Blocking Filter, Mini ERFs and Alternatives

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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Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Tue Mar 06, 2012 10:33 pm

I recently removed the rusted ERF from my coronado BF10 and discovere that while it gave me a significant reduction in exposure time, the resultant images were very low in contrast, like the gamma had been turned up, and also showed an increase in noise. Then Andrea on this forum stated he used a Baader neodymium filter on his modded setup as gave increased contrast. I tried this and found this to be very much the case. I suspected Infra Red was the culprit here but decided to pursue this further...

Consider the bandpass of the Coronado ERF as used on their SMxx series:


Cor-SM60-ERF by Mark Townley, on Flickr

Very Effective at cutting light
BF15 by Mark Townley, on Flickr

Made with 2 filters, the one at the eyepeice end has a very narrow bandpass at 656nm


Cor-BF15Ha-zoom blocker by Mark Townley, on Flickr

But if we zoom out can be seen this too is vbery leaky at IR wavelengths...


Cor-BF15Ha- EP Side by Mark Townley, on Flickr

So, to stop our eyeballs boiling and filters failing due to thermal loading the other filter (the mini ERF) on the blocker is employed, and will come as no suprise that this blocks at IR wavelengths, whilst letting through Ha...


Cor-BF15 MINI ERF by Mark Townley, on Flickr

The result is the nice safe view that we all love!

So, you can see by removing the mini ERF you are flooding the blocking filter and also your eyeball with Infra Red radiation - Nice! this accounts for the blooming and noise I was talking about in my images at the top of this thread. Also accounts for why using the Baader Neodymium in place of the mini ERF restores contrast; it is blocking IR. However only does so to ~1200nm and can be seen that the Coronado does so to >2500nm - i'm thinking about boiling eyeballs again folks! I'm also thinking that while my CCD may not be able to register an image with light at >1200nm it is still being subject to thermal loading, and as we know thermal loading in a CCd translates into noise, which we hate nearly as much as clouds!

So, what is the answer? Well I looked for IR cut filters, but struggled to find ones that are effective above about 1500nm. So instead turned my attention to different glass types, as different glass has different transmission properties. in the end i discovered Schott KG3 - this is of particular interest to us solar astronomers; it passes the all important Ha wavelengths with decent transmission, but crucially blocks IR from abou 1000nm upwards. Here is a data sheet for it that shows this...

http://www.stockoptics.com/media/datash ... ss/KG3.pdf

So I decided to look into buying some - on UK website found it was £15 for a 25mm diameter unmounted peice! What a bargain! Looking on a few websites stateside the $ price is very comparable. I've bought one and eagerly await its arrival to test in place of the Baader Neodymium.

I wonder if this may be the economical fix for a failed coronado blocker? I'm curious as to what you all think? Certainly all the curves add up, and if anything provides better IR blocking than the original mini ERF...

I shall report back with findings ;)

Mark
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by swisswalter » Wed Mar 07, 2012 4:49 am

Hi Mark

thank you very much for your very valuable investigation. I'm looking forward to your future findings
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Wed Mar 07, 2012 6:34 am

Mark,
Interesting stuff!
Why did you not consider just adding a UV-IR Cut filter?
Also, if you have a D-ERF up front (a la the PST Mod) doesn't this supperss the excess IR?
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Mar 07, 2012 7:12 am

Excellent Apollo! We're flying on parallel paths again - great minds think alike ;) Don't remember you talking about it in the CN days buy hey ho, interesting links thanks! :)

Hi Ken, I had considered various options including those you mention, problem is they don't block >1500nm, for instance a baader UV/IR filter...


Ba-UVIR-lin by Mark Townley, on Flickr

Transparent above 1300nm...

Even their C-ERF and D-ERFs are leaky above 1300nm...


Ba-CERF-lin by Mark Townley, on Flickr


Baader-D-ERF by Mark Townley, on Flickr

I'm not saying they're unsafe, just remvoving the mini ERF from the blocker you are letting through a load of IR that is not being let through when it is in place...

Using Schott KG3 this is well and truly blocked...


KG3-3-lin by Mark Townley, on Flickr

The Schott KG1 is another alternative, slightly higher transmission at 656nm, with pretty much as good blocking capabilities in the IR...


KG1-2-lin by Mark Townley, on Flickr

I think it gives us plenty of food for thought anyway.

