A New Spectroheliograph

I LOVE finding out about different ways to appreciate the Sun and light in general. Use this forum to post your info or questions about various outside the mainstream ways to appreciate our life giving star!
Post Reply
User avatar
p_zetner
Almost There...
Almost There...
Posts: 1163
Joined: Sun Mar 18, 2012 4:59 pm
Location: Winnipeg, Manitoba, Canada
Has thanked: 33 times
Been thanked: 194 times

A New Spectroheliograph

Post by p_zetner » Fri Feb 19, 2021 11:50 pm

Hello Everyone.

I've constructed a second spectroheliograph and thought I'd post some of the details. Why a second SHG do you ask? I'm modifying the 1st one to give me significantly better spectral resolution (bandpass < 0.1 angstroms) in order to study narrower solar lines. My first experiments with the modified high resolution design
viewtopic.php?f=8&t=10029&p=100493&hili ... ng#p100493
showed that operation in the uv was significantly diminished. As a result, I've built the new instrument to work primarily in the blue and uv.
Fig-1.jpg
Fig-1.jpg (375.72 KiB) Viewed 257 times
The new device is based on an EG&G / Bausch and Lomb monochromator that I picked up from eBay for USD160.00. It has a 2700 lines/mm grating (31mm clear aperture) blazed for the uv and a nice grating drive with counter to be able to dial to a desired wavelength. The grating and associated drive mechanics are fixed to a cover which is removable from the housing. Ultimately, this could allow for changing gratings in the field. The optics of the original device are shown in the figure from the original patent application.
Fig-2.jpg
Fig-2.jpg (271.14 KiB) Viewed 257 times
In the original device, a single concave mirror was employed to image the entrance slit, via reflection from the grating, on to the exit slit. As a result, the grating was illuminated with divergent light rather than collimated light; not ideal from the point of view of aberrations. There was a cylindrical lens element incorporated to reduce aberrations arising from this effect as well as from the effect of the unusual direction in which light is reflected from the grating. From the point of view of SHG operation (and most spectrographs), the ideal configuration involves reflection from the grating such that the grating grooves are perpendicular to the plane of incidence. The B&L design has the grating grooves lying parallel to the plane of incidence. As I show below, this distorts the image in a skew sense. For the purposes of a monochromator, this is not a huge drawback and the cylindrical lens addresses the problem by introducing a second skew in a spectral line to align it with the exit slit. This was unsatisfactory for image formation, though.

The first modification step was to replace the concave mirror with a flat one and remove the cylinder lens as well as the slit assemblies that were attached to the housing. Exit and entrance roles were reversed and external lenses were used to collimate the beam at the grating and refocus at the camera. In my previous instrument, Pentax telephoto lenses are used for collimation and refocusing. Here, in the interest of enhancing the uv operation, I’ve used fused silica, plano-convex, singlet lenses for collimation (500mm focal length) and refocusing (350mm focal length). The original B&L monochromator was designed to operate over the range of 180nm to 400nm which is a bit too far in the uv. The limitation here was a mechanical one arising from the drive bar used to steer the grating. By bending and displacing the drive bar, operation down to 500nm has been obtained with the uv limit yet to be determined.

The telescope is an Astro-Rubinar 1000mm f/10 catadioptric with Pentax M42x1 screw mount. The slit holder assembly screws on to the Astro-Rubinar (AR) and holds the slit at approximately the required back focus distance of 45.5mm. The other end of the slit assembly slide fits on a 40mm diameter carbon fibre tube. The slit holder assembly uses a Jarrell-Ash, bayonet type slit holder, comprising a flange with tube where the outer diameter of the tube is an extremely convenient 1.25 inches.
Fig-3.jpg
Fig-3.jpg (8.74 KiB) Viewed 257 times
A 300mm field lens is held inside the assembly near the slit. The AR is fixed to a carriage which allows it and the slit assembly to be translated back and forth with a rack and pinion. In this way, the slit can be brought to the collimator lens focus with the rack and pinion while the focus adjustment of the AR is independently used to project a sharp solar image onto the slit.
Fig-4.jpg
Fig-4.jpg (516.01 KiB) Viewed 257 times
The complete instrument is fixed to an ADM 31” D-Series universal dovetail bar. For a digital SHG which relies on the Sun’s transit across the spectrometer slit, the long dimension of the slit needs to be oriented parallel to the flat surface of the bar. To accomplish this, the original monochromator needs to be mounted so that its own base is perpendicular to the bar surface. A piece of 4” x 4” x 3/8” aluminum angle, visible in the photos, was used for this purpose. The spectrometer mounting / demounting was designed to be sturdy yet relatively easy and reproducible, in case the overall weight of the assembled instrument + bar made it too onerous and unwieldy to carry out and fix to the mount. In my current condition of decrepitude, though, the weight and size are manageable.
Fig-5.jpg
Fig-5.jpg (124.75 KiB) Viewed 257 times
Arrows in the photo show the rack and pinion knob (1), locking knob (2), slit holder assembly (3), grating (4) and modified grating drive bar (5).

