Blocking Filter, Mini ERFs and Alternatives

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by swisswalter »

Hi Mark

you are so right. I'm going to remove the ITF filter and instead place a 2" 15nm Ha filter. What do you think of that?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Hi Mark

you are so right. I'm going to remove the ITF filter and instead place a 2" 15nm Ha filter. What do you think of that?

I think all is good up until 1400nm when it will leak IR like a leaky bucket. Used in conjunction with some KG3 it will seal this leak and make imaging much less noisier. ;)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

Most of the “traditional” dielectric filters appear to have higher transmissions above 1200-1400 nm. That is why metallic silver is incorporated in to the ITF’s layers for IR blocking solar applications.

I believe it is much better to be conservative and have something that performs as well as the original ITF in the far IR beyond these nightime-use dielectric-only filters. After all, there must be some reason to use an ITF in the first place, and the only filter I have seen that accomplishes this far IR blocking is the KG heat absorbing glass filters (and again Lunt appears to now be using this or something similar).

Over the years I have noted a decrease in light sensitivity in my dominant eye, and the only reason I have been told this could be the result of is my 50 years of solar observation, which started out with the infamous “eyepiece cap” welders-glass type filters sold in the early 60's for use with small refractors.

So for visual use I would not and will not consider anything less than a replacement ITF or a KG type filter - perhaps in conjunction with a wide-band Ha or other similar filter. Besides, using an H alpha filter as an ITF replacement may expose it to IR degradation as well, and here the KG-like filter would be helpful placed ahead of any additional filters, especially the trim filter (a very expensive replacement)...

BTW here’s the transmission curve from Baader for their 7 nm H alpha filter:





And their 35 nm H alpha filter:





The 35 nm appears to be the better choice. I’m not sure which 15 nm H alpha filter you are considering Walter, but I would be sure to have a transmission curve to assure it’s safety for IR, in addition to the above considerations...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by swisswalter »

Hi Bob

thank you very much for that information. I'm not going to use it visually, I just place a CCD up front. My eyes are too valuable for experiments


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

Most of the “traditional” dielectric filters appear to have higher transmissions above 1200-1400 nm. That is why metallic silver is incorporated in to the ITF’s layers for IR blocking solar applications.

I believe it is much better to be conservative and have something that performs as well as the original ITF in the far IR beyond these nightime-use dielectric-only filters. After all, there must be some reason to use an ITF in the first place, and the only filter I have seen that accomplishes this far IR blocking is the KG heat absorbing glass filters (and again Lunt appears to now be using this or something similar).

Over the years I have noted a decrease in light sensitivity in my dominant eye, and the only reason I have been told this could be the result of is my nearly 50 years of solar observation, which started out with the infamous “eyepiece cap” welders-glass type filters sold in the 60's for use with small refractors.

So for visual use I would not and will not consider anything less than a replacement ITF or a KG type filter - perhaps in conjunction with a wide-band Ha or other similar filter. Besides, using an H alpha filter as an ITF may expose it to IR degradation as well, and here the KG-like filter would be helpful placed ahead of any additional filters, especially the trim filter (a very expensive replacement)...

BTW here’s the transmission curve from Baader for their 7 nm H alpha filter:





And their 35 nm H alpha filter:





The 35 nm appears to be the better choice. I’m not sure which 15 nm H alpha filter you are considering Walter, but I would be sure to have a transmission curve to assure it’s safety for IR, in addition to the above considerations...

For visual use the problem with these diagrams is they only show the bandpass to ND2 or 1%. We need ND5 for visual and UV and ND3 for IR. That would be .001% or .1% respectively.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

Thanks for that info Colin, which argues even more strongly against using wide-band Ha filters alone as an IR blocker for solar use. Going by your stated ND values, a KG 1 appears to be unsuitable for IR blocking (and exceeds the EU standards shown above):





The KG 3 appears to be better:





Although it doesn't go out far, this graph from Schott appears to indicate the KG 5 would be the best:




The Schott graph extended transmission KG 3:





The Schott graph extended transmission KG 5:




It appears the KG 5 rises above ND 3 from ~ 2300 to 2800 nm and around 3700 nm, peaking at ND 2 at 2700 nm. If we go by the apparent EU standards shown above, this would be the better filter r/t far IR transmitance. But even with this, near IR appears to need additional attenuation: ND 5.75 from < 750 to 800 nm, and ND 4.5 from 800 to 1400 nm...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

KG3 almost gets there. KG5 is better. All the glass shown starts to pass below (above depending on your terminology) 1500nm.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

We have studied thermal damage for a long time. Here is a good link about CO2 LASERs. It is a little geeky. CO2 LASER radiation is about 9000nm. We should remember here that the cornea has no neurons to detect thermal damage.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA191306


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

But even with this, near IR appears to need additional attenuation: ND 5.75 from

Some very interesting analysis going on here Colin & Bob! B)

Given your quote above Bob, Used with a typical 'hot mirror' ERF from Baader, the additional blocking that this provides in conjunction with KG3/5 should bring it down to the appropriate ND levels. If you look at the transmission curve for the ITF (way back at the very start of this thread), you will see it provides blocking to ND5 from >1000nm up to 2500nm...