Mark :)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Photonist » Wed Mar 07, 2012 8:08 am

Mark,

Good food, thanks.

Viljo

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Wed Mar 07, 2012 8:47 am

IMHO I don't think anything >1500 means much, so the usual D-ERF should be OK.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Mar 07, 2012 9:33 am

What about the front lenses? Does high IR pass?

Depends on what they're made of... ;)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Mar 07, 2012 9:43 am

IMHO I don't think anything >1500 means much, so the usual D-ERF should be OK.

Yes, if the mini ERF is in place and blocking it, but with removed everthing >1500nm is coming through, i'll bet beer on the fact this >1500nm IR puts a thermal loading on a ccd chip that results in extra noise. If IR >1500nm is inconsequential then why would manufacturers of solar telescopes block it? I'll have a look round for some info on IR and the human eye too. Either way, £15 for a peice of KG3 the sit in place of the removed mini ERF is worth it, and again will bet beer will result in better images than with it not in place... KG3 should be with me by weekend when I will feedback with findings... :)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Mar 07, 2012 10:21 am

IMHO I don't think anything >1500 means much, so the usual D-ERF should be OK.

From an eyeball point of view I thought this was interesting...

http://en.wikipedia.org/wiki/Laser_safety

All our 'normal' filters block the near infra red, but there are implications in the infra red itself >1400nm. With this in mind I don't think I would be visually observing in a coronado scope with the mini erf removed...
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Wed Mar 07, 2012 12:17 pm

Mark,
The laser beams are not similar to the solar energy distribution.
http://www.icnirp.de/documents/infrared.pdf
Check out page 640..
"Since wavelengths greater than 1,400 nm do not
contribute to the retinal hazard, this limit does not relate
to IR-C exposure."

IR-C above 1400 does not affect the retina.
If you look at the actual power loadings v's the solar spectrum, I think you will find they are very low ( I have some calcs somewhere I did a few years ago...I'll find them and upload..)- compared with the day to day IR lamps etc which can be harardous.
Found some published figures which show the total IR energy loading from the sun - no added filtering for the cornea/ eye is less than 10% the minimum safe ICN threashold....so basically > than a x 10 safety factor when IR-A is filtered out with an D-ERF or similar.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Mar 07, 2012 8:39 pm

Thanks Ken, an interesting read there. Wavelengths >1400nm definitely don't contribute to retinal damage as they're absorbed by the cornea and lens, this is my concern. I guess I probably am being over cautious here, but anyways... Postie tried to deliver the KG3 today but required a signature, so arrangements made for redelivery tomorrow. Should make things cloudy here in the UK for weeks to come so can't give it a test :P
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Pedro » Wed Mar 07, 2012 9:54 pm

I used a KG3 filter a lot for CCD deep-sky imaging with camera lenses. It improves the resolution considerably.

See this link:

http://www.astrosurf.com/audine/English/result/scan.htm

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by andreahaku » Wed Mar 07, 2012 10:07 pm

Ciao Mark,
Very interesting findings.
I'll try with one of the IRCUT fitlers I have and see of they give good results also.
Thanks for your analysis.
Andrea

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by peter drew » Wed Mar 07, 2012 10:10 pm

I've been using Ha telescopes for at least 10 years beginning with one of the first Coronao Helios1 70mm and several early PST's including a pair set up as a binocular. None of these had any form of extra ERF such as the current mini ERF and I can still see :) At no time can I recall any related discomfort or after effects despite prolonged use. I use the mini ERF in my new 150mm PST mod simply because it seems to give a darker background and slightly better contrast than without.

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Thu Mar 08, 2012 6:36 am

I agree Peter!
Remember that the etalon alone reduces ALL the energy - across the whole spectrum - by at least 90% ( the 0.8A bandpass every 10A)
I will find the other calcs and post them....
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Thu Mar 08, 2012 2:40 pm

Mark,
Have you seen any extended transmission curves for the Baader, Astronomik, or Astrodon DSO Ha filters?
I'd be interested to see if the 3nm/6nm/12nm filters also have an IR leak.....
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Thu Mar 08, 2012 7:36 pm

Mark,
Have you seen any extended transmission curves for the Baader, Astronomik, or Astrodon DSO Ha filters?
I'd be interested to see if the 3nm/6nm/12nm filters also have an IR leak.....

