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...
User avatar
marktownley
Librarian
Librarian
Posts: 24357
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Mon Apr 23, 2012 8:13 pm

Bit of confusion regards terminology here; what you're refering too in this post as an ITF is actually an ERF. What Ken is calling the mini-ERF is the ITF. What you're calling the 'peice of glass' is actually the ITF.

In a unmodded PST both the ITF & the Blocking filter MUST be used in conjunction with each other to isolate the narrowband of light at 656.28nm to give us the Ha image we know and love. Look at the transmission curve of the ITF shown below




If you look on the left hand side of the graph, there is an attenuation of light below 656nm down to 375nm with greatest attentuation of OD3.5 @ 500nm. I don't have a curve to hand for KG5, but lets look at KG3 which is very similar





It has little or no attenuation below 656nm all the way down into the UV. This is why you are seeing the overly bright greenish image that you observe...

But why you then may ask does removing the ITF work with a modded PST, or in my SM40, well, it is down to the primary ERF that is used; The Baader D-ERF blocks all the way down to OD4 below 600nm >




The same principle applies with the 7nm Baader Ha filterthat i am using as an ERF - before the objective.

Does this make sense?

Mark
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

User avatar
DSobserver
Almost There...
Almost There...
Posts: 542
Joined: Sun Feb 19, 2012 2:36 pm
Location: FRANCE

Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Mon Apr 23, 2012 9:42 pm

Right, some confusing.

So if I understood right, the ERF is only rejecting light below 600nm not like baader's one which also reject IR.

At the opposite ITF reject IR but not all light bellow 600nm

So here are the working solutions :

FP>ITF>BF = PST
SM with ERF>FP>ITF>BF = PST DS
SM with ERF>FP>BF : not IR 100% safe but interesting to take pictures (focus)
SM without ERF>FP>ITF>BF
SM with modify ERF (red/deep red filter, Ha filter baader ERF)>FP>ITF>BF
SM with modify ERF (red/deep red filter, Ha filter baader ERF)>FP>KG5>BF = more or less same as PST DS
SM with modify ERF (red/deep red filter, Ha filter baader ERF)>FP>BF : not IR 100% safe but interesting to take pictures (focus)

But I still have some questions :
- If ITF is blocking green light, it means that BF is not blocking it : do we have BF transmission bellow 655nm?
- why do we need on SM scopes an ERF (that is not blocking IR) when on PST it is not use?

User avatar
Merlin66
Librarian
Librarian
Posts: 3140
Joined: Tue Dec 13, 2011 1:23 pm
Location: St Leonards, Australia
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Tue Apr 24, 2012 5:15 am

Sorry,
I still prefer calling the "Mini-ERF" a "mini-ERF" than a ITF - I think an ITF could be any filter....
The ERF in the SM60 etc is basically just a red filter....blocks below 600nm or so and passes everything above.
The curve above (for the "mini-ERF" not the blocking filter -title is incorrect) shows that it effectively blocks everything above 750nm
When the two are combined you end up with a bandpass of approx 600-750nm.
The Baader D-ERF has a narrow bandwidth (600-700nm) and blocks everything else (up to 1500nm).
Hope this helps.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Tue Apr 24, 2012 6:18 am

If you want to replace the Mini-ERF(ITF) filter. I think a KG5 filter followed by a standard astro H-alpha filter would fit the bill..., right?

etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Tue Apr 24, 2012 4:30 pm

Merlin66, For blocking filters I see 6A quoted in literature, how did you decide 6-8A? Would a spec of 7 be optimal?

Bob, what are you going to use instead of the KG5?

User avatar
Merlin66
Librarian
Librarian
Posts: 3140
Joined: Tue Dec 13, 2011 1:23 pm
Location: St Leonards, Australia
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Tue Apr 24, 2012 5:01 pm

(what's your name?)
The 6-8A is based on the finesse and resonance of the various etalons used by amateurs....
The PST blocking filter(s) are very similar to the Lunt.....
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

User avatar
DSobserver
Almost There...
Almost There...
Posts: 542
Joined: Sun Feb 19, 2012 2:36 pm
Location: FRANCE

Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Tue Apr 24, 2012 5:10 pm

I just add a 23A red filter in front of my SM40 and now it works again using a KG5 filter!

