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colinsk
scholastic sledgehammer
Reged: 01/17/08
Posts: 901
Loc: CA
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I am almost finished building a spectroscope and I am wondering how I can create a scale to show the frequencies I am looking at and keep it calibrated?
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Mahalo.
Coulter 10.1" Dobsonian
Coronado PST
TV-76
Lunt LS60TDS50FTBF1200- On Order 
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1051
Loc: Fort Worth TX
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Well, do the one and only thing with a mercury light they're actually good for - a spectral line reference source. The mercury spectrum is loaded with very distinct lines across the visible spectrum. That's why they are SO popular with us!
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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colinsk
scholastic sledgehammer
Reged: 01/17/08
Posts: 901
Loc: CA
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How do I project a scale into the EP?
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1275
Loc: Hockessin, De
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You use a thin glass plate like a microscope slide to act like a beam splitter that is placed in the light cone from from the collimating telescope. You then use a small lens and place the scale at the focal point of the small lens and illuminate the scale from behind. The lens projects the image onto the beam splitter and the scale and spectrum are overlayed and view in the eyepiece of the spectroscope. The scale projection optics work the same way as a Telrad finder. If you use a Kellner eyepiece you can place the scale at the field stop, that will put the scale and image both at focus in the same time.
One thing to consider is the total dispersion of your spectroscope. If it is medium to high then your only going to able to view only a small section of the spectrum at one time. Your either going to have to have a moving scale, that tracks along with the tilt of the grating or do what most commerical units do, have a readout dial that shows the wavelength in the center of the eyepiece but the scale is not projected onto the eyepiece.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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colinsk
scholastic sledgehammer
Reged: 01/17/08
Posts: 901
Loc: CA
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Thank you Dave, Since I am using a grating and long fl optics the dispersion is high. However, are they linear?
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Mahalo.
Coulter 10.1" Dobsonian
Coronado PST
TV-76
Lunt LS60TDS50FTBF1200- On Order
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1275
Loc: Hockessin, De
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Quote:
Thank you Dave, Since I am using a grating and long fl optics the dispersion is high. However, are they linear?
A grating has a linear dispersion and prism does not, this is one reason why gratings are used over prisms. So your scale in your eyepiece would be linear. Remember that the movement of the grating as a function of angle is not linear with wavelength and follows a Sine function so any type of wavelength readout dial needs to correct for this.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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colinsk
scholastic sledgehammer
Reged: 01/17/08
Posts: 901
Loc: CA
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Is there a simple mechanical compensation for the sine function?
I am thinking if I make a scale that shows, say 1nm increments for 50nm and then use the angle of the grating to mark a center frequency on a dial I can make measurements to the accuracy I need.
Does this seem reasonable?
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Mahalo.
Coulter 10.1" Dobsonian
Coronado PST
TV-76
Lunt LS60TDS50FTBF1200- On Order
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gregj888
sage
Reged: 03/26/06
Posts: 203
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Colinsk,
I think the Hg lamp is better, but just about any gas discharge lamp will work. Even Neon lamps (from say Radio Shack). If you poke around the web you'll find the spectra.
Greg
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1275
Loc: Hockessin, De
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Quote:
Is there a simple mechanical compensation for the sine function?
I am thinking if I make a scale that shows, say 1nm increments for 50nm and then use the angle of the grating to mark a center frequency on a dial I can make measurements to the accuracy I need.
Does this seem reasonable?
Hi Colin,
The adjustment to the grating angle is made with a Sine Bar and threaded rod. By choosing the distance from the pivot and knowing the thread pitch of the rod, one can have a dial on the threaded rod with a linear scale that shows wavelength.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
Edited by DAVIDG (07/03/08 03:56 PM)
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1051
Loc: Fort Worth TX
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Here's one good reference for visible sources and spectral lines:
Spectral lines
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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colinsk
scholastic sledgehammer
Reged: 01/17/08
Posts: 901
Loc: CA
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Thank you Dave and Mike.
Am I correct in thinking that the drawing above is slightly in error and the pivot point needs to be the front surface of the grating at the center?
Then I can take, say, the two yellow lines from mercury and calibrate the spread per turn?
--------------------
Mahalo.
Coulter 10.1" Dobsonian
Coronado PST
TV-76
Lunt LS60TDS50FTBF1200- On Order
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1275
Loc: Hockessin, De
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The pivot needs to in the middle of the front surface of the grating. The arm can be attached on either the front or the back it makes no difference.
I would use the sodium doublet, no mistaking it, and the green mercury line at 546nm and again it stands out well. Both are almost alway present in any common energy saver lamp. You can always use the primary calibartion source, the Sun and Sodium lines and any of other Franunhofer lines.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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colinsk
scholastic sledgehammer
Reged: 01/17/08
Posts: 901
Loc: CA
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Thank you Dave!
--------------------
Mahalo.
Coulter 10.1" Dobsonian
Coronado PST
TV-76
Lunt LS60TDS50FTBF1200- On Order
|
|
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