I notice looking at the transmission curves for the KG glass, both on here and manufacturers sites, they quote figures based on a certain 'thickness' of glass. Given that KG glass is an absorption based filter, would using a thicker filter provide greater absorption than the same type less thick? Soooo, given discussion of the last half a dozen posts, if we went for the thicker KG filters we are going in the right direction ;)

*Edit*

I wonder how thick the KGx Lunt is using as an ITF in its blockers is? :whistle: Anybody want to measure this? I don't have a lunt blocker else would. The ITF from my coronado BF10 is 3mm thick, formed from 2 x 1.5mm pieces, with the coating between the 2 peices glass (well atleast this where the rusting is occuring).


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

Makes sense that a thicker absorption filter would have a decreased transmission, but I would like to verify this. Then there would be the possibility of “double stacking” (!). BTW Edmund offers the KG 5 in 3 mm thickness, as opposed to the 2 mm shown in the graphs above.

Whatever Lunt appears to be using, it also appears they are using special coatings on the BF mirror as well - a couple of recent CN posts indicated they are using a thicker mirror with a “long-pass” coating in addition to the “glass filter” that has resulted in a slight focus shift compared to previously used mirrors.

At this point (and until definitive information is available), I would remain cautious and consider KG glass to augment an existing BF to and protect it from larger aperture thermal loading and cycling issues (i.e. my CaK filter system), rather than a BF replacement.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

A very sensible appraisal of it all Bob. It doesn't take very long in any discussion like this before we are talking about double stacking :P


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

You can add ND filters ND1+ND1=ND2.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Mmmmm, a KG sandwich... ;)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

So if a 2mm filter is ND4 then a 3mm filter of the same glass would be ND6. 3mm seems very thin for an optical element that is going to heat up in the sun. Perhaps placed after the Baader DERF to reject some heat?

The old red filters on the DayStar systems were quite thick. I have not measured one but they were like 12mm or so and polished to 1/4 wave. Now DayStar and Solar Spectrum recommend the DERF.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

So if a 2mm filter is ND4 then a 3mm filter of the same glass would be ND6. 3mm seems very thin for an optical element that is going to heat up in the sun. Perhaps placed after the Baader DERF to reject some heat?

I'm using 3mm KG3 in conjunction with a (externally mounted) 2" 7nm Baader Ha filter as primary ERF on SM40 etalon. I guess much less thermal loading at this small aperture than larger ones...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

Ont op of this, what about atmospheric absorption :


Is that not enought as I expect?

Can someone explain me the link between ND value, % transmission and optical opacity?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by swisswalter »

Hi Joe

thank you very much for your explanations. Now I know why I only learned to add and substract :)

that was just ment as a little joke in a serious thread :oops: please forgive me


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

"Can someone explain me the link between ND value, % transmission and optical opacity?"

The ND value is the optical density (OD) of the filter, which is a measure of how much light is blocked by the filter. To determine the OD, take the negative log of the transmission value.
example: 1% transmission = OD2 = ND2
OD = -log(T/100) = -log(1/100) = 2

conversely, if you know the OD, the transmission is T = (10^-OD)*100 = (10^-2)*100 = 1%

Since the ND value are logs of the transmission, you can add them to find the total OD rather than multiply the transmission values; much easier to add than multiply.

joe

Thank you Joe!

So If I understood weel :

1% transmission = ND2
0,1% = ND3
0,01% = ND4
0,01% = ND5 (this is what today ITF is giving today up to 1000nm)


We can see here that with 3mm KG3 up to 1600nm, we are above ND3 that is recommended for IR according to Collin


But Schott curves are not giving same results for 2mm KG3 where we are always above ND3 (bellow 0.1% transmission), so safer



So from my understanding there is a mistake somewhere....

Still according schott, KG5 2mm is always above ND4 (bellow 0,01% transmission)

And apparently Baader ND5 is between 4 to 5 :


Collin, did you get confirmation for IR protection 'cause in fact that the main point!