Now I need good and clear sky to judge how much removing from filter, replacing (for the moment) with red filter and using KG5 instead of mini-ERF, it improve the picture.

"If you want to replace the Mini-ERF(ITF) filter. I think a KG5 filter followed by a standard astro H-alpha filter would fit"

Fully agree

Now regarding the BF, 6 or 7 or 8A doesn't care for the moment as the main point is to find them at a good price :whistle:

etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Tue Apr 24, 2012 5:18 pm

Cool, I was thinking instead of an expensive H-alpha filter maybe a red 25 would work as well so let us know how the 23A red does for you!

User avatar
DSobserver
Almost There...
Almost There...
Posts: 542
Joined: Sun Feb 19, 2012 2:36 pm
Location: FRANCE

Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Wed Apr 25, 2012 5:17 pm

So this morning some quick test (before work and ...clouds :angry: )

As I said I'm using a DS PST and when removing front SM ERF and replacing the mini ERF with KG5, with red 23A filter is much better than without : I can see Ha sun. But it's still not perfect. The sun is bit orange and as soon as you tilt too much the SM it became green (again.... :X )

Most probably 23A is not enough. Could be that a 25A would be better.

I also could saw that apparently the rust on my mini ERF is affecting transmission 'cause picture is brigher with KG5 filter...

So :
1- I have to replace 23A filter with a "darker" one. I'll most probably use an Ha one and like this adding on top a KG5 I should be 100% IR/UV safe.
2- Find a solution to install my KG5 filter before BF and not after (on the eyepiece) as it is now. Any idea?

User avatar
marktownley
Librarian
Librarian
Posts: 24357
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Apr 25, 2012 6:17 pm

So this morning some quick test (before work and ...clouds :angry: )

As I said I'm using a DS PST and when removing front SM ERF and replacing the mini ERF with KG5, with red 23A filter is much better than without : I can see Ha sun. But it's still not perfect. The sun is bit orange and as soon as you tilt too much the SM it became green (again.... :X )

Most probably 23A is not enough. Could be that a 25A would be better.

I also could saw that apparently the rust on my mini ERF is affecting transmission 'cause picture is brigher with KG5 filter...

So :
1- I have to replace 23A filter with a "darker" one. I'll most probably use an Ha one and like this adding on top a KG5 I should be 100% IR/UV safe.
2- Find a solution to install my KG5 filter before BF and not after (on the eyepiece) as it is now. Any idea?

What are the transmission curves for a 23a and 25a?
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

User avatar
DSobserver
Almost There...
Almost There...
Posts: 542
Joined: Sun Feb 19, 2012 2:36 pm
Location: FRANCE

Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Wed Apr 25, 2012 6:33 pm

Maybe this is of any use to you.
When I used my PST for photographic purposes I noticed that my mini-ERF (ITF) was heavily corroded, and found that a 35 nm H-alpha filter can be a good substitute. (Although I am not sure for visual use.)

ok for photo bud not safe for visual!

here is the 23A :


and here the 25A :

User avatar
DSobserver
Almost There...
Almost There...
Posts: 542
Joined: Sun Feb 19, 2012 2:36 pm
Location: FRANCE

Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Wed Apr 25, 2012 6:36 pm

Here are some filter curves.
Go to longpass filters and scroll down a bit to find the 23a and 25a curves.

faster :lol:

User avatar
marktownley
Librarian
Librarian
Posts: 24357
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Apr 25, 2012 7:28 pm

You'll get absolutely no difference with the 25a over the 23a.

Look carefully at the y - axis of the scales on the different graphs for the transmission curves for the 23a, 25a and ITF filters; the 23a & 25a are % transmission graphs, the ITF filter in an OD scale - the latter is logarithmic - considerably different magnitudes of transmission...

In a stock DS PST, I have to be honest here, not sure why or what the logic would be for removing the ITF unless it was rusted beyond use?
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

User avatar
DSobserver
Almost There...
Almost There...
Posts: 542
Joined: Sun Feb 19, 2012 2:36 pm
Location: FRANCE

Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Wed Apr 25, 2012 8:44 pm

Sorry but I don't follow both of your points :

Interest to replace mini ERF for a PST, a DS PST or a SM is the same I think....

Regarding transmission curve between ERF and red filter, you're right, but still don't understand why tilting the red one, the sun become greenish...

