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Spectral Joe
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Reged: 02/28/11

Loc: Livermore CA
Re: Single Vs. Double Stack new [Re: George9]
      #5595451 - 12/30/12 01:28 AM

George, your plots are correct, if you're still wondering. I've been lazy in using Gaussians to show the effect of stacking filters. I had a Gaussian spreadsheet already built and didn't want to take the time to do it right. I spent some time today to make a Lorentzian version and it agrees with yours.

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BYoesle
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Reged: 06/12/04

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595463 - 12/30/12 01:50 AM Attachment (34 downloads)

Wow Joe - that’s a spectacular video! I note how in the red and blue wings chromospheric disk details like filaments (as well as prominences) disappear, but the spicule forest of bright patches and mottles really pops out - just as they do (i.e. spicules) at the limb:

Quote:

...we can now see that Secchi’s needles (the “spicules”) are arrayed in long picket fences... Spicules lie at the borders of supergranule cells...

Zirker, J.B. (2002) Journey from the Center of the Sun (p. 142). New Jersey, Princeton University Press.




Hi George: The inside story on ‘matching’ of front mounted etalons can be found here:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=solar&...

Then see this post for my solution:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=solar&...


Quote:

In a phone discussion with one of the GONG design staff earlier this year the subject came up, relating to why imaging disk and proms was difficult with a single stack system. The NSO staffer (who's been doing this since before most of us were born) said that in single etalon systems the sideband leakage would present exactly the situation observed...

[Joe]




This is another reason for double stacking to reduce bandwidth (as noted above). Besides improving contrast, removing the light of the photospheric disk equalizes the brightness of the disk and the prominences, making capturing both in the same exposure much easier, and eliminates the need to combine separate exposures for each - note the filaprom @ ~ 10:30:

June 15, 2012 21:37 UTC


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BYoesle
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5595497 - 12/30/12 02:40 AM Attachment (30 downloads)

When you process a properly exposed 0.7 A disk filtergram to bring out disk detail with increased contrast, you usually end up wiping out the limb chromosphere and prominenecs. That’s why you may be required to make two separate exposures, or use more involved processing to obtain both disk and prominence details:

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David Knisely
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Reged: 04/19/04

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Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595528 - 12/30/12 03:36 AM

Spectral Joe wrote:

Quote:

Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!




I love that sig! A friend of mine lives up in the wilds of the Paul Bunyan State Forest in Minnesota on Mantrap Lake and I went to visit him one year. We had his 10 inch f/5 Newtonian out at the lake shore (three inch off-axis ERF and "Barlowed" to f/31) with his DayStar T-Scanner and were watching a post-flare arcade develop on the limb when some people came by on a boat and shouted out, "You'll go blind!!". We just smiled and went back to the scope. Clear skies to you.


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5595918 - 12/30/12 11:14 AM

Quote:

George, your plots are correct, if you're still wondering. I've been lazy in using Gaussians to show the effect of stacking filters. I had a Gaussian spreadsheet already built and didn't want to take the time to do it right. I spent some time today to make a Lorentzian version and it agrees with yours.




The thing is, when I incorrectly did the Gaussian the day before, I discovered that as you shift, it becomes dimmer but not wider. That is, when you normalize, all the curves coincide. I guess it's a moot point unless there are also Gaussian filters out there, but it's an interesting property. I knew it was supposed to get wider, but I didn't make the connection until seeing Valery's post. Bottom line, I guess, is that Gaussian turns out not to be a good shortcut.

Thanks for the double stacking links, Bob. So the matching is more mechanical. I remember seeing your great innovation when you first posted it. I just put up with the two dark ERFs and the TMax. Although when one of my filters decontacted after 12 years and I had to hacksaw the ASP-60 apart to fix it, I made it so I could leave one ERF off, and that brightened the double stack.

George


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BYoesle
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Re: Single Vs. Double Stack new [Re: George9]
      #5596152 - 12/30/12 01:14 PM Attachment (24 downloads)

Hacksaw?! You’re a brave man George!

Quote:

I made it so I could leave one ERF off, and that brightened the double stack.






Just in case others are as rusty on their math and statistics as I am, I thought I’d post what the shapes for Gaussian (“normal” or “Bell”) distributions verses Lorentzian (“Cauchy”) distributions look like (upper diagram).

It seems that unless you “do the math,” it can be hard to tell the difference given the variables of amplitude and deviation (lower diagram) - where the Gaussaian vs. the Lorentzian distribution looks almost identical to the DS v. SS profiles above...


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: BYoesle]
      #5596600 - 12/30/12 05:25 PM Attachment (23 downloads)

Your bottom curve really shows it well. I threw in a double and quadruple stack, all at the same FWHM. It looks suspiciously like a large number of Lorentzians will approach Gaussian.

(I had to hacksaw. Lunt said neither they, nor Meade, nor Isle of Man would be able to do it cost-effectively, so I had nothing to lose. Luckily the spacers remained stuck to one element.)

George


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bill1234
professor emeritus


Reged: 04/21/11

Loc: NJ, USA
Re: Single Vs. Double Stack new [Re: George9]
      #5596818 - 12/30/12 07:41 PM

excellent post...thanks for starting and continuing...

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BYoesle
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Re: Single Vs. Double Stack new [Re: George9]
      #5596876 - 12/30/12 08:18 PM Attachment (26 downloads)

Thanks for the plots George.

I was wondering if you might run the following bandpasses and post them as you have above at the same amplitude:

Lorentzian - 0.8, 0.7, 0.5, and 0.35 Angstroms FWHM respectively (this last value is what I assume results from a triple stacked series of 0.7 FWHM etalons). Is quad stacking actually a 0.24 A FWHM bandpass?

It would be nice to have a comparison for what Jim Ferreira is imaging at 0.8 A (which obviously shows a near-photospheric disk and lots of Doppler-shifted as well as on band prominences), as well as what I and others are seeing at 0.7 A and 0.5 A, and what Jesus Munoz has shown with his SS, DS, and TS images seen here:

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=solar&...

From what Jesus’ images show in the thread above, and interpolating a curve between your green Lorentzian ^2 curve (double stacking) and the red Lorentzian^4 curve (quad stacking), I’m not sure I see much benefit to triple stacking compared to double stacking - but then again the curves shown above are at the same FWHM. It would be nice to see some quantitative data or graphical representations, and to perhaps show what the narrower DayStar or Solar Spectrum filters actually accomplish - assuming these solid etalons provide similar shaped transmission curves.

Joe might press his awesome spectrohelioscope into service for showing some controlled comparison images to go along with these graphs - or perhaps he already has. It’s hard to find solid info like this anywhere...


Addendum

Finally had some sun today, but the seeing was horrible (1-2/10). Temp -1 C, solar altitude ~ 22 degrees (local noon), relative humidity 26%, winds SE 5-10 km/h. Close ups of the limb spicules off-band will have to wait for a much higher sun, and hopefully warmer temps. But I did get images of the chromosphere double stacked @ < 0.5 angstroms: top on-band @ 20:43 UTC, bottom ~ 0.7 A to the blue wing @ 20:44 UTC. Tuning was accomplished using the etalon closest to the objective (i.e both etalons were tilted together)...

The change off-band is obvious, showing the spicule (mottle) forest and much diminished general chromosphere features (Joe’s video is much better). Both images with identical exposures (5 ms), gain (575), gamma (1500), and processing. Paradoxically, the off-band image is dimmer - any ideas about this??? From the discussion above more continuum from the wing should be mkaing it through (especially considering tilting also widens the bandpass slightly), and therefore it should be brighter - no?

Quote:

The thing is, when I incorrectly did the Gaussian the day before, I discovered that as you shift, it becomes dimmer but not wider.

[George]




Hmmm.....


Edited by BYoesle (12/30/12 10:31 PM)


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: BYoesle]
      #5597171 - 12/30/12 11:46 PM Attachment (31 downloads)

Very good image despite the conditions. I gave it a visual try today, but seeing was bad for me, too.

Here is what I ran:

1. 0.8 FWHM of single stack
2. 0.7 FWHM of single staack
3. double stack of 0.7, which is 0.45052 FWHM
4. triple stack of 0.7, which is 0.35688 FWHM
5. quadruple stack of 0.7, which is 0.30449 FWHM

In the figure, "Lorentzian" is 0.7 unless otherwise specified. Remember that a real filter won't have 100% transmission, so each increase in stacking will actually be dimmer. I welcome someone to check my derived FWHM's. I just iterated to the answers. And very happy to replot if I misunderstood.

(On the Gaussian, yes it seemed odd. You could send one up to violet and one down to infrared and theoretically it would be just as narrow, but just very dim. Would need to double check that.)

George


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BYoesle
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Re: Single Vs. Double Stack new [Re: George9]
      #5597317 - 12/31/12 02:16 AM

Thank you very much George - these are very useful graphs and FWHM values!

As Joe noted, it looks as if the tail suppression is the more significant aspect compared to reduced bandwidth, although going from 0.7 to 0.45 A (FWHM - 0.5 on the verticle scale) with double stacking two etalons seems to give more “bang” compared to triple or quad stacking. As with most things it seems to be the law of diminishing returns... ERF removal would seem to be a must - hacksaw or no hacksaw.

I would think these shapes might possibly change a little bit for the solid etalons, although I don’t think they’re as susceptible to as much dimming with decreased bandwidth as via stacking multiple etalons...

Do you have any idea(s) as to why tilting a pair of DS etalons off-band would produce a dimmer image verses a brighter image? Going off-band would seem to allow more continuum in, and tilting an etalon should widen the bandpass (at least if it’s a Lorentzian), which would seem to be a double-whammy for giving you a brighter disk.

The front etalon (and attached D-ERF) was “pre-tilted” as usual to remove ghost images, and then the whole DS assembly was tilted to go off-band. Could it have something to do with how the D-ERF’s dielectric coatings act when tilted? They seem to be more reflective at an angle verses straight on (normal to the filter) - yet the tilting is not that great... total of a few degrees at the most.

Or would it more properly be inherent to the etalons themselves? For example, do their different initial tilts make them behave as if they are “shifted” rather than identical, thus producing the dimming you noted earlier for both Lorentzian and Gaussian transmissions? Would tilting them in unison somehow exacerbate the “shift- dimming”?

There certainly appears to be no significant brightening on-band, dimming off-band, or vice-versa, with Joe’s video via a hi-res diffraction grating. So it would seem to have something to do with the behavior of etalons, tilting, coatings, etc...


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marktownley
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Reged: 08/19/08

Loc: West Midlands, UK
Re: Single Vs. Double Stack new [Re: BYoesle]
      #5597452 - 12/31/12 06:34 AM

Quote:

I’m not sure I see much benefit to triple stacking compared to double stacking





Hi Bob et al,

I've had a bit of a play round with triple stacking. My results were very contrasty. This is with the TS40.


TS Full disk bw by Mark Townley, on Flickr

Banding and sweetspotting issues become an absolute pain and is tricky to get everything just right, so I agree that double stacking is a better and easier alternative. I would have explored this more this year but we've had the wettest year for over 100 years and solar moments have been at a premium...

Mark


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: BYoesle]
      #5597611 - 12/31/12 09:46 AM

Quote:

Do you have any idea(s) as to why tilting a pair of DS etalons off-band would produce a dimmer image verses a brighter image? Going off-band would seem to allow more continuum in, and tilting an etalon should widen the bandpass (at least if it’s a Lorentzian), which would seem to be a double-whammy for giving you a brighter disk.



Bob, I don't have a good explanation. My assumption was that tilting the pair inadvertently shifted them with respect to each other. E.g., if you tilt the front one to avoid the ghost (and you optimized the tilt for the best view), then tilting the pair could either untilt the front one with respect to the sun or further tilt it depending on the direction. My tilting double stack now belongs to a friend, so I can't try it, but I would just experiment with single stack tilting to note the behavior, and then add the second element to the already tilted first element, optimizing brightness. In other words, I suspect you had to do some fiddling to get your on-band double stack optimized, so to be fair you need to fiddle to optimize the off-band double stack.

For my current PT double stack, off band (continuum) is a little brighter than on band (H-alpha), but not enormously so.

George


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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5597620 - 12/31/12 09:50 AM

Excellent Mark!

Quote:

Banding and sweetspotting issues become an absolute pain and is tricky to get everything just right...




Looks like you nailed it

Any comments about prominences with TS? Jesus' image looks as if they lose a little intensity compared to SS and DS, but again processing needs to kept the same for valid comparisons - what’s your impression?


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BYoesle
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Re: Single Vs. Double Stack new [Re: George9]
      #5597719 - 12/31/12 10:51 AM

Quote:

My assumption was that tilting the pair inadvertently shifted them with respect to each other. E.g., if you tilt the front one to avoid the ghost (and you optimized the tilt for the best view), then tilting the pair could either untilt the front one with respect to the sun or further tilt it depending on the direction.... but I would just experiment with single stack tilting to note the behavior, and then add the second element to the already tilted first element, optimizing brightness. In other words, I suspect you had to do some fiddling to get your on-band double stack optimized, so to be fair you need to fiddle to optimize the off-band double stack.

George




Thanks George - that makes a lot of sense.

I'm looking forward to that experiment when it gets a little warmer - handling a frozen DS etalon assembly with my bare hands was a little unnerving. Fiddling with two frozen etalons and an separate ERF bare handed would probably result in a visit to the hospital ER...


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Bill Cowles
MacGyver
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Reged: 04/16/06

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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5598153 - 12/31/12 03:09 PM

Love your setup and images!

