Can anyone recommend a source for BVRI filters in Spain? I am working with a DSLR observer who lives near Madrid.
usually (I live between Spain and Germany) when I need somethin I contact www.telescoshop.com
The owner is Carlos Malagon,very serious guy.They are located in Malaga,south of Spain.They have Astrodon filters photometric.Tell my name,Stefano Padovan (Italian that lives close to Barcelona).
Hope it is helpfull.
Stefano T5 Padovan,PSD
Thank you so much, Stefano, both for the very helpful information and for your fast reply! I will tell the observer and say he may mention your name (thank you also for that).
best regards and good observing,
I may have misinterpreted your post but does your DSLR observer intend using BVRI filters on his DSLR? If so then it is not a good idea.
DSLRs have a number of filters in front of the CMOS sensor. Some can be removed or replaced to improve sensitivity at longer wavelengths, but the rgb Bayer array cannot be removed. So a DSLR will always be imaging through rgb filters. Adding a photometric filter, say B, will result in images with r+B, g+B and b+B spectral response - not B spectral response.
Roger Peri has more technical knowledge in this area and it would be good to hear his thoughts. But the bottom line is that using photometric filters on a DSLR camera is not recommended. Cheers,
Thanks very much, Mark, for the clarification! I will check with the observer again to see if he really meant DSLR (or if perhaps he is thinking of branching out beyond DSLR and I misinterpreted what he said).
good observing, Elizabeth
I second what you wrote Mark. Only one remark: a friend of mine just wrote me a few weeks ago that a guy from Portugal removed the Bayer-matrix filters from a Canon camera. I don't know wheter it is true or there was only the the IR block filter removed, but it sounds interesting.
I know of www.tecnospica.es , a good seller of astronomical material in Spain. Anyway, you can buy easily at online stores in the European Union. I've bought astrodon photometric filters at: www.robtics.nl (located in the Netherlands) and www.astroshop.com (germany), both have astrodon johnson-cousins filters.
And can be bought, too, directly from astrodon: www.astrodon.com. This company ships outside the US, but I have to warn you that every direct shipment from astrodon will be charged with taxes (including VAT) at customs, so be careful . (In stores you only pay for the price and shipping).
I only know of one case of baxer matrix removing, with not very good results, unfortunately.
Fully agree with your point, using Johnson-Cousins BVR filters with a DSLR is not a good idea !
The result is the math product of the transmissions of either Bj or Vj or Rc with the DSLR transmission at each wave length and that result is very different than the original photometric pass-band of the added filters. This is a question that has been addressed in our DSLR Manual discussions.
On idea (not recommended) was to use the Vj filter combined with the sum of the R+G+G+B channels (pure luminance) of the DSLR but the resulting pass-band requires a transformation that is worst than the "G" alone transformation ! Then the (precious !) SNR is significantly reduced by the Vj filter transmission and the fact to add the noises of the three channels (B and R being much worst than G).
Another idea was to combine the Bessell "photonic" Vj filter with a variant of my VSF technique using G+2xR (for recent Canon camera) . In this case the resulting pass-band is well similar to Vj but the poor DSLR "R" SNR strongly affects the end SRN.
The hereabove study has been done with the last Bessell photonic Vj pass-band, in fact the Astrodon and Baader filters have a significantly larger bandwidth than the Bessell V (to see the discussion under "Nova Del 2013 Photometry") and are not better than the DSLR "G" channel. There are filters from Optec that much better fit the Bessell photonic definition ( But there are also older Bessell definitions, based on glass prescription, that have a much larger bandwidth ! - similar to Baader )
Conclusion: Far best is to use the G channel either using a classical transformation to Vj or (better) my VSF technique. It's also possible to extract good B-V and V-R from the B/G and G/R ratios. As they are acquired at the same time in a single integrated chip they are very stable and reliable.
Recent DSLR are extremely accurate and stable photometers due to the very low power consumption of the CMOS technology (very low temperature rise) and the fact the photometric chain is fully integrated upto the digital level into a single silicon chip. The best results can be achieved using them without any extra filters or exotic tricks. All is in using a good photometry technique... nothing else.
Removing the DLSR RGB filters: I don't think it's possible, except maybe in a well equipped micro-electronics reverse engineering lab. And even in this case the sensor has all chances to be destroyed. That has nothing to do with the removing of the IR dye+IR cut+UV cut+Anti-Moiré filter stack. That stack is OUTSIDE the sensor packaging, in front of it, that stack can't be split in recent DSLR but can be removed as a wohle. The Bayer RGGB filters are INSIDE the sensor packaging. Those are made from pigments deposited onto the pixel surface, the silicon chip itself. There are below the microlens array and inside the sealed packaging of the sensor. Anyhow the cost of such an operation is extremely high in the proper lab and the risk would be very high too. Making such "reverse engineering like" operation is rarely used in the industry due to its very high cost. Much cheapper and less risky to buy an astro CCD camera ! (But between us, I prefer DSLR ! )
Clear Skies !
As some colleages are posting, I don't think that BVRI filters with a Bayer RGB sensor is very congruent.
It's obvious that that this observer wants to perform photometry. It would be better that he would learn DSLR photometry with the tutorial and reporting in R, G, B bands. And then, with more skills, buying only a V filter, and invest the cost of the other 3 filters in a modest monochrome cooled camera.
What stars would you target with a V filter alone? At best you're going to achieve 2-3% accuracy and if the colours very during a cycle this will be degraded a little more? Is it an improvement on DSLR photometry which, in spite of the sizable transformation values, still produces very good results on brighter stars? Or faint ones through a telescope.
Single filter photometry is really only an improvement on visual measures and unless it's on a large scale like ASAS3 where the light curves of many under- on non-observed stars are covered it doesn't really add much value to the measures. With Miras, the visual people have no trouble in obtaining acceptable accuracy. So stick with the DSLR or buy a CCD with at least BV filters.
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