Blockfilter performance and f-ratio

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Re: Blockfilter performance and f-ratio

Post by marktownley »

answering this as a 'late night question' just before I go to bed, in theory, yes it should, but, in practice I have found no. The loss in transmission (i have found) does not equal the gain in out of band blocking...


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Re: Blockfilter performance and f-ratio

Post by Merlin66 »

I don't think the f ratio is critical...
I use the BF10 and BF15 on the TS102 f11 and also on the ED80 f7.5 - no significant difference.


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Re: Blockfilter performance and f-ratio

Post by swisswalter »

Hi Theo

in practise I can not see a great difference. At my place the seeing on a given day seems to make a far bigger difference


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Re: Blockfilter performance and f-ratio

Post by marktownley »

interesting idea Neo, certainly one to try...


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Re: Blockfilter performance and f-ratio

Post by Valery »

Neo wrote:remove the 2nd collimating element behind the PST's etalon and place the BF15 directly after the etalon and then add a converging element to your choice to obtain the desired focal ratio, depending on the camera of your choice. I'm already in the process replacing the front collimating lens so why stop there... ;)
You likely will obtain overlapped dim solar images caused by reflections between etalon and BF filter.
This is the reason why the ERF filters on each Coronado and Lunt front mounted etalons are inclined to the
optical axis for about 2 degree.
So must be inclined the BF, but this will cause the bandpass shifting.


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Re: Blockfilter performance and f-ratio

Post by michael.h.f.wilkinson »

I gather the blocking filter has a bandpass width of >10nm (I might be wrong of course, but a 35nm H-alpha filter from Baader works as a replacement). Tilting it by 3 deg give a 0.8-1 nm or so shift in centre passband, but that hardly matters. It does matter on the etalon, but the blocking filter acts as an order-sorting filter in this case, so its wings should ideally be at quite a distance from the etalon centre passband, so a slight shift should not matter.


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Re: Blockfilter performance and f-ratio

Post by swisswalter »

Hi Michael

an interesting sight of the things, thank you very much


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Re: Blockfilter performance and f-ratio

Post by michael.h.f.wilkinson »

Neo wrote:
michael.h.f.wilkinson wrote:I gather the blocking filter has a bandpass width of >10nm (I might be wrong of course, but a 35nm H-alpha filter from Baader works as a replacement).

I don't quite understand, Michael. As far as I know the 35 nm Baader H-alpha is sometimes used as a replacement for the ITF to block IR up to 1500 nm, not as a replacement for the Block filter. I'm using the 35 nm Ha for this reason in my setup myself. It's bandpass of 35 nm means 350 Angstrom.
You are right about the replacement. As I understand it the blocking filter is used to block all light except precisely one interference maximum of the Fabry-Perot interferometer. The latter has a transmission spectrum shaped like a comb, with peaks at all (half) wavelengths that "fit" in the etalon gap. Therefore, any filter with a passband centred even roughly on the H-alpha line, and which has a narrow enough passband to block the two nearest unwanted maxima (above and below the desired one) from the interferometer is OK. If the flanks of the transmission band of the blocking filter are far enough from the etalon interference maximum, a slight shift should have little or no effect on the total transmission spectrum of the system. I would actually expect a blocking filter to have a fairly wide passband, because designing it with a very narrow band means tighter tolerances in manufacture, and possibly lower transmission at the passband centre. Typically, but not always the narrower the passband, the lower the central transmission. It can easily be shown that having a passband slightly smaller than the spacing between two consecutive maxima of the etalon guarantees that only a single maximum is passed. You might want to go a bit narrower to ensure better blocking of the wings of the etalon passbands, but not much.

The situation is very different when you put another etalon in the optical path (double stacking). In this case, the two narrow pass bands overlap eachother, and the centre passband is made a lot narrower.


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