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Tarnished Blocking Filter Lunt60THa

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#1 KaiserT

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Posted 07 February 2019 - 02:23 AM

A known problem of the Lunt Blocking Filter in the diagonal is the tarnishing of the glass surface. While Lunt has promised a replacement, I am not sure if the problem will recur on the new glass. I have read that it is due to salt-laden moisture (although I live far from the sea); some even claim it is some kind of fungus. Yet another post I came across said that the deterioration of acrylic (?) material is caused by IR and UV rays which it encounters over time.

Does anyone know what causes this tarnishing, and whether Lunt has licked this issue in the new products?
 

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  • Blocking Filter Blue Lens.jpg

Edited by KaiserT, 07 February 2019 - 02:58 AM.

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#2 PETER DREW

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Posted 07 February 2019 - 05:59 AM

I think this is the ITF filter rather than the blocking filter. These are included to reduce the radiation on the blocking filter. As they seem to be so prone to failure, Lunt are prepared to replace them free of charge.


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#3 KaiserT

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Posted 07 February 2019 - 07:26 AM

Peter, any idea what causes the deterioration? Beats me why a $3,628 telescope would be afflicted by this silly problem with a glass IR filter.

#4 bob71741

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Posted 07 February 2019 - 07:54 AM

The filter is just plain colored glass. It is NOT an ITF type filter, nor an acrylic filter(not plastic); I believe that the acrylic popped into the picture when acrylic cleaners were used to clean off the mess on the glass.

 

I believe that the problem is of an organic nature; fungi or materials used in filter processing that fungi or bacteria use. I unfortunately cleaned off my filter; I should have first looked at it with a microscope.

 

I live in the desert of Arizona and it took eight years for the problem to occur for my telescope while sister telescopes that had serial numbers 2 or 3 from mine had the problem five years before me. I also was an  everyday observer of the sun for the first five years; maybe the IR/UV did not allow the "stuff" to grow.


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#5 vincentv

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Posted 07 February 2019 - 11:30 AM

This filter sits behind the ERF, there should be little to no UV and NIR at this point.

We don't know the exact type of filter. From Schott's literature:

 

For the optical filter glass types BG42, UG5, UG11, BG39, S8612, S8022 and S8023 a change in the glass surface is possible after a few months of normal storage.

I believe the problem is humidity. If it happened once it will happen again been there done that. My solution consists of sticking the whole scope in a sealed case (like a pelican) along with reusable silica. It has been a month so no long term results yet.

 

Edit: Lunt refers to this filter as the BG filter and it matches Schott's BG line of glass. Some BG types are more resilient to humidity.

It is easy to polish with PlastX.


Edited by vincentv, 07 February 2019 - 11:33 AM.

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#6 KaiserT

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Posted 07 February 2019 - 06:56 PM

Vincentv: Lunt has confirmed to me in a mail that the glass is type BG38.
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#7 BYoesle

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Posted 07 February 2019 - 09:09 PM

This link may provide some enlightenment ;-)

 

Ultimately the BG38 element was intended to replace the ITF originally used by Coronado and then Lunt. Some people have been able to clean, and then use gentle polishing to restore the filter for a period of time.

 

Lunt offers a free drop-in replacement BG38 filter for these as well. Easy to do.

 

The BelOptik ITF replacement filter might be the ultimate replacement, but is not inexpensive.


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#8 vincentv

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Posted 08 February 2019 - 10:16 AM

After going through a couple of datasheets I conclude:

-With 50% transmission in halpha the bg filter clearly has a role controlling image brightness

-It blocks some IR so it might also have a safety aspect

-It's one of the more resilient BG offerings and still fails when uncoated

 

As for replacements beloptik's smaller ITF alternative is smaller than the opening, it will simply fall through. Oliver might be able to cut the larger one to size or custom fab one -> $$. His uv/ir on kg3 sounds tempting though, especially with double stack and/or binoviewers. I would contact him first to make sure it is safe. I haven't heard of anyone taking this route so safety is unkown.

 

I see edmund sells a coated bg-38: https://www.edmundop...s-Filter/40246/

I'll post the lunt filter's size later to compare.

 

Any one knows why Lunt has chosen the uncoated version?

 

Edit: Lunt's BG is 20x2.5mm. Edmund's option appears to be too large and too thick


Edited by vincentv, 08 February 2019 - 12:08 PM.


#9 KaiserT

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Posted 08 February 2019 - 12:26 PM

New Picture 2.jpg

A few points about the BG38 IR filter in Lunt solar scopes. 1) It measures 20mm in diameter and 2mm in thickness (personally measured with  Vernier Calipers). 2) The deposit can be easily removed by a non-abrasive cleaner. I used Silvo cleaner (good for metals incl gold, mirrors, etc) on my terribly tarnished filter. It is as good as new (See attached picture). 3) The tarnish is actually a deposit of microscopic salts/minerals in moisture, thoroughly 'cooked' by IR radiation. I doubt if coatings would help. 4) The deposit is likely to recur sooner or later, though moisture control might help. 5) Schott's BG60, BG64 and BG67 are 'high humidity resistant' filters but apparently do not have the right specs required for use in solar scopes. 6) Lunt would do well to include two or three spare BG38 filters with the telescope (along with fixing instructions). Each costs $3.75 only.


