Tests of Astrodon 5nm Continuum Filter

[avandonk]

Astrodon make a 5nm continuum filter that does not give you any emission data. Spectrum plot below. It is used to get a star field you can subtract from your emission images to get rid of star clutter. It does this with surgical precision and you have full control over the attenuation. I used RegiStar.

It is very useful when combining SII, HA, OIII etc. Annoying coloured stars and haloes can be eliminated.

This is the HH in 3nm NII with stars. 6MB

http://d1355990.i49.quadrahosting.com.au/2013_01/HH_NII_N.jpg


This is the HH with 140% continuum subtracted. 6MB


http://d1355990.i49.quadrahosting.com.au/2013_01/HH-140stars_NII.jpg


I used half the exposure for the continuum filter data as it has about twice the bandwidth of the 3nm NB filters.

This should be very useful when combining NB and RGB data. It is most useful for very faint nebular detail.

This is a bit of the Vela SNR showing far more very faint detail without the star clutter. 9MB


http://d1355990.i49.quadrahosting.com.au/2013_01/VSNR_NII-95stars.jpg

I subtracted 95% of the star intensity for the VSNR image.


What is even more useful if you take the continuum data just before or after or both the NB data it markedly reduces gradients due to light pollution and the Moon! This is more noticeable with wider fields.

A quick coloured image of the VSNR NII to red, OIII to green and blue 7MB

http://d1355990.i49.quadrahosting.com.au/2013_01/VSNR_NOO.jpg


Note all this data was collected in the last two nights with a near full Moon! Last night was punctuated by a lot of noisy fireworks. Have I missed something?


It will take practice to get it better. Moonless nights will get better results for the really dim stuff.


Bert

[Merlin66]

Bert,
Very interesting....
I'm sure you could similiar results with other filter combinations which absorb at the emission wavelengths you want and give good transmission in the other regions.....??????

[avandonk]

It is a matter of matching the whole optical train and filters for meaningful subtraction.

It just cannot be done with filters that are not close optically to NB filters.

If this was possible it would have been done by now

Star intensities are thousands of times higher than very dim emission nebulae. That is why there are still quite bright remnants of some 'subtracted' stars. The spectral emission of stars are also very different that leads to these remnants.

Once the colour data is plugged into the holes or dimmer remnants of the 'subtracted' stars the result would be far better than dealing with colour haloes of stars of overstretched narrow band data due to a paucity of inherent faint nebular signal to noise. Very weak SII causes those purple haloes in Hubble Palettes.


Bert

[Merlin66]

Bert,
I think the real issue isn't the optical accuracy of the filters- there are many good ones out there - it comes down to the availablity of the filter bandwidths...
Also, using very narrow band emission filters would limit the amount of "additional" starlight which would be recorded. Ha emission (other than Doppler shift issues) is only a few Angstrom wide......

[avandonk]

I think we are at crossed purposes here. What do you mean exactly?

Bert

[Merlin66]

Bert,
1. Using very narrow band filters <4nm will allow the recording of emission wavelengths without allowing to much extraneous starlight getting through.
2. The equivalent of the Astrondon Red Continuum, may be available centred on other wavelengths - blue? green?
Using these would allow more control over the corrections applied to the stellar light by suppressing at blue/ green wavelengths as well as the red end....
Just my 2c

[Peter.M]

I might be misunderstanding you but wouldn't the continuum filter be at the red end of the spectrum because the only broadband light getting through the narrowband HA filter is the red light. Subtracting blue and green continuum light from the HA image would over subtract the stars.

Would I be right thinking that if the red continuum has been subtracted from the HA frame then any stars remaining would be the HA emission from the star?

I have a background in spectroscopy (ICP specs) but havn't thought about this idea before. Very interesting.

[Merlin66]

Peter,
Yes you are correct, if the others were applied to the Ha frame...
I was thinking of the other narrowband filters OIII, Hb etc where the same corrections could be made to suppress the "extraneous" continuum.
When the images are combined (after correction) then a significant amount of the starlight across the broader spectrum would be suppressed.
I should have made this more clear....

[avandonk]

The star field from the continuum filter can be subtracted from any NB filter image or any image for that matter. It just works better if the optics are the same or very similar. The delta lambda over lambda is about the same for anything with a continuous spectrum. It is just a matter of controlling the amount by exposure and weighting for the subtraction.

The continuum filter will work for all NB filters.

You did see OIII data in one image below? Maybe this may tell you something!

Bert