Re: atmosphärische Refraktion
AVA 1/10TH WAVE ANTI-REFLECTION COATED PLANETARY ATMOSPHERIC DISPERSION CORRECTOR (PADC)
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/padcimassembly.jpg
" A Commercial First for the Amateur Astronomy Market From Adirondack Video Astronomy!"
Be sure to read the article " Coping with Atmospheric Dispersion" by Thomas A. Dobbins in the August 2003 Sky & Telescope, pp124-127.
PADC will realign the Red and Blue wavelengths of objects 40 degrees and lower in the sky.
PADC will work for all telescope types.
PADC will show the most dramatic correction in long focal length telescopes, this is because the telescope is looking at a much smaller field of view.
PADC can be used visually, for film, with video, webcams and conventional ccd cameras.
PADC can be used for White Light Solar, Lunar, Planetary and Planetary moon imaging.
PADC possible uses for Astrometric work, low elevation CCD work including LRGB imaging
PADC for possible spectroscopy work at low sky altitudes.
PADC can be used for double star visual observations.
PADC can be used for photometric work.
WHAT IS ATMOSPHERIC DISPERSION?
Refraction, or the bending of light is a function of wavelength. This is referred to as dispersion. In refraction the longer wavelengths (red) slow down the least and shorter wavelengths (blue) slow the most. Red light bends the least, violet the most. The lead crystal prisms that you hang in your window or car to cast "rainbows" are a demonstration of the dispersion of light. Notice that the red light is on the inside, as red light bends the least. Blue is on the outside as it bends the most.
Refractive Index, or the amount that light is bent , is slightly different for the different colors of light. This variation of the refractive index with the wavelength or frequency of the light is called dispersion. Dispersion is a property of all transparent materials.
The color of the solar green flash observed over the ocean is due to the dispersion of air, which makes atmospheric refraction slightly different for different parts of the spectrum. The dispersion of air, like that of water, glass, clear plastics, and most other materials, is very small: the refractivity (n - 1) varies by about 1% across the visible spectrum.
Because dispersion is so small, it is negligible for many purposes. Only in special situations, such as astronomical telescopic observations, is the dispersion of air visible to the naked eye. Visual, Film, Video, or CCD observations through a telescope are performed at high magnifications. Very small fields of view are being observed visually or recorded on various media. Atmospheric Dispersion can become a problem under those conditions. It can lead to a decreased ability to visually view fine planetary features or to capture fine features with Video, Film or with CCD cameras, especially color systems.. In fact, if viewing planetary objects 40 % or less above the horizon the atmospheric dispersion becomes quite evident. You will see a red cast on one side of a planet and a blue on the other side of the planet. The red and blue images may not even align with the brighter central planetary image.
See the image below:
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/diffRefrac.gif
0 Degrees - Zenith 90 degrees above the horizon, 0.00" arc seconds of dispersion
30 Degrees - 60 degrees above the horizon, 0.35 arc seconds of dispersion
45 Degrees - 45 degrees above the horizon, 0.60 arc seconds of dispersion
60 Degrees - 30 degrees above the horizon, 1.04 arc seconds dispersion
75 Degrees - 15 degrees above the horizon, 2.24 arc seconds dispersion
Dispersion is the variation in refraction as a function of wavelength. A star viewed at a large zenith distance may appear elongated by this effect. The figure illustrates what may be seen where s is the zenith angle and is the separation between the blue and red images (the assumed star size here is 1 arc second). Clearly this effect can also become a problem for astrometry (position measurement) as well as spectroscopy where some of the wavelengths of interest may not even pass through the spectrograph entrance aperture.
Please note: If the seeing is poor or there is very heavy amount of moisture in the air the amount of atmospheric dispersion may appear greater and may not be able to be corrected as well.