Hi Ken,

Only thing i could find on the Astrodons was this





On their website the quote blocking to 1150nm to 4 OD. Looking at that curve the transmission is starting to rise above 1200nm... I think the only way to block out IR >1500nm is with the type of glass used rather than filter coating...

Good news is the KG3 arrived today - looks like a bog standard clear peice of glass apart from it has a greeny tinge to it. Hoping the sun comes out this weekend so I can get chance to play with it! :) First light report hopefully sooner rather than later...
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle » Fri Mar 09, 2012 3:13 am

Hi Mark,

FYI, I think Lunt may have changed to using KG glass as a replacement for the traditional ITF in the blocking filter. They had some recent isolated blocking filter OEM coating failures, and Rikki H. stated the problem was a failure of the "anti-reflection" coating on a "clear glass" filter element. This element used to be labled as the ITF in their production drawings...and I too have noted the similarities of the transmission curves.

I'm looking to see where I can obtain a ~ 50 mm KG filter with optical quality surfaces B)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Fri Mar 09, 2012 7:27 am

Woohoo! Bob Yoesle is here! Welcome Bob!!! B) B) B) You gonna love this place ;)

VERY interesting about Lunt potentially using KG3, I remember when the coatings failed on those blockers over on CN...

In terms of sourcing KG3, certainly the places I was looking at in the UK for sourcing mine all quoted the 'flatness' value lambda/4 or 6 seemed to be the norm.

There's a 50mm peice here for not much http://www.ebay.co.uk/itm/110522395714? ... 1423.l2649 and I think Edmundoptical/scientific (sp?) in the states carry stock too.

You should check out schott ug11 http://www.uqgoptics.com/pdf/Schott%20UG11.pdf to use a blocker in your CaK module ;)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Derek Klepp » Fri Mar 09, 2012 12:01 pm

Thanks for this wealth of info and welcome Bob.

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Fri Mar 09, 2012 7:40 pm

Bob,
good to see you on board. I look forward to your contributions.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle » Fri Mar 09, 2012 7:45 pm

Hi and thanks Ken, Derek, and Mark!

Nice to be here (been lurking for a while).

Checking the transmission curve I'm not sure that UG filter would make too good a blocking filter for use at 394 nm. Right now I'm using a Baader CaK filter as a blocking filter located just behind the Coroanado CaK PST primary filter set, and I'm using a Baader Blue CCD filter as an ERF, which has an excellent transmission aat 394 nm: http://www.sonnen-filter.de/Filter-1/BAA-blau-G.gif





The KG galsses all appear to have good transmission at 394 nm, so I'm thinking of adding a KG filter in order to allow use of my CaK module with greater than 100 mm of aperture - for example my AP 130. I would add it just ahead of the Baader Blue CCD to protect all the "downstream" filters...





The KG filter would also be excellent for protecting an existing H alpha ITF & trim filter, as well as be a potential replacement for a failed ITF... Edmund Optics offeres them in a variety of sizes that might be readily adapted to astronomical use: http://www.edmundoptics.com/products/di ... uctid=1934

I'm also working on protecting the OEM filters from moisture - stay tuned.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Fri Mar 09, 2012 11:04 pm

I was more looking for the 35nm Ha Baader filter



As you can see, it looks much better with no transmission up to 1200nm. After, did anybody made an analysis?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Fri Mar 09, 2012 11:41 pm

Hi Bob,

KG3 and the Baader blue would indeed make an excellent combination for the CaK module - I was looking at the 'cheapo' options with the UG11 :P

DSObserver, I considered the 35nm alternative over the the 7nm however went for the 7nm as gave less thermal loading on the the blocker... I don't really think there is much in it with either filters regards their transmission >1150nm, the fact that all the Baader curves cut off at 1200nm suggests to me that the flat response ends abruptly and transmission increases rapidly in the IR - if it didn't they would be showing it in the response curves to shout about it. I've not come across any dielectric coated filters that cut above ~1500nm effectively - to do this you need a heat absorbing glass such as the KG3...

Its nearly midnight here now and have clear skies, I hope these remain in the morning - if it does I will be able to get the pics to demonstrate from an imaging perspective that the use of KG3 in place of a failed ITF in a Coronado blocking filter is a cost effective and viable alternative to replacing a blocker directly from Coronado...

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