BF transmission values

[A McEwan]

I was just wondering if anyone had seen transmission % values for the Lunt and Coronado blocking filter diagonals. I wondered if there was a difference between them.

Reason? To increase brightness of my double-stacked Lunt scope. I have been told that the image brightness drops because of the double-etalon reducing the bandwidth and therefore brightness, along with a 10mm aperture drop (50mm DS filter on 60mm scope), but it's also been suggested that not all blocking filters share the same transmission performance.

Thanks,

Ant

[Peter Drew]

I doubt if there would be a noticeable difference and there is always a chance of incompatibility by mixing different manufactures components. The reduction in transmission due to double stacking seems to be the accepted price to pay visually for the enhanced surface detail effect, photographically this is addressed by longer exposures. You could try <www.solarastronomy.org> for an authoritative answer.

[E621Keith]

It is questionable whether the blocking filter actually blocks light from around Ha. If you consider the visible spectrum is 310nm wide and your DS scope is letting through 0.05nm, that's a reduction of 99.984%. While Baader film reduces white light by 99.999% (i.e. it let through 15 times less light), a factor of 15 is light gathering the difference between a 8" and a 50mm. It is reasonable to assume the blocking filter do no brightness reduction around Ha at all, especially after you considered the fact that most energy in sun light is concentrated in the green part of the spectrum.

The blocking filter's main job is to block the secondary wavelengths let through by the Etalon. It's a band pass filter rather than a boradband ND filter. I think when people talk about transmission, they are talking in terms of bandpass.

Unlike other Ha, solar Ha is an absorption line (except promineces), so the aim of a Ha scope is to see the shadows cast by hydrogen absorption in the chromosphere. When you look at the sun's spectrum, it's a continuous white light black body spectrum with a number of dark absorption lines, one of which is Ha. It's the reason why filaments appears as dark lines against the sun disk (It's essentially greater concentration of hydrogen). Double stacking enhances contrast by reducing the bandpass and removing even more white light around the Ha line, so the absorption line features will stand out better. Basically that brightness reduction is what you paid for.

If I am right about blocking filter not blocking any Ha, then the only way to increase brightness of DS Ha scope is to buy a scope with a bigger aperture.

[A McEwan]

If I am right about blocking filter not blocking any Ha, then the only way to increase brightness of DS Ha scope is to buy a scope with a bigger aperture.

There's the rub! And to be honest that is what I thought the conclusion might be.

Brilliant explanation though Keith, very succint explanation of what H-alpha scopes actually do. Given that there is probably no way to visually brighten the image I've got, and I'm not going to splash out on a 80mm DS setup, then I shall be happy with the view I'm getting. The increase in disc detail is indeed impressive, and is indeed what I paid the money for.

Thanks,

Ant

[Merlin66]

At Ha the blocking filter is about 50% transmission..the Coronado and Lunt are about the same.

If you are imaging, you can increase the amount of light getting through the system by removing the "mini-erf" from the front of the blocking filter - need to check my records but it's around a 20% improvement. The up front ERF on the 50mm DS then provides the protection etc.

[A McEwan]

At Ha the blocking filter is about 50% transmission..the Coronado and Lunt are about the same.

If you are imaging, you can increase the amount of light getting through the system by removing the "mini-erf" from the front of the blocking filter - need to check my records but it's around a 20% improvement. The up front ERF on the 50mm DS then provides the protection etc.

Presumably that is only for imaging? Not safe for visual use then?

Ant

[Merlin66]

It will also work safely for visual providing the front DS etalon with it's ERF is always in place.

[Uplooker]

You could perform a Stage 1 or Stage 2 PST Mod. Just do a search on here or anyother astro forum for "PST Mod". Ideal scope to use is an F/10 with an apperture of 70mm plus to make the mod worth the expense. Essentially you could end up with a 4" H-alpha scope that if you were to buy off the shelf would cost in the region of £6-7000.

Not done the mod myself, as yet, just assembling required components.

Merlin66 and Peter Drew are the experts as they have modded more than a few scopes

Ian

[Merlin66]

The Lunt 60 can also be modded to a larger aperture....similar (but different) process.