40 replies to this topic

[hopskipson]

Hi Stuart,

 

You are fine.  As Bob pointed out "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."  If you want to keep unwanted IR/UV from entering your telescope, especially for Cassegrain systems, you need to have an ERF before the objective.

He also pointed out "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." So the next best place is before the Quark as far up the light cone without restricting it.  So in front of the diagonal is good.  Close to the Quark is not as good, but since you have it in front of the diagonal, you're fine.

[StuartT]

Hi Stuart,

 

You are fine.  As Bob pointed out "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."  If you want to keep unwanted IR/UV from entering your telescope, especially for Cassegrain systems, you need to have an ERF before the objective.

He also pointed out "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." So the next best place is before the Quark as far up the light cone without restricting it.  So in front of the diagonal is good.  Close to the Quark is not as good, but since you have it in front of the diagonal, you're fine.

Great! Thanks so much

[BYoesle]

An even better filter might be a standard nighttime H alpha filter like the Baader 35 nm. It will block all wavelengths below about 630 nm, where a significant amount of solar energy output lies.

 

 

For ultimate protection, the BelOptik ITF replacement filter will provide a similar UV-VIS cutoff to the 35nm Ha filter, but incorporates a KG3 element for long IR absorption, in addition to dielectric coating for near IR:

 

 

Be advised however that the transmission is lower, reducing both the image brightness and the heat load to the filter system, which may affect the temperature setting for being on-band and possibly requiring additional adjustment of the temperature setting.

 

 

 

[StuartT]

An even better filter might be a standard nighttime H alpha filter like the Baader 35 nm. It will block all wavelengths below about 630 nm, where a significant amount of solar energy output lies.

 

656 35nm.jpg

11592.jpg

 

For ultimate protection, the BelOptik ITF replacement filter will provide a similar UV-VIS cutoff to the 35nm Ha filter, but incorporates a KG3 element for long IR absorption, in addition to dielectric coating for near IR:

 

ITF_Ha_1.jpg

 

Be advised however that the transmission is lower, reducing both the image brightness and the heat load to the filter system, which may affect the temperature setting for being on-band and possibly requiring additional adjustment of the temperature setting.

Interesting. I guess I was neglecting the heating caused by visible wavelengths! Thank you

Edited by StuartT, 11 March 2022 - 10:53 AM.

[David Knisely]

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?

 

An etalon is a pair of very flat plane-parallel partially reflective plates of a certain separation that produces a large number of very narrow transmission spikes at different wavelengths (sometimes referred to as a "comb filter" for obvious reasons).  The blocking filter is used to select only one of those peaks and block the others.  In some cases (like the DayStar filters), they use blocking and trimming filters to select the H-alpha wavelength, with one passing light to the red side of the selected wavelength spike and the other passing only wavelengths to the blue side of the particular spike.  I like to use the diagram below to illustrate what an etalon and the blocking filter may do.

Attached Thumbnails

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[TareqPhoto]

I am almost going to buy a filter that i hope it is the right filter to be as ERF or blocking filter whatever, but i think i can't do the same with CaK or call it K-Line filter then.

[StuartT]

An etalon is a pair of very flat plane-parallel partially reflective plates of a certain separation that produces a large number of very narrow transmission spikes at different wavelengths (sometimes referred to as a "comb filter" for obvious reasons).  The blocking filter is used to select only one of those peaks and block the others.  In some cases (like the DayStar filters), they use blocking and trimming filters to select the H-alpha wavelength, with one passing light to the red side of the selected wavelength spike and the other passing only wavelengths to the blue side of the particular spike.  I like to use the diagram below to illustrate what an etalon and the blocking filter may do.

Great image! Thanks.

 

So if I buy the Baader 35nm recommended by BYoesle that kind of makes the blocking filter in the Quark redundant? Because the other harmonics will already have been blocked

[TareqPhoto]

Great image! Thanks.

