I've recently obtained M67 data from a system consisting of a CDK20, QHY268M camera, and Baader BVRI filters. The B filter does not have a red leak, based on V838 Mon data. However, I'm a bit disappointed in the transformation coefficients, as calculated by TG.
Tb_bv = 0.211
Tv_bv = -0.017
Tr_vi = -0.130
Ti_vi = -0.051
Tbv = 1.269
Tvr = 1.242
Tri = 0.868
Tvi = 1.037
with typical fitting errors around 0.01 from 3 independent calculations of the coefficients.
So I'd consider V and Ic to be close to the standard system, and Rc to be slightly farther off than I'd like. The one that seems worse is B, at least compared to the original Astrodon filters. That said, using a coefficient of 0.211 will transform ok to the standard system.
I also tried using a Baader Sloan u' filter as a replacement for the Baader Johnson U filter, and the transformations were reasonable:
Tu_ub = 0.008
Tub = 0.886
I like using the Sloan u' filter because it is blueward of the Balmer limit (364nm), while the Johnson U filter straddles it.
I don't have independent spectral scans of these filters, but they have some on their website:
https://www.baader-planetarium.com/en/blog/results-of-the-tests-made-by-galli-gianni-about-the-new-baader-ubvri-bessel-filters/
I would not worry about the red leak that Galli shows, as the transmission is beyond the response limit of silicon (usually given as 1.08 microns). The filters do show some bumps and wiggles compared with standard glass filters, but that probably won't affect photometry.
This particular telescope has only Baader filters, so I can't compare actual throughput of the filters compared to my reference set of Astrodon filters.
I have equivalent data for other filter vendors that I'll post in the coming days.
Arne
Hello, Arne,
Which transformation coefficients would be ideal, or at least acceptable, for a set of BVRI filters?
I have a mixed set - Baader V, Astrodons B & Rc - and I am about to buy a Baader Ic.
Ari
Hi Ari,
The ideal coefficients are as close to 0.0 for magnitudes and 1.0 for colors as possible. I think values up to +/- 0.25 from these ideals are fine with linear coefficients, but above these offsets, I'd consider moving to higher-order terms. This is a hand-waving answer - I have not calculated what effect a large coefficient would have on transformation.
Usually non-red stars are no problem, especially if the comp star(s) are chosen to have similar colors. Where transformation has difficulty is for very red stars. The problem is twofold: (1) your original coefficient calculation is unlikely to have very red stars, so transforming red stars will be an extrapolation rather than an interpolation; and (2) very red stars typically have a spectrum that is heavily influenced by molecular bands, so slight differences in the spectral response of a filter can make a large difference in the magnitude estimate. You can see the difficulty most easily by transforming V with respect to (B-V) using Tv_bv, and doing the same thing using Tv_vi. As the star gets redder, the difference between these two transformed values increases.
Arne
Arne,
Thank you for detailing the matter. I will be doing some reading so as to fully understand this fundamental subject.
Ari
I suppose the "Appendix D: Supporting Calculations for Color Transformations" in the AAVSO Guide to CCD/CMOS Photometry with Monochrome Cameras (2022) would be an appropriate and practical guide for the characterization of a filter set.
Am I on the right path?
Hello Ari,
I am currently using an Astrodon V filter and I am looking to add a B filter to my observations. I was planning to use an Astrodon B filter. Unfortunately, Astrodon has no B filters in stock. I am considering purchasing a Baader B filter instead. Is there anything that I should consider before mixing a Baaber filter with my Astodon filter?
thanks
I have Baader B & V filters. Here are the coefficients that resulted from averaging two standard fields (NGC 7790 & SA41).
[Coefficients]
Tbv= 1.119
Tb_bv= 0.088
Tv_bv= 0.001
[Error]
Tbv= 0.049
Tb_bv= 0.031
Tv_bv= 0.027
[R Squared Values]
Tbv= 0.996
Tb_bv= 0.535
Tv_bv= 0.130
I don't know enough about this to know if I should be "disappointed" or not. Thoughts?
What camera are you using? These transform coefficients are fine. I would not be disappointed.
Arne
This is with an ASI2600MM Pro camera attached to a Takahashi FC-100DF refractor (100mm doublet, no flattener). Glad to hear these are reasonable.
I achieved similar results calculating transformation coefficients on my Baader BVRI filter set paired with a CDK14 and IMX461 sensor in a Moravian C5 camera - a larger version of the sensor(s) found in the QHY268 and QHY600 cameras.
