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Tilt tuning Etalon

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#1 sam9148

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Posted 14 May 2016 - 03:11 PM

Hi all

I recently acquired a front mounted solarmax 90 II bf30 filter.

I use it with my fs102 refractor and the more I use it the more I love it.

Since this is a tilt tuning Etalon,views I see on the perimeter of the sun's disk,fade out when I change the orientation of the scope,so,I have to readjust the tuning.

I believe that this phenomenon is due to different angles between the sun and the filter.

My question is this:when using a pressure tuned Etalon (like Lunt),which as people say,have a more uniform field of view,does the angle of different points on the sun's disk,to the filter,have the same influence on the view ?

Thanks

sam



#2 BYoesle

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Posted 14 May 2016 - 10:14 PM

Hi Sam,

 

I'm not sure what you mean by "change the orientation of the scope."  Could you be more specific.

 

The views of the sun with an objective mounted filter will be the most uniform possible, as there are no instrument angles, and no field angle magnification.  Therefore you should be able to get the entire disc of the sun and prominences in an on-band "sweet-spot," which should be over 1 degree or so in diameter.  The "sweet spot" will actually be smaller with an internal mounted etalon, despite the use of pressure tuning.  But if you center a limb prominence in the field of view with an objective mounted filter, the opposite limb may begin to lie outside of the uniform band-pass region and have some reduced brightness.



#3 GUS.K

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Posted 14 May 2016 - 11:37 PM

The pressure tuned scopes have a circular sweet spot and if the sun is centered the whole disc and all proms around the perimeter are visible at the same time. As the disk is moved closer to the edge of the FOV, you start to loose detail.With my LS100 single stack the sweet spot is slightly larger than the suns disk and I can usually see all proms visible at once. My LS60 has a larger sweet spot and there is a bit more room to move the disk around before I start loosing detail at the edge. 



#4 sam9148

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Posted 16 May 2016 - 05:00 PM

You are both correct

Sorry for my English. Changing orientation means that I move the scope half a degree or one degree in any direction.

In other words,when the whol disc is located in the middle field of view,I may see prominences arround the disk.

If I move the telescope half a degree down,so,a prominence located in the upper side of the sun,is now in the middle(let say for imaging),the prominence almost disappears and I have to readjust the Ethalon.

I hope this time I expressed myself better.

By the way,what is "solar continuum" ?

Thank you again



#5 markthais

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Posted 20 May 2016 - 09:05 PM

What you are describing could be caused by the etalon uniformity. The plates have a small angle or the flatness at that point have shifted  the wavelength one way or the other. The new area could be shifted red and when you tilt it, it goes back on band. A good check would be to take a filament and move it to different areas in the eyepiece. If it changes in contrast  then there is a wedge. This does not mean you can't get good images. Just adjust for the area your looking at.

Mark W.  



#6 sam9148

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Posted 21 May 2016 - 06:11 AM

Thanks for your answer.

I shall perform what you have suggested.



#7 BYoesle

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Posted 21 May 2016 - 08:56 AM

 

By the way,what is "solar continuum" ?

 

Solar continuum is generally considered to be light coming from the photosphere.  In the context of what we are discussing, it is where your Ha narrow-band filter goes off-band, and you begin to lose disc detail and prominences begin to fade or disappear.

 

All H alpha narrow-band filters have an "acceptance angle," which defines how much of an angle can pass through an etalon before the design wavelength (or center wave length - CWL) shifts un-acceptably "off-band."  This is naturally one-half the diameter of the Jacquinot Spot diameter, other wise known as the "sweet spot."   The Jacquinot spot is defined as the field about the optical axis within which the the peak wavelength variation [ Δλ ] with field angle does not exceed √2 of the etalon bandpass.  This angular field can be used to perform close to monochromatic imaging.  

 

So you can see that tilt-tuning the etalon can only be accomplished to the degree in which the tilt doesn't exceed the acceptance angle.