The spectral resolution of the instrument was measured by separating the telescope from the slit assembly and illuminating the slit with a mercury discharge lamp.
Fig-6.jpg
Fig-6.jpg (588.87 KiB) Viewed 257 times
Fitting the 10, 25 and 50 micron slit profiles with Gaussian lineshapes gave corresponding fwhm values of 0.15, 0.18 and 0.24 angstroms respectively. The measured change in fwhm from narrowest to widest slit suggests using the 50 micron slit for greatest light throughput without sacrificing too much spectral resolution.
Fig-7.jpg
Fig-7.jpg (389.92 KiB) Viewed 257 times
Comparing all four slit widths (10, 25, 50 and 150 microns) to their image slit widths on the camera sensor gave magnification factors of 2.40, 1.17, 0.73 and 0.74 respectively, confirming that diffraction (and aberrations) played a significant role for the two narrowest slits. The magnifications measured for the widest two slits are consistent with the expected magnification f_camera / f_collimator = 350 / 500 = 0.70.

I also wanted to test the imaging characteristics of the SHG. This was done by reinserting the AR telescope and focusing on a distant (~ 5.5 m) spectral tube source. Here, I used a helium discharge tube and set the wavelength of the spectrometer to centre the 4471 angstrom line and, in separate measurements, the 5015 angstrom line. The slit was then removed and images captured.
Fig-8.jpg
Fig-8.jpg (490.04 KiB) Viewed 257 times
In the photographs, (a) shows one end of the He discharge tube while (b) and (c) are images obtained with the SHG. Image (c) is corrected for distortion as described below. Determination of the angular size of the object in the testing setup, gave a field of view of about 1.1 degrees across the 2736 pixels in the long direction of the camera sensor (Pt. Grey GS3-U3-60QS6M-C Sony ICX694).

Image (b) shows the image of the spectral tube (object) without any adjustments (aside from levels and a slight sharpen). The image is well focused but exhibits a geometrical distortion with respect to the object. It’s predominantly a skew distortion arising from the grating reflection scheme used in this particular spectrometer.
Fig-9.jpg
Fig-9.jpg (7.59 KiB) Viewed 181 times
With an SHG, this type of image deformation would have to be corrected in the video file such that the spectrum in each frame of the video is treated. Luckily, there is a Virtual Dub filter which can accomplish this (General Quadrilateral Transform). The image in (c) was corrected using this Vdub filter. This doesn’t add much more complexity to the workflow as a number of filters are already used to correct rotation and “smile” of spectra. The image quality in (c) looks quite acceptable and promising for the creation of spectroheliograms.

It remains to see how well this instrument performs in the field. I was hoping to enhance the uv operation using fused silica singlet lenses but the AR telescope itself is probably quite limited in its uv performance. That being said, an earlier SHG using the AR did perform reasonably well for the CaII H and K lines as well as the CN bandhead at 3883 angstroms. More light throughput with the 50 micron slit should definitely offer an improvement. A limitation of employing singlet lenses is that they are quite chromatic. Tuning the instrument to different solar spectral regions will require more frequent refocusing than with my previous instrument built around camera lenses.

I'm looking forward to the start of my observing season!
Cheers.
Peter

PS. Any resemblance to the Discovery spacecraft of 2001 A Space Odyssey is purely coincidental?
Fig-10.jpg
Fig-10.jpg (20.78 KiB) Viewed 254 times
Last edited by p_zetner on Sat Feb 20, 2021 4:41 pm, edited 2 times in total.



User avatar
Carbon60
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 8382
Joined: Wed Mar 07, 2012 12:33 pm
Location: Lancashire, UK
Has thanked: 1388 times
Been thanked: 1222 times

Re: A New Spectroheliograph

Post by Carbon60 » Sat Feb 20, 2021 8:30 am

It looks awesome, Peter. I hope it gives you the performance you’re looking for.

