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Baader Solar Continuum filter: single or double stack

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#1 CrossoverManiac

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Posted 18 September 2019 - 02:18 AM

I recently purchased a Baader Solar Film (3.8) for my 6" Newtonian-Maksutov, specifically for the Mercury transit as well as observing sunspots and flares later.  My question is which of the two 1 1/4" Baader Solar Continuum filters would be better for taking time elapses images (lucky imaging) of the Mercury transit with a monochrome camera?

 

https://agenaastro.c...uum-filter.html

 

https://agenaastro.c...uum-filter.html

 

If having both is preferable, then I can buy the other one later after the transit.

 


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#2 MalVeauX

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Posted 18 September 2019 - 06:44 AM

There's literally no point to double stacking wide bandpass filters (it's 10nm bandpass) like that other than to grind down transmission. If you're imaging, you do not want to lose transmission, you want high transmission. If you want a 540nm like that, get a single one. There's really nothing special about 540nm in general, it's mostly because today's scopes are highly corrected for that wavelength, it has less to do with the actual emission/absorption lines of light spectrum. Sodium D for example is 589nm. Otherwise, if you want to save some money, just get a Baader Green 1.25" 500nm imaging filter. Practically the same thing for $49, it passes 500~575nm and still is in that best corrected range for optics. Shorter wavelengths increase contrast of convection cells nicely. You could go even shorter, if seeing permits.

 

Very best,


Edited by MalVeauX, 18 September 2019 - 08:55 AM.


#3 PilotAstronomy

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Posted 18 September 2019 - 11:04 AM

Marty, are you telling me I can use my Optolong 2" green filter that I've been using for night as a good enough stand-in for a continuum filter? I literally just added a continuum filter to a shopping cart...guess I can take it out? Don't know why I haven't thought of this.



#4 MalVeauX

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Posted 18 September 2019 - 11:32 AM

Marty, are you telling me I can use my Optolong 2" green filter that I've been using for night as a good enough stand-in for a continuum filter? I literally just added a continuum filter to a shopping cart...guess I can take it out? Don't know why I haven't thought of this.

There's nothing special to it. Just look at the centered wavelength and the bandpass. None of these are really that narrow band and include different wavelengths, they're not targeting a specific wavelength based on solar emission/absorption lines. You'll find the closest thing to 540nm is 546nm which is a defined absorption line based on Mercury (Hg), after that is 587nm based on Helium (He) and before all of those is 527nm which is based on Iron (Fe). 540nm itself doesn't correspond to anything specific other than its what most optics are highest corrected for as our eyes are most sensitive to green. Using it as a narrowband filter, well, it's not narrowband enough here (10nm) to represent anything specific really. Yes, it's more narrow than a typical green imaging filter. But again, you're not viewing something unique in 540nm since it has a bandpass of 10nm deviating from that centered wavelength and none of those wavelengths correspond to a specific emission/absorption line.

 

Here's the defined classic lines:

 

https://en.wikipedia...raunhofer_lines

 

Here's more info on different lines:

 

http://www.columbia....pectrum Ex.html

 

And here's some interesting tests comparing different filters.

 

https://astro.ecuado...-color-filters/

 

I have both the Solar Continuum filter, Baader green filter, 395nm filter, 430nm filter, etc. And I rarely use the Solar Continuum filter when imaging. 430nm & 395nm give more contrast on cells than 500+nm does. And when it comes to sunspots, these shorter wavelengths actually darken the inside of a sunspot, so it's actually better if you like sunspots to image them with long red filters like 610nm~656nm.

 

Very best,


Edited by MalVeauX, 18 September 2019 - 11:58 AM.


#5 CrossoverManiac

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Posted 18 September 2019 - 12:54 PM

Sodium D for example is 589nm. Otherwise, if you want to save some money, just get a Baader Green 1.25" 500nm imaging filter. Practically the same thing for $49, it passes 500~575nm and still is in that best corrected range for optics. Shorter wavelengths increase contrast of convection cells nicely. You could go even shorter, if seeing permits.

 

Very best,

I have a 1.25" Baader SII filter and a ZWO 1.25" Green (LRGB) filter.  Would those be good enough?



#6 MalVeauX

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Posted 18 September 2019 - 01:14 PM

I have a 1.25" Baader SII filter and a ZWO 1.25" Green (LRGB) filter.  Would those be good enough?

Yes, your S2 will be best when seeing is poor as the longer wavelength will calm the seeing. The ZWO green will be good if seeing is good and/or small image scale is being used to increase contrast of cells so that you'll see granulation better.