 

Equation 1: Δλ = √2 x FWHM

 

The tilt (field) angle verses wavelength change can be found with formula for the CWL shift:

 

Equation 2: Δλ = ½ (CWL / n^2) θ^2

 

We can now solve for θ:

 

√2 x FWHM = ½ (CWL / n^2) θ^2

 

θ^2 = √2 x FWHM ÷ ½ (CWL / n^2)

 

For an air spaced etalon (n = 1.00) with a FWHM of 0.7 Å at the H alpha line (6563 Å), with θ in radians (1 radian = 57.2957795 degrees):

 

θ^2 = 1.4142 x 0.7 ÷ ½ (6563 / 1.00)

 

θ^2 = 0.98994 ÷ 3281.5

 

θ^2 = 0.000301673

 

θ = √0.000301673

 

θ = 0.017368736 (radians) x 57.2957795 degrees

 

θ = 0.9951553 degree

 

Therefore the Jacquinot spot is ~ 1.0 degree, and the “acceptance angle” (field angle) for this size a spot would be ~ 0.5 degree, as is the frequently cited acceptance value for a 0.7 Å FWHM etalon.  Outside this "sweet spot" radius H alpha detail will begin to fade.

 

Next, for a double stacked etalon system with a 0.5 Å FWHM, and assuming a DS system follows the same rules, we get the following;

 

θ^2 = 1.4142 x 0.5 ÷ ½ (6563 / 1.00)

 

θ^2 = 0.7071 ÷ 3281.5

 

θ^2 = 0.00021548

 

θ = √0.00021548

 

θ = 0.0146792 (radians) x 57.2957795 degrees

 

θ = 0.841058 degree

 

Therefore the Jacquinot spot is ~ 0.84 degree, and the “acceptance angle” (field angle) for this size a spot would be ~ 0.42 degree.  Christian Valadrich produced a great graph of these relationships for various band-passes and refractive index materials which is quite useful:

 

Jacquinot-Fig.JPG

Used with permission.

 

You can see that for higher index of refraction materials (mica focuser-based filters) the acceptance angle (Jacquinot spot diameter) is larger than for air-spaced stalon filters, and they are more tolerant of tilting and field angle magnification and instrument angles.  This also explains why pressure tuning an air-spaced etalon is vastly superior to tilt tuning for internally etalons located within the collimator optics.

 

Using my double stacked SM90/90 filter system, and using the first ghost image placed directly in contact with the primary image, you can see how the detail begins to go off-band at the ~ 0.4 degree radius, and the filter further shifts outside of this radius to the blue wing and loses prominence and surface detail, and by the radius of the outside limb of the ghost image the filter is essentially completely off-band:

 

Sweet spot sm.jpg

 

 


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#8 ASTROTRUCK

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Posted 21 May 2016 - 11:16 AM

" If I move the telescope half a degree down,so,a prominence located in the upper side of the sun,is now in the middle(let say for imaging),the prominence almost disappears and I have to readjust the Ethalon "  - - quote from above.

 

i am using a 100mm f9 meade coronado 60mm with 1500 blocking filter.

 

i do not experience this loss of detail when i shift ( as described ) the image from top to middle - i actually gain detail as my sweet spot is nicely centered - just luck.

 

when i set up my system i do adjust the tilt to best tilt position giving both ! proms & disk detail - at this tilt i do photograph both disk & proms separately but no need to readjust the tilt for either.

however - if i wish -  i do readjust the tilt for either disk shots or prom shots to enhance either for photos -

but as i say it is not necessary with my system - i sometimes do readjust the tilt to give best detail for photographing either the disk or proms when i photograph them separately.  for full disk merging of both to create a full disk image of both detail of disk & prom i use the program "imerge" after processing the images in registax.

.



#9 sam9148

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Posted 21 May 2016 - 02:51 PM

What an educated answer.

Very  interesting.

So,I have a lot of homework to make this coming week.

I believe after understanding all the theory above,I shall be able to determine if there is any problem with my filter.

How can you tell when the image shifts to the blue wing or to the red wing ?

The image I get is more like the right image.Perhaps everything is ok and it's only lake of expiriance.

May I ask what kind of camera are you using ?



#10 sam9148

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Posted 21 May 2016 - 03:25 PM

Since I have a blocking filter of 30mm and the telescope has a f.l. Of 820mm,which means that the image at the focal plain is 7.1mm,this image is about 1/3 of the blocking filter.

I think,this can make me deviate from the acceptance angle.

am I wright ?



#11 sam9148

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Posted 21 May 2016 - 11:51 PM

Sorry,the image is about 1/4 of the blocking filter.



#12 BYoesle

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Posted 22 May 2016 - 12:02 PM

Hi Sam,

 

The blocking does not really come into play with regard to the etalon acceptance angle or tilt tuning.

 

How can you tell when the image shifts to the blue wing or to the red wing ?

