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Filter Conservation

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#1 BYoesle

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Posted 28 November 2011 - 06:15 PM

As many of us have discovered, hi - end solar filters are subject to degradation due to extremes of environmental thermal loading, thermal cycling, and environmental moisture. These conditions can lead to filter coating deterioration or even failure.

While I have recently implemented a stratagem of heat blocking ERF’s to minimize the thermal loading and cycling issues for both H alpha and CaK filter systems, moisture remains an issue. In order to minimize moisture deterioration, I have come up with the following scheme, especially for the long humid winter months here in the Pacific Northwest: Vacuum sealing and the use of high grade molecular sieve desiccants. This technique seems to be in common use in the optics and electronics industry, and is quite suitable for home use as well.

Photo - “rusting” Coronado BF30 ITF (now repair/replaced x 2), and recently discovered deterioration of my CaK PST primary filter:

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#2 BYoesle

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Posted 28 November 2011 - 06:15 PM

Use of a kitchen food preserving vacuum bag sealing system and desiccant to retard moisture from the individual BF30 and CaK filters:

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#3 BYoesle

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Posted 28 November 2011 - 06:16 PM

Next, to enhance the protection, and to protect the main H alpha filter stack, a “space bag” clothing vacuum system is used - with additional desiccant:

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#4 BYoesle

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Posted 28 November 2011 - 06:17 PM

My hope is that because the filters are in storage for the majority of the time when not in use, moisture deterioration can be reduced if not halted to a significant degree, greatly prolonging the life span of the filters. Give the relative inexpensiveness of the materials involved, and the ease of use (total packaging time was 10 minutes), this hopefully will become routine in my use of all my sensitive solar equipment filters. ;)

#5 JJK

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Posted 28 November 2011 - 07:04 PM

Bob,

What kind of plastic is used to make the bags? Is it poly(ethylene) or another kind that might outgas?

Best,
John

#6 colinsk

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Posted 28 November 2011 - 08:54 PM

Good point, I would choose food grade Mylar. I use them for hops and can not taste a flavor after 4 years of storage. O2 ingress over that time period would cause a major change in flavor.

#7 BYoesle

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Posted 28 November 2011 - 09:11 PM

John: I'm not sure, but I think the standard food starage bags are PE. There was a slight smell when I first opened them up, so I let them "air out" for two weeks before using them (until no odor detectable). The "space bags" had no odor of any kind (they are packaged open). Good to be sure...

Colin: Thanks for the advice, I will look into mylar substitutes. :waytogo:

#8 Chris Schur

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Posted 28 November 2011 - 10:15 PM

Gee Bob, I hope your not in a hurry when that next giant prom shoots up! ;)

#9 BYoesle

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Posted 29 November 2011 - 01:57 AM

Not a problem Chris!

The space bag simply zips open, and the vacuum sealed bag is just cut open – and both of the above sealing methods are either reusable or resealable.

I hardly ever get out of bed without this - it has saved my bacon on numerous occasions ;-)

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#10 bob71741

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Posted 29 November 2011 - 10:38 AM

Bob -Nice solution for an old problem!

I was not aware that there are many types of desiccants other than silica gel; but after looking up what you are using, molecular sieve, I wonder what pore size you choose as I see that they can be purchased in 3, 4, 5 and 10 Angstroms sizes, at least from Sorbent Systems?

#11 BYoesle

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Posted 29 November 2011 - 11:26 AM

Hi Bob,

3 or 4 A molecular sieve is the ideal for water absorption. Also be sure to specify Tyvek packaging to eliminate any dust or fiber contamination -- MIL spec, etc.

#12 markthais

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Posted 01 December 2011 - 03:06 AM

HI Bob,
I am going to disagree about using a vacuum bag with a desiccant in side.
What I have found is, that when you put it under vacuum the water that is already in the desiccant will out gas. Now you have a contained system where the water can not leave. If you plan to use a vacuum bag, I have had better luck with just using the vacuum food sealer with out the desiccant.
If you are going to use a desiccant it will work better just in a sealed o-ring container.
Mark

#13 JJK

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Posted 01 December 2011 - 08:20 AM

HI Bob,
I am going to disagree about using a vacuum bag with a desiccant in side.
What I have found is, that when you put it under vacuum the water that is already in the desiccant will out gas. Now you have a contained system where the water can not leave. If you plan to use a vacuum bag, I have had better luck with just using the vacuum food sealer with out the desiccant.
If you are going to use a desiccant it will work better just in a sealed o-ring container.
Mark


In my lab at work, I've also observed that Drierite tends to emit a puff of dust when first pulled under vacuum. I wasn't sure if it was water or the part of the powder of material itself, which inevitably forms when the dessicant particles bang into each other in shipment, etc.

I suppose one might avoid either issue by first putting the dessicant under vacuum in another bag, vent it to atmospheric pressure and soon thereafter put it in the bag with the optics. But, I don't know the kinetics of vapor absorption by these dessicants.

#14 swisswalter

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Posted 01 December 2011 - 12:28 PM

Hi Bob and all

I see your points and think there are good ideas to prolonge the life time of a Ha filter, but it can not be the task of the amateur astronomer to find out what to do to get a 10 or 20 year life time out of a filter or filterstack. That's up to the manufacturer. There are filters on the market which survive 10 years and more in ambient conditions.

