Telescope Filters: Is Something Missing?

[randa177]

I have a question about the colored filters used for the telescopes. Here I will give an example for the filters of the big telescopes, but I assume that it applies also to the smaller personal telescopes.

The range of the visible light goes from about 400nm to about 750 nm. Where the "blue" has a range of 400–475nm.

Now, when we look at the list of some "blue" blocking filters for the Blanco 4m telescope for example, we see that it allows a transmission in the http://www.ctio.noao.edu/spectrographs/filters/blocking/Schott/wg345.gif"

which would not really be blocking the blue...

Is there anything that I'm missing here?

 

[Drakkith]

If the transmission range is 350-1000 nm, then it isn't a blue filter, and it really isn't a filter for any visible light.

 

[randa177]

But that's what it's used by the RC spectrograph for Blanco 4m telescope:
http://www.ctio.noao.edu/spectrographs/filters/blocking/Schott/wg345.gif

 

[Drakkith]

I really don't know, sorry.

 

[PAllen]

I have a question about the colored filters used for the telescopes. Here I will give an example for the filters of the big telescopes, but I assume that it applies also to the smaller personal telescopes.

The range of the visible light goes from about 400nm to about 750 nm. Where the "blue" has a range of 400–475nm.

Now, when we look at the list of some "blue" blocking filters for the Blanco 4m telescope for example, we see that it allows a transmission in the http://www.ctio.noao.edu/spectrographs/filters/blocking/Schott/wg345.gif"

which would not really be blocking the blue...

Is there anything that I'm missing here?

What I see is that following up from your link to the explanatory materials and the complete list of filters is:

1) They have a whole range of blue blocking filters, each blocking more wavelengths. Many of them clearly block all of visible blue light.

2) This is a spectrograph, not an observational telescope. What matters is the frequency range of the whatever medium is used to record the spectrum, not the human eye response. I would not be surprised if the cameras used can detect shorter wavelengths than the human eye.

 

[randa177]

2) This is a spectrograph, not an observational telescope. What matters is the frequency range of the whatever medium is used to record the spectrum, not the human eye response. I would not be surprised if the cameras used can detect shorter wavelengths than the human eye.

Can you explain this part more please? I didn't really get it.. sorry

 

[turbo]

Perhaps you can Google on "ugriz". This is an acronym based on the wavelengths that each filter will pass. SDSS observations rely on such separation to discriminate between objects, and sort out quasars at various wavelengths from really red stars, etc. My explanation is a huge oversimplification, but the SDSS program is quite well documented, so you can check in with them and learn probably more than you ever wanted to know about filters.

Without filters, the sensor(s) on such survey scopes would be swamped with photons most copiously-produced by galaxies and other objects, leaving the extremes in wavelengths "washed out" in the data.

 

[PAllen]

Can you explain this part more please? I didn't really get it.. sorry

I don't understand what you don't understand. This instrument takes pictures of spectra. The filters are designed to be used with the CCD camera sensors, not with the human eye. If the camera responds to shorter wavelengths than the human eye, then 'bluer' filters are useful to accentuate the shortest wavelengths that can be recorded.

[EDIT: I just verified that the CCD's used in this type of device respond well into the UV region, so the filters that eliminate essentially all of the visible spectrum are highly useful.]

 

[Redbelly98]

I suspect that "blue blocking" refers to the filter blocking wavelengths on the short-wave side of some cutoff wavelength. Just as "red shift" refers to a Doppler shift towards longer wavelengths -- if the wavelengths are say i.r. to begin with, they are not shifted towards visible red, but we still refer to a red shift.