Blocking Filter, Mini ERFs and Alternatives

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by poita » Wed Aug 01, 2012 12:54 am

I may need to try this out for my B600 that has a horrible film and destroyed coatings on the itf at the from of the B600.

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle » Fri Oct 05, 2012 5:17 pm

As noted earlier in this thread, I have had a number of issues with blocking filter / ITF failures, and most recently have noted possible deterioration of my PST-based CaK filter module. One possible solution that presented itself is to both reduce thermal cycling via the use of additional filtering (i.e. “mini” ERF’s) and elimination of atmospheric moisture.

So I designed a pair of identical modules that incorporate an additional Baader CCD filter to serve as a mini-ERF, and an optical window sealed chamber that is purged and sealed with Argon gas:





H alpha unit with Baader’s Red CCD filter:





Module in place and requires only 12 mm (½ inch) of back focus (much less than the OEM BF30 itself):





Preliminary views and imaging from yesterday reveal no contrast anomalies or spurious reflections:





Next hurdle will be assembling the “glove box” for the Argon purging - will report as soon as a few issues are solved. But so far so good.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk » Fri Oct 05, 2012 10:52 pm

Very nice work! :bow:
Thank you, Here are some references I have collected.



Etalon Article Part I

Etalon Article Part II

Solar Patents of interest

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Sat Oct 06, 2012 6:54 am

Damned impressive Bob! :bow:
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by GreatAttractor » Sun Jun 02, 2013 9:51 pm

Excuse my slightly off-topic question: DSobserver, does your KG5 filter from stockoptics.com have any negative impact on image sharpness (beside the red leakage problem you have to solve)? I'm considering it for my own build.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Sun Jun 09, 2013 5:21 pm

well, to be honest I could found a new ITF to replace the original PST one so my KG5 stay in its box...

What scared me to replace PST ITF is that using an ITF like KG5+night Ha filter or a IF UV/IR cut on KG3 from bel optik+night Ha filter, BF will not be anymore IR protected.

Could that deteriorate it?

Any idea?

Otherwise we can use 2" filters to screw on front PST...

By the way I recently test a Lumicon Ha filter and I feel that it slightly improve contrast. Could it be that it reduce bandwidth or it's only a question of %transmission?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Mon Jun 10, 2013 12:24 am

Oliver's UV-IR/KG3 filter will block all the IR.
Combined with a narrow band Ha filter ( either up front as an ERF) or combined with the UV-IR will work very well, and protected both you and the BF.
Which Lumicon Ha filter? The one I have is just a red filter - not a narrow band like the Baader.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Mon Jun 10, 2013 5:32 am

Oliver's UV-IR/KG3 filter will block all the IR.
Combined with a narrow band Ha filter ( either up front as an ERF) or combined with the UV-IR will work very well, and protected both you and the BF.

except that for PST I don't think you can screw them is the ITF tube, so you'll screw them on the eyepiece : BF will not be protected anymore...

I don't think that for PST, UV/IR block filter is good enough as you also need to block light bellow 650nm. For SM filter it should work 'cause there is a ERF/deep red filter on front.


Which Lumicon Ha filter? The one I have is just a red filter - not a narrow band like the Baader.

I know that it's a deep deep red filter, but visually there's a slight improvement...
Could it be that it reduce bandwidth or it's only a question of %transmission?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Mon Jun 10, 2013 5:55 am

I wasn't thinking of screwing them into the eyepeiece...
I was thinking about removing the spacer/ retaining ring at the existing "mini-erf" and using a dab of hot glue to hold the 1.25" filter(s) in place.
The OD of a 1.25" filter cell is 30.97mm and the retainer is 29.47- the thread to the back box is 32.89mm - these figures (could) allow a standard filter to be placed in the "recess" available after removal of the ITF surround, and the filter would sit just inside the black box. (there's sufficient clearance to the top of the prism.) The BF section would fit as normal.

