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Jim Lafferty
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Reged: 06/30/07

Loc: Southern California
Re: Single Vs. Double Stack new [Re: BYoesle]
      #5603066 - 01/03/13 01:57 PM

Wow! Excellent discssion!

Jim


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marktownley
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5603454 - 01/03/13 06:08 PM

Quote:

(or evidently even three triple stacked front etalons as shown by Mark's SM40's)




This was with 2 external double stacked 40mm etalons and a internal 20mm etalon


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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5603589 - 01/03/13 07:42 PM

Ah-ha. Thanks Mark. This explains your comment related to "sweetspotting issues" being "tricky to get right" - and makes your results even more extraordinary.

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Scott BeithAdministrator
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5604922 - 01/04/13 02:31 PM

This has been a great thread and the results are pretty darn impressive!

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Bill Cowles
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Re: Single Vs. Double Stack new [Re: Scott Beith]
      #5605184 - 01/04/13 04:44 PM

I just hope there is not going to be a test.

Bill


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Spectral Joe
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Reged: 02/28/11

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Re: Single Vs. Double Stack new [Re: Bill Cowles]
      #5605405 - 01/04/13 07:14 PM

Tuning any combination of bandpass filters is going to be tricky, especially if you aren't sure of what to tune to. If it's of any use to anyone, this is what the core of H alpha looks like at 0.35 Angstrom bandpass:



Taken during a rare moment of good seeing on 4/24/09, 1.1 arcsec per pixel.

The hydrogen lamp looks like this in use:



It's easily mounted in any sort of enclosure you prefer. I use it for alignment and bandwidth checking.


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: BYoesle]
      #5605526 - 01/04/13 08:28 PM

Quote:

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.





Nice shots, Bob and Joe.

The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding. The weak point is the reflections, covered on other threads. The SS is pretty close to perfect in terms of sweet spot, banding, and reflections.

My external Coronado ASP-60s were free of sweet spots or banding, just as Bob describes, until I bought a binoviewer. Then I had severe banding. I never quite figured it out. Perhaps the Denkmeier OCS (more or less a Barlow that goes in front of the blocking filter so you can achieve focus) changed the light cone so that the blocking filter induced banding. Tuning the blocking filter did move the band up and down (the blocking filters used to be tilt tuned).

George


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marktownley
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Reged: 08/19/08

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Re: Single Vs. Double Stack new [Re: George9]
      #5605999 - 01/05/13 04:06 AM

Quote:

The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding.




Is this a result of the etalons being 'oversized' to what you might expect? ie. only the central area of an etalon is being utilised...


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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5606592 - 01/05/13 01:07 PM

Ditto, great close up detail and discharge tube pic Joe.


Quote:

Perhaps the Denkmeier OCS (more or less a Barlow that goes in front of the blocking filter so you can achieve focus) changed the light cone so that the blocking filter induced banding. Tuning the blocking filter did move the band up and down (the blocking filters used to be tilt tuned).




The blocking filter is also a interference filter, therefore the incident angle of the rays it receives will affect it's performance. Per David Lunt (in an old Coronado blog):

"...the focal ratio ideally should be greater than f10 to maintain performance of the blocking filter....but in fact you will see good results at considerably shorter f ratios than this."

My understanding is that a standard barlow will increase the field angles (bad). This could be made worse with tilting as described above, and might indeed produce banding. Using a TeleVue Powermate would be better, since it has "telecentric" properties which will minimize the increase in field angles.


Quote:

"The DS internal pressure-tuned etalons also achieve large sweet spots (by large I mean good; i.e., bigger than the sun) and no banding."

Is this a result of the etalons being 'oversized' to what you might expect?




I’m in no way an optics expert, but from what I have been able to learn here’s how etalon size relates to sweet spots:

Typically the sweet spot with a collimated etalon is caused by a magnification of the field angles of an extended object. If the field angle exceeds the acceptance angle of the etalon (i.e the angle needed to meet the filter's band pass specification), that portion of the image (in this case the limb of the sun) will begin to fall off-band. A decease in contrast (a loss of chromosphere detail) or prominence visibility will be noted outside the sweet spot.

For the sun’s limb, the field angle starts out being ~ 0.25 degree (i.e 1/2 the sun's angular diameter, or when optically centered the center-to-limb angle). For an etalon mounted on front of the objective, this is the field angle, and amounts to an f 108 optical cone convergence. Generally there is a very wide sweet spot at this angle and configuration.

In a collimator based internal etalon system, the field angle magnification is the ratio of the objective to collimator lens focal lengths. Because of the optical geometry, this is also roughly proportional the ratio of objective diameter to the etalon (working) diameter. Therefore, the larger the internal etalon, the less the magnification of the field angles, and the better the on-band performance.

