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Blocking filter - how it works and what does it do?

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#1 adios

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Posted 28 October 2020 - 11:10 AM

Hi

 

I have been trying to find some information what exactly is the function of blocking filter, and how it is done.

Because if it's only to block some ir/uv it makes no sense, why it can't be replaced by set of suitable filters. So it has to be something more.

 

It all started with recent purchase of ZWO ASI 174 MM, which i'm very pleased with. Finally no artifacts, grid pattern etc. But, B600 blocking filter from Lunt does not cover entire sensor.

Obviously with quark, or regular imaging, let's say of the Moon, there is no problem with vignetting. So B600 is too small. Clear.

 

But, why do i need it for imaging anyway?

 

I was thinking, what if i replace it with regular diagonal, and try imaging this way? I'm afraid to try it, because it may fry the camera sensor 😆

I want to understand how it works, and what is the reason for it in visual use vs imaging. For me saying, that it's necessary part of h-a scope is not enough.

 

Can someone with such knowledge please explain it?



#2 jfrech14

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Posted 28 October 2020 - 11:37 AM

In terms of solar equipment, usually the front filter of a solar telescope is an etalon filter. This works on interference by having two optical components a specific distance apart. Since light has a wavelength, any wavelength of light that "fits" between those two optical components will go through the filter as long as the wavelength is an integer of the distance. So, while hydrogen alpha will go through, it isn't the only wavelength that will go through. The blocking filters are used to block the rest of the wavelengths that go through. So, the etalon is a very narrow filter that gives you incredibly high contrast of the hydrogen alpha line, but the other wavelengths of light will ruin that contrast and also possibly cause the sensor to saturate or something else that is undesired or dangerous. Etalon transmission is typically very high, so if a lot of light is still going through that etalon, it is still harmful.

The blocking filter is a filter that is only narrow enough to pass the hydrogen alpha but block the other wavelengths. There are other ways that this can be done, like using an energy rejection filter, but those are usually only used on larger telescopes that aren't made for solar astronomy like SCTs and they will most likely not work nearly as well.


This link is to an image that shows how a bunch of wavelengths go through the etalon. The blocking filter is to select only one of them.
https://www.cloudyni...ml_143c4c83.png


Edited by jfrech14, 28 October 2020 - 11:50 AM.

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#3 TOMDEY

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Posted 28 October 2020 - 12:06 PM

The blocking filter removes the unwanted harmonics passed by the Etalon, only permitting the H-Alpha (or other selected transition) through.    Tom



#4 Gregory Gross

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Posted 28 October 2020 - 12:23 PM

See BYoesle's excellent overview of how etalons work: part one and especially part two, the latter having a nice graph which illustrates how the blocking filter participates in a chain of optical elements in an H-alpha solar telescope.


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#5 Gregory Gross

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Posted 28 October 2020 - 12:30 PM

Ian Morrison's H-alpha Solar Telescopes: An In-depth Discussion and Survey is also an excellent source of information.


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#6 rkinnett

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Posted 28 October 2020 - 12:51 PM

Regarding the question of why fancy solar imaging equipment is needed when imaging the sun, understand that if you point your scope at the sun with no filter at your aperture, you collect an immense amount of solar energy.. 1000 W/m2!  For a 100mm refractor, for example, that equates to 30W entering the scope.  Roughly half of that 30W is UV/IR while the other half is in the visible range.  If you were to try a standard UV/IR-cut filter designed for DSO work and nothing else in front of your camera, that UV/IR filter will absorb or reject 99% of the UV/IR energy, converting it into heat at the filter and in the scope tube.  The filter itself will get very hot and will likely be ruined, and your sensor would still receive about 15W (passed visible light) concentrated onto the sensor.  That's a lot of heat!  That will indeed very quickly fry your sensor. 