Edited by KaiserT, 08 February 2019 - 11:39 PM.

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#10 vincentv

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Posted 08 February 2019 - 12:30 PM

The spares would probably go bad during storage. Schott's documentation does say that coatings help with longevity.



#11 KaiserT

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Posted 08 February 2019 - 05:19 PM

The spare BG38 IR filter must be stored with silica gel to prevent/minimise tarnishing.

#12 davidmcgo

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Posted 08 February 2019 - 05:56 PM

Just pop it out and polish with Meguireā€™s PlastX

 

I do it as soon as I see anything on the edge.  The BG is hygroscopic and leaves mineral deposits on the surface.

 

Dave


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#13 KaiserT

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Posted 09 February 2019 - 09:12 AM

Quote from Lunt Filter Safety Systems:
https://luntsolarsys...equipment-care/

"To ensure long term stability and filter safety that is required in the production of a solar telescope system, Lunt Solar Systems is pleased to announce newly designed, Induced Transmission Filters (ITF). A vital part of the safety and performance of these systems, the ITF is used to block unwanted out of band transmissions. Notably providing a high transmission at the desired wavelength."
 

"The metal layer used in the filter works to block IR to the safety levels required for visual use. Without the induced part (matching metal oxide layers), the metal layer would simply act as a neutral density filter. Generally these filters were used primarily in laser systems where thermal loading and cycling are kept to a minimum. Under these ideal conditions the filter can provide years of service."
 

Do I correctly understand that: BG38 IR Blocking Filter in the ITF has 'metal oxide layers'? If so, won't these layers get scrubbed off if a cleanser is used (as I and many others have done)? Would DIY cleansing render this filter into a simple 'neutral density filter', and thus be unsafe?


 


Edited by KaiserT, 09 February 2019 - 10:07 AM.


#14 KaiserT

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Posted 09 February 2019 - 10:55 AM

An old Cloudy Nights post by Andy Lunt:

"An ITF is a cemented filter. By definition it is a matched dielectric layer on opposite sides of a very thin metal layer (DMD filter). ITF's are designed to be fairly narrow band filters, but by default, due to their metal layer, are IR blockers. They are used to block the out of band transmissions of the blocking filter which blocks the out of band harmonics of the etalon.

Typically ITFs have been used in climate controlled laser applications where heat is not an issue. I have found that standard ITF's will degrade at the glue interface due to expansion and contraction of the optical elements if exposed to a heat load.

In the LS60T system the first filter behind the objective is in fact a UV/IR rejection filter (a TRUE ERF!). The filter glass (RG630) absorbs UV radiation. This filter glass is coated with what is basically a multi-layer dielectric hot mirror coating. This is a proprietary coating which reflects ~99% of all harmful IR radiation. The hot mirror is a "hard" coating and is non-degrading.

We do not rely on an ITF filter system to remove IR radiation. Although we do use what one could call an ITF in the system, we place it in a position that allows it to do its designed job, narrow band blocking. We do not utilize it in a position that also requires that it reflect and absorb IR.

ITF's are generally "soft" coatings and it is my opinion that these filters should be used in a non-heat load application. Our "ITF" has been designed to meet or exceed MIL Spec for environmental stability. It was designed for Lunt specifically for our application.

We do utilize IR absorptive filter glass, but the glass is in a position where it only sees ~1% of the total heat load. This filter glass covers to a much deeper portion of the spectrum than an IR dielectric could. It is important to note however, that this filter glass could accept 100% of the designed heat load for an extended period of time if the front filter should fail.

Safety. Lunt telescopes and filters have redundant IR and double UV blocking systems. Our filters and coating will never degrade.

The placement of filters in a solar system is very important. It is important to understand where and how filters are used in a solar telescope when safety is a fundamental concern."

#15 BYoesle

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Posted 09 February 2019 - 05:38 PM

 

Do I understand correctly that the BG38 IR Blocking Filter in the ITF has 'metal oxide layers'? If so, won't these layers get scrubbed off if a cleanser is used (as I and many others have done)? Would DIY cleansing render this filter into a simple 'neutral density filter', and thus be unsafe?

 

No. The BG38 filter glass REPLACED the ITF filter in the Lunt blocking filter systems several years ago, and the first post from Andy Lunt you have cited is no longer applicable.

 

The "what one could call and ITF in the system... for narrow band blocking" referred to in the second post seems to be referring to the order selection blocking filter which removes the out-of-band harmonics of the etalon.


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#16 KaiserT

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Posted 09 February 2019 - 10:20 PM

"In the LS60T system the first filter behind the objective is in fact a UV/IR rejection filter (a TRUE ERF!). The filter glass (RG630) absorbs UV radiation. This filter glass is coated with what is basically a multi-layer dielectric hot mirror coating. This is a proprietary coating which reflects ~99% of all harmful IR radiation. The hot mirror is a "hard" coating and is non-degrading."