The image I get is more like the right image.Perhaps everything is ok and it's only lake of expiriance.

 

This movie by Spectral Joe shows him tuning his spectrohelioscope from the blue wing, on-band, and then into the red wing:

 

http://s106.photobuc...df8aa6.mp4.html

 

Your on-band image should look like the middle of the movie, with maximum filament density.  Certainly if your etalon is not properly tuned you may have increased difficulty seeing features further off-axis.  Tilt-tuned etalons are designed to have the CWL to the red side of the H alpha emission, since tilting can only shift the CWL blue ward.  But the SM90II also is a mechanical pressure-tuned etalon.  As you increase the pressure (decrease the gap thickness), it will also shift to the blue.

 

The transmission peak wavelength changes in FP etalons can be found using the basic etalon theory formula:

 

λp = 2 n t cos θ

 

These are represented as:

 

etalon math.jpg

 

These parameters are defined as:

 

n = gap refractive index
t = gap thickness
θ = angle of incidence (tilt)

 

Therefore we can see that:

 

1. As refractive index of the gap increases, the wavelength will increase (shift red-ward);

2. As the gap thickness increases, the wavelength will increase (shift red-ward);

3. As the tilt angle increases, the cosine decreases, and the wavelength decreases (shift blue-ward).

 

So you may have to adjust the SM90II to be a just slightly to the red side via decreased pressure (increased gap thickness), and then apply a minimal tilt (shift blue-ward) to be exactly on band, while maintaining the ability to make slight tuning adjustments for changes in barometric pressure and temperature.

 

The above image was made with a Pentax DSLR, but this was for the limited purpose of imaging the retro-reflection with the on-axis primary image.  Most imagers use an industrial video camera, and I use a PGR Cahmeleon and use mosaics to for a full-disc image:

 

Sept 27, 2015 full disk SM labeled.jpg

 

Tutorial here: http://www.cloudynig...s/#entry6426147


Edited by BYoesle, 22 May 2016 - 12:21 PM.


#13 sam9148

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Posted 22 May 2016 - 05:08 PM

I really appreciate your efforts to explain the theory in such a comprehensive way.

It is easy to see the tilt,since turning the small wheel,I can see the angle changes.When I change the pressure,(by turning the large wheel,)looking from the focuser,which direction decreases pressure ?



#14 BYoesle

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Posted 23 May 2016 - 10:30 AM

I have no experience with the "rich view" Coronado tuning, so I do not know.  You will know if you "rich view" tune the etalon a bit off-band and can tune it back on-band by using tilt, that you rich view tuned to the red side.  If on the other hand you tune it slightly off-band , and tilt tuning gets it farther off-band, you rich view tuned to the blue side.



#15 sam9148

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Posted 23 May 2016 - 03:35 PM

The way Coronado suggest to tune is ,first to tilt adjust till I get the brightest image,then use the "rich view" to get the most contrast image.

i tried that way but did not see any difference in image brightness.



#16 BYoesle

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Posted 23 May 2016 - 06:14 PM

Again I've never used a rich-view tuned scope or filter, so I can't have a definitive answer.  However, when the filter is off-band the image would be a little brighter, as more continuum form the photosphere on the side of the H alpha line would usually get through the blocking filter....  :hmm:   Hopefully someone who has a rich-view tuned scope will chime in.

 

Minimizing tilt gives you the most uniform contrast, as the angles through the filter are the most normalized over the entire filter.  Therefore - for a single filter that does not need to be slightly tilted to remove the ghost reflections that occur when double stacking - just as with Lunt pressure tuning, the ideal would be to have zero tilt to the filter, and only use the rich-view pressure tuning to get on-band. 


Edited by BYoesle, 23 May 2016 - 10:39 PM.


#17 sam9148

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Posted 24 May 2016 - 06:16 AM

OK. I shall check everything as soon as weather permits.

Thank you and others for all the explanations.



#18 BYoesle

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Posted 25 May 2016 - 08:40 AM

You're very welcome; be sure to check out my post on Etalon Basics, and the benefits of double-stacking, which will give you a vast improvement in in H alpha disc detail by suppressing parasitic continuum leakage from the photosphere:

 

http://www.cloudynig...-etalon-basics/

 

The view through a bino-viewed and double stacked filter system is a jaw-dropping experience for most people...



#19 sam9148

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Posted 25 May 2016 - 04:42 PM

Yes ,I am using a bino viewer and there is a big difference from one eye.




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