WEDGE PRISMS:
One way to correct atmospheric dispersion is to use wedge prisms made of Schott BK7 glass. The reason the Schott BK-7 glass is well suited is that it very closely resembles the refractive index of the Earth's atmosphere. Therefore, having the same visible properties of the Earth's atmosphere, BK7 glass is not going to degrade the image. This is the same reason binoculars also use BK7 glass prisms.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/wedgeprismsmallforweb.jpg
Schott BK7 Glass - The wedge prisms in the PADC are of the highest quality. They are 1/10th wave, BK7 Schott Glass, Anti-Reflection coated. We tested several different prisms and incorporated the best we could obtain. The angled lighting shows the AR coatings on the wedge prism on the right. The wedge prisms are AR coated on both sides.
Here is the dispersion curve of air at standard conditions:
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/disp_air.gif
The values plotted are taken from
E. R. Peck and K. Reeder
Dispersion of air
JOSA 62, 958-962 (1972)
Similarly, here's the dispersion curve for a commonly used optical glass (Schott BK 7) similar to ordinary window glass:
Notice that, apart from the scaling, these dispersion curves are much alike.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/disp_BK7.gif
© 2002 Andrew T. Young.
ROTATION OF TWO WEDGE PRISMS ( RISLEY PRISMS)
The AVA PADC incorporates the Risley Prism design, using two circular wedge prisms. This allows the observer or imager to correct for the amount of dispersion present based upon the height of the astronomical object above the horizon.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/risley%20prisms.gif
The direction of light refracted after passing through the wedge can be controlled by rotating the circular prism. In many cases, two wedges are paired and the light path is altered to an even greater degree over a 360-degree range by rotating the wedges in opposite directions. Wedge prisms act as versatile beam splitters to prevent ghost images, remove atmospheric dispersion, and to steer light beams through adjustable pathways in optical systems.
Contributing Authors
Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
AVA PADC SYSTEM COMPONENTS:
A.1/10 the Wave 25mm Circular BK-7 AR Coated Wedge Prisms.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/wedgeprismsmallforweb.jpg
Visual & Imaging Use 4 degree prism.
Advanced Imaging use Two 2 degree prisms for variable 0-4 degree adjustment.
B. 1.25 inch Filter Thread Prism Holder Assembly screws into the filter threads of eyepieces, video camera adapters, webcam adapters, ccd cameras.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/prismassembly.jpg
C. 1.25 inch eyepiece holder for eyepieces, video cameras, webcam, ccd cameras. Visually also permits the use of multiple eyepieces without screwing and unscrewing the prism assembly for each eyepiece used.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/holdersmall_small.jpg
D. 1.25 inch locking ring for visual use Prevents the prism assembly from extending too far in some short diagonals. This is is more important if you are screwing filters onto the end of the prism assembly.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/lockringsmall_small.jpg
E. Instructions
Please Note:
It is not absolutely necessary to use an IR Blocking filter, however at the telescope testing did show an improvement with the IR Blocking filter for Color Webcams , Video Cameras and One Shot Color CCD Cameras.
INSTRUCTIONS:
Caution: Do not over tighten prism holders, tighten lightly until prism is held in place. Over tightening may result in damage to the prism.
Do not use on the Sun without proper safe solar filtration.
VISUAL USE: PERFECT FOR MARS
The wedge prism is an extremely useful visual tool, especially for this Mars apparition. Even though Mars will be close it will not be very high in the sky. Because the AVA PADC can be adjusted you can accommodate for the effects of atmospheric dispersion. More and finer planetary detail will be present with the use of the prism than without.
VISUAL & IMAGING USE OPTIONS
PERFECT FOR MARS
The wedge prism is an extremely useful visual and imaging tool, especially for this Mars apparition. Even though Mars will be close it will not be very high in the sky. The AVA PADC can be adjusted to reduce the effects of atmospheric dispersion. More and finer planetary detail will be present with the use of the prism than without.
Caution: Do not over tighten prism holders, tighten lightly until prism is held in place. Over tightening may result in damage to the prism.
OPTION A: ( ITEM# PADC- V4) VISUAL USE
Visual use of the wedge prism with one eyepiece at a time. (Screws onto the bottom of a 1.25 inch eyepiece)
Includes;
One two piece prism holder assembly
One 4 degree 1/10 wave 25 mm circular wedge prism
1.25 inch locking ring
Prism Assembly & 4 Degree 1/10 Wave AR Coated Circular Wedge Prism
Locking Ring
The Wedge Prisms were mounted in the holders at AVA.