 

In short, Etalons, Quarks, blocking filters, Herschel wedges, etc, are all designed to safely reject and/or absorb all that extra energy beyond what you can safely focus onto your camera sensor.  For Ha/Ca/etc narrowband filtering, it's no small feat of engineering to safely reject >99% of total solar energy, passing only light within a very narrow band of interest and attenuating that sliver of light to safe yet usable intensity levels for your camera.. that's why solar gear is so expensive.


Edited by rkinnett, 28 October 2020 - 02:37 PM.


#7 Stardust Dave

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Posted 29 October 2020 - 03:58 AM

"See BYoesle's excellent overview of how etalons work: part one and especially part two, the latter having a nice graph which illustrates how the blocking filter participates in a chain of optical elements in an H-alpha solar telescope"

 

Thanks for the share , I'd often wished it explained in that way.

Good topic .smile.gif


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#8 adios

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Posted 29 October 2020 - 03:15 PM

Thanks, it's now much clearer how this stuff works. I don't like to not know how devices i use work :)


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#9 briansalomon1

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Posted 30 October 2020 - 08:31 AM

Thanks, it's now much clearer how this stuff works. I don't like to not know how devices i use work smile.gif

I agree. For me it's almost as interesting to understand how these systems work as looking at the images themselves. BYoseley's explanation was about as thorough as you'll get. Manufacturers won't share very much information on their blocking filters because their design and how they are made is an expensive process and competitors may want to copy them.



#10 ASTROTRUCK

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Posted 31 October 2020 - 11:25 PM

i was of the impression that using an external ha etelon at the objective end of my refractor allows me to place the uvir filter either ahead of the bf or after it - am i wrong that either/or is correct  or am i wrong ??



#11 bigdob24

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Posted 01 November 2020 - 08:11 AM

Question while we’re on BF.

‘Is the BG38 “blue filter” the only filter in the BF system that filters the light coming in ?

Is it just a regular diagonal with a BG38 filter in front or is there more magic that happens after the BG38 in the BF as the light passes thru.



#12 BYoesle

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Posted 02 November 2020 - 08:13 AM

The BF also contains the essential "order selection filter." This is the ~ 4-8 Angstrom bandpass blocking filter for the etalon harmonic side-peaks.



#13 bigdob24

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Posted 02 November 2020 - 08:49 AM

So the BG38 filters IR/UV to a safe level for observing and determines the brightness of the view thru the BF in a nutshell.

Im just glad it all works and allows me to observe our day star.



#14 ASTROTRUCK

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Posted 02 November 2020 - 12:44 PM

I was of the impression that using an external ha etelon at the objective end of my refractor allows me to place the uvir filter either ahead of the bf or after it - am i wrong that either/or is correct  or am i wrong ??

 

above is a repost

 

seen both on sites but want to be sure - i have always placed after the bf but balance becomes an issue - can  i place the ircut before the bf without worry of heat cracking the ircut ??

 

thanks



#15 briansalomon1

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Posted 02 November 2020 - 04:41 PM

My quark manual says "for telescopes under ~120mm aperture a UV/IR cut filter can be employed in front of the diagonal. The UV/IR cut filter reflects light back out the front of the telescope, reducing temperatures inside."

 

It also says "The UV/IR cut filter must be the first optical element to receive concentrated light".

 

Here's a technical note from Daystar http://www.daystarfi...-for-Refractors

 

I don't see why you would need (or want) to add a UV/IR cut filter in an etalon/blocking filter system.



#16 bigdob24

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Posted 02 November 2020 - 08:09 PM

My quark manual says "for telescopes under ~120mm aperture a UV/IR cut filter can be employed in front of the diagonal. The UV/IR cut filter reflects light back out the front of the telescope, reducing temperatures inside."

 

It also says "The UV/IR cut filter must be the first optical element to receive concentrated light".

 

Here's a technical note from Daystar http://www.daystarfi...-for-Refractors

 

I don't see why you would need (or want) to add a UV/IR cut filter in an etalon/blocking filter system.