 

So if I buy the Baader 35nm recommended by BYoesle that kind of makes the blocking filter in the Quark redundant? Because the other harmonics will already have been blocked

That is why i will go with something else, once i get things and i start to use my Quark then i can talk about what i have or got, for now i keep reading and watching, you want to buy Baader 35nm go ahead and let's know the result.

[BYoesle]

So if I buy the Baader 35nm recommended by BYoesle that kind of makes the blocking filter in the Quark redundant? Because the other harmonics will already have been blocked

NO ABSOLUTELY NOT!!! The diagram posted by David is a very simplistic view of an H alpha filter system, and in NO WAY represents a real filter system!

 

The Baader 35nm filter is acting as an internal sub-diameter ERF (as an alternative to an internal sub-diameter UV/IR filter ERF) - NOT A BLOCKING FILTER! The blocking filter itself has an order selection filter usually with a bandpass only 6-10 Angstroms wide, along with additional UV and IR blocking. The Baader 35 nm H alpha filter has a 350 Angstrom bandpass, and will have no long-IR blocking!

 

The free spectral range (FSR) is the distance between the etalon harmonic peaks. The blocking filter contains an order selection filter that has a bandpass narrow enough to block off-band harmonic peaks, only allowing the peak of interest through and blocking the others:

 

 

David: You may also want to modify your diagram by combining "changing the plate mirror spacing" and "changing the temperature of the filter," which are essentially the same thing. The third way is to change the refractive index of the etalon gap, which is commercially done by changing the air pressure inside the etalon chamber. 

 

I am almost going to buy a filter that i hope it is the right filter to be as ERF or blocking filter whatever, but i think i can't do the same with CaK or call it K-Line filter then.

That is correct, but it is not a "ERF or Blocking filter whatever."

 

Listen up everyone. If you don't fully understand what these filters are called, what these filters do, and where they need to be placed, you shouldn't be mucking around with these systems in the first place.

 

The ERF for a H alpha filter generally will block UV at 394 nm, and will therefore not work for CaK or K line use. In that case you'd need a KG3 filter for IR blocking, and a 400nm + short-pass filter.

Edited by BYoesle, 11 March 2022 - 06:58 PM.

[David Knisely]

Great image! Thanks.

 

So if I buy the Baader 35nm recommended by BYoesle that kind of makes the blocking filter in the Quark redundant? Because the other harmonics will already have been blocked

Because I don't know what the separation of the peaks of the DayStar etalons are (what is known as "the Free Spectral Range" (FSR)), I can't tell you whether a 35 nm passband filter would be narrow enough to block the other peaks or not.  I would stay with whatever blocker/trimmer filters that are used in the Quark (plus a potential UV/IR ERF if the aperture is over 80mm).  Clear skies to you.

[David Knisely]

NO ABSOLUTELY NOT!!! The diagram posted by David is a very simplistic view of an H alpha filter system, and in NO WAY represents a real filter system!

 

The Baader 35nm filter is acting as an internal sub-diameter ERF (as an alternative to an internal sub-diameter UV/IR filter ERF) - NOT A BLOCKING FILTER! The blocking filter itself has an order selection filter usually with a bandpass only 6-10 Angstroms wide, along with additional UV and IR blocking. The Baader 35 nm H alpha filter has a 350 Angstrom bandpass, and will have no long-IR blocking!

 

The free spectral range (FSR) is the distance between the etalon harmonic peaks. The blocking filter contains an order selection filter that has a bandpass narrow enough to block off-band harmonic peaks, only allowing the peak of interest through and blocking the others:

 

Image6.jpg

 

David: You may also want to modify your diagram by combining "changing the plate mirror spacing" and "changing the temperature of the filter," which are essentially the same thing. The third way is to change the refractive index of the etalon gap, which is commercially done by changing the air pressure inside the etalon chamber. 

 

That is correct, but it is not a "ERF or Blocking filter whatever."