I try to investigate. Here it's what I found on NASA web site : "A safe solar filter should transmit less than 0.003% (density~4.5)[1] of visible light (380 to 780 nm) and no more than 0.5% (density~2.3) of the near-infrared radiation (780 to 1400 nm). Figure 24 shows the spectral response for a selection of safe solar filters." where you can found this interesting chart :


I also found that french security institut recommend to use welder's glass with SN between 12 to 16

SN = 1 + (7/3) OD means >ND4.7 (again.....)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

The figures I quote are for 8 hours per day occupational exposures. I choose them for the obsessed of us. The Nasa diagram was created for eclipse viewing only.

Really where the solar designers with 40 years of experience go is to a silver coated ITF filter for far IR. It is the most fragile filter in the system yet they all include one for h-alpha observing. If they could avoid it I think they would.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

For sure. I only try to make sure if we need ND 3 or 5 for IR. At least for me it's not clear with previous discussions...

If on top of this there's some mistakes in the graph,the story really start to be complicate...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

Ont op of this, what about atmospheric absorption :


Is that not enought as I expect?

Can someone explain me the link between ND value, % transmission and optical opacity?

Merlin66 addresses this in his article but it is not online now. I am working to get it published again.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Really where the solar designers with 40 years of experience go is to a silver coated ITF filter for far IR. It is the most fragile filter in the system yet they all include one for h-alpha observing. If they could avoid it I think they would.

Very good point Colin!


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

As an after thought...

I gues it would be possible to use 'other' filters or glass stacked to give the desired transmission in the IR, however the more elements are stacked, the more transmission at 656nm will be reduced - hence why the silver coatings are used...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

“The figures I quote are for 8 hours per day occupational exposures. I choose them for the obsessed of us.”

Actually Collin, I’m not sure I would trust even these exposure levels, as in my experience with sound levels, OSHA 8 hour tolerances (90 dBA) can an do result in hearing loss.

And as with any form of radiation, individual susceptibility can vary and is unknown until damage occurs...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

As a thought, we talk about blocking as much continuum leakage as possible; what would be the effect of stacking 2 blocking filters like we do with etalons???


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

I don't think it's the same problem... we need the blocking filter to have a large enough bandwidth to allow chasing the doppler shift...as long as the blocking FWHM is around 6-8A it will do its job very well.
( I'm hoping Colin will do a spreadsheet based on the actual blocking filter bandwidth...I think it will show very little sideband passing. Each etalon blocks the other and as long as the blocking filter works within the finesse - there's nothing else left??!!)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Hmmm, I keep thinking about this continuum leakage and ways to reduce it... I have to say ever since double stacking I rarely find myself hunting around the doppler shift, I get a pretty decent view of proms and disk simultaneously, my full disk pics are a pretty darn good representation of what i see at the ep. Both etalons are locked down in optimum position and other than a lil' twist of my dew shield which acts like a very fine tuner I find no need to retune - counter intuitive I know; air pressure and temperature et al, but this is the reality of what I find... I do periodically hunt around incase there is a better view, but i seem to have etalon positions nailed.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by peter drew »

I've tried double ERF's and double mini ERF's as well as double BF's, no improvement unfortunately, but if you don't try...... :)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I've tried double ERF's and double mini ERF's as well as double BF's, no improvement unfortunately, but if you don't try...... :)

fair dinkum, thanks for that Peter! :)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

The most interesting thing from Joe's spreadsheet was the double peaks from the side bands that leaked through the blocking filter from mis-tuned etalons. I really need to run through the math in that spreadsheet to understand what Joe was doing because his last post differs from my change to the spreadsheet and it is not fair for me to hack his spreadsheet without checking all the math. Joe did a great job and I just hacked in for a quick answer.

As far as the blocking filter specs they are now public knowledge.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

Colin,
I thought the bandwidth of the blocking filter in the spreadsheet was slightly greater than the 6-8A we currently have, hence the side band pass?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

It is going to take me a week to go through my notes form the etalon articles and see what spec I have. I know I have Coronado and DayStar's specs.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I was thinking about blocking filters (again!)...

If the blocking filter is tilted with respect to the optical axis - say as a result of focuser droop / sag, would it's response be slightly blue shifted compared to it's reference value?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

I was thinking about blocking filters (again!)...

If the blocking filter is tilted with respect to the optical axis - say as a result of focuser droop / sag, would it's response be slightly blue shifted compared to it's reference value?

Yes, When you specify a design you also specify the tilt. I look to see if I have some Dobroski articles. He is the coating god for the last 70 years!


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

I spelled his name wrong. This is the paper that started the whole mica etalon revolution.