Re the Lumicon - it can only be due to the increased transmission at Ha wavelength...
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by markthais » Mon Jun 10, 2013 8:06 am

All this talk about removing the IFT and replacing it with KG glass is fine if the bandpass is designed to be blocked to 1200nm.
A 2 cavity standard 6 ang filter will turn on again for a peak bandpass at about 850-900nm on the long side. The KG-3 needs to be blocked to at least 1100nm before it gets down to a low transmission level. So on the long side you have a leak or some energy between 900-1100nm. This is why you use the ITF, to block from 700nm-FIR
On the short side the 6 ang filter turn back on around 560nm with a narrow bandpass then in the blue green it really doesn't block much. This is what the red glass is for.The closer your cut wavelength to Ha the better. That is why we use RG610 or RG630.
The ITF will look silver and the bandpass will look gold.
But all this depends on how the filter is made.
When the ITF fails it get what is called rusted of foggy. When the bandpass fails it shifts the center wavelength from center to edge. So if you scan the center it will be at a different wavelength then the edge. The problem with looking at the bandpass is it can look fine and still have shifted. Usually you will find a loss in contrast or the filter will be dimmer. Two main thing are happening, it is letting the next peak leak through or has moved off the the peak it was designed for.
Just to review because there are a mixture of terms.
The ERF is to keep the heat out of the scope. If you put the ERF just in front of the etalon it is to keep the etalon cooler.
The blocking of the inter-order peaks of the etalon are blocked with filter glass(red), ITF and bandpass or a combination of bandpass, filter glass(red) and IR glass as long as the bandpass is blocked to 1200nm.
You can also have the ITF coated on top of the bandpass and red glass. This is also used.
Now, for just the ITF replacement.
The idea of using a Baader 7nm and the KG-3 is still not a bad idea to replace the ITF. The Baader 35nm all depends on the bandpass. It may or may not work. If you have a spectrometer then it's easy to tell.
The Baader filters are hard coated and are blocked passed 1200nm in the red. The red glass of the filter will take care of the short side.
The KG glass is not the best IR glass out there, but it is probably the easiest to find for the amateur.
All this is for standard filters. Hard coated bandpass filters can be different depending on the design.
But don't take a chance with the IR blocking. If your not sure then send it back to be repaired.
Mark W.

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Mon Jun 10, 2013 5:03 pm

Great summary and very insightful, thanks Mark :)

You know then, picking up on your very last point; I know the KG glass isn't the best option in terms of quality, but have to ask the question, if were going for something better what would it be?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Mon Jun 10, 2013 5:15 pm


Re the Lumicon - it can only be due to the increased transmission at Ha wavelength...

increased transmission at Ha wavelength? How can it be 'cause I simply screw this filter in front of the PST including ITF filter?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Mon Jun 10, 2013 11:00 pm

Sorry, I misunderstood...I thought you were using it as a part replacement for the ITF...
I can only assume then that there's an improvement in contrast? It doesn't act in any way (that I know of) to reduce the bandwidth.

Mark,
The transmission curves of the Baader 35nm filter are given at the front of this thread, the UV-IR/KG3 filter curves (above) show good blocking up into the IR....
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by markthais » Tue Jun 11, 2013 6:54 pm

OK, stand corrected, the Baader 35nm filter looks good. It gets down before 700nm and stays down pass 1200nm. Depending how the bandpass was made. There should not be a leak before 700nm.

The KG glass has a problem with water vapor. And don't be fooled in thinking that coating the glass will keep in out.
Water vapor is a tricky thing. Is it important to the amateur market, probably not. If it starts to fail just get another one.
The better glass would be the Hoya HA30. This glass is made for them by Isuzu Glass. I was told that Isuzu will soon have all the military market for this type of IR glass. It holds up better then the KG series to weather and other requirements.
But for us,if you can get the KG already in the size you need go for it. Just use it at the eyepiece end of the scope because it will not work well at the objective end.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Tue Jun 11, 2013 7:38 pm

here are the hoya info

At 656nm transmission is around 68% and the KG3 was 60%....not so bad!

price around 65$ here but not easy to find

What about HA50 with 75% transmission?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by markthais » Sat Jun 15, 2013 2:17 am

The HA-50 would probably be fine. It just turns back on in the IR, but it is out there at 1500nm. I really don't think that less then 1% transmission out there at 1500nm matters. The blocking of the IR depends on the thickness of the glass. The thicker the glass the better blocking but it also drops the transmission of the visible. Most of the glass is 2 to 3mm thick.
The use of IR glass is old school. But it still a good way to block unwanted light.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by ronatu » Mon Sep 24, 2018 3:33 pm

Excuse me, but does anyone try to replace blocking filter?
Please share.
PS Would like to change size of it. From BF 5mm to 10-15mm.

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by MalVeauX » Sat Sep 29, 2018 8:43 pm

Sorry to maybe dig up something old, but some filters mentioned already in this thread could hold interest.

