# What is the bluest narrow band filter mounted on ground based telescopes?

If we want to check a QSO's lyman alpha narrow band image, could a ground based telescope do that?

I mean under the circumstance that its lyman alpha shifts to 3300A.

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Are you asking what is the bluest commercially available filter, or the bluest ever commissioned by a professional observatory to make special observations? The former might be feasible to answer, but I cannot imagine that the latter will be, short of someone happening to know every single filter available to astronomers at every facility. I also don't quite understand the reason for your question. People are imaging ly-alpha sources and studying them spectrographically, so yes that is feasible. But why are you qualifying the question for ly-alpha sources red-shifted to 3000 Angstroms? –  Jeremy Apr 10 '14 at 20:33
There are only certain windows of visibility for ly-alpha due to the atmosphere, requiring satellite/probe observations for frequencies outside those windows. –  Jeremy Apr 10 '14 at 20:37
Just want to do a narrow band photometry. I find Gemini-N (gemini.edu/?q=node/10420) has a poor response at 3300A and the u-band filter is no longer available now.What is the bluest commercially available filter？ As far as you know, what is the bluest ever commissioned by a professional observatory? –  questionhang Apr 11 '14 at 1:26
Do you want to do photometry with your own equipment, or do you want to approach a facility to use theirs? Are you trying to source a filter for yourself (and if you find a place using a suitable filter, you will endeavour to also source one from where they did?) I think most U filters will be weak at 330nm. However, if you have special needs for a filter centred on 330nm, and you can fund it, I'm sure you could commission something... Who knows what others might have commissioned. –  Jeremy Apr 11 '14 at 5:28
I find someone bought a suitable filter for their Keck observation. I know the absorption by the atmosphere at 3300A on the earth is large. If we want to get a 3300A narrow image,a space telescope,i.e. hst, is necessary? –  questionhang Apr 11 '14 at 7:43

You probably will have difficulty imaging Ly$\alpha$ from a z=1.7 quasar at 330nm.

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Yes, the lowest redshift in their table 1 is 1.7. However their surveys are spectroscopic, a redshift lower than 1.7 is OK too.They can use other lines except lyman alpha. –  questionhang Apr 20 '14 at 13:38
And isn't it ly-a you wanted to image? –  Jeremy Apr 20 '14 at 20:17
Of course we talk about this problem under the premise that 3300A falls in the wavelength range. I should say larger than 1.7. 3300A low a response at a CCD's blue edge. It is marginal and maybe be not feasible. I do not know whether it is worth a try. I need to know how much feasibility is left. –  questionhang Apr 21 '14 at 1:27
Why not use the T17 telescope at iTelescope.net using the U filter, and image a target of interest. See how it comes out, maybe post the result here in an answer. –  Jeremy Apr 21 '14 at 7:34
I tried to register but failed. U band is too wide. I need a narrow band image. sigh, maybe hst is necessary. –  questionhang Apr 21 '14 at 12:20

Stromgren u just about incorporates 330nm, as does Bessel U, RGO U (all filter responses found here). But the bluest one appears to be the Gunn u-band filter (response reproduced below).

Of course this is the response without taking into account the atmosphere, which is sharply falling at wavelengths of 330nm, the response of the CCD and efficiency of the optics. The efficiency at 330nm of the Sloan survey using a Gunn u filter is about 9%; with a factor of two lost in the atmosphere. (http://arxiv.org/abs/1002.3701) and other significant losses caused by the CCD and mirrors.

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