Real time observation of stellar and galactic sources using a third generation image intensified optical system
4 August 1998
This report will familiarize the reader with the optical frequency spectrum of astronomical objects for observation using the I 3 intensified optical system and the system performance as it relates to the objects spectrum.
2) GALAXY SPECTRA
It is generally agreed that the spectral range of human vision is between ~ 380 to ~760 nanometers (nm= billionths of a meter). Referring to Chart 1, the A curve represents the spectral response of a typical gen 3 intensifier (as used in the I 3 piece). One can immediately see that the tube response extends to 900 nm with the peak at ~ 775 nm. This region of the spectrum between 760 and 900 nm is included in the near infrared portion of the electro magnetic spectrum and is not visible to the eye in real time without the assistance of a device such as an image intensifier. Now referring to Chart 2 Curve B, this is a curve fitted plot of galaxy types (spiral, elliptical, irregular). The slope of Curve B is a good fit to tube response of Curve A, particularly between 550 and 800 nm. Also note that the majority of spectral output falls above 700 nm and extends to 900 nm with good uniformity. (Galactic spectra extend well beyond 900 nm; however, this report concerns itself with the spectrum of tube operation < 900 nm.) Also note that the spectrum of curve B actually begins below the threshold of sensitivity of curve A,Generation 3 devices are essentially blind to this (violet) portion of the visible spectrum.Fortunately this narrow band between 400 and 450 nm represents a small percentage of the entire galactic spectra that is visible.
Curve B represents the average spectrum of the entire galactic mass for the 3 galaxy types. Within individual galactic types, the spectrum and hence the intensifier response can be further quantified. Ellipticals, which are classified by Hubble category from E0 to E7 depending on how round (E0) or elliptical (E7) they appear are symmetrical in shape. M87 is an example of an elliptical. The stellar population of ellipticals is called Population 1 from the work of Walter Baade at the 100 inch Mount Wilson Telescope. Ellipticals in fact are comprised of "old" Population 1 stars. These are "metal rich" and predominately M Class including red giants, most of which exceed 10 billion years in age. From a spectral standpoint, ellipticals are very energetic sources in the red and infrared portions of the spectrum. Ellipticals display a uniform spectral curve across their entirety and when taken individually, their spectra are similar to the S2 curve of Chart 2 (an M Class star). This far red/infrared spectrum makes ellipticals an excellent match to the image intensifier response (Curve A).
Spiral galaxies can be normal (Hubble Type S) or barred (Hubble Type SB). Both types are also classified A, B, or C, as to the tightness of spiral structure that they display, with A being tightest and C being most open. Also the size of the nucleus relative to the spiral structure is A, the largest to C, the smallest, some galaxies show disk like structure without spiral structure and are termed S0. Within the nucleus of spiral types, old Population 1 stars predominate as in ellipticals. The nuclear bulge is therefore also an excellent match for the intensifier spectral response. As we look into the spiral structure, gas, dust and young (Population 2) stars are most prominent. This makes the spiral structure more skewed towards the blue portion of the spectrum, hence making the spiral structure less visible using image intensification than the nuclear central bulge. This can be confirmed observationally by noting the increase in luminosity between the nuclear and the spiral structure. The image intensifier response to the spiral structure independent from the nuclear bulge is very dependent on the averaged spectrum that comprises the entire spiral structure.To clarify this important point,spiral galaxies that present their structure to us without oblique perspective such as M101 will appear highly intensified in the nucleus and will show little difference from visual observation in their spiral structure due to the predominately blue response in the spiral arm region. As the observers plane of view to the galaxy becomes more oblique, the dust lanes becomes more prominent. Galaxies such as M107 and NGC 4565 present an "edge on" appearance. The dust lanes have a strong infrared signature making these galaxy types ideal for image intensified observation.