For example, the Lunt LS80 uses an internal 50 mm etalon, and dividing 80 by 50 = 1.6. Hence the internal field angle of the suns limb can be generally determined to be 0.25 x 1.6 = 0.4 degree. For the LS100, which uses the same 50 mm etalon, the collimated field angle is 0.25 x (100/50) = 0.5 degree, and so on. So we can see that the smaller the etalon is compared to the objective (i.e. the collimator lens FL and geometry) the greater the field angle magnification, hence the smaller the sweet spot of good on-band performance.

Of note for narrower bandwidth filters is that they also have a narrower acceptance angle to remain on-band. This perhaps can explain why a single stacked system with no evidence of a sweet spot may develop a sweet spot (and possibly banding from tilting the second etalon) when double stacked, due to the constraints of the system to have a narrower acceptance angle for proper on-band performance.

Again I’m not an expert, so if I’ve erred, I hope someone will correct me.


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Spectral Joe
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5607601 - 01/06/13 12:12 AM

I'm trying to figure out what is causing the banding while using the binoviewer. Nothing past the etalon will affect its bandpass. Is it actually a band or a central hot spot? If it a spot I suspect it's a relay imaging issue with the design of the binoviewer.

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marktownley
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Reged: 08/19/08

Loc: West Midlands, UK
Re: Single Vs. Double Stack new [Re: Spectral Joe]
      #5607772 - 01/06/13 04:59 AM

Quote:

I'm trying to figure out what is causing the banding while using the binoviewer. Nothing past the etalon will affect its bandpass. Is it actually a band or a central hot spot? If it a spot I suspect it's a relay imaging issue with the design of the binoviewer.




Newtons rings maybe?


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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5607836 - 01/06/13 07:27 AM

Perhaps the design of the blocking filter used with the ASP 60 might be part of the banding issue George noted when used with the accessory optics...

Quote:

The [ASP-60] etalon is in the front element. The diagonal (PROM-15T) contains the blocking/prominence filter. This does not 'tune' the whole filter. The narrow element (etalon) has a temperature coefficient of passband shift of <1 Angstrom/2000 degrees C. However, the blocking filter is somewhat temperature sensitive and needs compensating, - by tilt, - for large differences in ambient temperature. This filter isolates the H-alpha passband in the etalon and eliminates all the others. If it moves with temperature, without being compensated, it would allow one of the other etalon orders to come through and, thereby, lower the contrast.

[Old Coronado blog - emphasis added]






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marktownley
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Reged: 08/19/08

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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5608380 - 01/06/13 01:29 PM

Quote:



I’m in no way an optics expert, but from what I have been able to learn here’s how etalon size relates to sweet spots:

Typically the sweet spot with a collimated etalon is caused by a magnification of the field angles of an extended object. If the field angle exceeds the acceptance angle of the etalon (i.e the angle needed to meet the filter's band pass specification), that portion of the image (in this case the limb of the sun) will begin to fall off-band. A decease in contrast (a loss of chromosphere detail) or prominence visibility will be noted outside the sweet spot.

For the sun’s limb, the field angle starts out being ~ 0.25 degree (i.e 1/2 the sun's angular diameter, or when optically centered the center-to-limb angle). For an etalon mounted on front of the objective, this is the field angle, and amounts to an f 108 optical cone convergence. Generally there is a very wide sweet spot at this angle and configuration.

In a collimator based internal etalon system, the field angle magnification is the ratio of the objective to collimator lens focal lengths. Because of the optical geometry, this is also roughly proportional the ratio of objective diameter to the etalon (working) diameter. Therefore, the larger the internal etalon, the less the magnification of the field angles, and the better the on-band performance.

For example, the Lunt LS80 uses an internal 50 mm etalon, and dividing 80 by 50 = 1.6. Hence the internal field angle of the suns limb can be generally determined to be 0.25 x 1.6 = 0.4 degree. For the LS100, which uses the same 50 mm etalon, the collimated field angle is 0.25 x (100/50) = 0.5 degree, and so on. So we can see that the smaller the etalon is compared to the objective (i.e. the collimator lens FL and geometry) the greater the field angle magnification, hence the smaller the sweet spot of good on-band performance.

Of note for narrower bandwidth filters is that they also have a narrower acceptance angle to remain on-band. This perhaps can explain why a single stacked system with no evidence of a sweet spot may develop a sweet spot (and possibly banding from tilting the second etalon) when double stacked, due to the constraints of the system to have a narrower acceptance angle for proper on-band performance.

Again I’m not an expert, so if I’ve erred, I hope someone will correct me.




