Blocking Filter, Mini ERFs and Alternatives

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

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 »

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 »

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 »

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 »

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 »

[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 »

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 »

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).


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

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.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

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 »

[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 »

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 »

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 »

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 »

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?


colinsk

Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »



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.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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 »

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 »

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


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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 »

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 »

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 »

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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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

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.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

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?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

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?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

(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.....


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

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!


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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 :


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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

faster :lol:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

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...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

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:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

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.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

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


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

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.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

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...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

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.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by markthais »

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


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

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.....


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

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 :)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

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.

I found this out long ago for my PST etalon. Unless you use a broadband mirror for the etalon coatings it will peak. I used a reflection grating and looked through mine at an incandescent bulb. The dark lines were confined to red and by green it was clearly broadband transmission. The peaks also got narrower and narrower as it moved from the design wavelength. This is of course easy to model in Excel if you enter a reflectivity curve because the finesse is reflectivity dependent.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Check this out...

[ebay]150809975052[/ebay]

In particular the transmission curves - blocking out to 3000nm to ND5. This could well be an affordable and serious contender for replacing rusted ITF filters...


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