The Four
BIG Myths of Spectroscopy
MYTH 1. A Blazed Grating is the Best Type of Grating to use with a CCD camera!
MYTH 2. A Gratings Efficiency Rating %, or Effeciency Curve, is extremely useful!
MYTH 3. For Visual Spectrum Observations a Single Cylindrical Lens will Greatly Enhance the Views!
MYTH 4. 500 l/mm Gratings can't do Red Shift measurements because they don't have enough resolution!
MYTH 1. A Blazed Grating is the Best Type of Grating to use with a CCD camera!
This Myth originated years ago when all that was available to amateurs were commercial gratings that had been manufactured for some commercial use. The commercial uses varied but generally required response at a particular single wavelength or a set of closely spaced wavelengths. The manufacturers blazed their gratings to produce a brighter spectrum in the specific area of interest. This produces a grating that has a very asymmetrical output response. Non-blazed gratings have a much more linear response curve and produce cleaner spectrums.
Blazed gratings have five inherent short-comings when it comes to using them for CCD astrophotography. They are:
1. They allow more intensity to pass in the center of the visual region at the expense of having decreased efficiency over the rest of the visual region.
2. They generally only have a useable band width from 420 nm to 670 nm, when blazed for 550 nm (or 500 nm either). By their nature the UV and the IR regions are cut off in order for a slight increase in efficiency where our eyes are most sensitive.
3. Blazed gratings are designed for a parallel light beam passing directly perpendicular to the normal grating surface. If the light reaches the grating at angles other than 90 degrees then the blaze angle changes and its band width warps. This is generally not a problem for telescopes with a F/D > 20 (a F/D 10 SCT with a 2X Barlow). However, for short scopes or scopes with focal reducers there is a problem. For F/D <6 the incoming light rays hit the grating blazing at two widely differing angles. This skews the blaze point wavelength and narrows the band width.
4. In addition to the skewing of the blaze point and narrowing of the band width there is another problem. The light coming in from the half of the objective that is perpendicular to the blaze surface is refracted into a bright spectrum and the light from the other half of the objective hits the blazing on its edge. This edge illumination is mostly lost because of the steep angle of attack at the blazing surface. The bottom line is that 40% of the objective's light is lost from the spectrum for all short F/D ratio telescopes using a blazed grating.
5. For CCD work using a camera that has the IR blocking filter removed, using a blazed grating will drastically cut down the width of the spectrum available. If a blazed grating is used on a CCD camera that has the IR Blocking filter in place there will be little gained out side of the visual region.
Manufacturers blaze their gratings for several very specific reasons (money being the biggest reason - selling to the industrial market).
Blazed gratings are specifically made to respond to a small specific region of light. For amateur astronomy use this would be for observing star spectral in the visible region (as your eyes can't see outside of the visible region). The blazing is generally for 550 nm which is the region our eyes are most sensitive to.
The draw back here is that a blazed grating only passes a very small part of an objects spectrum. If all you are interested in doing is visual observation of spectrums then a blazed grating will give you a little more colored light but not very much more. Additionally, blazed gratings that are designed for visual work have gratings of around 200 l/mm. This type of grating produces a very short spectrum compared to much longer spectrums produced by more dense gratings, like the ones we manufacture.
A typical blazed grating has an efficiency of around 60% at 550 nm (depending on the manufacturer), with a spectral width of 420 nm to 670 nm where the efficiency falls off rapidly as the wavelength departs from 550 nm. A non-blazed grating has a fairly flat efficiency curve of around 50% over the spectral bandwidth of 370 nm to 970 nm. Do not confuse Efficiency with Transmission as they are Not the same thing.
For wide bandwidth CCD work the non-blazed grating is far superior to any blazed grating. For visual work the blazed grating gives a bit more light at 550 nm then our Compact grating does. However, blazed gratings generally cost several times what a non-blazed grating cost to manufacturer.
MYTH 2. A Gratings Efficiency Rating %, or Effeciency Curve, is extremely useful!
This Myth orignated from optics design engineer requirements (who design special purpose spectroscopes that will only be sensitive to an extremely small part of the spectrum of 10 Angstroms or Less). They generally require very high transmissions efficiencies around a specific laser wavelength.
Diffraction Efficiency is what most manufacturers of commercial equipment specify for their gratings as their gratings are usually sold for narrow band commercial use not wide band amateur use. This is a good thing to know if you are designing a narrow band system but if you are using a wide band system in the field it is of little use. Efficiency percentages numbers tend to be rather high compared to Absolute Transmission percentage numbers, which looks good to the average amateur consumer (nothing but a selling ploy).
Diffraction Efficiency compares the amount of light that is diffracted by a grating, at a specific wavelength(s), to the amount of light that is diffracted by a spectral mirror for the same wavelength(s), for the same diffraction order and the same surface coating. Diffraction Efficiency doesn't have much practical meaning for end users, unless they want to tell everyone that their grating stands up well against an equivalent spectral mirror...
