Home Built Solar Spectroscope v6.5 -- focusing the sun and tracking it

[thesmiths]

I am omitting what might be called our "sixth attempt" from our on-going series of home built and modified solar spectroscopes. Version 6 enabled us to finally mount our a spectroscope on our EQ5 mount. This greatly increased our ability to actually capture sunlight by being able to point at the sun and to track it, instead of constantly adjusting a mirror. Our current apparatus, Version 6.5, adds an additional camera lens to focus an image of the sun onto the slit. This is basically what night-time astronomers do when focusing stars onto the slit using a telescope. Here's what v6.5 looks like:



We mounted everything on an ADM bar that is normally used to mount two telescopes side by side (e.g. hydrogen alpha and white light). A pair of BPM bellows pieces, used in a previous spectroscope, hold a 135mm camera lens (for imaging) and a 55mm lens (for collimating). The 1800 l/mm grating is mounted on a Baader tangent assembly that allows 3-axis adjustment (normally used for a finder scope). Some other ADM hardware holds everything together but allows lots of degrees of freedom to adjust distances and angles.

This time we used an Imaging Source DMK51 (our normal solar camera) to record the spectra. It has slightly smaller pixels than the DMK41 and quite a bit larger frame size to give a greater spectral range. A big advancement is the addition of the small 100mm f4 camera lens as a kind of telescope to focus the image of the sun onto the 5 micron wide slit. The extra light is very helpful in certain parts of the spectrum where the CCD is not as sensitive (i.e. ultraviolet). With the 100m lens focusing is very easy (built in helical focuser) as is stopping down the aperture (built in iris) so in many ways it is superior to a telescope. The foam board disk attached to the lens blocks out some of the direct sunlight as well as acting like a solar finder. The whole setup gets covered by a film changing bag (they still make those!) which has little openings for the arms that are useful for making adjustments like focus and grating rotation while keeping everything in the dark.



The image above is an relatively unprocessed image of the sodium portion of the solar spectrum (near 589nm). What's interesting here is that the image of the 3mm long slit is slightly larger than the height of the DMK51 chip. What you're seeing is actually the focused image of the sun on the slit. The angular size of the sun is approximately 0.5 degrees and a simple calculation shows that the diameter of the sun at focus is give by the lens focal length divided by 107, or about 0.9mm in this case (or about 1/3 of the image height, as observed).

You may ask, isn't it dangerous to focus the sun onto the slit? A quick calculation shows that the maximum energy is less than 1W per square millimetre. Thorlabs states that the slit is rated up to 1000W per square millimetre (they are used with high power lasers).



Above is a processed image of the Magnesium part of the solar spectrum, near 520nm. Compared to our previous attempts, it was much easier to get consistent results with everything placed on the equatorial mount and the lighting was much more even because of the 100mm front lens.



Here are the Calcium H and K lines near 400nm. These were previously hard to resolve since in this part of the spectrum there are relatively fewer photons and the CCD is also significantly less sensitive.



Finally, here are the A-band oxygen lines near 760nm. These were also previously hard to resolve without a front mounted "telescope" pointed at the sun.

For those interested, here are some of the technical specifications of the setup shown in the first photo. The angle between the collimator and imaging lenses was 18 degrees. The distance of the imaging lens from the grating was 220mm. The grating was oriented to -1. The measured dispersion was 14.2pm per pixel.

[swisswalter]

Hi Douglas

what a fine setup , great gear and mod, congratulations

[marktownley]

Wow! That is cool! 8)

[Merlin66]

Very good!
Excellent results!
I can see it's quickly reaching the stage of development were a separate spectroscope "housing" would be very beneficial.
The Classical design, as you see, works very well...when you get to the wavelength calibration stage - the non-linearity of the Classic is significant when compared to a Littrow.

[Montana]

Well done :

Alexandra

[solarchat]

this is so cool man. I love it and the spectrum looks as good as my $5500 LHIRES, definitely.

[Montana]

Congratulations on Pic of the day

Alexandra

[thesmiths]

Thanks for the encouragement. We've definitely learned a lot by building these instruments ourselves. I had no clue about this stuff before a few months ago. Like with anything, when you build it yourself, you start to really understand what the important parameters are. We learned a lot about bidding for stuff on eBay too.

[swisswalter]

Hi Douglas


congratulations on the pic of the day