Tb_bv = 0.159
Tv_bv = -0.012
Tr_vi = -0.078
Ti_vi = 0.006
Tbv = 1.206
Tvr = 1.181
Tri = 0.845
Tvi = 0.999
I had a smaller set of these filters (36 mm) tested on a spectrophotometer last July, and the results agreed pretty closely with the information posted on the Baader site.
The modest negative coefficients Arne got make it seem like the R filter is in fact not far from Sloan r rather than Cousins R, i.e. the filter is shifted blueward from the nominal standard. This might also be the case for the I filter. Could you use the ATLAS 'refcat2' gri photometry in the M67 field to make a trial of this.
Seems like where one runs into trouble with the passband mismatches is with reddened stars, or where you have a mix of unreddened and much reddened comp stars. To some extent it may be possible to use transformations involving a quadratic or with two colors (probably need that for u ---> U anyways).
\Brian
Arne,
is there a minimum focal ratio required for the Baader or other filters currently available?
I am looking to use a 200mm f2.8 camera lens with a CMOS monochrome camera and filter wheel for bright star photometry. The fov will be about 6 x4 degrees and I don’t want filter band pass to vary across the image. Cheers,
Mark
Here is the basic idea of an interference filter: Bandpass Filter Kits (thorlabs.com)
As the angle at the edge of the light cone gets steeper, light hits those layers at less than normal incidence and distance traveled by the ray becomes more than 1/4 wavelength.
That becomes important when using a very narrow band filter like a bandpass of 5 nm around a laser line or a favored emission line as in solar filters.
For example: AOI_Technical_Note.pdf (omegafilters.com) Has handy F ratios, cone angle, NA data. NA data is more useful for NB filters fed by very steep cones like you may see going into or coming out of fiber.
And: Infrared Optical Filters - Angular Shift - Electrical Optical Components, Inc. (eoc-inc.com) If you know your light your light cone angles, you can calculate the shift from formulas here. The shift goes to longer wavelengths because the longer path through the 1/4 wave layers becomes a quarter wave at a longer wavelength.
Generally, angles of incidence of less than about 3 degrees produce only about a nanometer of shift on a narrow band filter. Probably more noticeable on a Sloan filter than a Bessel filter, and probably not really noticeable on either except at the band edge of a Sloan and then only if the edge shift happens to fall on an important emission line.
Ray
Hi Mark,
Baader says their J/C filters are good from f/15 to f/1.8. They produce two versions of their narrowband filters, depending on the focal ratio.
Don Goldman said that his Astrodon wide-band filters were ok at f/3.6, but was concerned at much faster than this. So my personal feeling, without testing, is that f/2.8 is a questionable focal ratio. This was one reason why I chose the ASA N8 telescopes instead of the Takahashi E-180 for APASS. You can estimate the bandpass degradation for the center of the field of view from the website Ray gave, but ray-tracing to the edge of the field of view is beyond my expertise.
The way professionals did it in the past was to insert extra optics that collimated the beam so that the filter saw parallel light (and was also temperature controlled). Then optics were added to refocus the beam onto the sensor. You can't do it in this case, so the best that you can do is either find someone with the right software, or do the experiment yourself, imaging an area in the Milky Way with lots of stars and doing a transformation in rings to see the change and whether it is measurable for wide-band filters.
Arne
Thanks Ray and Arne,
very useful information. Canon have documentation detailing the optical path of the lens I plan to use.
I could stop down the lens to, say, f4 but diffraction spikes start becoming prominent at slower f ratios. Cheers,
Mark
I've followed this thread with interest and followed through on Brian's suggestion. Using TychoTracker I calculated transformation coefficients for the Baader VRI filters, but used Sloan gri photometry data from ATLAS for each corresponding filter. Here are my results. On the left are my original BVRI transformations, and on the right, the same data transformed using Sloan gri.
Now to go start digging and understanding these results better than I do.
J/C VRI Sloan gri
Tv_bv = -0.012
Tr_vi = -0.078 Tr_vi = 0.022
Ti_vi = 0.006 Ti_vi = 0.103
Tbv = 1.206
Tvr = 1.181 Tvr = 1.896
Tri = 0.845 Tri = 0.848
Tvi = 0.999 Tvi = 1.339
Mike
Thanks for these checks, Mike. I'm not sure how to interpret the figures either. For the Tr_vi case, for instance, are you giving the coefficient as a function of V-I or actually g-i? In this instance it does seem as though the Baader Rc filter is in fact closer to Sloan r. Another case is Tvr, with the value 1.896. If this is for g-r, then the large value may in fact be about what one expects (rather than being close to the ideal 1.0).
\Brian
Coefficients…
Hey Brian,
Coefficients were/are calculated in that test as g-i and g-r. Data was collected using the BVRI filters, but when I did the transformations I used the applicable Sloan bands (g', r', i') for each filter.
