|The Spectroscopy of Filters
I have always enjoyed spectroscopy in Chemistry, and
have wanted to know more about the filters used in
Astronomy. In Chemistry UV/Visible spectroscopy is
used to determine the concentration of materials in
various solutions without having to isolate them. A
general procedure would go something like this.
Dissolve your sample up in a solvent and place it in a
glass cuvette (square testtube) of known optical path,
usually 1 cm. Take a 2nd matching cuvette and fill
it with the same solvent that your sample is in. This is
your "standard". Using various methods, take a reading of
each cuvette at each wavelength of interest. By
dividing the quantity of transmitted light of your sample
by the transmitted light of your standard, the absorption
of the sample is measured. The "standard" is
considered to be 100% transmission, and the absorption
curve is displayed as a percentage or fraction of
Of course, the transmission of the standard is not really 100%. The glass absorbs light, and the solvent absorbs light. The purpose of using the standard is to eliminate those variables and isolate the amount of absorption by the sample.
|In order to
learn more about astronomical filters, I purchased a
UV/Visible/IR spectrometer capable of providing spectra in
the 300-1000nm range. In general, the light source I
use has no UV so my normal spectra are from 400-1000nm,
but I have a light source capable of delivering radiation
in the 300-400nm range should I need it. When I started
taking spectra, something became immediately apparent from
the experiment I describe above. In the case of an
astronomical filter, what
is the appropriate standard?
At first I gave it very little thought. Since I wanted to know how the filter worked in my telescope, I chose air (essentially nothing) as my standard. Before I put the filter in my telescope, air is what is there. I would like to know what changes the filter makes in the optical path and so measuring the total changes appear to be the correct way to approach this. I quickly found out that the spectra I took did not quite match the manufacturers samples. Orion's spectrum for their ultrablock filter is below left (400-700nm); my spectrum (400-1000nm) is on the right. They are quite similar in the 400-700nm (visible range) with the exception that I am reading a maximum transmission of 0.65 (65%) vs their 90%.
|While I am
using an Orion filter here, I am not trying to single them
out. This behavior proved to be pretty generally
true. I was seeing 10-25% lower transmissions on all
of the filters I measured. Clearly I
was encountering some sort of systematic
problem . In fact, none of the 40 or 50 samples I
have taken ever measure above 80%-85%.
How can this be? Well I can only see 2
choices. My new spectrometer doesn't work right, or
the folks doing these spectra are using a different
standard than air for filter spectroscopy. I
realized at this point one could argue, just as in the
Chemistry description above, that one should separate the
effects of the filter itself from the glass that holds the
filter. After all, from Dave Hall's and my earlier
study of eyepiece transmission we knew that the more
air-glass interfaces there are, the more absorption of
light there will be.
At this point I decided I should contact the manufacturers and ask them to tell me how they take their spectra. I contacted Orion on May 21, 2012 requesting information on what standard they use when taking these spectra. On May 25, I contacted Baader, Celestron, Hands On Optics, GSO, Lumicon, and Sirius Optics requesting the same information. Meade does not accept email requests, and I have not yet bothered to write them a letter. Thus far I have not heard back from any of them.
While waiting for replies, I decided to try an
experiment. What would happen if I tried some simple
uncoated clear glass as my standard. A microscope
slide came to mind and I bought some. Since they are
uncoated, one would expect them to overestimate the
correction with respect to a coated piece of glass. The
purpose of the coating is to minimize light losses. Here
are the results for the two filters above.
my spectrometer can show above 80% (0.8)
transmission. My spectra now look very much like the
manufacturer curves, perhaps the correction is a little
bit large as they look to be slightly higher than the
manufacturer curves. The Orion ultrablock shows I
can still get down to 0% transmission also. This
leaves us with a question. Which is the "correct"
standard? In my mind, using a coated piece
of glass as a standard would be correct if I constantly
put one of those on the end of every eyepiece. Thus
when I removed it and put a filter in its place
these curves would accurately represent the reduction in
the light transmission of the system. Personally I
don't do that. In fact, I don't know where to buy
such a coated optical dust cover for my lenses or I would
buy one to use as a standard, perhaps they are available
On my telescopes, there is nothing but air at the end of my eyepieces and the stronger absorptions seen with my air standard ought to represent that case well. Unfortunately that means that the light losses for using a filter are significantly larger that the manufacturer curves seem to indicate. Of course all of this is subject to modification when the manufacturers tell me how they run their spectrometers and what standards they use for setting 100% transmission. When they tell me, I will post it here (see below).
|Direct comparison of the
above data for the Mars 2003 filter. In
this comparison, the spectra of the manufacturer, the air
standard and glass standard are superimposed on the same
graph. Since the manufacturer gives no IR data, the
spectra were stopped at the point the manufacturer data
The grey is the manufacturer spectrum. The yellow is my spectrum when using glass as a standard. The red is when air is used as the standard. This implies that in your telescope where you are replacing air with this filter, your transmissions will resemble the air standard most closely.
Hands On Optics:
Dear Dr. Duchek,
We are factory representatives for several manufacturers. The statistics
that we publish are generated by the manufacturers.
We do no spectral testing here.
I then requested that the contact their manufacturer with my request.
Celestron responded to tell me that they have referred my request to their R&D group.
for your email to Orion. The
transmission graphs are provided by the
-Since I wrote this article I have talked to some
knowledgeable amateurs who agree me about air being the
logical and reasonable standard. I have also run
across a couple of filters that do match the manufacturer
standards given in that a certain wavelength be 95%
transmission. On some filters where I have 3
different "identical" filters, I find they vary in this
respect. I have revised the database to only
included the air standard, and will let the transmissions
fall where they may. (The spectroscope will be
calibrated to an air standard before any spectra are