider blocking range resulted in (a bit) worse behavior (typically more waving and less transparency) in visible light and quite a bit of increase of the cost. IMHO Astrodon found a sweet spot. :-)
Update. Good point from our lab engineer about scattering was: scattered light from surfaces is almost always faint in such setup and because only monochromatic light exits monochromator, the contribution from scattered light (it's intensity x very low QE of sensor) is very low, next to zero.
I looked on the Chroma web site, and their photometric filters are bandpass flat topped. That is not the Bessel formula, but they label them that way. There is no data that confirms that this will not leed to errors at this time. Arne is evaluating some of these, but he has not blessed them and from what I understand it is unlikely that he will ok the flat topped ones. Chroma may have reformulated them to match the Bessel curves, but I am not aware of that.
Right. The transmission spectra that Chroma call "Bessel" (as on their own website as of today) are not modeled on Bessel or Johnson-Cousins passbands.
An observer using these filters might be able to transform his images to the standard passbands--but the transforms will be substantial and might have to be quadratic. In any case the burden would rest squarely on the observer. Before risking a purchase, I would ask Chroma if they offer full refund should transformations fail.
Chroma's strength is clearly in making filters with sharp-cutoff spectra. So it's a mystery to me why they don't just make Sloan filters, which should fit their existing technologies well (and which are the future anyway) rather than these "faux-Bessel" filters.
did you found that document from Chroma: https://www.chroma.com/sites/default/files/Astronomy-2015.1.pdf ? According to that, their Bessell filters look pretty realistic, though a bit "opaque". Maybe they have just wrong data shown in web shop?
Chroma's faux-Bessel filter spectra are on their website, in fact right on the page from which you buy the filters: https://www.chroma.com/products/sets/27103-bessell-ubvri
Their "Bessel" filters with rounder spectra are listed as Discontinued.
Hm... I was too optimistic and thought, that maybe that is some kind of copy-paste error. Pity that they don't have "true" Bessell ones anymore.
I can confirm that if you order one of these "Bessell" filters from Chroma, you will get the flat-topped version, at least if the data sheet that comes with the filter is correct. I ordered and received it last month, well before hearing Phil's warning about them during yesterday's webinar, then finding this thread. From seeing the webinar Q&A, I am not the only one in this boat.
As a beginner, I'm not sure where to turn from here. I currently have no other filters. My Starizona filter slider takes 2" filters; I have a 1-1/4" adapter for it but have had trouble with filter cell height in this size. (The adapter adds just enough height to make it impossible to slide the drawer in or out if I wanted to change filters, though it would be fine if I used only one filter.) I've seen Sloan filters in both sizes made by Chroma available through resellers, but Chroma has no information about them on their own website. Wondered about swapping the "Bessell" V for their Sloan g but not sure that's a good idea, especially if I haven't seen a transmission curve for the latter. I e-mailed Chroma this weekend looking for options and will see what they say.
Yesterday Phil mentioned Optolong as a viable option. I found a set of 2" UBVRI filters from them for sale, but as I'm just starting, I am reluctant to spend that kind of dough on a set, and apparently Optolong does not sell individual filters. Their transmission curves also appear a bit flat-topped, though they do slope down much less sharply on the long-wavelength end compared to the Chroma "Bessell." I'd want to make sure they'd absolutely work before ordering them.
I suppose I could just stick a 1-1/4" Astrodon Johnson V into this thing and call it good. Any other thoughts or advice?
You mention that you don't have transmission curves for Sloan filters. Here are figures from two very reputable manufacturers:
(My own solution to these problems is to migrate entirely from Johnson-Cousins to Sloan, planned for 2Q 2021. All the new catalogs are in Sloan-clone passbands anyway.)
To clarify, I've seen the Sloan passbands, but not the transmission curves for the Chroma Sloan filters. Given the issues with their "Bessell" filters, I'd certainly like to see some data on their Sloans before I'd consider getting one.
Given that newcomers should start on Sloan, if I remember correctly from a link above, the g would be the one to get first?
Be careful in comparing Sloan g to J-C V, Sloan r to Rc, etc. The spectra don't match in wavelength they way one might think they would. And very careful when reading these transmission spectra: they often color the Sloan g curve blue and the Sloan r curve green! Indeed Sloan g is relatively blue, which may or may not be quite what you want.
And yes, personally I do think it reasonable to ask for transmission spectra from any prospective filter supplier, especially at the costs we're talking about.
For an amateur I wouldn't recommend sloans.
Don't give up on the bogus Bessells yet. I do think you really need to transform your measurements. The B-V transforms in these filters are large, e.g. the color transform T bv is 1.329 (0.008) with my KAF 3200 CCD. The V-I transforms are not too bad. Arne says this could be an indication of a red leak in the B filter, a common isssue in interference filters.
I would be very courious to see how well your transforms do. You may be okay, but red stars could still be a problem.
It seems to me that if the primary problem is a red leak in B, you may only need to replace the B. There still is the issue of the non-standard transmission curves for all the filters, but I think the question of a red leak needs to be cleared up first.
Would you be interested in helping me? If so, contact me off line and we can discuss it. I have a 1 1/4 inch filter slider adapter you could borrow.
Arne advised observing a really red star in B and V. A red leak would cause an excessively bright measurement in B.
I'm going up to the observatory tonight to take some test exposures of T Lyr.
I noticed a day or two ago that Custom Scientific is now selling the same bogus bessell filters under their name. I wonder if that means the SBIG BVRI filters are the same. ...hope not.
Since the Johnson V filters are currently scarce, what is a good alternative from the readily available filters for CCD photometry?
L-RGB and the narrowbands (Ha, SII, OIII), UHC, are generally available in most sizes both on their own and in sets.
The green filter from an RGB tricolor astrophotography set is a good alternative. Many people getting started in photometry already have one of these. When using a green filter with the V magnitudes of the comps you would report the filter as TG in the AAVSO photometry report.
If you look at some dense lightcurves you will see that the TG data points match pretty well with the V's.
"If you look at some dense lightcurves you will see that the TG data points match pretty well with the V's."
But be aware that some of the LPV light curves show the TG data being consistently about 0.2 mag brighter than the Vis observations.