Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_dslr_mirrorlesss_winners.thumb.jpg.9deb4a8db27e7485a7bb99d98667c94e.jpg

vlaiv

Spectrum isolation filter recommendation?

Recommended Posts

What filter would you recommend for filtering out second and higher order spectra with SA200?

So far my choice would be 495nm long pass from Baader - that will let me cover full 300-900nm in "two pass" recording - first unfiltered for 300-600 range, and second with filter from 500 to 900nm.

I'm probably about to press the button on it, but wanted to check if there are better options out there that I'm not aware of?

Share this post


Link to post
Share on other sites

Hi Vlaiv

There is no light in astronomical spectra below 3100A (The atmospheric ozone cut off) so you can go for a higher cut off than that if you like . I used a 610nm Baader filter which I happened to have which has the advantage that the Sodium light pollution is kept out of  the IR spectrum.

http://www.threehillsobservatory.co.uk/astro/Classifying_red_stars_using_a_Star_Analyser_VdS_poster.pdf

In practise though you can typically measure up to ~7000A with the Star Analyser unfiltered as the signal in the second order is very low below ~3500A because of the low sensitivity of the camera and the high blaze efficiency of the grating

Cheers

Robin

Edited by robin_astro
  • Thanks 1

Share this post


Link to post
Share on other sites

Alternatively you can use a wedge prism to separate the orders vertically (like an echelle spectrograph) This does have the disadvantage that the spectrum is slightly curved though

Robin

Edited by robin_astro

Share this post


Link to post
Share on other sites

I did not know that lower limit is at 310nm - that is good to know, so any overlap starts at 620 and upwards - this means any Baader long pass will do, be that yellow 495, orange 570 or red one like you are using at 610nm.

I also have Baader UV/IR cut filter and one from ZWO. I was reluctant to use Baader one because transmission curve shows peak below 400nm (small UV leak) that would pollute 600-700 range - but it looks like this would not be an issue then?

Hopefully my camera is sensitive enough in range of up to 9000A - looking forward to play around with red shifts of quasars.

3 minutes ago, robin_astro said:

Alternatively you can use a wedge prism to separate the orders vertically (like an echelle spectrograph) This does have the disadvantage that the spectrum is slightly curved though

Robin

Oh, I've seen that one on some dedicated spectroscopy site - nice idea.

I was searching for "algorithmic" way to remove higher order spectra - but it looks like response of grating might not be linear - so that would not work (or not easily anyway).

Share this post


Link to post
Share on other sites

Vlaiv,

The grating dispersion is non-linear with wavelength, but usually so small it can be ignored.

Robin's given you some useful answers. I'd just use what you have and see how the results pan out.

 

  • Thanks 1

Share this post


Link to post
Share on other sites
2 hours ago, vlaiv said:

Hopefully my camera is sensitive enough in range of up to 9000A - looking forward to play around with red shifts of quasars.

Depending on what lines you are looking for you dont need to go that far into the IR. For example at high redshifts the Lyman jump in the UV at 1216A is shifted into the visible eg as here at z=3.87

http://www.threehillsobservatory.co.uk/astro/spectra_22.htm

and at low redshifts the H Balmer lines are still measurable without an order filter eg as here

http://www.threehillsobservatory.co.uk/astro/spectra_12.htm  (bottom of the page)

http://www.threehillsobservatory.co.uk/astro/spectra_21.htm

 

Robin

 

 

Edited by robin_astro
  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.