I was looking at Baader's Red CCD filter (IR block) as an ERF. Seems Bob has used one already for HA.

I'm currently using the Blue CCD filter (IR block) as an ERF with a white light & calcium setup, and it works very well with a 150mm aperture refractor to reflect the heat back out. I use a 395nm +/-20nm filter as a 2nd ERF in the train and most of the heat is eliminated to the point where I can put my hand behind the system and not get melted.

I'm curious about maybe using a Daystar yellow ERF (passes IR, but blocks UV) that is full aperture (150mm in this case) in conjunction with a Baader Red CCD filter (IR block). I'm only considering it because I can get ahold of a 150mm Daystar yellow ERF for a very low price and if I combine it with the Baader Red filter, it seems like it would block most of the thermal load from UV & IR and still have high transmission. Comparison would be a 150mm Baader D-ERF in a cell, which is greater than $1500+ on its own.

Just curious as I'm looking to cut out more heat in my 150mm refractor with a budget solution, while saving for a good 200mm filter for the future.

Very best,

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by christian viladrich » Sat Sep 29, 2018 9:03 pm

Hi Marty,
The Daystar ERF are now a bit outdated. There are not dielectric filters. This means that they aborb light and heat. In other words you will have a piece of very warm glass in front of your refractor. Not good for HR ;-)
Further more, there is a huge gap in optical quality beteween the Daystar ERF and the Baader ERF.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Sat Sep 29, 2018 9:26 pm

Personally I wouldn't bother with the yellow daystar erf. I don't see any gain in using the sub erf you are using already. Save your money!
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by MalVeauX » Sun Sep 30, 2018 12:25 am

Thanks, very appreciated, I was thinking about that and wondered. It makes a lot of sense that absorption filters are just going to keep heat in the line of the optics versus filters that reflect everything.

Do you guys think a Baader Red CCD Filter (IR block) would be ok with a 6 inch refractor? I have used the Blue CCD filter this way, with the 6 inch refractor for white light & calcium light, and the thermal load seems fine. I'm curious if the Baader Red CCD filter would be much improvement over just using an UV/IR block filter with my Quark with my 6 inch refractor.

Very best,

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by krakatoa1883 » Sun Sep 30, 2018 6:53 am

MalVeauX wrote:
Sun Sep 30, 2018 12:25 am
Do you guys think a Baader Red CCD Filter (IR block) would be ok with a 6 inch refractor?
I broke one just few months ago. Probably the fault is mine, may be the filter was too close to focus, however since then I make use of a H alpha CCD filter instead, no problems at all.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by christian viladrich » Sun Sep 30, 2018 8:46 am

Thanks for the feeback Raf. Can you confirm it was a Baader Red CCD filter ?

Which Ha filter are you using now ? With what instrument, diameter, F/D ratio and filter diameter ?

We should make a list of the success / failed attempts ;-)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by krakatoa1883 » Sun Sep 30, 2018 10:05 am

Yes, Christian, it was a red CCD filter, I removed the telecentric from my Quark and was experimenting some different solutions (which all proved to work better...) the filter was mounted just before an amplifier with my 150 mm achromat reduced to 110 mm i,e, @ f/6.8. The mistake was to mount the filter before the negative lens, I think, where heat is more concentrated. Or it may be (it was during winter) the glass broke when suddenly exposed to the ambient air, anyway I do not intend to repeat the experiment in summer :lol:

Since then I purchased a Baader D-ERF however its cell fits only my 150 mm, for my other scopes I am using a 35nm H-alpha filter as internal ERF with apertures not greater than 90 mm from f/5 (Taka Sky 90) to f/11.7 (TMB). Not tried greater apertures yet.

Based on what I read here I am tempted to purchase a 3.5nm....
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by MalVeauX » Sun Sep 30, 2018 12:27 pm

krakatoa1883 wrote:
Sun Sep 30, 2018 6:53 am
MalVeauX wrote:
Sun Sep 30, 2018 12:25 am
Do you guys think a Baader Red CCD Filter (IR block) would be ok with a 6 inch refractor?
I broke one just few months ago. Probably the fault is mine, may be the filter was too close to focus, however since then I make use of a H alpha CCD filter instead, no problems at all.
Interesting, is that with a 6 inch refractor? Was that in the focuser or some where else? Mind sharing a link to the H Alpha CCD filter you're using?

Appreciate it!

Very best,

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