Wed, 06/13/2018 - 15:20
Hello,
I have some people interested in Pluto opposition surge observing campaign, but most of them don't have any photometric filters. But they have Baader CCD filters instead, because they are mostly interested in astrophotography. Is there any use of those for us? They have a little different wavelength transmission. But in my opnion, those measurements are still more rewarding than non-filtered (clear). What are your opinions about that?
Also, what about any other non-photometric filter that have more narrow spectrum (IR-pass, IR850 etc.)? There's no way they can get a true photometric filter in such short time, though.
Regards,
Gabriel
Hello! It interesting for me too. Now i choose V/G filter. V is most expensive than G. But spectral response is most identical. Realy use Baader G-filter for photometry like with V? Is very big differense?
Eugene,
It is acceptable to use a tricolor Green filter instead of a V filter. In this case, if you are reporting your results to the AAVSO, you should use the TG filter designation in your report.
After looking at lots of lightcurves, I have noticed that usually the TG magnitudes match very well with the V magnitudes.
Phil
Phil, thank you for answer! Very much!
My uncertainty is stoped now...
Those deep sky imaging filters are very different than DSLR filters often reported as TG. They have a 'rectangular' response shape instead the more complex one of DSLR (designed to comply with human vision standard).
I have applied my photometry simulation software to that 'rectangular' filters, just curious (I usually work only with DSLR). This is a simulation I did develop to evaluate my VSF color correction technique, specific to the DSLR case. It is based on the response curves of the DSLR, an atmosphere model at various air-masses, and the spectra of the Pickles library. It calculates the error for the various spectral types included in Pickles. The results are provided for V, B-V and V-R (not B and R alone at time being, sorry).
For that last test at AM=1.5 I combined the typical response of those Baader filters with a QE3200ME CCD response. The results are attached: first graph shows the error in mmag in function of B-V, excluding the M giants (that are a serious problem in any case). The correction coefficients of the VSF have been optimised in between B-V of -0.4 to 1.2 . Same for the classical transform coefficient. A standard Vj filter is also shown but without air-mass correction (against, the optimization of the VSF and transform coefficients includes it).
The result for V is very good with the VSF technique from B-V -0.4 to 1.3 ( green curve, Gcm). Number of redder stars show large errors. The classical transform (red curve, Gtm) is somewhat worst in particular for blue stars, red stars above 1.3 also show large errors.
A second graph includes the red M giants, several have very large errors, this is usual as soon the response curve shapes are somewhat different than the standard Johnson / Bessell 2012 reference.
B-V and V-R results are not in the graphs but can be found in the attached result table. Gm is the V non-corrected output error. Vjm is a standard V system error (air-mass not corrected). Gm-Rm is the V-R from the sytem under simulation. Bm-Gm same for B-V. Bz-Vz is B-V from the standard filter at AM=0. All mmag. B3-V and V-Rc are the reference from Pickles (mag instead mmag).
Hope it helps,
Clear Skies !
Roger
Roger, you have very interesting comparisons! And it seems to be useful for decision about Pluto photometry with those rectangular filters, too.
IF you used Baader pretty-picture-V filter transmission curve for your simulation, then measurements of Pluto should be fine (B-V around solar B-V, probably Pluto is slightly redder, though) when compstars also have B-V<0.9. They may have systematic offset but IMHO correct transformation should correct for it to some extent.
Even more - if you look at Nova Del 2013 in AAVSO LCGv2 and plot only V-band measurements, then you see multiple parallel lightcurves, in some cases having generous separation... They are from different observers with different instruments transforming or not doing that with their data. Even in the case of AAVSO database where observations are done mostly with Johnson V or DSLR G filters, something needs to be done in order to use that data for scientific research. U. Munari has published a description of secondary(?) calibration procedure called Lightcurve Merging Method (see https://arxiv.org/pdf/1209.4692) to overcome such issues.
Best wishes,
Tõnis
Tonis,
I've attached a VStar plot showing only V reported CCD data points for the section of the Nova Del 2013 lightcurve that appears by default in LCGv2. This is an example of the "parallel light curves" which you mentioned. The green data points are untransformed V mags The blue points are transformed V.
The AAVSO is certainly making a serious effort to encourage and help CCD observers to transform their data.
The difference here between tranformed and untransformed V maginutes is quite large (~0.5 mag), much larger than I'm used to seeing for stars of this color. Perhaps this has something to do with the ejecta.
Phil
Phil, you found definitely the worst case example :-D
I was referring to autumn 2013 to end 2014 when there was plenty of observations and few observers provided long series. Roughly what I added below. Anyway, I think that AAVSO observations of this nova are very educative for any photometrist, showing several possible issues to deal or take into account. :-)
Note: I'm not complaining at all, AAVSO's Nova Del 2013 lightcurve is great achievement!
Tõnis
Summary, "to be or not to be"? :)
Or i need accumulate more $$ and buy Johnson/Sloan for various tasks with varstars?
Hi Eugene,
The tricolor G filter can be used as a surrogate for the Johnson V filter, with some exceptions. As mentioned by Phil, if you just use it by itself, you should report your results as "TG". If you transform, you can then report it as V, but transformation takes two filters and the determination of coeffcients.
Where filter issues are most prevalent are for pathological stars. Two prime examples are novae or other emission-line stars, where the emission lines make filters extremely difficult to transform/match, and LPVs, where the molecular absorptions and huge red flux not only make transformation difficult, but highlight any red leak that may be present in your filter. Avoid at least those stars, and you should have pretty good luck (and plenty of projects to work on). Pluto photometry, for example, would be ok since you are dealing primarily with a reflected G-star spectrum, and can find equivalent-color comparison stars.
Using any filter is better than no filter at all (except for very specific projects like CBA, and even there, you'll find the pros use filters). Using a photometric filter is better than RGB, but kinda secondary importance.
Arne
For photometry of events thar are „grey“ in the sense that there is a variation in brightness across the spectrum, but no change in color, any filter (even no filter at all for b/w cameras) should do. Classical examples are eclipses. I am not familiar enough with the Pluto opposition effect campaign mentioned in the original message, but I would think that it falls within this category.
For events/projects where the exact color matters, you either have to invest in photometric filters or try conversion of filters which can be tricky and might not work at all for some targets.
I guess one should not be discouraged too easily by the lack of photometric filters. Try to collect data as best as you can, and then see what can be done with it and how it compares to other observers´ data. It Is not uncommon for us amateurs to be limited by equipment, and making the best of what we have is most of the fun, isn’t it?
cs
hb