 

Take a look at the above links, there are examples of what you can expect at course image scale with various filters.

 

It's up to you, if you think it looks like its worth it.

 

Personally I don't find an advantage with my solar continuum filter. I tend to use 430nm or 610nm or 656nm for the photosphere. 430nm to get high contrast on cells and the long 600+nm lengths for calming the seeing at a fine image scale. Just my take on it.

 

Very best,



#7 BYoesle

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Posted 18 September 2019 - 11:02 PM

The "double stacked" Continuum has been around a while. Some references go back to 2010 or so. It provides a less bright image; and is usually sold for use with the Baader OD 3.8 "photographic" density astrosolar film.

 

 

Shorter wavelengths increase contrast of convection cells nicely.

Hi Marty,

 

I think the improvement is due to increased resolution based on shorter wavelength (when seeing permits), versus improved contrast. The Continuum filter is designed for and excellent for use with refractors, which are usually best corrected for spherochromatic aberration at the e line of 546 nm. This holds true even for APO refractors, which while they have less longitudinal chromatic aberration than an ACHRO, can have as great or even larger amounts of spherochromatic aberration. The Continuum filter does however pass a large amount of contrast-reducing near IR, which should be filtered with a suitable IR blocking filter, and it then functions to its best.

 

Continuum filter spectrum. Top - single filter, middle - double stacked filter.

 

Continuum single v. double.jpg

 

 

Here's the solar spectrum centered on 540 nm (~ Continuum filter) and 10 nm wide. Double stacking might narrow the transmission profile "bandpass" to 7 nm, and would not appear to accomplish much, other than reducing image brightness - which may be of no advantage depending on your imaging devices and processes:

 

Solar Spectrum 540 nm + - 5 nm.jpg


Edited by BYoesle, 18 September 2019 - 11:20 PM.

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#8 MalVeauX

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Posted 19 September 2019 - 07:22 AM

Hi Marty,

 

I think the improvement is due to increased resolution based on shorter wavelength (when seeing permits), versus improved contrast. 

 

 Double stacking might narrow the transmission profile "bandpass" to 7 nm, and would not appear to accomplish much, other than reducing image brightness - which may be of no advantage depending on your imaging devices and processes:

Hey Bob,

 

Agreed, the improvement is secondary to increased angular resolution (reducing diffraction), also the narrower the FWHM increases contrast (not in the case of the 540nm though, apparently closer to 2nm this effect is seen) of the bright areas of a granule versus the darker cooler temperature areas surrounding the edges of the granules themselves.

 

And as you pointed out, stacking the 540nm filter doesn't do much other than grind down transmission.

 

Separate from this though, there's a big difference when imaging sunspots between shorter wavelengths and longer red wavelengths with respect to what can be seen within the umbra of the sunspot. Shorter wavelengths darken it even more, while a red wavelength can show the structures within the umbra. It's interesting to say the least! I wish I understood why this is.

 

Very best,


Edited by MalVeauX, 19 September 2019 - 07:30 AM.


#9 steve t

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Posted 20 September 2019 - 02:52 PM

I recently purchased a Baader Solar Film (3.8) for my 6" Newtonian-Maksutov, specifically for the Mercury transit as well as observing sunspots and flares later.  My question is which of the two 1 1/4" Baader Solar Continuum filters would be better for taking time elapses images (lucky imaging) of the Mercury transit with a monochrome camera?

 

https://agenaastro.c...uum-filter.html

 

https://agenaastro.c...uum-filter.html

 

If having both is preferable, then I can buy the other one later after the transit.

Hi,

Visually on my 6" F8 newtonian, with Baader (5.0) full aperture solar filter. I've tried both a #56 green and the baader solar continuum filters on my 6" F/8 Newtonian.  At 50X - 75X, to my eye, I can just detect a hair more detail in the photosphere using the baader SC. I'm not sure if this effect is more due to just the reduction of light

Regards 

Steve T 



#10 Great Attractor

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Posted 20 September 2019 - 03:59 PM

The Continuum filter does however pass a large amount of contrast-reducing near IR, which should be filtered with a suitable IR blocking filter, and it then functions to its best.


Bob, I believe this is no longer an issue with the current version of Solar Continuum. (Mine from 2011 indeed does leak.)
Here's a reference to a 2015 filter with IR-blocking https://solarchatfor...pic.php?t=16944

#11 BYoesle

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Posted 21 September 2019 - 02:14 AM

Good to know it looks like Baader addressed the issue waytogo.gif


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