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 »

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 »

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 »

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 »

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 »

Mark,

Good food, thanks.

Viljo


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

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 »

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 »

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 »

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 »

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 »

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 »

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

best


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by andreahaku »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

Thanks for this wealth of info and welcome Bob.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

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 »

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 »

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 »

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...

Mark


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I suspect I have just had my half hour solar window for the day, and made full use of it to record some frames with the KG3 in place of the removed ITF. Not processed anything but can report the live view on the laptop screen was the best I have ever got; it is going to be very interesting to see the outcome when I process them. There was significantly more contrast on the disk and the noise levels at the black end of the histogram were reduced.

The eyepeice view is exactly the same as usual, but then again our eyes aren't sensitive to the wavelengths the KG3 is working it's magic on, unlike the DMK31.

Will say more about this later...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

Very good to hear Mark! Looking forward to seeing your results. The KG transmission curves make them look like a very suitable replacement for a failed ITF...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Very good to hear Mark! Looking forward to seeing your results. The KG transmission curves make them look like a very suitable replacement for a failed ITF...

Cheers Bob! My thoughts aswell - and at a price that is quite considerably less than a full replacement of the blocker... :)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

look's really interesting mark B)

I'm following all your experiences to improve my DS PST....

how did you manage to install the KG filter in an eyepiece?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

look's really interesting mark B)

I'm following all your experiences to improve my DS PST....

how did you manage to install the KG filter in an eyepiece?

Thanks! :)

KG filter was installed in a spare 1.25" filter holder I had then just screwed into the nosepeice of the blocking filter (objective side).


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Thought I would add todays full disk images taken with the KG3, and elaborate a bit more on the changes it has brought... First off the pics...


Ha Full Disk B&W by Mark Townley, on Flickr


Ha Full Disk Colour by Mark Townley, on Flickr

Just to clarify, when i'm doing my 'comparison' it is in the context of the coronado blocker with the mini-ERF or ITF filter removed.

The most obvious difference was the increased midtone contrast on the disk, really stood out on the lappy screen. This was also an improvement compared to my rusted ITF which I had removed. Next off was the complete absence of the pseudo spicule ring that is apparent with the ITF-less blocker; with my first images with the ITF less blocker I initially thought I was getting a great view of the spicule ring, really, it was a defocussed IR signature of the solar disk superimposed on the in focus disk. With the KG3 if you look closely on the images above you can see plenty of small scale prom detail close to the limb - bear in mind this is with 40mm aperture so resolution is always going to be my limiting factor... Next thing that struck me in the 'sky' around the sun was the lack of noise, looking at my ITF less images the sky was always speckled with noise - I figure if the sky has this faint snow of noise, then this snow is also present on the disk. Post processing was easier in the sense when stretching levels or doing I-USM it generates less noise in this process.

I'm going to do some more imaging with it tomorrow, weather permitting, initial findings are what I expected - in an ITF-less blocker IR >1500nm imparts a thermal loading on the CCD that manifests itself as noise; using KG3 blocks this and results in Ha images that aren't as noisy. Now, i wonder if this has a parallel in WL or CaK imaging??? ;)

Mark


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

Mark,
Certainly an interesting outcome....
The QE curves I have (DMK)show that the response out at the >1000nm is well below 1.8%...
The thermal effects/ noise would require the CCD to "warm up" I wonder if this is happening...
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Mark,
Certainly an interesting outcome....
The QE curves I have (DMK)show that the response out at the >1000nm is well below 1.8%...
The thermal effects/ noise would require the CCD to "warm up" I wonder if this is happening...
Onwards and Upwards

That's my gut feeling as to what's happening. It would have to be the (nearly) focussed IR directly on the CCD chip itself that causes the 'noise'.

Hmmm, more things for me to explore :P


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »



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...

Mark, so why not using only a KG3 filter in front of the SM instead of Baader Ha filter as SM ERF + KG3 filter as "BF" ERF?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

[quote]

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...

Mark, so why not using only a KG3 filter in front of the SM instead of Baader Ha filter as SM ERF + KG3 filter as "BF" ERF?

That is another option too - I got the 25mm peice of KG3 as was only £15(!!!) - shall have to look how much the 2" version is...

Todays full disk is a good one btw - no haze and better seeing, the contrast improvement is significant... Watch this space...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

just check
from edmund optic

25mm : 28,5€ and 50mm = 53,2€

and found here a B+W 489 filter in 43mm same as a KG3 ( ok to screw in front of a SM40) for 33€ :
[ebay]370590694120[/ebay]

But at the end, there is something that I can't understand : why do we need one ERF in front of SM and one before BF? :?

Mark, topic's title is BF & ERF alternatives. Any idea on BF alternative? :whistle: ..... :mrgreen:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

just check
from edmund optic

25mm : 28,5€ and 50mm = 53,2€

and found here a B+W 489 filter in 43mm same as a KG3 ( ok to screw in front of a SM40) for 33€ :
[ebay]370590694120[/ebay]

But at the end, there is something that I can't understand : why do we need one ERF in front of SM and one before BF? :?

