Spectrum isolation filter recommendation?

[vlaiv]

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?

[robin_astro]

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

[robin_astro]

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

[vlaiv]

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).

[Merlin66]

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.

 

[robin_astro]

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