Blocking Filter, Mini ERFs and Alternatives

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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

I don't think it's the same problem... we need the blocking filter to have a large enough bandwidth to allow chasing the doppler shift...as long as the blocking FWHM is around 6-8A it will do its job very well.
( I'm hoping Colin will do a spreadsheet based on the actual blocking filter bandwidth...I think it will show very little sideband passing. Each etalon blocks the other and as long as the blocking filter works within the finesse - there's nothing else left??!!)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Hmmm, I keep thinking about this continuum leakage and ways to reduce it... I have to say ever since double stacking I rarely find myself hunting around the doppler shift, I get a pretty decent view of proms and disk simultaneously, my full disk pics are a pretty darn good representation of what i see at the ep. Both etalons are locked down in optimum position and other than a lil' twist of my dew shield which acts like a very fine tuner I find no need to retune - counter intuitive I know; air pressure and temperature et al, but this is the reality of what I find... I do periodically hunt around incase there is a better view, but i seem to have etalon positions nailed.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by peter drew »

I've tried double ERF's and double mini ERF's as well as double BF's, no improvement unfortunately, but if you don't try...... :)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I've tried double ERF's and double mini ERF's as well as double BF's, no improvement unfortunately, but if you don't try...... :)

fair dinkum, thanks for that Peter! :)


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

The most interesting thing from Joe's spreadsheet was the double peaks from the side bands that leaked through the blocking filter from mis-tuned etalons. I really need to run through the math in that spreadsheet to understand what Joe was doing because his last post differs from my change to the spreadsheet and it is not fair for me to hack his spreadsheet without checking all the math. Joe did a great job and I just hacked in for a quick answer.

As far as the blocking filter specs they are now public knowledge.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

Colin,
I thought the bandwidth of the blocking filter in the spreadsheet was slightly greater than the 6-8A we currently have, hence the side band pass?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

It is going to take me a week to go through my notes form the etalon articles and see what spec I have. I know I have Coronado and DayStar's specs.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I was thinking about blocking filters (again!)...

If the blocking filter is tilted with respect to the optical axis - say as a result of focuser droop / sag, would it's response be slightly blue shifted compared to it's reference value?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

I was thinking about blocking filters (again!)...

If the blocking filter is tilted with respect to the optical axis - say as a result of focuser droop / sag, would it's response be slightly blue shifted compared to it's reference value?

Yes, When you specify a design you also specify the tilt. I look to see if I have some Dobroski articles. He is the coating god for the last 70 years!


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

I spelled his name wrong. This is the paper that started the whole mica etalon revolution.

“Mica Interference Filters with Transmission Bands of Very Narrow Half-Widths” by J.
A. Dobrowolski


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Yes, When you specify a design you also specify the tilt.

Hmmmm. Gives us another factor to consider when getting the most out of out Ha systems... :whistle:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Bob Yoesle »

I'm nearing completion of my new Ha blocking filter and CaK modules, which will be evacuated and filled with Nitrogen. These are essentially identical units with a sealed inner chamber intended to remove moisture from the primary filter elements and keep thermal stresses to a minimum. Therefore I'm hoping to reduce filter degradation/failure issues as much as possible. They incorporate a "mini ERF" in the form of Baader Red and Blue CCD filters ahead of the enclosed Ha BF assembly and PST/Baader CaK filters (the Ha system also uses a Baader DERF for the SM90's):




I'm trying to obtain optically polished KG5 filters to add in front of the Baader CCD filters.

Here's some of the components - Top row left - completed unit with Blue Baader CCD "mini ERF" - third row left to right - Coronado BF 30, PST/Baader Cak assembly, optical window in cell, bottom right - red Baader CCD "mini ERF." The windows (right) were obtained quite inexpensively from Surplus Shed - 50 mm in diameter and 10 mm thick, both surfaces 1/10th wave ( http://www.surplusshed.com/pages/item/pm1106.html ). Two pairs were AR coated by Majestic ( http://www.majestic-coatings.com/ ) specifically for 656 nm and 395 nm:




The modules will fit inside the AP 2.7 focusers with minimal backfocus issues - machining by Teleskop Service in Germany (very fine workmanship - http://www.telescope-service.com/ ):



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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

Very professional Bob! :bow:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

A new solar scope brand is born : Bobnado :lol:


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

New to the forum, first want to say thanks! I was SERIOUSLY distressed seeing other sites use monitoring / censorship on random topics. Really ruins the whole forum concept. Sure hope the trend eventually reverses some day and people grow their spines back!!

Anyway, Bob that filter looks great.

I'm interested in making my own "BF25". Since the blocking filter is basically 2 filters, was wondering if it's possible to just screw 2 filters onto the bottom of my camera? If so can you guys point me to the correct filters?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by jjk »

Mark,

What is the spectral response of your camera? Is it very sensitive to IR > 1400 nm?

Best,
John


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by colinsk »

You can order your own blocking filters. One element is a Notch filter centered on the H-alpha bandpass that matches the Free Spectral Range of your etalon. The out of band blocking is about ND5. The other filter is an ITF and is usually made from silver layers and is designed as a step filter to block IR all the way to IR-C. There are a number of custom filter coaters out there. Some of them will be able to know if there designs are rated for use in solar work.

Somewhere I have the part numbers for the Daystar blocking filters but it is not handy and there is no guarantee that the FSR will match your etalon.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

You guys talk of replacing the itf with a B+W 489 filter.

http://diglloyd.com/articles/Filters/sp ... W-489.html

Would this followed by a Baader 7nm H-alpha filter work?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

B+W489 is same as KG3 from schott and we saw that it was not enough. Best is KG5 from schott double stack with one other IR cut filter.

Now regarding Ha filter whatever the one you take, they are made for night sky and not enough selective for solar shots.


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by etatsolarchat »

OK, so KG5 filter, then some standard UV/IR blockerd followed by a notch.

Anyone know where I can get KG5, preferably 1.25 or 2in filter?

colinsk, wouldn't the free spectral range be the same for any .7nm etalon? Sorry not an optices engineer, but isn't this just a h-alpha filter with a really tight band of say 1.5nm? Which is why a baader at 7nm won't work?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by DSobserver »

For KG5 filter I bought mine here : www.stockoptics.com

Highly professional guys and reactive, but you have to mont them (as soon as it arrive I'll make a picture)

regarding Ha filter, don't forget that we are looking for <1A ones and that 7nm=70A!

I also have one question : ERF is always i front of BF what would happen if I put my new KG filter after it? would it damage it?


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by marktownley »

I also have one question : ERF is always i front of BF what would happen if I put my new KG filter after it? would it damage it?

Yes, The KG5 keeps the thermal load off the blocking filter, if the KG5 was placed behind thermal cycling / loading could damage the blocker...


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Re: Blocking Filter, Mini ERFs and Alternatives

Post by Merlin66 »

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 »

so what about this? :)


looks already better than standard BF


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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...


Catalin Fus

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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