The PADC prism holders are scribed so that the thin side of the wedge prism is centered on the scribe mark.
Using the wedge prism screw it into the base of your eyepiece where a filter would go.
Place the eyepiece / PADC assembly into the telescope and rotate the entire unit so that the scribe mark on the prism holder is perpendicular to the horizon. That is, the entire unit should be placed into the telescope so that the scribe mark is at the 12 O'clock Position as you look through the eyepiece.
We have also supplied a lock ring so that the collar of the locking ring can be placed around the eyepiece tube. This will accommodate for the extra length from the wedge prism assembly and will prevent the eyepiece from hitting the diagonal mirror in some telescopes. This is especially useful if you are adding filters to the prism assembly.
Once you have the prism and eyepiece at a secure depth in the 1.25 inch holder on your telescope you can tighten the locking ring.
The locking ring will also give you a base to rotate your eyepiece upon.
Focus the planet in the eyepiece.
Once in focus you should be able to see the atmospheric dispersion. There will most likely be a blurry blue fringing on one side and a blurry red fringing on the other side.
Rotate the eyepiece with its attached wedge prism assembly.
You can move rotate the eyepiece / PADC assembly either clockwise or counter-clockwise.
As you rotate the eyepiece / PADC assembly the planet will remain in the field of view.
As you rotate the eyepiece / PADC you will see that the planet travels in a 360 degree circle some distance from the center of the eyepiece.
Once rotated to the proper angle and you see that the dispersion is minimized and more planetary detail will become apparent.
You can center the image in the center of the eyepiece and refocus.
You will need to screw in the PADC prisms holder directly to the base of the each eyepiece used for your observations.. If you want to use the PADC prism assembly with another eyepiece you will have to unscrew the PADC prism holder from the current eyepiece and screw it into the next eyepiece you are going to use. While this is not the most versatile option it is the most cost effective.
In some Newtonians with limited back focus and low profile focusers this may be the best option.
Caution: Do not over tighten prism holders, tighten lightly until prism is held in place. Over tightening may result in damage to the prism.
OPTION B: ( ITEM# PADC-V4H) - For VISUAL USE & IMAGING
imaging and visual use of the wedge prism with multiple eyepieces. Includes,
One two piece wedge prism holder assembly
One 4 degree 1/10th wave circular AR coated wedge prism
1.25 inch holder to hold various eyepieces
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/holderwithpadcprismassemblyscribemark.jpg
Prism assembly with 4 Degree prism screwed onto the bottom of the eyepiece holder. Scribe marks the position of the thin side of the prism.
METHOD ONE: VISUAL
The Wedge Prisms were mounted in the holders at AVA.
The prism holders are scribed so that the thin side of the wedge prism is centered on the scribe mark.
Using the accompanied 1.25 inch holder with wedge prism assembly screwed into the base of it, place the holder/ PADC unit into the telescope and rotate the entire unit so that the scribe mark on the prism holder or 1.25 inch holder is perpendicular to the horizon. That is, the entire unit should be placed into the telescope so that the scribe mark is at the 12 O'clock Position as you look through the eyepiece.
Place your favorite planetary eyepiece into the 1.25 inch holder.
Focus the planet in the eyepiece.
Once in focus, you should be able to see the atmospheric dispersion. There will most likely be a blurry blue fringing on one side and a blurry red fringing on the other side.
Rotate the holder unit with its attached PADC wedge prism assembly.
You can move rotate the holder / PADC assembly either clockwise or counter-clockwise.
As you rotate the holder / PADC assembly, the planet will remain in the field of view.
As you rotate the holder / PADC assembly, you will see that the planet travels in a 360 degree circle some distance from the center of the eyepiece.
Once rotated to the proper angle, you will see that the dispersion is minimized and more planetary detail will become apparent.
You can center the planetary image in the center of the eyepiece and refocus.
You will be able to use various eyepieces with the PADC / eyepiece holder option.
This is the best visual option.
You can also purchase additional optional prism assemblies.
