Different Types Of Glass @ 656.28nm

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marktownley
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Different Types Of Glass @ 656.28nm

Post by marktownley »

Hi all.

It is clear from the CaK threads that certain types of glass are better suited / have better transmission at these shorter wavelengths.

My question is are certain types of glass better performers at 656nm for Ha? I have googled around but nothing is jumping out at me.

Thanks.

Mark


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Re: Different Types Of Glass @ 656.28nm

Post by brianb11213 »

AFAIK all the glasses sold for general optical use are very similar in transmission in the yellow / red end of the visible spectrum ... of course they have differing refractive index & dispersion, but the situation is not the same as it is in the extreme violet / near UV where many optical glasses (especially flints) absorb quite strongly.


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Re: Different Types Of Glass @ 656.28nm

Post by Merlin66 »

Mark,
Brian is correct. the issues with "standard" glass is the high frequency UV region.
Most/ all give good transmission in the red, and beyond.
I downloaded the Schott glass catalogue somewhere - this gave transmissions/ refractive index etc etc.
I'll see if I can find it....


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Re: Different Types Of Glass @ 656.28nm

Post by swisswalter »

Hi Mark

ISTAR is offering Ha optimised FH lenses. www.istar-optical.com


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Re: Different Types Of Glass @ 656.28nm

Post by brianb11213 »

But doesn't "Ha optimised" mean correction for minimum spherical aberration at 656 nm and/or optimum coating efficiency at that wavelength? The transmission is unlikely to be much different ... the real benefit of optimising in the red, rather than in the green, is likely to be that the objective works better in the infra-red.

This will very likely have a severe impact on optical performance in the violet.

My WO FLT 110 actually works quite well at 393 nm (CaK) but seems to be compromised with an infra red long pass filter fitted, even quite a mild one like the Astronomik Planet Pro 742. I'd expect a "Ha optimised" objective to perform very poorly with a solar CaK filter set fitted. Maybe this doesn't matter to the target market for these objectives ... but it's a factor solar observers should at least be aware of.


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Re: Different Types Of Glass @ 656.28nm

Post by swisswalter »

Hi Brian

I'm sure you are absolutely right. ISTAR even states that Ha optimised lenses are not suitable for other then Ha work


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Re: Different Types Of Glass @ 656.28nm

Post by marktownley »

Glad this is prompting a bit of debate.

Point i'm getting to with my original post I think; certainly for the collimating / relay lenses in a PST mod, these pieces of glass are available in a variety of flavours and coatings, i'm wondering if there is a 'best of the rest' alternative that could be used?


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