What does a "blocking filter" consist of... ?

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
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Macavity
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What does a "blocking filter" consist of... ?

Post by Macavity » Wed Aug 30, 2017 9:58 am

Bad English grammar maybe? :lol: But can I not MAKE my own? :cool:

The *important* safety issues noted(!), I see the Lunt diagonals
*appear* to consists of a 10^-5 IR block... A prism (obviously!)
a SIX Angstrom H-Alpha Filter (the substrate of which seems?)
to provide 10^-5 UV blocking...

LuntPic.jpg
LuntPic.jpg (202.27 KiB) Viewed 1166 times

If I had one desire it would be to possess a *Straight Through*
blocking filter of greater field stop than my modest BF600! I
feel *some* of the limitations of Lunt scopes are the need to
collimate ("clock") the redundant (for imaging!) diagonal. :|

For my (in progress) Frankenscope - It's for *imaging only* -
I will use an "Objective" Baader ERF to exclude UV and IR!
Can I not try to *make* my own blocking filter from a 1.25"
H-Alpha filter of requisite bandwidth? I will certainly add
a "thick" ND filter + UV IR filter for any first camera test! :P

This seems no more "hazardous" than using e.g. Coronardo
"blocking filters" in a Lunt system with very few details on
the *precise* transmission etc. properties of the thing... :?

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Re: What does a "blocking filter" consist of... ?

Post by Bob Yoesle » Wed Aug 30, 2017 3:22 pm

Can I not try to *make* my own blocking filter from a 1.25"
H-Alpha filter of requisite bandwidth?
Hmm, "requisite bandwidth" - nope, since even the narrowest nighttime H alpha 1.25" filter has a FWHM (bandpass) of a couple of nanometers, which equals 20 + angstroms. So where will you get this "requisite bandwidth filter"? It will be quite expensive if purchased from most optical filter vendors. You need a filter with about 5-10 angstroms FWHM, which is 0.5 to 1.0 nm. Since your primary H alpha etalon would have a free spectral range of 10 - 15 angstroms, a nighttime 2 nm filter would let side-band etalon peaks through, making the view essentially a red continuum view due to the parasitic off-band continuum leaking through the "blocking" filter.

You would also need a secondary IR filter that blocks fairly far out - not the typical IR/UV filter - despite your stating it's for "imaging only." This usually consists of an ITF type filter, or use a KG3 or KG5 filter with appropriate coatings, but it would be necessary to also have an RG630 filter and/or ERF eleswhere in the system. BelOptik can supply these.

Honestly, if you haven't already figured this out, you should probably stick with a OEM blocking filter. You can order any of the Lunt blocking filters as straight through, and the cost is the same. That's the way I'd go.
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Re: What does a "blocking filter" consist of... ?

Post by Macavity » Wed Aug 30, 2017 6:50 pm

Many Thanks, Bob!

Mea Culpa! A certain amount of embarrassment to cover... :oops:

In part mitigation, it may be not *totally* about an inability to
"figure things out" but a mix of SI and "traditional" units on the
diagram, which is at best hard to read? I genuinely "saw" the BF
as having a bandwidth of 6nm rather than the SIX Angstroms!
That no one does something is suggestive? It's a "Fair Cop"! ;)

Thank you for additional information. Some splendid pointers
to information which I will check out -- Probably not use in any
practical way. <G> My Frankenscope thread illustrates what I
will do. Pretty confident re. that. "No news is good news"? :D

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Re: What does a "blocking filter" consist of... ?

Post by antonello » Fri Sep 01, 2017 5:25 am

Please read this my message in a italian forum:
https://www.astronomia.com/forum/showth ... RIMA-PARTE
It is in Italian language, but you should be able to understand something with google translator ...

Antonello

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Re: What does a "blocking filter" consist of... ?

Post by highfnum » Fri Sep 01, 2017 9:59 pm

i found this fascinating nice idea so I tried it
camera 1nm(10A) andover Ha filter - and orion variable polarized filters
blockcamS.jpg
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Re: What does a "blocking filter" consist of... ?

Post by highfnum » Fri Sep 01, 2017 10:02 pm

take out lunt block and in my case I added diagonal - if you want straight thru you may need extension tube
diagnols.jpg
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Re: What does a "blocking filter" consist of... ?

Post by highfnum » Fri Sep 01, 2017 10:04 pm

a quick test - results OK not quite as good as lunt filter
it does work
i would use only for capture since I have no idea if there is IR/UV leakage
fc2_savespecial_2017-09-01-165657-0000.jpg
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Re: What does a "blocking filter" consist of... ?

Post by highfnum » Fri Sep 01, 2017 10:17 pm

variable filter set at about 20%

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Re: What does a "blocking filter" consist of... ?