These have Hubble classification IRR1 (mostly O and B type stars and HII regions) and a general lack of dust clouds and IRR2 (not resolvable into stars, no HII regions) and prominent dust lanes. Of these two, IRR2 types have a more red/infrared spectrum (dust lane infrared signature) and may be a better match to the the intensifier spectral sensitivity.
Two additional galaxy types not easily classified are Seyfert (1 and 2) and BL Lacertae objects. Both Seyfert types have unusually small and optically intense (star like) nuclei. Of the two types, Seyfert 2 have a more energetic infrared spectrum. BL Lacertae objects have rapid intensity variations in visible and infrared wavelengths and may be a good candidate for image intensified observation.
3) STELLAR SPECTRA
Referring to chart 2 (below), Curves S1 and S2. Curve S1 is a star with spectral class G such as our sun or Capella. Notice the distribution of spectral energy with the majority in the visible spectrum and decreasing (although still significant) in the infrared. These main sequence stars have surface temperatures of ~ 5000 degrees Kelvin producing the spectral distribution curve of S1. Looking at Curve S2 in chart 1, we see a spectral distribution shifted more towards the red-infrared portion of the spectrum. This would fall into spectral class M that includes red supergiant stars such as Betelgeuse in Orion or Anteres. M stars such as these are a fine match to the spectral response of the imaging tube. M stars have surface temperatures in the 3000 degree Kelvin range causing their red shifted spectrum. Spectral class types B, A and F are not shown. These hotter, bluer stars have spectrums shifted towards blue and ultraviolet (the spectral region at the 400 nm end of chart 1) these star types may show modest or no intensity increase when viewed with a gen 3 intensifier due to their spectrum falling in the region near the tubes minimum response. K types fall between G and M and are also not shown.Understanding where a stars spectral class falls within the intensifiers effective spectral range(curve A),will allow the user to predict the effectiveness from an image intensification standpoint. M giants and supergiants give the greatest potential for image intensified observability.
A) Emission nebulae
Refer to chart 2, (D) nebulae along the top section.Notice that the spectral distribution ranges across the entire spectrum shown.The most predominate frequency for nebulae is centered at the HII line in chart 2.This is the H-alpha line at 656.32 nm and is the result of spontaneous photon emission from the ionized hydrogen gas present in the nebula as electrons decay from the 3rd to 2nd energy level.Other gasses present within the nebula may also be ionized as is the case with the great nebula in Orion in which ionized helium and oxygen are also present. .These optical recombination lines give rise to other characteristic spectra causing emission lines at other wavelengths.In the case of M42 ionized oxygen at 500.7 and 495.9 nm) produces the green light present with HII emission producing the greater part of the red emission.
Emission nebulae will show greatly enhanced observability using gen 3 intensification when most of their emission spectra occur within the HII region.This is the first emission line in the Balmer series of hydrogen emission lines.As electrons decay from higher valence levels within the hydrogen atom,they emit photons at higher frequencies.This gives rise to the Balmer series of visible emission spectra with the first line (known as hydrogen alpha or HII).There are 5 emission lines in the Balmer series that are present in the visible spectrum at 656 nm,486 nm,434 nm,410 nm and 397 nm..As previously stated,the HII line is most observable with gen 3 intensification,with hydrogen beta (486 nm) also visible.Wc may therefore predict the image intensified observability of emission nebulae by first knowing what ionized gasses constitute their observable spectra and their corresponding emission line frequencies.These emission lines can then be plotted in relation to the tube response curve and their potential for amplification predicted.
B) Planetary nebulae
As with emission nebulae,the ionized spectra present are due to their proximity to a star(s),and in the case of planetary types,their surrounding a hot star (30k to 100k degrees kelvin).The Ring Nebula in Lyra is a good example with a characteristic circular shell (hence planetary by Herschel)surrounding the central star with a temperature of 70,000 degrees kelvin.The strong ionizing radiation gives rise to hydrogen (Balmer series) and oxygen lines at 500.7 and 495.9 nm.The shell of expanding gas in M57 is an excellent choice for gen 3 intensification because of its HII abundance and to a lesser extent its oxygen lines.The central star although observable
C) Reflection Nebulae
Certain nebulae are simply clouds of dust that are illuminated by nearby stars and reflect the stellar spectra present.The Pleiades are a good example of a reflection nebula in the presence of young (hot, blue) stars.The nebulosity present in the cluster reflects the blue spectrum present in these stars.The potential for intensified observability can be determined by the characteristic spectrum of the stars that illuminate reflection nebulae.