You may not be an expert Bob, but you can be on my pub quiz team anyday

Taken me a while to ingest all this and get my head around it, alongside some other stuff about it elsewhere on the web. I've never really given the whole field angles thing much thought, but it really does make a whole lot of sense and gives an interesting viewpoint to look at Ha systems:

Some of the best images i'm seeing to date on the web are coming out of LS80s - the pictures are stunning in terms of nice tight and even bandpass across some quite large chipped CCDs. Maybe our field angles have a lot more to do with it than we think.

I'm curious now as to the etalon diameters in the Coronado equipment... Hmmm


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BYoesle
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Re: Single Vs. Double Stack new [Re: marktownley]
      #5608627 - 01/06/13 03:23 PM

Thanks Mark!

I found this article and blog discussion from several years ago by David Lunt be very helpful in understanding how important the issue of angles (among other factors) are to narrow band interference filter performance:

link


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: BYoesle]
      #5609073 - 01/06/13 07:17 PM

Wow, this thread just keeps staying interesting.

Joe, it’s a band, not a spot. Maybe a quarter or third FOV high, and it is as wide as the FOV. I can shift the band up or down in the field of view by tuning the blocking filter. Therefore, I don’t think it is actually the binoviewer. If I had the filter, I could just throw an eyepiece in place of the binoviewer with the OCS in it and try it; not sure why I never tried that before. The other thing I just realized is that the band was always horizontal (like the tilt of the blocking filter), so it wasn’t the etalon (whose angle varied depending on how tightly I screwed in the TMax).

Bob, I think your explanation is right on. The OCS is magnifying the field angles. Great job finding that document.

Coronado’s first solar filters basically took a prominence filter, which was an ERF plus a PROM15T (prominence, 15mm, maybe T for tilting?) and inserted an etalon behind the ERF. The prominence filter by itself showed no solar disk details. I remember David Lunt commenting that I should not need to tune the blocking filter with the etalon; just leave it untilted because the temperature effect was not that big. This worked until I got the binoviewer.

Over time I guess they switched to a blocking filter designed for the etalon. I don’t know, but I suspect that the new blocking filters were wider: still narrow enough to avoid other etalon orders but wide enough to avoid having to tune it with temperature variation (and perhaps cheaper to produce).

In your document, David Lunt mentions the dark element in front of the blocking filter to reduce reflections, and how they later brightened the view with a lighter one. I remember he had me send my blocking filter to Isle of Man to swap in the brighter version. Makes we wonder about the DSII and whether an element could be put between the two etalons to reduce the reflections (reflections would pass three times, but the primary would pass only once), but I guess the view would become too dim.

George


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack new [Re: George9]
      #5609205 - 01/06/13 08:42 PM

Bob, I think the 1A/2000C was correct. Your finding that document prompted me to look at the old coronado solar blogs circa 2000 (via the Internet Wayback Machine). Here was David Lunt's answer to my question about temperature sensitivity (5/10/2000):


"First it should be said that no normally achievable temperature will adversely affect the filter on a permanent basis. The AS1 series will easily withstand 200 degC.
The construction of these filters is such that, when they are optimised for operation in vacuum, the configuration of the assembly has an effective coefficient of expansion of 8x10e-8, corresponding to a passband shift in wavelength of ~1Angstrom/2,000degC. This design was originally conceived for a space project in which the satellite went in and out of the Earth's shadow encountering wildly varying temperatures.
The performance in air is more complicated. There are two factors involved,- the physical expansion of the etalon support structure and the change in refractive index of the air with temperature,- dn/dt. This value is -9.5x10e-7 and, as it is negative, means that the air gap becomes effectively narrower with increasing temperature. To combat this, a material is used for the support structure with a higher coefficient of expansion than that used for vacuum,- approximately 5x10e-7. As this value is positive, it can be chosen to balance the effect of changing refractive index of air. In the ASP-60 and AS1-90 models, this balance results in a coefficient of passband shift of approximately 1A/350degC. Thus, it can be seen that, provided the filter comes to equilibrium with ambient conditions, the shift of the passband is negligible for any normally experienced temperature.
However, there is a side effect that can be observed under certain conditions. Any change in temperature will affect the refractive index of the air more rapidly than the larger mass of the structure of the filter. Thus, if the ambient temperature is ramping quickly, the structural mass will lag behind the refractive index change of the air and the filter will drift slightly to shorter wavelength. It will resume its normal performance when it comes to equilibrium. It should be noted, however, that it could be possible for the ambient temperature to be changing enough that this point of equilibrium is never fully achieved.
The crux of the matter is that the filter is extremely thermally stable for a wide range of temperatures,- but not quite so in a rapidly changing temperature. Even so, this effect is not great;- amounting to a drift of about 0.25A in 40degC (72degF)."