User avatar
marktownley
Librarian
Librarian
Posts: 24357
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Apr 25, 2012 9:37 pm

Sorry but I don't follow both of your points :

Interest to replace mini ERF for a PST, a DS PST or a SM is the same I think....

Regarding transmission curve between ERF and red filter, you're right, but still don't understand why tilting the red one, the sun become greenish...

I think the absolute minimum if you want to replace the ITF is a nighttime Ha filter, the narrower the bandpass the better and appropriate KG glass.

If you tilt any filter, you blueshift it's bandpass, which is why I suspect you seeing your greenish sun when you tilt your red filter...
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Wed Apr 25, 2012 9:51 pm

I can see tilting a thin film filter changing characteristics, but how is that for a basic #23 red glass filter?

Stupid question #2, what machine is used to create these nice transmission plots? Never mind, Carl Zeiss M4-QIII Lab Spectrophotometer :cheer:

colinsk
Almost There...
Almost There...
Posts: 507
Joined: Sat Mar 10, 2012 9:30 pm

Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk » Thu Apr 26, 2012 1:06 am

We really should describe a homemade spectrophotometer. It would not be that hard if you had a nice blazed grating. The rest could be salvaged or cheap parts.
Thank you, Here are some references I have collected.



Etalon Article Part I

Etalon Article Part II

Solar Patents of interest

etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Thu Apr 26, 2012 2:32 am

Ok, I started a new thread for you :bow:

User avatar
Bob Yoesle
Im an EXPERT!
Im an EXPERT!
Posts: 449
Joined: Thu Mar 08, 2012 7:24 pm

Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle » Thu Apr 26, 2012 12:36 pm

Bob, what are you going to use instead of the KG5?

Hi Etat,

First, if I had a failed ITF, I'd get it replaced with another ITF, as they are relatively cheap. And this is BY FAR the safest route to take in protecting ones eyesight from far IR... I would only consider alternatives for imaging - not visual - use. But even then, why risk a mistake in use?

Trying to replace (or in my case protect) the ITF with a optically poslished 2" KG5 has resulted in quotes of $425-$750 (uncoated) and it would cost another $100 + per filter for AR coating, and then additional funds for mounting in a tilted cell, whereas a new replacement BF30 ITF would be around $100-$200, be coated, and fit the existing tilted cell.

My reason to consider KG5 glass was to protect the ITF itself from any residual short and medium IR (thermal loading/cycling heat) passed by a sub-aperture Baader DERF in my future 152 mm scope, and to protect the CaK module's PST filters for the same purpose in my 130 mm APO. For H alpha my alternative will be a Baader Red CCD filter. It too is way cheaper than a polished KG5 (optical polishing is a neccessity IMO), and likely is a much better quality substrate.

For my CaK module (again imaging only) I'll employ the currently used Badder Blue CCD filter, with a Baader ND 3.8 film filter on the objective - I will have to see what the CaK exposures are with this set up to know if it is practical. If not, the "bare" Baader Blue CCD filter will have to do, and may have to be replaced if it fails down the road from excessive heat. Same cost issue as above - e.g. way cheaper than a polished KG5.
Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Dark-Sky Defenders

etatsolarchat

Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Fri Apr 27, 2012 4:46 am

Awesome, thanks Bob. I'll follow your lead on the 152mm so eager to hear know how it goes!

Maybe if a bunch of us start pestering Baader, they's do a polished/coated KG5 and sell it for $100 ;)

Oh, the E, is for Eric maybe I can change that...

User avatar
marktownley
Librarian
Librarian
Posts: 24357
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Sun Jun 24, 2012 7:19 pm

Just as a bit of further information on this post you may be interested in...

I recently compared the ITF from my BF10 with the ITF from my PST. They are different. The view through the PST one is brighter, and comparing views holding them up to the sky and viewing with naked eye, the PST one seems to pass more shorter wavelengths. Physically they also appear to be different filters; even though they are the same size, the PST one has a black cell that it sits in (similar I have to say to some of the Omega filters I have seen). Using either in combination with my PST mod results in reflections galore that reduces contrast. The best results I am getting with my PST mod is with the ITF removed and a peice of KG3 and Baader Neodymium in place...
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

colinsk
Almost There...
Almost There...
Posts: 507
Joined: Sat Mar 10, 2012 9:30 pm

Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk » Mon Jun 25, 2012 2:51 am

OK
Let's walk through a PST...
The front objective is just AR coated - nothing special
The comes the tuneable PST etalon (Fabry-Perot etalon filter) - the heart of the instrument.
In the bottom of the eyepiece holder there's a small "Mini-ERF" to reduce the energy loading on the...
Blocking filter -positioned in the top eyepiece adaptor. This "sorts" the "comb" from the etalon and only allows the Ha bandwidth to pass through....
(In the Coronado BF diagonal there's also the "mini-ERF" and final blocking filter.)
Hope this helps.

