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

<p>Says P. Veron: <a href="http://adsabs.harvard.edu/full/1986A&amp;A...170...37V" rel="nofollow">The lower limit [z=1.8] is set by the ultraviolet transmission of the atmosphere... between 3300 and 3600Å</a>.</p>

CC BY-SA 3.0
2014-04-20T13:38:58.780Z
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
CC BY-SA 3.0
2014-04-20T20:17:44.843Z
And isn't it ly-a you wanted to image?
Jeremy
CC BY-SA 3.0
2014-04-21T01:27:50.243Z
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
CC BY-SA 3.0
2014-04-21T07:34:06.707Z
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
CC BY-SA 3.0
2014-04-21T12:20:46.290Z
I tried to register but failed. U band is too wide. I need a narrow band image. sigh, maybe hst is necessary.
questionhang
CC BY-SA 3.0
2014-04-21T19:54:01.710Z
If you'd still like to give those scopes a go, find them on Facebook and mention your registration problems - they are very helpful. So far you have only mentioned sensitivity at 330nm, so what bandwidth are you interested in? How wide do you want the passband of the filter?
Jeremy
CC BY-SA 3.0
2014-04-22T09:11:06.733Z
You might like this: http://www.stevedawson.org/thesis/thesis.pdf
Jeremy
CC BY-SA 3.0
2014-08-22T19:45:09.573Z
Do you know the equivalent width of the Ly$\alpha$? If it's broad enough, you may be able to use a filter with a few hundred angstrom passband.
Aaron
CC BY-SA 3.0
2014-10-17T10:08:03.130Z
The issue doesn't appear to be whether a filter could be made, they don't seem to exist off the shelf because there is no point, as the atmosphere doesn't permit observations at the desired frequency
Jeremy
CC BY-SA 3.0
2014-04-20T04:36:18.120Z
2014-04-20T04:36:18.120Z
Jeremy
<p>If we want to check a QSO's lyman alpha narrow band image, could a ground based telescope do that?<br/></p>

<p>I mean under the circumstance that its lyman alpha shifts to 3300A.</p>

CC BY-SA 3.0
2014-04-10T20:33:44.293Z
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
CC BY-SA 3.0
2014-04-10T20:37:45.107Z
There are only certain windows of visibility for ly-alpha due to the atmosphere, requiring satellite/probe observations for frequencies outside those windows.
Jeremy
CC BY-SA 3.0
2014-04-11T01:26:47.363Z
Just want to do a narrow band photometry. I find Gemini-N (http://www.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
CC BY-SA 3.0
2014-04-11T05:28:29.203Z
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
CC BY-SA 3.0
2014-04-11T07:43:40.363Z
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
CC BY-SA 3.0
2014-04-12T02:53:02.900Z
Yeah, 330nm is pretty much the starting point for visible light, given the blocking below that. Yes, to avoid that, a space telescope is required. That isn't to say that you cannot accomplish a lot with a standard U filter ground based, but you'll find that 330nm is about the lower point in the usable spectrum, rather than a point you can target easily. But there are ly-alpha targets redshifted to longer wavelengths than 330nm, if those are of special interest.
Jeremy
CC BY-SA 3.0
2014-04-17T20:41:02.420Z
Can you identify an example target you are keen to measure?
Jeremy
CC BY-SA 3.0
2014-04-18T01:51:17.660Z
a quasar with z=1.7 is ok.
questionhang
CC BY-SA 3.0
2014-12-16T12:32:19.507Z
The atmosphere does not cut-off at 330nm. The transmission is still of order 50% (per airmass) (and of course depends on height), though it is dropping sharply towards shorter wavelengths but is still about 5% at 300nm. I guess that's why sunscreen is required.
ProfRob
<telescope>
What is the bluest narrow band filter mounted on ground based telescopes?
questionhang
# What is the bluest narrow band filter mounted on ground based telescopes?

(Asked by: username_0 on 2014-04-09)

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.

**Tags:** telescope

> **Comments:**
> **username_1**: 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?  
> **username_1**: There are only certain windows of visibility for ly-alpha due to the atmosphere, requiring satellite/probe observations for frequencies outside those windows.  
> **username_0**: Just want to do a narrow band photometry. I find Gemini-N (http://www.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?  
> **username_1**: 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.  
> **username_0**: 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?  
> **username_1**: Yeah, 330nm is pretty much the starting point for visible light, given the blocking below that. Yes, to avoid that, a space telescope is required. That isn't to say that you cannot accomplish a lot with a standard U filter ground based, but you'll find that 330nm is about the lower point in the usable spectrum, rather than a point you can target easily. But there are ly-alpha targets redshifted to longer wavelengths than 330nm, if those are of special interest.  
> **username_1**: Can you identify an example target you are keen to measure?  
> **username_0**: a quasar with z=1.7 is ok.  
> **username_2**: The atmosphere does not cut-off at 330nm. The transmission is still of order 50% (per airmass) (and of course depends on height), though it is dropping sharply towards shorter wavelengths but is still about 5% at 300nm. I guess that's why sunscreen is required.  

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## 1 answers

### Answer by username_1 on 2014-04-20. Score: 2

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

Says P. Veron: [The lower limit [z=1.8] is set by the ultraviolet transmission of the atmosphere... between 3300 and 3600Å](http://adsabs.harvard.edu/full/1986A&A...170...37V).

> **Comments:**
> **username_0**: 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.  
> **username_1**: And isn't it ly-a you wanted to image?  
> **username_0**: 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.  
> **username_1**: 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.  
> **username_0**: I tried to register but failed. U band is too wide. I need a narrow band image. sigh, maybe hst is necessary.  
> **username_1**: If you'd still like to give those scopes a go, find them on Facebook and mention your registration problems - they are very helpful. So far you have only mentioned sensitivity at 330nm, so what bandwidth are you interested in? How wide do you want the passband of the filter?  
> **username_1**: You might like this: http://www.stevedawson.org/thesis/thesis.pdf  
> **username_3**: Do you know the equivalent width of the Ly$\alpha$? If it's broad enough, you may be able to use a filter with a few hundred angstrom passband.  
> **username_1**: The issue doesn't appear to be whether a filter could be made, they don't seem to exist off the shelf because there is no point, as the atmosphere doesn't permit observations at the desired frequency  

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