If you have an eyepiece that does not fit our holder ask us about our 1.25 inch extension tubes for your eyepiece. Because planetary viewing will take place at very long effective focal lengths and high F/ratios, we kept the tolerance tight so as not to have any play in the optical alignment and some eyepieces may not fit in the holder. Our low profile extension tubes screw onto the bottom of the eyepiece and will fit into the prism holder.
METHOD TWO: IMAGING - works very well, easiest imaging option
The Wedge Prisms were mounted in the holders at AVA.
The prism holders are scribed so that the thin side of the wedge prism is centered on the scribe mark.
Using the accompanied 1.25 inch holder with wedge prism assembly screwed into the base of it, place the holder/ PADC unit into the telescope and rotate the entire unit so that the scribe mark on the prism holder or 1.25 inch holder is perpendicular to the horizon. That is, the entire unit should be placed into the telescope so that the scribe mark is at the 12 O'clock Position as you look through the eyepiece.
Place your favorite planetary eyepiece into the 1.25 inch holder. Since you are going to be imaging, you may find it easier to use a fairly wide field eyepiece as it may be easier to find the center of the field and easier to place the planet in the center of the field.
Focus the planet in the eyepiece.
Once in focus, you should be able to see the atmospheric dispersion. There will most likely be a blurry blue fringing on one side and a blurry red fringing on the other side.
Rotate the entire holder unit with the eyepiece with its attached PADC wedge prism assembly.
You can move rotate the holder / PADC assembly either clockwise or counter-clockwise.
As you rotate the holder / PADC assembly, the planet will remain in the field of view.
As you rotate the holder / PADC assembly, you will see that the planet travels in a 360 degree circle some distance from the center of the eyepiece.
Once rotated to the proper angle, you will see that the dispersion is minimized. That is the dispersion will move closer to the planet and almost disappear. More planetary detail will become apparent.
Center the planetary image in the center of the eyepiece and refocus.
Carefully remove the eyepiece without moving the telescope.
Place the camera, webcam, or video camera with its mounted IR blocking filter into the holder. Since you re-centered the planet prior to removing the eyepiece the planetary image should fall on the CCD chip.
Refocus your camera.
You can re-adjust your camera by rotating only the camera to orient the planetary image.
If you have an eyepiece that does not fit our holder ask us about our 1.25 inch extension tubes for your eyepiece. Because planetary viewing will take place at very long effective focal lengths and high F/ratios, we kept the tolerance tight so as not to have any play in the optical alignment and some eyepieces may not fit in the holder. Our low profile extension tubes screw onto the bottom of the eyepiece and will fit into the prism holder.
ADVANCED IMAGING OPTION: - 1st run sold out!
Caution: Do not over tighten prism holders, tighten lightly until prism is held in place. Over tightening may result in damage to the prism.
OPTION A: (ITEM# PADC-I) Complete Variable Prism Adjustable Imaging System for Video, CCD, Webcam use.
Includes;
Two two-piece wedge prism assembly holders
Two 2 degree 25mm 1/10th wave circular wedge prisms
One 1.25 inch holder
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/padcimassembly.jpg
For imaging, you will find it easiest to work with a 90 degree diagonal.
The Wedge Prisms were mounted in the holders at AVA.
The prism holders are scribed so that the thin side of the 1/10th wave circular wedge prisms are centered on the scribe marks.
Using the accompanied 1.25 inch holder with wedge prism screwed into the base of it, place the holder in the telescope and rotate the entire unit so that the scribe mark on the prism holder or 1.25 inch holder is perpendicular to the horizon. That is, the entire unit should be placed into the telescope diagonal so that the scribe mark is at the 12 O'clock Position as you look through the eyepiece.
Screw the other holder with its prism onto the filter threads of your, finder eyepiece, webcam adapter, video camera c to 1.25 inch adapter, or CCD camera 1.25 inch adapter.
Note where the position of the scribe mark is on this holder.
Line up the two scribe marks so that they are on the same side lined up with each other. This will represent the 4 degree angle for the two 2 degree prisms and the light will pass through refracted.
If the two prisms scribe marks are 180 degrees from each other this position this will represent them minimum refraction angle of 0 degrees for the two x 2 degree prisms. Light will pass through unrefracted.