Mike
I hope you all don't mind me breaking in to ask a four-year-old question: what two filters should I get for the upcoming Choice Photometry course?
I apologize I don't understand the data presented well enough to interpret it for my needs.
I'll be using a 6 inch SCT and Asi178 cooled mono camera if that is pertinent.
Thanks!
Terry
The course…
Terry,
The course description says B and V or V and I.
I would probably go with V and I, but as they say: your mileage may vary! Either are good choices. It seems that most advice I've seen suggests V then B then I then R when expanding ones filter set.
Peter
BPEC
The Baader…
Thanks!
The Baader Bessel V I?
Terry
I would suggest you ask the class instructor, Ed Wiley, before your purchase filters. You can look him up in this web page and send him a PM. Consider the QE of your imager and compare its efficiency at the B and I bandpass. Page 17 of this document I think: https://astronomy-imaging-camera.com/manuals/ASI178_Manual_EN.pdf
Also consider that with a 6" SCT OTA you will benefit from the higher QE of the B bandpass. As someone new to photometry I would recommend you begin with V and B filters, but certainly do ask Ed.
Welcome!
John
I was fortunate enough to purchase Astrodon Johnson/Cousins V, B, & Rc ("I" wasn't available) just before they went out of stock. They have been everything I needed for the past three years. I have been using V, B,Rc for overcontact binary research and Rc for exoplanet transits. V & B in support of various Alert requests. But, as such I am not qualified to answer which manufacturer's filters you purchase. To be specific I am suggesting your compare the B and I (near infrared) filter's bandpass to the QE of your sensor and in consideration of your current OTA. IMO: You will better served with V & B filters to start. But do ask Ed Wiley (WEY), he has vastly more experience than I and he is teaching your class. I'm also hopeful that others in this group with more experience than I correct any misconceptions I might have.
John
Thank you!
Filter Band Coefficients
Tb_bv = 0.134
Tv_bv = -0.09
Tr_vi = -0.083
Ti_vi = -0.001
Tbv = 1.287
Tvr = 0.993
Tri = 0.849
Tvi = 0.922
These are with Chroma (classic) BVRI filters and QHY600 CMOS camera, understanding them is beyond my pay grade at this point.
Any thoughts?
Steve - HSTG
Steve,
This information is helpful. Which version of the Chroma filters are you calling "classic", the older flat top, square shoulders version or the newer version with pass bands more closely resembling the UBVRI standards?
Phil SPP
The "classic" ones are those that have passbands that closely resemble the traditional UVBRI filters, not the square ones that seem to resemble Sloan filters.
Tom (rth)
Chroma calls their original Bessel prescription Johnson-Cousins filters with the very high flat-top transmission "Bessell" filters. Their "classic UBVRI" set is their later set of filters where they worked to get closer to the original Bessel prescription JC filter transmission curves. At least that is how Chroma is using the terminology. It's easy for people to confuse their naming though.
-Walt
FWIW, so far as I know, the last word from Bessell (two l's, not one) himself on the nominal UBVRI filter passbands is this paper:
https://ui.adsabs.harvard.edu/abs/2012PASP..124..140B/abstract
...see Figure 9 here. Since many ordinary colored-glass filters require non-linear transformations with sundry detectors to recover the Landolt system, it would not be surprising if the filters giving high throughput in the rectangular passbands also need non-linear (or bi-linear) transformations.
\Brian
Brian,
Actually, these Chroma (so called) "Bessell" filters are not Bessell filters at all. They are interference filters! Why Chroma, with people who presumably should know better, would what to call their quality UBVRI interference filters "Bessell" filters mystifies me. This leads to confusion between people, such as yourself, who know what Bessell filters are, and many newer AAVSO observers who don't. To make matters worse, now they have come up with "classic" Bessell filters. Good Grief!
Chroma makes good filters, and I suppose they have a right to call them anything they want, but I think we should not enable this misnomer. To prevent further confusion I suggest that we all abstain from discussing Chroma "Bessell" filters (classic or not) and call them Chroma UBVRI or Chroma J/C filters.
As for "classic" Bessell filters, many old timer CCD observers (such as myself) probably have them scattered around in the backs of their filter drawers. The B,R,I filters may still be good, but by now the V's are all certainly fubar.
Phil
Perhaps you might like to reach out to Chroma and discuss this with them so the filter taxonomy can be made consistent?
Ann Stanley, an account manager at Chroma, represented them at their booth at NEAIC this past April. I would be happy to give you her contact details off her business card.
Yes, please use the forum link to contact me.
Phil