Post by Merlin66 » Sat Sep 02, 2017 12:48 am

John,
An interesting experiment...
The surface and proms show up reasonable well...
I assume you were using a Lunt etalon?

Bob, I know this has been raised before...
""You would also need a secondary IR filter that blocks fairly far out - not the typical IR/UV filter"
I understand some filters may have resonance transmissions away from the target wavelengths - at either side (towards the blue and towards the IR) and that these should be blocked to avoid "leakage"
My question is:
Do we have any documented evidence that IR leakage is a real issue/ problem ????.
1. The human eye is only sensitive to wavelengths below 800nm
2. The standard CMOS/ CCD chip in amateur cameras is only sensitive to wavelengths below 1100nm
3. Most if not all the filter transmission curves show data up to around 1500nm, well beyond the sensitivities of the eye or camera.
Unfortunately with my spectrograph which uses a CCD camera (ATiK 314L) I can't get any useful data beyond 1100nm to measure any NIR/IR effects.
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Re: What does a "blocking filter" consist of... ?

Post by highfnum » Sat Sep 02, 2017 9:06 am

yes my old trusted and true 60mm lunt manual tilt Ha scope

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Re: What does a "blocking filter" consist of... ?

Post by Bob Yoesle » Sat Sep 02, 2017 3:21 pm

Do we have any documented evidence that IR leakage is a real issue/ problem ????.
1. The human eye is only sensitive to wavelengths below 800nm
2. The standard CMOS/ CCD chip in amateur cameras is only sensitive to wavelengths below 1100nm
3. Most if not all the filter transmission curves show data up to around 1500nm, well beyond the sensitivities of the eye or camera.
Hi Ken,

Admittedly, most of the information regarding the hazards of IR came from the laser industry, where flux densities are usually higher than those from our solar telescopes (except maybe those C8's and C14's now being placed into service!) I'm not sure if there would be a "real" verses theoretical issue or problem with IR B/C. I'm just in the "a gram of prevention is worth a kilogram of cure" camp - especially when it comes to my own eyesight ;-) Moreover, an individuals susceptibility to harm from low levels of radiation of any kind can vary based on genetic and other factors.
Retinal Hazard Range.jpg
Retinal Hazard Range.jpg (127.08 KiB) Viewed 1019 times
For IR B/C (1400 + nm), it is not a question of retinal sensitivity, rather a long-term exposure issue for the cornea/lens:
In general, ultraviolet and far-infrared laser radiation is absorbed at the cornea or lens, and its effect depends upon the intensity and exposure duration. At high intensities, immediate thermal burns occur, while lower exposures may lead to the development of cataracts over a period of years. (Emphasis added).
https://www.microscopyu.com/microscopy- ... ser-safety

The energy from the Sun itself does seem significant at 800 nm + :
11592.jpg
11592.jpg (65.28 KiB) Viewed 1005 times
Again, don't know if I'd call this documented evidence. I know you have written a thorough treatise on the subject. I just prefer not to be the guinea pig with my eyesight. Because the long term effects are cumulative and probably come from multiple sources, solar observation without additional blocking to beyond 1400 nm might possibly be adding "insult to injury."

Best wishes,

Bob

RA
eye of ra sm.jpg
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Re: What does a "blocking filter" consist of... ?

Post by Merlin66 » Sun Sep 03, 2017 6:19 am

The graphs I think show that at >1500nm the risk is non existent/ minimal - for semi-regular visual observers....
(Bear in mind most will be using Baader Solar Film, or suitable other ERF type optical systems)
The extended exposures required to accumulate retinal damage are far in excess of "normal solar amateur observations"
On that basis I feel the manufacturers transmission graphs which show significant attenuation out to at least 1500nm are valid.

Re Imaging - has anyone got some images which show the impact of NIR/IR wavelengths on their images???
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Re: What does a "blocking filter" consist of... ?

Post by Bob Yoesle » Fri Sep 22, 2017 2:29 pm

With mid and long IR B & C one is not risking retinal damage, it is rather the cornea and lens which will absorb these wavelengths, and the risk is for development of cataracts:
eye-light-interaction.png
eye-light-interaction.png (54.64 KiB) Viewed 796 times
Below is the effect of near IR A leakage through a SlocarScope blocking filter. It represents itself as out-of-focus continuum.
IR leak.jpg
IR leak.jpg (218.44 KiB) Viewed 796 times
Top: near IR leak, bottom: blocked near IR by placing a Baader Red CCD filter ahead of the blocking filter:
BA_RGB_Kurve.jpg
BA_RGB_Kurve.jpg (38.8 KiB) Viewed 796 times
SolarScope has since corrected this issue ;-)
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Re: What does a "blocking filter" consist of... ?

Post by Merlin66 » Fri Sep 22, 2017 11:10 pm

Re IR leakage..
As you say, SolarScope have corrected the issue.
Are there any other examples of similar IR leakage?
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