5) VISUAL V.S. SILICON BASED SPECTRAL SENSITIVITY
When used for visual astronomy purposes, image intensified devices are often met with questions such as, "why is the image green?". The logical reasoning behind this is as follows:
Therefore, at 505 nm, the minimum threshold of perceptible vision is 1, yellow light requires 100 times the intensity to produce the equivalent visual response, orange 1000x, red incredibly 10000x, blue 2x and violet 20x. This visual spectral sensitivity is based on scotopic (rod vision). During photopic (cone) vision (light levels above approximately 10 LUX), the peak sensitivity shifts upwards to 555 nm. The image intensifier phosphor screen spectral frequency is centered at 530 nm. With the phosphor screen output illumination level at 2.25 foot lamberts maximum, the visual response falls within the threshold region between scotopic and photopic visual sensitivity. Therefore, 530 nm represents the ideal median frequency for the typical level of visual adaptation that occurs when using a Generation 3 image intensifier. Also and very importantly, as the intensifier illumination level drops (when imaging low surface brightness galactic objects for example), the eyes response becomes predominantly scotopic and the perception of color will actually disappear because of the retinal rods insensitivity to color and the visual transition to grey scale. Therefore, the green image present at higher illuminated image levels will become less apparent as the objects level of illumination decreases to the point of showing little or no color as the tube output approaches the equivalent background illumination of the tube (EBI) . Green frequency phosphor also greatly reduces the power requirement necessary by the tubes power supply because of the much greater visual sensitivity to green 530 nm lisght which in turn, extends the operating hours with a given (battery) power source.
A) Tube spectral response
Refer now to Chart 1, Gen 3 photo response.
The peak spectral response of the tube is at 775 nanometers. The gain of the tube which is output illumination / input illumination = gain is independent from photo response. The gain setting for the generation 3 tube is 50,000.This gain is present across the spectrum of photo response.
This brings us to one of the most important concepts concerning the use of a Generation 3 intensifier for astronomical objects. That is the ability to dynamically amplify the optical spectrum of a star,nebula or galaxy is directly related to the integrated spectrum of the object. The same statement applies to human vision except that the peak response is literally at the other end of the spectrum.
An excellent example of the differences between visual and intensifier response is apparent with SC galaxy types such as M33. The naked eye response does not give the appearance of the galaxy nucleus as being brighter than the spiral arms to the magnitude that is actually measured with instrumentation with bolometric response. This is due to the eye responding with much greater sensitivity to the spiral arm section made up of much bluer stars than the nucleus which, although much more energetic than the spiral arms, is nevertheless comprised of much older red M class stars and large HII regions to which the eyes response is much less sensitive to (Ref Chart 3).
The intensifier responds in a much more linear fashion in comparison to the eye. Lets look at the bandwidth between 505 and 605 nm for the eye and the intensifier. First for the eye, at 505 nm the response is 1, at 605, it is 1000 ( in this case 1000 is 1/1000 or .001 as sensitive as at 505 nm). For the intensifier...at 505 nm = 135 ma/w at 605 nm = 205 ma/w, a ration of 205/135 = 1.52 : 1 versus 1000 : 1 for the eye. This illustrates an additional important point: The Generation 3 intensifier provides a more linear and therefore more accurate response to astronomical objects over a large portion of the spectral response of human vision, as compared to the eye over the same spectral range.
1)Erhardt,Louis,1977,Radiation Light and Illumination