The above quote is from Andy Lunt, in which he describes the dual function ERF that blocks upto 99% UV & IR radiation, the latter through 'hot mirror hard coating'. I wonder why Lunt cannot use a similar, non-degrading coating on the secondary IR BG38 filter glass in the diagonal? That might lick the tarnishing problem altogether.



#17 BYoesle

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Posted 10 February 2019 - 12:15 PM

The Lunt RG630 ERF is IR blocked, but I have seen no specs on it. IR "A" is defined as 780 - 1400 nm,  IR "B" needs to be blocked as well 1400 - 3000 nm, and this IR B is what the original ITF was used for (along with being on RG630 for additional UV blocking).

 

The Scott BG38 seems to be used more for controlling image brightness, with a transmission of about 45% at 656 nm. It also has some IR A blocking, but little IR B blocking.

 

One of the best ways to prolong blocking filter life is proper storage. I use laboratory grade MIL spec desiccants and vacuum sealing bags, which seem to have worked well so far.



#18 vincentv

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Posted 12 February 2019 - 09:12 PM

The Scott BG38 seems to be used more for controlling image brightness, with a transmission of about 45% at 656 nm. It also has some IR A blocking, but little IR B blocking.

I took another look at the datasheet this morning. The specs are for 1mm thick glass but Lunt uses 2 or 2.5mm*. That means the transmission is somewhere between 15% and 22%.

Blocking from 1200nm to 2700nm is pretty much non-existent.

 

 

*: KaisetT measured 2, I measured 2.5.


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#19 bob71741

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Posted 13 February 2019 - 12:17 PM

"...KaisetT measured 2, I measured 2.5."

 

My old filter measured 3.0 mm; this filter was used in the second production run Lunt made in 2008, which was shipped at the end of November. Seems like they put in whatever is available without regards to consistent brightness in their production runs.


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#20 rigel123

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Posted 13 February 2019 - 03:38 PM

After seeing this thread I decided to check my Lunt LS60T that I got in 2012 and so far so good! I do keep the pictured dessicant in the case with it whenever not in use.

Lunt Filter.JPG

#21 rigel123

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Posted 13 February 2019 - 03:39 PM

And the dessicant I use:

Dessicant.JPG

#22 KaiserT

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Posted 15 February 2019 - 04:44 AM

In Lunt solar telescopes, 99% UV radiation is said to be blocked by the ERF (RG630 glass), and the remaining 1% or so is blocked by the UV filter just above the Blocking Filter that screens out the unwanted harmonics. Does anyone know what type is the 'secondary' UV filter inside the diagonal (Filter Glass 2 in diagram)? I am talking of the red glass visible through the eyepiece. Does it have a Schott glass code like BG38 and RG630?
 

A related question: Is this 1% UV radiation reacting with humidity sufficient to tarnish the BG38 IR filter located inside the diagonal? If the residual 1% UV is the cause of all tarnishing woes, why isn't the secondary UV filter located BEFORE the BG38 (Filter Glass 1)?

 

Attached Thumbnails

  • lunt-solar-tel-design.jpg

Edited by KaiserT, 15 February 2019 - 05:12 AM.

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#23 vincentv

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Posted 15 February 2019 - 09:38 AM

In Lunt solar telescopes, 99% UV radiation is said to be blocked by the ERF (RG630 glass)

The datasheet for RG630 shows that a 3mm thick piece has an optical density of 5+ all the way to 580nm. If the ERF is RG630 (likely) and if it is at least 3mm thick (likely, I haven't measured), then there is no UV hitting the BG38 filter.

 

The diagram is missing the ERF so it may be an older design. Also the mirror doesn't look like a normal star diagonal IIRC and is likely a cold mirror, again, not in the diagram. Even if it is a cold mirror I suspect it's not a critical filter since the straight through assemblies aren't labelled as imaging only.



#24 KaiserT

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Posted 15 February 2019 - 09:59 AM

The datasheet for RG630 shows that a 3mm thick piece has an optical density of 5+ all the way to 580nm. If the ERF is RG630 (likely) and if it is at least 3mm thick (likely, I haven't measured), then there is no UV hitting the BG38 filter.

I quote from a previous post by BYoesle:

"The current Lunt blocking filters, and the SolarScope filters use soda-lime glass filters as a replacement for the ITF's mid to long IR blocking. These are either BG filters, KG filters, or a combination of both, and some may even have a short IR blocking applied as well (BelOptik). The deterioration of these filters comes mostly from UV and moisture exposure."

 

Where does this UV come from if "there is no UV hitting the BG38 filter?"

 



#25 vincentv

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Posted 15 February 2019 - 11:20 AM

Most likely you don't need both to have the degradation, humidity alone is enough. That would explain why some members never complain about the filter and why Lunt doesn't see it as a big problem. As long as your storage is dry enough you won't have issues.




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