 

Listen up everyone. If you don't fully understand what these filters are called, what these filters do, and where they need to be placed, you shouldn't be mucking around with these systems in the first place.

 

The ERF for a H alpha filter generally will block UV at 394 nm, and will therefore not work for CaK or K line use. In that case you'd need a KG3 filter for IR blocking, and a 400nm + short-pass filter.

My diagram was never intended to show anything close to what the layout of an actual ultra-narrowband etalon-based solar filtering system was to be, but merely to explain what the role of the etalon is and how a blocking filter selects the one desired passband (I had another different image that I used to describe a full observational setup in my presentations on H-alpha observing).  While changing the temperature indeed does change the plate spacing, the spacing of the etalon plates can also be adjusted mechanically.   The Fabry-Perot interferometer I used in my lab courses when I was a Physics undergraduate tuned by fine physical adjustment of the plate spacing, which is the reason I stated things in the above slide the way I did.  I'm not certain, but I believe that the etalon in my PST is mechanically compressed to change the plate spacing, but right now, I don't have the desire to take the thing apart and potentially ruin it just to verify that speculation.  At one point, Daystar offered its T-Scanner that was tuned via tilt mechanism of the entire filter stack to compensate for both temperature variations and Doppler shifts (I have the 0.7 angstrom T-Scanner, but I no longer use it due to blocker/trim filter failures).  With Daystar's other higher-end H-alpha filters, they only used heating (well above ambient temperature) to establish and maintain the location of the passband long-term.  At one point, they actually combined the two methods into one unit, so the temperature could be varied to help keep the unit on-band, and the tilt could be used to allow much more rapid tuning than changing the temperature yielded.  I was disappointed when they didn't incorporate this into the Quark, but I suppose they had enough trouble just getting the cost of the unit down in the first place.  Clear skies to you.  

Edited by David Knisely, 11 March 2022 - 07:44 PM.

[BYoesle]

Hi David,

 

The FSR of the DayStar filters per their White Paper on the Quantum is about 26 A, or 2.6 nm. Their blocking filter is stated to have a 10 A bandpass = 1 nm. So one can see that a nighttime H alpha filter could not be employed as an order selection filter the way Stuart was thinking.

 

You are of course correct about mechanical gap spacing changes. Mechanical pressure tuning (patented by Coronado in the early 2000's) is used in the PST, and is called "RichView" tuning in the other Coronado etalons.

 

Therefore the three tuning methods are tilt, etalon gap refractive index changing (air density changes via pressure changes), and etalon gap space changing - which includes mechanical pressure changes and temperature changes as subcategories of etalon gap spacing change methods. The "RichView" etalons also often incorporate both tilt and mechanical pressure tuning methods. The Solar Spectrum Suna filter uses both tilt and heating for tuning, which has the advantage of both temperature stability and the ability to rapidly shift the central wavelength of the filter for Doppler shifted events.

 

Their is also piezoelectric etalon tuning which minutely changes the etalon gap spacing (and likely what was used in the lab classes your referred to), but this is not found in commercial H alpha solar etalons. They are used extensively in professional optical etalons.

Edited by BYoesle, 11 March 2022 - 08:41 PM.

[David Knisely]

Hi David,

 

The FSR of the DayStar filters per their White Paper on the Quantum is about 26 A, or 2.6 nm. Their blocking filter is stated to have a 10 A bandpass = 1 nm. So one can see that a nighttime H alpha filter could not be employed as an order selection filter the way Stuart was thinking.

 

You are of course correct about mechanical gap spacing changes. Mechanical pressure tuning (patented by Coronado in the early 2000's) is used in the PST, and is called "RichView" tuning in the other Coronado etalons.