They must have changed if it says that? I used the UV/IR cut filter with my quark on a Williams Optics 132mm and never had any trouble. Spoke with Jen in depth about this before I bought it and was assured it was safe for me and my equipment.

I was thinking she told me they work up to around 150mm

I think these manufacturers are leaning on the safe side with filtration , my DS has an ERF built in and my scope has one also, I guess the only thing wrong with being redundant with these filters is loss of light transmission.

I do understand and appreciate safety when selling a filter to stare at the sun with.



#17 briansalomon1

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Posted 02 November 2020 - 08:37 PM

I have a fairly new quark. Many users report using UV/IR cut filters on refractors well above the 120mm spec without any issues. Since UV/IR cut filters are essentially mirrors they are going to function regardless of aperture. I think the limiting factor is the heat load the filter can carry without damage to the coatings or substrate, and after all, the UV/IR cut filter can be replaced...


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#18 BYoesle

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Posted 03 November 2020 - 12:30 PM

 

I don't see why you would need (or want) to add a UV/IR cut filter in an etalon/blocking filter system.

 

You do this for several reasons, not the least of which is to protect the downstream filters and optics from thermal loading and cycling. This is especially important for the blocking filter ITF and order selection filter longevity. The rust issues with ITF's is a direct result of thermal loading and cycling.

 

The best place for the ERF and UV/IR blocking is ahead of the objective and/or etalon. These will also obviously be the most expensive, but keep these unwanted wavelengths - especially IR - out of the optical system. The next best place is after the objective using a sub-diameter ERF with UV/IR blocking, but making sure any reflected energy is not refocused at the objective. The next best place is to place the UV/IR blocking filter before the DayStar or Solar Spectrum filter. Here it will receive more concentrated energy, and possibly create local heating effects from the non-reflected wavelengths. For the quark it might be optional at smaller apertures given the cost of replacement. But for a $4000.00 + filter system, it is my opinion that a gram of prevention is worth a kilogram of cure.

 

Most UV/IR blocking filters have over 90% transmission at 656 nm, and therefore minimal effect on image brightness.



#19 geothomas

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Posted 05 November 2020 - 01:36 PM

I have a 130mm EON triplet. Could I put something like a Thousand Oaks full aperture solarlite filter up front of the scope and use that as my blocking filter for the daystar quark to get a higher focal ratio?



#20 BYoesle

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Posted 05 November 2020 - 03:00 PM

No. That is a continuum filter for observing the photosphere, and will completely prevent you from seeing the chromosphere. See here. The filter alone does not affect the focal ratio. That is solely the result of aperture and effective focal length.


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#21 geothomas

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Posted 05 November 2020 - 03:44 PM

No. That is a continuum filter for observing the photosphere, and will completely prevent you from seeing the chromosphere. See here. The filter alone does not affect the focal ratio. That is solely the result of aperture and effective focal length.

At 130mm, I want to prevent too much heat entering the scope, is it sufficient to build a blocker with a smaller aperture to place up front or do you have another suggestion?



#22 BYoesle

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Posted 05 November 2020 - 04:25 PM

Go full aperture with this.



#23 ASTROTRUCK

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Posted 07 November 2020 - 11:14 AM

i use the uvir to prevent rust - my coronado etelon 60 is over 5 years so not sure if it includes uvir.

placing it before the bf leads to balance issues as all the weight is after the filter which can throw the system off balance at later part of day when bf is sideways leading to offside weight -

but before is safer so who needs rust



#24 BYoesle

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Posted 07 November 2020 - 06:50 PM

The RG630 ERF glass of the Coronado's (and DayStar's red and yellow ERFs) absorbs UV (and some visible wavelengths below about 600 nm). It does nothing to reduce IR since it doesn't have IR blocking coatings like Lunt and BelOptik ERFs provide. The Baader DERF is clear BK7 with both UV/IR blocking, but doesn't block much in the visible wavelengths.




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