Spectrometer study of H-beta, H-alpha filters and H-alpha etalon

[thesmiths]

I was assembling a new spectrometer using a grism (not meant for solar imaging) but decided to use sunlight for testing purposes. While I was at it, I decided to also test some filters. Here is an image of the spectral region near H-beta (in the blue) and the Magnesium triplet (in the green):

H-beta spectral region. 20 micron wide by 3 mm Thorlabs slit. 600 l/mm Paton Hawksley grism. ZWO 224MC camera.

h-beta no filter.jpg (47.58 KiB) Viewed 1378 times

Here is the same region with a Baader 8.5 nm H-beta CCD filter:

H-beta region with Baader 8.5nm filter

h-beta with filter.jpg (15.44 KiB) Viewed 1378 times

Similarly, here is an image of the region near H-alpha (on the right in the red) and the Sodium doublet (on the left in the yellow):

H-alpha spectral region

h-alpha no filter.jpg (40.22 KiB) Viewed 1378 times

Here is the same region with a Baader 7 nm H-alpha CCD filter:

H-alpha region with Baader 7m filter

h-alpha with filter.jpg (14.05 KiB) Viewed 1378 times

I should mention that it was possible to easily observe the shift of the filter region when tilting the filters. Since I was holding the filter with my fingers, I could shift the filter window by around 4 nm to the blue by tilting the filters to a high angle.

Finally, I put a PST etalon in front of the spectrometer. Here is an image of the H-alpha spectral region with the PST etalon a bit tilted so the interference fringes are not strongly visible:

H-alpha PST etalon (tilted and not aligned)

etalon not aligned.jpg (37.54 KiB) Viewed 1378 times

Here is the etalon aligned so that one of the interference fringes coincides with the H-alpha line. The fringes are not as sharp as usual (due to issues like proper collimation of the light beam) but gives a good idea how the H-alpha etalon functions.

PST etalon aligned with the H-alpha line

etalon aligned.jpg (79.84 KiB) Viewed 1378 times

Typically, a blocking filter would be used to remove the extra fringes and result in a spectrum that could be used for imaging.

[solarchat]

Beautiful post! I love my Shellac LHIRES III

[Montana]

Wow!! I love this, it demonstrates exactly how filters work and also why a wide bandpass is bad



Alexandra

[marktownley]

Excllent thread Douglas!

[thesmiths]

The etalon image above I found quite interesting so I decided to do a higher resolution version. I used our SHG setup (shown here: download/file.php?id=73934) and used the front filter of our Lunt 60 PT (a LS60FHa, used for double stacking). The spectrometer has a 2400 l/mm grating and a ZWO 183MM camera. The exposure was 2 sec at a gain of 264 (58%). I created a GIF with images with and without the etalon held in front of the telescope objective.

H-alpha line with and without H-alpha etalon

161122_H-alpha.gif (605.61 KiB) Viewed 1291 times

The telescope was pointed towards the Sun, which was behind some clouds, that provided a fairly strong but diffuse light. I could turn the tuning wheel on the side of the etalon and see how the dark line would get brighter towards both ends of the tuning range as the interference line became more off band. The middle of the tuning range was pretty much exactly overlapping the centre of the H-alpha line.

[christian viladrich]

Very nice !

Just for the fun of it, here is the transmission of a PST etalon observed at a higher spectral resolution.
First line is the solar spectrum.
Second line is the solar spectrum transmitted by the PST etalon tuned to Ha.
Third line is the solar spectrum transmitted by the PST etalon at normal incidence.

http://astrosurf.com/viladrich/astro/in ... -tuned.jpg

[thesmiths]

Just for the fun of it, here is the transmission of a PST etalon observed at a higher spectral resolution.

Hi Christian, that's also nice. I guess you removed the PST etalon from the normal front and rear lenses? And you did tuning via tilting rather than what is I think the normal compression tuning? Does the finesse really drop so much when tilting (as the last two photos seem to suggest)?

[Montana]

That's incredible!! I was wondering if I could use these images in a talk I need to prepare? they explain what the filter is doing so incredibly well.

Alexandra

[thesmiths]

I was wondering if I could use these images in a talk I need to prepare? they explain what the filter is doing so incredibly well.

Of course. The typical H-alpha blocking filter is something like a 1nm bandpass filter, I think?

There's a similar discussion of CaK filters here: viewtopic.php?t=37306

[highfnum]

interesting
i tried with beloptik Hb filter

Capture 2022-11-19T11_27_40hbbeloptikfikter.jpg (42.45 KiB) Viewed 1211 times

[KMH]

It's very interesting to see that - thanks for posting!

Kevin