This is for the new or repaired PSTs. The Gold objectives were:

ERF>Objective>Etalon>pentaprism>BF>EP

The first repair was the dark blue objectives and I don't know if the second piece of glass was added then. Then the second repair was

Objective>Etalon>Pentaprism>ITF>BF





I should note that I have seen both silvered pentaprisms and narrow band reflection coated Pentaprisms.

The narrow band reflection prism would leave green and blue out of focus suns inside the black box.
Thank you, Here are some references I have collected.



Etalon Article Part I

Etalon Article Part II

Solar Patents of interest

markthais
Oh, I get it now!
Oh, I get it now!
Posts: 44
Joined: Fri Jun 01, 2012 5:26 am

Re: Blocking Filter, Mini ERFs and Alternatives

Post by markthais » Mon Jun 25, 2012 7:50 am

KG glass is good for cutting the IR. That is what it was designed for. The problem is that it is not like BK-7, it has strains and homogenous problems. But on the other hand if you keep it near the eyepiece end it should work fine. It is not a good choice at the objective end. You would be hard pressed to find a piece that would pass 1/4 wave on transmission at 650nm larger then 2".
They also mention that the etalon has band passes across the whole spectrum. Depending on the design of the mirror the etalon may only have peaks +/- 30nm from the center wavelength of Ha then it is just transparent to the rest of the visible spectrum.
The blocker + red glass cuts the short side. The long side the filter can start transmit again around 900nm. If you only have a KG glass there will be a leak in the red.
The idea of using one of Baaders 7nm filters and the KG glass used near the eyepiece end should block any IR.

The scan of the BF15 should look the same on both sides at the base. It should go to zero(OD4-5) on both sides. Either it's the scan or they missed and thickness on the layers.

The attachment shows an etalon and a two cavity 1nm blocker,
Mark

Attached files

User avatar
Merlin66
Librarian
Librarian
Posts: 3140
Joined: Tue Dec 13, 2011 1:23 pm
Location: St Leonards, Australia
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Mon Jun 25, 2012 8:03 am

Colin, et al,
I believe when the change was made to the "blue" objective, the "mini-erf" was either added (for warranty repairs) or incorporated into new builds. I haven't come across a "blue" PST without the "mini-erf"
Interestly, I also haven't come across a version where the pentaprism is silvered....
We/I need to get organised to take a wider spectrum (ie 400- NIR) of the various elements.
The data I published previously (see library files) has a limited wavelength coverage and doesn't (similar to Mark's) show the full spectral extent. Next job on the list!
(I also think the mirror in the Coronado BF diagonal is a "cold mirror" which adds another complication.....
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

User avatar
marktownley
Librarian
Librarian
Posts: 24357
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Contact:

Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Mon Jun 25, 2012 8:38 am

KG glass is good for cutting the IR. That is what it was designed for. The problem is that it is not like BK-7, it has strains and homogenous problems. But on the other hand if you keep it near the eyepiece end it should work fine. It is not a good choice at the objective end. You would be hard pressed to find a piece that would pass 1/4 wave on transmission at 650nm larger then 2".
They also mention that the etalon has band passes across the whole spectrum. Depending on the design of the mirror the etalon may only have peaks +/- 30nm from the center wavelength of Ha then it is just transparent to the rest of the visible spectrum.
The blocker + red glass cuts the short side. The long side the filter can start transmit again around 900nm. If you only have a KG glass there will be a leak in the red.
The idea of using one of Baaders 7nm filters and the KG glass used near the eyepiece end should block any IR.

The scan of the BF15 should look the same on both sides at the base. It should go to zero(OD4-5) on both sides. Either it's the scan or they missed and thickness on the layers.

The attachment shows an etalon and a two cavity 1nm blocker,
Mark

Very interesting input, thanks for that Mark :)
Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests