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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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Sat Apr 14, 2012 8:51 am

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 » Sat Apr 14, 2012 10:37 am

so what about this? :)


looks already better than standard BF

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Sat Apr 14, 2012 10:41 am

Hmmmm
You're getting confused between nm and Angstrom...
The band width of 10nm is equal to 100A....this is about 15 times too wide for a suitable BF (target 6A = 0.6nm)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Sat Apr 14, 2012 11:03 am

Hmmmm
You're getting confused between nm and Angstrom...
The band width of 10nm is equal to 100A....this is about 15 times too wide for a suitable BF (target 6A = 0.6nm)

:angry: same mistake as always :angry: :angry: :angry: :angry: :angry:

By the way I found this test on CN : "Just wanted to add that the standard 7nm Baader H-Alpha deep sky filter does not even work as a Coronado's block filter. Too wide bandpass, no details are seen. "

Now it's clear! :P

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Spectral Joe » Sat Apr 14, 2012 5:12 pm

I just discovered this thread, not paying attention I guess. Two things,first, the behavior of the BF in the skirts is at least (and maybe more) important as the basic bandpass. Good attenuation near Ha but poor 100A away won't help, since the modes of an etalon reach over a wide range of wavelengths, depending on the coatings. Second, for those wanting to stack color glass filters there is a very useful calculator on the Schott site: http://www.us.schott.com/advanced_optic ... index.html It's an Excel spreadsheet and has curves for all their filters in it. Takes a while to get used to but very powerful. You can enter data for your own filters as well, and come up with a properly convolved transmission curve for the system.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Sat Apr 14, 2012 7:08 pm

Merlin66 so a ITF filter with less than 6A BW is to little you think...and > 8 is too much..

Another issue, flatness and quality, obviously better can't hurt, does anyone know what we can get away with?

I found some with a 60/40 quality but no flatness spec...

Bob you have any luck finding a 1/10 KG-5 filter?

BTW, In my coronado, seems the ERF (objective side) has a slight tilt, but the ITF not.

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Sun Apr 15, 2012 9:17 am

With the filter position so close to the focus - "standard" quality - probably 1 wave/ inch would work.
The 6A would work, but so would a 8A, subject to the above comments about out-of-band leakage..
(On the Coronado BF diagonals the mini-ERF (ITF)is slightly tilted, but the final blocking filter element is not)
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Catalin Fus » Sun Apr 15, 2012 4:11 pm

[quote]Hmmmm
You're getting confused between nm and Angstrom...
The band width of 10nm is equal to 100A....this is about 15 times too wide for a suitable BF (target 6A = 0.6nm)

:angry: same mistake as always :angry: :angry: :angry: :angry: :angry:

By the way I found this test on CN : "Just wanted to add that the standard 7nm Baader H-Alpha deep sky filter does not even work as a Coronado's block filter. Too wide bandpass, no details are seen. "

Now it's clear! :P

What about this one :

http://tinyurl.com/6vx8jus

is in discussion over another thread and the bandpass is 0.15nm = 1.5A. This should be a good one I guess, in your quest for a DIY BF.

I have read this particular thread a couple of time and I understood that the BF (blocking filter) is comprised of 2 filters and a mirror.
From what I can see, a KG3 like filter (objective end) + gold coated mirror + H'alpha filter (eyepiece end)....please correct me if I'm wrong.

Thanks
Cata
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat » Sun Apr 15, 2012 5:05 pm

Cata, That sounds right, only confusion for me was which is called ITF..I thought it meant integrated thin film and was how the eyepiece filter was made but it is the objective side and means, Induced Transmission Filter.. :?

To clarify, According to Lunt,

1st filter (objective),
Induced Transmission Filter
Infra-Red Blocking Filter
IR Blocking E-5
Military Spec 810C

2nd Filter (eyepiece),
Blocking Filter
Ultra Precision Coated Narrow Band Filter
Optimized at 656.28
Bandwidth 6 Angstroms

Well Merlin66, theirs your 6A, and DSobserver I'm afraid the 1.5A IS to tight, I was hoping that would work...anyone know where I can buy a "cheap" 6A filter? The best Halpha filters are still at 30A at this point...

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Catalin Fus » Sun Apr 15, 2012 7:00 pm

There are two options from Lot Oriel and Cheshire Optics, with 10A (1nm) band. 12.5 or 25mm round, but they are pricey (~200$ / piece).
Regards,



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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk » Mon Apr 16, 2012 1:25 am

The bandpass of a coated notch filter can either be square or peaked depending on the number of layers. Also, the out of band blocking needed for night time work is much less than daytime. So, all 1.5A filters are not the same. It would be really helpful to have a diffraction grating around when looking at these parts. Before I bought a nice blazed grating I used a DVD.

With a grating, a slit formed from two razor blades on a magnet and a light power meter and you can make some basic measurements. If a filter is 1.5A wide and the out of band blocking is OD1 it is not as useful as if it is OD3. Without buying a $30K piece of equipment you can make some basic measurements.

I have read some measurements on the web but I have never bothered to collect them. We really should make a database. I know PSTs used at least three different systems over the years.

Induced Transmission Filters are usually made from silver. At least all the ones before Lunt. I am not sure what Lunt is doing. One day I'll have to measure my LS60DS as see how it was made. I like the factory setup so much I have not wanted to open it.

The acceptance angles of mica make a mica etalon much more attractive to me. A 40mm mica etalon placed at the focal plane of a telecentric F/30 light cone would allow full disk views of a 140mm objective. The etalon would have its bandpass widened by about .2A. So, a .5A etalon would render full disk views at .7A. If we stop down the aperture so the scope is operating at F/45 we can do much better, likely .5A across the visible field. Since we have not changed the focal length we can use the same telecentric system.
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 Bob Yoesle » Mon Apr 16, 2012 1:58 am

Bob you have any luck finding a 1/10 KG-5 filter?


Yes, I have a quote for two 47 mm diameter KG5 filters to ~ 1/10 lambda for $1500.00 USD. At this point I'm still waiting for a coating quote for UV blocking, as KG glass is stated to have "transmission changes" with "intense UV" as could be expected for the CaK scope.

But to be honest, replacing a deteriorated Baader CCD filter used as a "mini ERF," or even a failed ITF, would be much cheaper.

Moreover, Mark Wagner has advised that KG glass probably has poor optical uniformity... Therefoer I doubt at this point I will pursue the KG glass much further.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Tue Apr 17, 2012 5:58 pm

[quote]

Moreover, Mark Wagner has advised that KG glass probably has poor optical uniformity... Therefoer I doubt at this point I will pursue the KG glass much further.

well I'll be able to tell you this really soon! I just receive a KG5 filter that I can visually compare with a baader IR cut.

I let you know asap.

I've one new question. What would happen if in front of a sm we tilt an Ha night filter let say 6nm. Would it not slightly close the Ha window?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Tue Apr 17, 2012 6:55 pm

Well i'm perfectly happy with the results from my KG3 I have to say, no signs of any aberations with it in place...
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by sullij1 » Wed Apr 18, 2012 4:04 am

Thanks for saying so. I have been following along and now have a 25mm X 3mm KG3 in hand that I intend to add into my stack.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Wed Apr 18, 2012 5:28 pm

If anything, I have to say i think my images are BETTER with the KG3 in place than the ITF, not sure why but i think they are...
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Sun Apr 22, 2012 3:28 pm

I just tested my KG5 filter, and bad news. Apparently it doesn't works :

From my PST I remove the ERF and for the test screw directly the BF + eyepiece with KG5. But picture is completely greenish out of Halpha and too luminous :unsure:

The strange point is that I already saw that both ERF and BF are tilt and apparently it's on purpose.....

Any idea?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk » Mon Apr 23, 2012 3:46 am



The strange point is that I already saw that both ERF and BF are tilt and apparently it's on purpose.....

Any idea?

Flat optics are tilted enough to move the reflections out of the field of view. Otherwise the reflections would degrade the view.
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 DSobserver » Mon Apr 23, 2012 5:19 am

but here the sun is clearly to bright and green without any detail

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Mon Apr 23, 2012 5:56 am

I just tested my KG5 filter, and bad news. Apparently it doesn't works :

From my PST I remove the ERF and for the test screw directly the BF + eyepiece with KG5. But picture is completely greenish out of Halpha and too luminous :unsure:

The strange point is that I already saw that both ERF and BF are tilt and apparently it's on purpose.....

Any idea?

My KG3 is a clear peice of glass - rather weird :unsure: Is yours the B&W486(?) filter?
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Mon Apr 23, 2012 4:31 pm

mine is an official schott KG5 clear and bit grey....

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley » Mon Apr 23, 2012 6:00 pm

I just tested my KG5 filter, and bad news. Apparently it doesn't works :

From my PST I remove the ERF and for the test screw directly the BF + eyepiece with KG5. But picture is completely greenish out of Halpha and too luminous :unsure:

The strange point is that I already saw that both ERF and BF are tilt and apparently it's on purpose.....

Any idea?

PST's and blocking filters aren't the same thing, probably why doesn't have the result you're after... :(
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Mon Apr 23, 2012 6:37 pm

We're all agree that a PST BF is same as SM BF

So the question is : what is the piece of glass between the fabry perrot and the BF.

For your info, my PST was double stack.

So we have : SM40 without ITF>fabry perrot>piece of glass>BF> eyepiece : picture ok

After modification : SM40 without ITF>fabry perrot>BF> KG5>eyepiece : picture bright and green

So not too different from SM40 without ITF>BF> KG5>eyepiece that works, isn't it?

The piece of glass is same looking as the ITF : reddish. I remember now that in the first PST version this glass was integrated in the front lens. They change this point after rust issue.... :woohoo: ...could it be that my problem is that in my new modification I don't have any more one kind of ITF that blocks light bellow 600nm and I'm now collecting many rays from blue to red that mask the Ha one!

I remember that you replace ITF with Ha filter. Can you try removing it with original ERF and your KG3?

What's looking your ERF in front of the BF?

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 » Mon Apr 23, 2012 7:16 pm

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.
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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver » Mon Apr 23, 2012 7:42 pm

So apparently this ERF is not not only IR blocking but also bloking light below 600nm and can't be replace only with an IR blocking filter like a KG5

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