Bill


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Spectral Joe
super member


Reged: 02/28/11

Loc: Livermore CA
Re: Single Vs. Double Stack new [Re: Bill Cowles]
      #5598933 - 12/31/12 11:56 PM

With the recurring question of how to set filters (single, double, or any multiple) onto H alpha, I have a suggestion. Get a source of hydrogen emission that you have control of. You can get a hydrogen discharge tube (commonly referred to as a "spectrum tube") for between $20 and $40, depending on the vendor. You can get the power supply and mount for $150 to $200, or you can build your own. So, $170 to $240 sounds like a lot? Some people spend that on eyepieces. With the spectral source there is no question where you are tuned. This seems to be an issue with double stacking, I see images that clearly show chromospheric network features that are see in the wings of H alpha, the stack is tuned red or blue of the line core. As the passband gets narrower (with a properly tuned double stack) the details seen in the line wings fades, and different details are seen, ones that were hidden before. Tilt tuned systems may have trouble finding a tilt value that doesn't give annoying reflections, pressure tuning is better for stacked systems for that reason. But even pressure tuned systems will show a wavelength variation across the field, such are the woes of filter systems. Oh, one last thing before I get off my soapbox - go lightly with the Photoshop or Registax rolling pin, bakers know that overworking the dough or batter can render the result less tasty.

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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5599717 - 01/01/13 02:21 PM

Interesting suggestion, Joe. And it's always fun to have another device to work with. I guess you could learn how to optimize the filters, but you would presumably still need to do in-the-field tweaking (e.g., as temperature changes or as air leaks slowly out of the PT).

George


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Spectral Joe
super member


Reged: 02/28/11

Loc: Livermore CA
Re: Single Vs. Double Stack new [Re: George9]
      #5599763 - 01/01/13 02:50 PM

The thing is portable, and can be taken to the scope. One of the drawbacks to etalons is not knowing exactly where you're tuned, this is a way to find out. An yes, monitoring etalon conditions is a great idea, and achievable for the gadgeteer.

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BYoesle
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Re: Single Vs. Double Stack [Re: Spectral Joe]
      #5602306 - 01/03/13 02:33 AM Attachment (14 downloads)

Bill

Quote:

With the recurring question of how to set filters (single, double, or any multiple) onto H alpha, I have a suggestion. Get a source of hydrogen emission that you have control of... With the spectral source there is no question where you are tuned...

[Joe]




Hi Joe -- excellent suggestions - the discharge tube would be a relatively inexpensive H alpha “accessory.” Perhaps a “discharge tube light box” (using more than one tube?) could be used in optimizing filter tilts and rotations the way a flat-fielding light box is used. This could help to not only get the filter system on-band, but to also help optimize field uniformity as noted by Mark for his triple stacking endeavor.


Quote:

Tilt tuned systems may have trouble finding a tilt value that doesn't give annoying reflections, pressure tuning is better for stacked systems for that reason. But even pressure tuned systems will show a wavelength variation across the field, such are the woes of filter systems.




Tilt verses pressure tuning is usually more a problem of tuning the primary internal etalon to be on-band. Tilting can introduce “banding” - creating a band of good contrast, and the area outside of it which falls off-band. Tilt-tuned induced banding is more problematic with internal etalons - Andy Lunt stated “I would guess that internal Etalons are about 5-6 times more sensitive to tilt [banding] than external Etalons...” http://luntsolarsystems.com/blog/internal-etalon-performance

Pressure tuning eliminates the problem of banding due to tilt tuning with internal etalons.

Internal etalons are also more sensitive to circular “sweet spots “ from the internal collimating (or telecentric) optics they use. This is due to the difficulty in getting field and instrument angles within the acceptance angle of the etalon for good across-the-filter bandpass performance. Adding “banding” via tilt-tuning exacerbates a sweet spot issue.

On the other hand, ghost reflections are created between multiple etalons (i.e. double stacking). These reflections are relatively easily removed from the field of view with a little extra tilting to the DS etalon. But this again can introduce banding. Pressure tuning would be great for the second etalon, as when it is tilted it goes slightly off-band, and pressure tuning could bring it back exactly on-band, but probably would not do much for reducing "banding." No one is making a pressure tuned front etalon, although at one time Lunt was looking at the possibility. However, one could design a sealed enclosure with pressure tuning to house an existing etalon, using the ERF for the front “window” and optical glass for the second...

Because of the aforementioned sweet spot issues that can occur with internal etalons - and greater susceptibility to banding if tilt-tuned - they seem more likely to exhibit banding and/or sweet spots when double stacked (e.g. the bandwidth is reduced making any sweet spot and banding more evident.)

Using two external (front) etalons seems to minimize the magnitude of both sweet spots and banding, as there are no instrument angles, and the field angles are just those of the sun itself ~ 0.25 degree. Of course it's the most expensive way to go, and the aperture of commercial etalons is currently limited to 100 mm. Tilting the etalons (front to remove ghosts, objective to tune), can produce some banding, which is a more subtle gradient than is seen with internal etalon based systems. However, when properly adjusted, double stacking two front etalons (or evidently even three triple stacked front etalons as shown by Mark's SM40's) can give good contrast uniformity with little evidence of banding or a sweet spot.

DS SM90-SM90/BF30:


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