George


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BYoesle
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Re: Single Vs. Double Stack new [Re: George9]
      #5609386 - 01/06/13 10:40 PM

Thanks George, I will “re-correct” the text back to the original. The reason I thought it might have been a typo is that I recalled - and then confirmed - that the SM series of Tucson etalons made in the early 2000's, and the Isle of Man etalons currently made by Solarscope, have a specification for thermal stability stated to be < 1 Angstrom / 200 degrees C.

So good find - and great info!

Quote:

Coronado’s first solar filters basically took a prominence filter, which was an ERF plus a PROM15T (prominence, 15mm, maybe T for tilting?) and inserted an etalon behind the ERF. The prominence filter by itself showed no solar disk details... Over time I guess they switched to a blocking filter designed for the etalon. I don’t know, but I suspect that the new blocking filters were wider: still narrow enough to avoid other etalon orders but wide enough to avoid having to tune it with temperature variation (and perhaps cheaper to produce).




Yes, the Coronado blog confirms the PROM15T used a 2A bandpass prominence filter as described, while the later blocking filters used a 6A bandpass blocker, which apparently is well within the range needed for an etalon with a FSR (i.e. free spectral range - the distance between the side-band harmonic peaks) of 10A.


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BYoesle
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Re: Single Vs. Double Stack new [Re: BYoesle]
      #5613776 - 01/09/13 01:56 PM Attachment (20 downloads)

... To be honest, when I double stacked my filter system, I was puzzled as to why my close up images of the limb of the sun didn’t show the “double limb” I was seeing in so many other close up images posted here. For a while I honestly thought my filter system may not be performing as well as it should. It wasn’t until Joe’s original question regarding double stacking that I realized what was actually taking place - a narrower bandpass eliminates continuum light coming through the filter system. This was confirmed by the additional research as shown above, and looking back it seemed quite obvious.

Quote:

I would definitely want to "Try Before I Buy" due to the cost of a DS. It would have to be a huge improvement for me to spend that kind of $$$$.




In my opinion the substantial improvement in overall performance from reducing the bandwidth via double stacking filters is quite worth the added expense. This thread has covered a lot of ground, but I’d like to get back to and summarize the essence of what happens when you double stack air-spaced H alpha solar filters.

1. When you DS two 0.7 Angstrom filters, the “full width half modulation” (FWHM) bandpass is reduced form 0.7 Angstrom for the single filter to less than 0.5 Angstrom - per George’s graphs and research ~ 0.45 Angstrom.

2. The “tails” of the transmission curve of the filter system are greatly suppressed. This reduces to invisibility out-of-band continuum light coming from the photosphere. Again reference George’s graph’s.

The effect of these changes is to increase contrast of the features seen on the chomospheric disk, which is exceptionally obvious visually (represented by the top image shown below.)

Second, a “pure” view of the limb and prominence or filament/prominence detail is achieved without interference from the brighter “double limb” and disk of the photosphere (bottom image below). If one wishes to observe off-band features such as spicules in more detail, this can be easily achieved by tuning the filter system slightly off-band.

Third, and perhaps more subjectively, I and others have observed the scatter of light seems reduced, rendering the sky background darker. This may also represent a reduction in continuum light transmittance.

A disadvantage of double stacking is the additional expense to get the improved performance. I don’t know of many endeavors where this does not apply.

Another disadvantage is an overall decrease in image brightness. This is due to two factors - the peak transmission is reduced via the two etalons in series, and the presence of two energy rejection filters. The latter cause can be corrected relatively easily by judicious removal of one of the ERFs without any safety concerns - but may void your warranty.* To be honest the decrease in disk brightness was a blessing to me for visual use compared to a single filter (once I remove at least one of the ERF’s), and I do not notice any decrease in the visibility of prominences which are not Doppler-shifted.

Speaking of which - another possible disadvantage for some might be the decrease in the visibility of Doppler-shifted events due to the narrower band-pass. This again can be remedied by a slight de-tuning of the filter system. If this is your main interest, you might be better off with a wider bandpass system or a prominence only filter.

But for most of those who have tried it - “once you’ve double stacked, you won’t go back.”

* Note - Solarscope DS etalons may not have the redundant ERF. If so they can’t be used as a stand alone filter. Solarscope also supplies a second blocking filter with less ND filtering for use when double stacking due to the decreased peak transmission of two etalons, which is nice for imaging as well as observing. I found that removing the RG (red galss) ERF's and replacing them with a single Baader D-ERF (which uses dielectric coatings instead of colored glass) likely does the same thing due to its increased transmission.


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frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: Single Vs. Double Stack new [Re: BYoesle]
      #5614141 - 01/09/13 05:35 PM

Quote:

“once you’ve double stacked, you won’t go back.”



I agree wholeheartedly!


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George9
sage


Reged: 12/11/04

Re: Single Vs. Double Stack [Re: frolinmod]
      #5614463 - 01/09/13 08:26 PM

Nice summary, and I love those photos. George

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