#15 markthais

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Posted 01 December 2011 - 08:10 PM

Hi Bob,
That would be unrealistic. The etalon should last because there are not that many layers to form the mirrors. The blockers are more complicated and a lot more layers. If you are talking about narrow hard coated bandpass filters, they haven't been around for ten years yet. The other thing, they will cost 2 to 3 times more than a standard design.
In the field of coating they have come a long way in the last 5 years with new advances, most for the military an IR market.
Like I have said before, think of the blockers as closer to electronic then glass. How many of you have lap top computers that run every day that are more then 10 years old or plasma TV's that are that old. There may be some but most have long failed.
Mark

#16 BYoesle

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Posted 02 December 2011 - 08:42 AM

Hi Mark and John - thanks so much for your input!

My thinking was that the out-gassing of water vapor and dust would be of low concern, as 1) the desiccant - if relatively “fresh” - should not be doing much out-gassing of water vapor under the actual partial pressures achieved by a home vacuum device (i.e. more of an air removal system and not a laboratory grade vacuum); 2) the MIL grade Tyvek packets are designed not to allow desiccant material/dust to escape; and 3) these products are used in similar applications for military airborne optical systems to prevent condensation.

But you have got me thinking about another possible storage method - using Nitrogen purging instead of a vacuum - to help eliminate moisture...

Hi Walter - I agree the manufactures should be addressing the filter longevity issue, and they appear to have done so in some instances. But for me I’m always looking for ways to improve things. For example, the Coronado ERF’s do not block IR - which can lead to premature ITF failure, and simply replacing them with a Baader or Lunt IR blocking ERF is a straightforward solution to reduce or prevent thermal loading and cycling (and in the case of the Baader DERF increase image brightness). Likewise, my CaK PST filter is showing signs of deterioration, and posts here on CN state that some have become useless. MEADE no longer makes these and will not replace them if they are out of warranty. Therefore my choice right now is to try and prevent further deterioration...

Mark: With regard to using hard coated bandpass filters instead of traditional blockers, I for one would like to consider these - all my filters seem to have deteriorated way before ten years (one has actually failed twice), and it would be worth it to me just to get that long a filter life. Could you recommend any sources? (PM me if you prefer ;) )

#17 swisswalter

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Posted 03 December 2011 - 10:12 AM

Hi Bob

thank you. Regarding storage under a nitrogen blanket. That's a good idea. But you are even better of with argon or carbondioxide as they are heavier then air and are poured into bags very easely

#18 marktownley

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Posted 03 December 2011 - 11:17 AM

Interesting thread guys, thanks! :)

#19 bob71741

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Posted 03 December 2011 - 11:45 AM

SwissWalter - You may have hit on a nice solution for the manufacturers to ensure longer lifetime for their blocking filters; purge w/nitrogen and seal. Is this not what Explore Scientific does with their eyepieces?

#20 colinsk

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Posted 03 December 2011 - 08:02 PM

I would not use CO2 but N2 or Argon would be good choices.

#21 JJK

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Posted 03 December 2011 - 11:44 PM

Hi Mark and John - thanks so much for your input!

My thinking was that the out-gassing of water vapor and dust would be of low concern, as 1) the desiccant - if relatively “fresh” - should not be doing much out-gassing of water vapor under the actual partial pressures achieved by a home vacuum device (i.e. more of an air removal system and not a laboratory grade vacuum); 2) the MIL grade Tyvek packets are designed not to allow desiccant material/dust to escape; and 3) these products are used in similar applications for military airborne optical systems to prevent condensation.

But you have got me thinking about another possible storage method - using Nitrogen purging instead of a vacuum - to help eliminate moisture...

Hi Walter - I agree the manufactures should be addressing the filter longevity issue, and they appear to have done so in some instances. But for me I’m always looking for ways to improve things. For example, the Coronado ERF’s do not block IR - which can lead to premature ITF failure, and simply replacing them with a Baader or Lunt IR blocking ERF is a straightforward solution to reduce or prevent thermal loading and cycling (and in the case of the Baader DERF increase image brightness). Likewise, my CaK PST filter is showing signs of deterioration, and posts here on CN state that some have become useless. MEADE no longer makes these and will not replace them if they are out of warranty. Therefore my choice right now is to try and prevent further deterioration...

Mark: With regard to using hard coated bandpass filters instead of traditional blockers, I for one would like to consider these - all my filters seem to have deteriorated way before ten years (one has actually failed twice), and it would be worth it to me just to get that long a filter life. Could you recommend any sources? (PM me if you prefer ;) )


Is it oxygen that causes the coatings to fail?

#22 swisswalter

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Posted 04 December 2011 - 03:23 AM

Hi Colin

thank you. Why not carbondioxide, LUNT even uses it

#23 Bullwinkle

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Posted 04 December 2011 - 08:47 AM

Just a point but where is a convenient place to easily get dry nitrogen or argon gas to purge bags? I mean I've not seen gas supply houses dotting the countryside. Seems to me that the most simple device is the desiccant in a glass container like a jar rather than a plastic bag.

#24 EddWen

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Posted 04 December 2011 - 11:55 AM

I get my high pressure N2 tank filled at the local welding supply shop.

#25 swisswalter

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Posted 04 December 2011 - 12:38 PM

Hi Edd

me too, I get the CO2, N2 and Ar pressure bottles filled also at the local welding supply shop


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