 

Therefore the three tuning methods are tilt, etalon gap refractive index changing (air density changes via pressure changes), and etalon gap space changing - which includes mechanical pressure changes and temperature changes as subcategories of etalon gap spacing change methods. The "RichView" etalons also often incorporate both tilt and mechanical pressure tuning methods. The Solar Spectrum Suna filter uses both tilt and heating for tuning, which has the advantage of both temperature stability and the ability to rapidly shift the central wavelength of the filter for Doppler shifted events.

 

Their is also piezoelectric etalon tuning which minutely changes the etalon gap spacing (and likely what was used in the lab classes your referred to), but this is not found in commercial H alpha solar etalons. They are used extensively in professional optical etalons.

Well, no, the Fabry-Perot interferometer I used in my undergraduate labs in the 1970's was an almost antique unit that dated back probably well before 1960, and used a very fine mechanical adjustment for the plate spacing (Piezoelectric was way way in the future).  I never really appreciated the F-P Interferometer until much later when I finally learned how the DayStar H-alpha filters actually worked back in the 1980's just before I bought my first T-Scanner.  I would get the blockers and trimmers on my T-Scanner replaced except that to do that would cost almost as much as a brand new Quark would.  Clear skies to you.

[StuartT]

 

Listen up everyone. If you don't fully understand what these filters are called, what these filters do, and where they need to be placed, you shouldn't be mucking around with these systems in the first place.

 

Bob, thanks for explaining. But I am sorry I provoked such a reaction. Let me be clear, I was not for one moment suggesting dismantling the Quark! I am mechanically inept and wouldn't dream of doing anything of the sort! I was merely asking if the Baader filter you suggested would block the unwanted harmonics (to check my understanding, not to go 'mucking about' as you put it). But I now have an answer.

Edited by StuartT, 12 March 2022 - 07:34 AM.

[BYoesle]

Hi Stuart,

 

Sorry if I came across a bit brusquely; I did not intend my comments to be taken personally. However, in concert with possibly taking David's conceptual diagram literally, the other "i hope it is the right filter to be as ERF or blocking filter whatever" comment, made it clear there was a lack of understanding of these filter systems and their components.

 

While you may not have actually considered removing the "redundant" blocking filter from your Quark, others who lack sufficient knowledge and understanding of how such filter systems function might decide to "experiment" with such an implementation.

 

For example, similar to folks who think they can do solar H alpha on-the-cheap by using a white light solar filter combined with a nighttime H alpha filter - if someone purchased a double stacking etalon and decided not to purchase a more expensive blocking filter and instead use a much less expensive nighttime H alpha filter as a "blocking filter whatever," they would potentially encounter permanent eye damage at the first look through such a system:

 

 

 

 

Solar filter systems are inherently dangerous if they are not properly thought out and implemented.

Edited by BYoesle, 12 March 2022 - 12:49 PM.

[StuartT]

Hi Stuart,

 

Sorry if I came across a bit brusquely; I did not intend my comments to be taken personally. However, in concert with possibly taking David's conceptual diagram literally, the other "i hope it is the right filter to be as ERF or blocking filter whatever" comment, made it clear there was a lack of understanding of these filter systems and their components.

 

While you may not have actually considered removing the "redundant" blocking filter from your Quark, others who lack sufficient knowledge and understanding of how such filter systems function might decide to "experiment" with such an implementation.

 

For example, similar to folks who think they can do solar H alpha on-the-cheap by using a white light solar filter combined with a nighttime H alpha filter - if someone purchased a double stacking etalon and decided not to purchase a more expensive blocking filter and instead use a much less expensive nighttime H alpha filter as a "blocking filter whatever," they would potentially encounter permanent eye damage at the first look through such a system:

 

DK crop.jpg

 

Safety.jpg

 

 

Solar filter systems are inherently dangerous if they are not properly thought out and implemented.

No worries Bob.

 

I really appreciate all your posts on this subject. I have a lot to learn here.

 

But safety is, of course, paramount in this branch of the hobby, and you are right to emphasize it. Better over zealous, than under zealous!

Edited by StuartT, 12 March 2022 - 02:36 PM.