Absolute Transmission on the other hand measures the actual amount of light that gets diffracted through the grating for a specific wavelength(s). This is the amount of light that is available for your camera to use. Manufacturers either specify Relative Transmission or Absolute Transmission. Relative Transmission usually gives a larger percentage so its used a lot of the time to make low transmission gratings look good.
Absolute Transmission is easy to use. If a grating has an Absolute Transmission of 40% at 650 nm then that means that the light available from the grating (for use by a camera or any other light sensor) will be one (1) magnitude dimmer than the total light coming from the object (16% would be 2 magnitudes, 6.4% would be 3 magnitudes, 2.6% would be 4 magnitudes, etc). Remember that a difference of one magnitude is 2.5:1 and 1/2.5 = 40%. So if you want to photograph the Hydrogen Alpha line in Denebola's spectrum you will need a camera/telescope system that will be able to capture 3.1 magnitude objects (Denebola's magnitude is 2.1).
The Transmission Graphs for our Diffraction Gratings are Here if you would like to see what one looks like. They were measured using a commercial photo-optical spectrometer.
MYTH 3. For Visual Spectrum Observations a Single Cylindrical Lens will Greatly Enhance the Views!
This Myth originated from the various inexpensive spectroscope manufacturers over the years as a way to enhance their product at the expense of image intensity. These products were not optomized, to keep price down, and therefore produced very short spectrums that were hard to see. The cylindrical lens spreads the spectrum out wider so its easier to see at the expense of making it very dim. The famous GOTO spectroscope came with three different cylindrical lens to be used on different sized scopes.
No visual spectrum filter made by any manufacturer, no matter what they might say, will produce spectrums of dim stars when using a cylindrical lens. When a spectrum filter is combined with a cylindrical lens the spectral image is so dim that 3 rd magnitude stars become the limiting magnitude when using an 8" to 10" telescope. With our High Effeciency D3 Spectrum Filter mounted on a 90 mm telescope, without using any cylindrical lens, the spectrum of 6 magnitude stars can be seen (this is about the limiting magnitude for a 90 mm scope used for visual spectrum analysis).
When using a cylindrical lens its very difficult to actually see any of the spectral lines present much less identify any of them. Without a cylindrical lens its extremely difficult to see any spectral line information except the very wide ones, but without using a cylindrical lens you can see the spectrums of stars several magnitudes dimmer then is possible when using a cylindrical lens.
Additionally, a single cylindrical lens won't work for many different combinations of telescopes, eyepieces and stars. All Cylindrical lens need to be matched to the specific eyepiece and telescope you are using. This is one of the things that other manufacturers don't tell you as they only supply one cylindrical lens. The older GOTO Star Spectroscope supplied three different cylindrical lens with their unit and had instructions on which one to use with different optical systems. To supply a single cylindrical lens that would only work well for 10% of your customers and leave the other 90% to their own devices is not a good way to do business.
No spectrum filter that mounts on an eyepiece for strictly visual use, no matter who makes them or how much they cost, can take advantage of analytical software tools. That's why none of our competitors offer anything but a simple user manual with their units, as for visual use there is very little that one can actually do or see with enough precision to use/need any analytical software.
Using a Type D Filter for visual use is fine at a star party where people would like a colorful visual treat or would like to see what a spectrum looks like. But the real use comes when you combine this filter with a camera and produce the image of an object's spectrum. Then you can start to do something with the information available in the spectrum. Then you will need the software on our CD. That's why none of our competitors offer you anything with their spectrum filters as their units were not designed to be used with a camera. A few amateurs have adapted their devices for camera use and have had varying degrees of success.
If you are really into visual spectroscopy then you should consider our Type D3 Spectrum Filter as it was designed to produce Extraordinary Breathtaking Spectral Views (at half of everyone else' price). One young woman, at a Star Party, looking through a small scope equipped with a D3 Filter, at a star field, Exclaimed - "WOW! With this you don't need drugs to Hallucinate!"
Cylindrical lens Selection Criteria.
First, you will need a lens that will cover most of the exit lens diameter at the top of the eyepiece that you are going to use it on. Second, is the focal length of the cylindrical lens: Focal lengths from 150 mm to 250 mm is good for smaller diameter telescopes (< 4" in Diameter) and bright stars, 90 mm to 130 mm is good for larger telescopes and 1 to 2 magnitude stars, and 50 mm to 80 mm is good for dim stars (3rd+ magnitude) on larger telescopes.
Anchor Optical Surplus and Edmund Optics sells cylindrical lenses. Edmund has a better selection but they are much more expensive than Anchor Optics. Once you have your cylindrical lens you will need to build a small hat to mount it on. The hat can be made of a cardboard disk with a short cardboard skirt around it. The skirt needs to be able to fit over the eyepiece that you want to use it with.