Mark, topic's title is BF & ERF alternatives. Any idea on BF alternative? :whistle: ..... :mrgreen:

B&W 489 seems to be another viable alternative...

In terms of why there is a an ERF on the front of the SMxx etalons and one (the ITF) on the blocking filter unit - both block different wavelengths, so to effectively block across the spectrum both are needed in Coronados configuration.

If you're wanting to improve your DS-PST, the easiest and cheapest upgrade you could do would be to remove the ERF from the SM40 unit - the PST is safe optically (obviouslly!) - so extra ERF filtering is not actually needed on the DS bit...

As for an alternative to the actual blocker part, that's something else to look out for... :P


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »





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

I investigate and I just gate Baader's answer : "The filter will be open above 1200 nm because CCD´s are becoming insensitive at 1150 nm."

I think that it's clear enought... :(

Mark, I'm still fighting with my SM40 to unscew the Tmax : either I'm really :S or something wrong's happening. It's a quite old coronado SM40, could it be that they were "sticked" at that time?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

Not an unusual response. The QE of the typical CCD drops to almost zero beyond 1200nm, so any transmission will not affect the imaging.
(This is being investigated by Mark...)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

my question to them was what was Ha 35nm filter transmission above 1200nm and they clearly answer that above transmission drop; but as those filters are normally use for CCD (and not for visual as an ERF ;) ) it doesn't matter 'cause CCD are not sensitive above 1200nm


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

IMHO I'd think the extended transmission curve would be similar to the Baader UV-IR curve shown above - similar coating process??


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

my question to them was what was Ha 35nm filter transmission above 1200nm and they clearly answer that above transmission drop; but as those filters are normally use for CCD (and not for visual as an ERF ;) ) it doesn't matter 'cause CCD are not sensitive above 1200nm

The Baader ERF will not provide enough safety for Far IR. An ITF is needed for visual use. See "Glass Blowers Cataracts".


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

Hi Colin,

I agree completely that a DERF is not adequate by itself, but for far IR blocking the KG glasses look to be a possible alternative to an ITF as shown in their transmission blocking out to 2500 nm. Lunt appears to have gone this route as well, as noted above.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

I am working on getting an online reference posted but I think the EU says ND3 to 4000nm.

Edit:

I found this good resource but Ken wrote a more readable one.

http://www.icnirp.net/documents/broadband.pdf

It describes the effects to 3000nm.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

This site is the source of most of the solar filter transmission curves posted here, and has some references to the German/EU standards: http://www.sonnen-filter.de/

Translated - “EU standards, guidelines and recommendations presented in graphical form”:




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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

This is the chart I have worked from:




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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

I do not believe there is any possiblity of eye damage from solar radiation above 1400nm for the Baader D-ERF.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

I am working on getting an online reference posted but I think the EU says ND3 to 4000nm.

Edit:

I found this good resource but Ken wrote a more readable one.

http://www.icnirp.net/documents/broadband.pdf

It describes the effects to 3000nm.

Interesting....so according to schott curves, KG3 looks an alternative : http://www.schott.com/advanced_optics/e ... d_text=kg3


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Not an unusual response. The QE of the typical CCD drops to almost zero beyond 1200nm, so any transmission will not affect the imaging.
(This is being investigated by Mark...)

It's an unusual result; i've been looking back at my images with and without the KG3 in place. I wholeheartedly agree that the QE of a CCD drops to above zero above ~1200nm so all of the transmission above this should not affect the image - but it very clearly and most definitely does! It manifests itself as noise - this is really noticeable in the far left end of the shadows curve... This give noisier proms and a lighter sky background. I also find contrast better on the disk too. It is no difference at all visually with or without the KG3 in place, but imaging wise the difference is night and day. I would recommend anyone who has already removed their ITF from a Coronado blocker and is an imager to replace it with a peice of KG3 - you will be impressed with the results! Given the transmission, at 656nm, certainly of my peice of KG3 is higher than the original ITF, I would even go as far as to suggest it is a very worthwhile mod to replace anyway - shorter exposure times, higher mags etc.

What is going on with all this? I think (and it's only Townley theory!) Is that IR >1400nm imparts thermal loading on the CCD (chip, electronics - I don't know?) - whilst the CCD for imaging purposes are insensitive at these long wavelengths they are not immune noise wise to the effects...

Mark


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Coming back to this post...

(Started my response then had to go to work)

Regards eye safety an IR, i'm certainly no expert in this field, however I do take on board the energy levels of the IR involved are obviouslly a key factor. Whatever the details i will be leaving the KG3 in place all the time simply on grounds of improvements it makes to imaging and the fact it's easier leave on than taking on and off. Even if IR levels are safe already reducing thm further can only be a positive thing...


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