“Mica Interference Filters with Transmission Bands of Very Narrow Half-Widths” by J.
A. Dobrowolski


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Yes, When you specify a design you also specify the tilt.

Hmmmm. Gives us another factor to consider when getting the most out of out Ha systems... :whistle:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

I'm nearing completion of my new Ha blocking filter and CaK modules, which will be evacuated and filled with Nitrogen. These are essentially identical units with a sealed inner chamber intended to remove moisture from the primary filter elements and keep thermal stresses to a minimum. Therefore I'm hoping to reduce filter degradation/failure issues as much as possible. They incorporate a "mini ERF" in the form of Baader Red and Blue CCD filters ahead of the enclosed Ha BF assembly and PST/Baader CaK filters (the Ha system also uses a Baader DERF for the SM90's):




I'm trying to obtain optically polished KG5 filters to add in front of the Baader CCD filters.

Here's some of the components - Top row left - completed unit with Blue Baader CCD "mini ERF" - third row left to right - Coronado BF 30, PST/Baader Cak assembly, optical window in cell, bottom right - red Baader CCD "mini ERF." The windows (right) were obtained quite inexpensively from Surplus Shed - 50 mm in diameter and 10 mm thick, both surfaces 1/10th wave ( http://www.surplusshed.com/pages/item/pm1106.html ). Two pairs were AR coated by Majestic ( http://www.majestic-coatings.com/ ) specifically for 656 nm and 395 nm:




The modules will fit inside the AP 2.7 focusers with minimal backfocus issues - machining by Teleskop Service in Germany (very fine workmanship - http://www.telescope-service.com/ ):



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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Very professional Bob! :bow:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

A new solar scope brand is born : Bobnado :lol:


etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

New to the forum, first want to say thanks! I was SERIOUSLY distressed seeing other sites use monitoring / censorship on random topics. Really ruins the whole forum concept. Sure hope the trend eventually reverses some day and people grow their spines back!!

Anyway, Bob that filter looks great.

I'm interested in making my own "BF25". Since the blocking filter is basically 2 filters, was wondering if it's possible to just screw 2 filters onto the bottom of my camera? If so can you guys point me to the correct filters?


jjk

Re: Blocking Filter, Mini ERFs and Alternatives

Post by jjk »

Mark,

What is the spectral response of your camera? Is it very sensitive to IR > 1400 nm?

Best,
John


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

You can order your own blocking filters. One element is a Notch filter centered on the H-alpha bandpass that matches the Free Spectral Range of your etalon. The out of band blocking is about ND5. The other filter is an ITF and is usually made from silver layers and is designed as a step filter to block IR all the way to IR-C. There are a number of custom filter coaters out there. Some of them will be able to know if there designs are rated for use in solar work.

Somewhere I have the part numbers for the Daystar blocking filters but it is not handy and there is no guarantee that the FSR will match your etalon.


etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

You guys talk of replacing the itf with a B+W 489 filter.

http://diglloyd.com/articles/Filters/sp ... W-489.html

Would this followed by a Baader 7nm H-alpha filter work?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

B+W489 is same as KG3 from schott and we saw that it was not enough. Best is KG5 from schott double stack with one other IR cut filter.

Now regarding Ha filter whatever the one you take, they are made for night sky and not enough selective for solar shots.


etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

OK, so KG5 filter, then some standard UV/IR blockerd followed by a notch.

Anyone know where I can get KG5, preferably 1.25 or 2in filter?

colinsk, wouldn't the free spectral range be the same for any .7nm etalon? Sorry not an optices engineer, but isn't this just a h-alpha filter with a really tight band of say 1.5nm? Which is why a baader at 7nm won't work?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

For KG5 filter I bought mine here : www.stockoptics.com

Highly professional guys and reactive, but you have to mont them (as soon as it arrive I'll make a picture)

regarding Ha filter, don't forget that we are looking for <1A ones and that 7nm=70A!

I also have one question : ERF is always i front of BF what would happen if I put my new KG filter after it? would it damage it?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I also have one question : ERF is always i front of BF what would happen if I put my new KG filter after it? would it damage it?

Yes, The KG5 keeps the thermal load off the blocking filter, if the KG5 was placed behind thermal cycling / loading could damage the blocker...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

It's always best to keep as much energy away from the blocking filter as possible. So, the ERF should be placed somewhere in front of it.

The blocking filter, as Colin said, needs to be "matched" to the etalon performance. We've been lucky to find both the Coronado and Lunt etalons can work with the same blocking filters!
A bandwidth of 6A FWHM centred on the Ha wavelength would be a great start for DIY BF's.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

so what about this? :)


looks already better than standard BF


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