Rotating one prism 90 degrees from the other will give you the approximate equivalent of a 3 degree prism.
Please read the following two imaging methods below.
Method A: 4 Degree Prism Setup. Aided WITH the Use of an Eyepiece. (Recommended for ease of use)
In this method you will line up the two scribe marks on both the prism / holder unit and camera / prism unit so that they are on the same side lined up with each other. This will represent the 4 degree prism angle.
Link zur Grafik:
http://www.astrovid.com/padc%20corrector/padcimassemblySCRIBE.jpg
Scribe marks lined up on the same side. 4 degree prism alignment.
Place the holder unit with its attached prism assembly into the telescope. (You are going to rotate the whole unit later in the procedure.)
With the holder / prism unit in the telescope, Screw a prism assembly onto the bottom of your favorite planetary eyepiece. You may find it easier to use a fairly wide field eyepiece as it may be easier to find the center of the field and easier to place the planet in the center of the field when imaging.
Place the Eyepiece / Prism into the top of the holder.
Line up the two scribe marks as shown in the picture above.
Lock the eyepiece / prism assembly in the holder with the brass set screw.
Focus the planet in the eyepiece.
Once in focus, you should be able to see the atmospheric dispersion. There will most likely be a blurry blue fringing on one side and a blurry red fringing on the other side.
Rotate the entire 2 prism assembly / eyepiece/ holder unit.
You can move rotate the holder / PADC assembly either clockwise or counter-clockwise. I find it easier to rotate it clockwise.
As you rotate the holder / PADC assembly, the planet will remain in the field of view.
As you rotate the holder / PADC assembly, you will see that the planet travels in a 360 degree circle some distance from the center of the eyepiece.
Once rotated to the proper angle, you will see that the dispersion is minimized. That is, the red and blue dispersion will rotate around the planet and also move closer to the planet. The red and blue dispersion may completely disappear. More planetary detail will become apparent.
Center the planetary image in the center of the eyepiece and refocus.
Remove the eyepiece
Insert the camera, webcam, or video camera with its mounted IR blocking filter into the holder. Since you re-centered the planet prior to removing the eyepiece the planetary image should fall on the CCD chip.
You can readjust the position of the camera only to orient the planetary image.
Lock the camera in place with the set screw.
You can initially set the prism to a lesser angle and also use the method above. For example, Rotating one of the prisms 45-90 degrees from the 4 degree prism position will give you the approximate equivalent of a 3 degree prism. You will then follow steps 5-17 again.
CARE AND CLEANING OF THE PADC UNIT:
Effort should be made to keep the PADC unit as clean as possible.
The anti-reflection coatings on the wedge prisms are quite durable and if properly cared for should last a long time.
If bringing the PADC unit in from cold damp temperatures both ends of the PADC should be capped. Do not uncap them until the unit has reached room temperature.
If possible, gradually warm the PADC unit to room temperature by wrapping in a towel.
If the prisms are dusty you can try first removing the dust with a soft camel hair brush.
Canned air can be used but may sure only air is coming out prior to spraying the prisms. I always make a short burst prior to cleaning lenses. Care must also be exercised to keep the can upright so as not to force any liquid out.
If the prisms are be dusty or dirty you can also clean them with: , either 70% or 90% Isopropyl Alcohol ; Methanol, or Acetone. Use only lintless lens cleaning tissue.
IR BLOCKING FILTERS:
It is not absolutely necessary to purchase a separate IR blocking filter with the PADC system. However, the use of an IR blocking filter may further prevent the transmission of IR light . At the scope testing with an IR blocking filter on the Color PlanetCam and TouCam webcam did show an improvement. An IR Blocking filter is useful when imaging with one shot color CCD, Color Video cameras, or color webcams.
For Further Information Please Visit the Following Websites: kindly provided to me by Thomas A. Dobbins
http://www.ls.eso.org/lasilla/sciops/feros/Projects/ADC/
http://www.eso.org/instruments/vimos/documents/atmos_paper.pdf
http://www.astro.lu.se/~torben/euro50/technical_notes/EURO50_atm_disp.pdf
