Comparing 0.3A and 0.6 A bandpass in Ha

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Re: Comparing 0.3A and 0.6 A bandpass in Ha

Post by jp-brahic » Fri Jul 15, 2016 7:42 am

It's completely abnormal to see the photosphere ( solar granulation ) by the transparency with such a filter!!!! Filter Daystar has generally a problem of transparency of the chromosphéric material, how to explain that a standard Etalon PST ( at 1A° ) shows much more chromosphéric material than a 0.3A ° Daystar?



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Re: Comparing 0.3A and 0.6 A bandpass in Ha

Post by Valery » Fri Jul 15, 2016 9:47 am

jp-brahic wrote: how to explain that a standard Etalon PST ( at 1A° ) shows much more chromosphéric material than a 0.3A ° Daystar?

PST etalons are VERY VARY in their bandwidth! I saw 0,9A - 1,0A and I saw as narrrow as <0,3A.

Similar variability in their bandwidth one can find in Quarks, Lunt (LS35 and LS50) and in Coronado etalons.

Your PST etalon sample is at least as narrow as <0,3A - hence your excellent images in H-a.

BTW. Christian confirmed by the math how much is a photospheric part in the tolal energy transmission in such a
filter like 0,6A.


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Re: Comparing 0.3A and 0.6 A bandpass in Ha

Post by christian viladrich » Sat Jul 16, 2016 6:15 pm

Hello Jean-Pierre,

You raised at interesting question. Here is my present understanding of the situation. It is based on some calculations, some tests and observations. Of course, I may be wrong somewhere :

For a given FWHM :
1) the transmission profile is exactly the same for an air-spaced Fabry-Perot (Coronado, Lunt, etc) and for a mica-spaced F-P (Solar spectrum, Daystar). There is no doubt about this ;-)
The bandpass at 10% transmission is 3 x FWHM, the band pass at 1% transmission is 9.95 x FWHM.
2) With a frontal step up (or in collimated light), the diameter of the sweet spot is smaller with an air-spaced F-P compared to a mica-spaced F-P.
3) In a telecentric set-up, an air-spaced F-P required a larger F/D ratio to reach the same resulting FWHM than a mica-spaced .

Another important point, is than the "contrast" of an Ha filter (ie its capacity to cut photosphere light, and let only chromosphere light through) is very badly defined by its FWHM. It would be more relevant to use the band pass at 10% transmission. This explains why stacking F-P gives very good results.

So ... back to the PE 0.6 A versus PST 1A. I guess the answer has been given by Valery ;)

I have looked through some PE0.6 or 0.5A, and also through some PST. There is a lot of variability on PST (and also on Quark). Some are very bad, a lot are average, a very few of them are excellent. There is less variability on PE.

For sure, a "good" PST doesn't have 1 A FWHM, and far from all PST have FWHM 0.3A.

I think Valery told us he tried about 10 (?) PST to nail an excellent one. I have no statictics there. Maybe, we can launch a pool on "how many PST you have to test before finding an excellent one" ;-)

This is not a critic to Coronado, since they adverstise 1 A FWHM. Indeed, we should be happy to find out a few PST with very narrow FWHM. The same holds for Quark.

There is also the question of the BF. All are not equal ...

Just my five cents on this interesting subject.

Christian Viladrich
Co-author of "Astronomie Planétaire"
Co-author of "Astronomie Solaire"

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