To use your cylindrical lens, after you have it mounted so it fits over the top of your eyepiece. First, without the cylindrical lens over the eyepiece, find a spectrum that you would like to see and the focus it until the spectrum is a thin straight line or the best that you can. Then put the cylindrical lens over the top of your eyepiece. You will need to rotate the cylindrical lens until its axis is lines up with the axis of the spectrum's image. Then you will see the spectrum still stretched out but you will also see that the spectrum now has height to it.
With the proper cylindrical lens adjusted correctly the thin line spectrum will now be a rectangle with the same length as the thin spectrum had. If the cylindrical lens is too powerful for the telescope or the star is too dim then the stretched spectrum will be too dim to see well. If the cylindrical lens is not strong enough then the spectrum won't be stretched very much from the initial long thin line.
Cylindrical lens need to be matched with the size of the telescope, the eyepiece, and the magnitude of star to be observed. As these parameters change you will need to go to a different focal length cylindrical lens. So when you buy your first cylindrical lens you should buy a range of different focal length ones. The manufacturer of the famous GOTO Star Spectroscope use to supply three different cylindrical lens with their unit.
MYTH 4. 500 l/mm Gratings can't do Red Shift measurements because they don't have enough resolution!
This Myth originates from the thought that the Resolution of a Grating, as determined by the Grating Resolution Equation, determines the finest resolution of wavelength measurement that can be made using that Grating.
However, the grating equations only predict what the minimum spectral line resolution is for a given grating line spacing. This means how close together spectral lines can be before they merge with other lines near them. These grating equations Do Not predict what the minimum limit of wavelength resolution is. Spectral line wavelength resolution is a statistical function that is not related to the standard resolution equation.
The Grating Resolution Equation is defined as:
The spectral resolution of an instrument is determined by the separation between two spectral peaks that can just barely be detected as separate with the instrument.
A theoretical treatment of the instrumental resolution shows that the properties of the grating sets the ultimate limit for the resolution. The gratings described by its "resolving power" which is a dimensionless number, R.
The mathematical definition is (this is the Grating Resolution Equation): R = m N = Lambda / Delta(Lambda)
where, m is the diffraction order (one in our case - the 1st order image) and N is the total number of grooves on the entire grating surface (for our A1 and B1 DG units, N = 500 x 10 mm wide = 5000). For our 500 l/mm Type A1 and B1 DG Filter units R = 1 x 5000 = 5000, and computing the static resolution in wavelengths we find Delta(Lambda) = 500 nm / 5000 = 0.1 nm or 1 Angstrom Static Resolution (here Lambda is 500 nm - Green Light). For our Type D2 DG 1000 l/mm grating the Static Resolution is 0.25 Angstroms. Note, however, that the grating equation places restrictions on the possible combinations of m and N as both m and N have to be integers.
What is Static Resolution?
Static Resolution is the defination of resolution from the grating equation (the resolution required to just separate two lines of equal strength). However, in practice when you are measuring the wavelength of a line you are using dynamic resolution which is an optical function of the picture not the number of grating lines or pixel separation. While the minimum separation between the spectral lines can't be any closer than predicted by the grating equation the determination of a lines wavelength can be made to much greater precision.
What is Dynamic Resolution?
Dynamic Resolution is the precision that the wavelength of a spectral line can be measured. If there are no other spectral lines that are very close to the line of interest then the position of that line can be measured using optical measurement techniques extremely accurately. Dynamic resolution can easily exceed the static grating resolution by a factor of Ten and with extreme care can be extended out to a Hundred times the static resolution or more.
Static Resolution comes into play BEFORE you take a Spectrum Picture. Dynamic Resolution comes into play AFTER the picture is taken and you are interested in measuring the wavelength of the lines you photographed.
In most cases the Static Resolution of a spectroscope system isn't limited by the Static Resolution of the Grating being used but by the CCD camera that takes the picture. Even very expensive CCD cameras rarely have less than one Angstrom/pixel Static Resolution.
There is a question on our FAQ Page, Q: How can your DG Filters measure wavelengths to 0.01 Angstroms for Red Shift measurements?, that goes into more detail about this subject.
Typical Field Resolutions are Actually Ten Times (10x) Better than what the Resolution Equation Predicts!
Don't get too carried away worring about the Resolution of our units as the practical resolution typically seen in the field is generally Ten Times (10x), or more, better than the resolution predicted by the Resolution Equation. The question Q: Why do I get 10x the Resolution with your unit then is predicted by the Resolution Equation? on our FAQ Page goes into great detail (with actual examples) about what level of detail you can expect to see. While resolution is important you need to know what you are talking about when you talk about limiting resolution of a grating spectroscope, remember - Apples and Oranges.
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