Thank you for your detailed and comprehensive description.

Do you make your own slits, or buy them? I’m still playing around with my device, but the slit I’m using is pathetic. Two pencil sharpener blades back to back, but they must be poorly made since the image I’m getting is full of dark bands, and I don’t mean the diffraction lines I’m looking for. The diffraction lines are relatively weak, running orthogonal to the much stronger black bands. I can only assume the black bands are coming from surface irregularities at the slit.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/

User avatar
marktownley
Librarian
Librarian
Posts: 30296
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 3528 times
Been thanked: 1460 times
Contact:

Re: A New Spectroheliograph

Post by marktownley » Sat Feb 20, 2021 10:13 am

Loving it Peter! Keep us updated, and very much looking forward to the results!


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

christian viladrich
Almost There...
Almost There...
Posts: 790
Joined: Sun Jun 14, 2015 4:46 pm
Location: France
Been thanked: 211 times
Contact:

Re: A New Spectroheliograph

Post by christian viladrich » Sat Feb 20, 2021 10:48 am

Great Peter ! Thanks for sharing.
Some inspiration for when I am retired :-)


Christian Viladrich
Co-author of "Planetary Astronomy"
http://planetary-astronomy.com/
Co-author of "Astronomie Solaire"
http://www.astronomiesolaire.com/

User avatar
Bob Yoesle
Almost There...
Almost There...
Posts: 644
Joined: Thu Mar 08, 2012 7:24 pm
Has thanked: 20 times
Been thanked: 96 times

Re: A New Spectroheliograph

Post by Bob Yoesle » Sat Feb 20, 2021 12:43 pm

That's really cool Peter!

And it does remind me of Discovery ;-)


Diagonally parked in a parallel universe.

Curiosity is the father of knowledge; uncertainty is the mother of wisdom.

Goldendale Observatory

Dark-Sky Defenders

justapictureposter
Ohhhhhh My!
Ohhhhhh My!
Posts: 116
Joined: Thu Dec 24, 2020 3:29 pm
Has thanked: 102 times
Been thanked: 205 times

Re: A New Spectroheliograph

Post by justapictureposter » Sat Feb 20, 2021 1:13 pm

Have you tested the rubinar with your calcium filter on the sun, before the building everything?

About 10 years ago, mark attempted to use a meade etx90(mak-cass) and the solar image would produce poorly(

Around the same time I was using a meade etx125 (mak-cass) and had also was getting poor results on calcium filter,

The rubinar does have a nice meniscus in the back end where the etx series did not; and this maybe the life saver as a camera lens?

I know you tested image sharpness with a mercury lamp, and I did happen my first tests with a black light bulb initially; However I notice that although I am able to get a great picture of the light bulb as you did; when I pointed my system at the sun it had no details. The focus was sharp with a well defined disc, but the calcium details were gone. Something was blocking the solar calcium line, quite possibly the meade UHTC coating;

I do not believe a lamp is a replacement for the actual sun, due to its distance from the optics. (but i may be wrong)
Just some food for thought!

edit: upon second reading i see you have used the rubinar in the past on a previous device?



LTHB
Im an EXPERT!
Im an EXPERT!
Posts: 311
Joined: Thu Mar 03, 2016 7:31 pm
Been thanked: 299 times

Re: A New Spectroheliograph

Post by LTHB » Sat Feb 20, 2021 3:31 pm

Extremely cool project! Thanks for showing!

Frank



User avatar
p_zetner
Almost There...
Almost There...
Posts: 1163
Joined: Sun Mar 18, 2012 4:59 pm
Location: Winnipeg, Manitoba, Canada
Has thanked: 33 times
Been thanked: 194 times

Re: A New Spectroheliograph

Post by p_zetner » Sat Feb 20, 2021 6:35 pm

Thanks for the feedback, everyone

Stuart: I'll agree that slits are a problem. The bands you see ("transversalium") running along the grating dispersion direction (orthogonal to the spectral lines) are indeed due to dust and irregularities at the slit opening. Would razor blades be a better choice than pencil sharpener blades for this? I have kept my eye on eBay for many years, looking for spectrometer slits with some luck on rare occasions. There are currently Chinese air spaced slits available of length 3mm and various slit widths. They are quoting totally exorbitant shipping prices to Canada but maybe the situation with the UK is different.
https://www.ebay.ca/itm/Custom-Precisio ... Swd5xcbAOi
The 3mm length is sufficient for general spectroscopy but I prefer longer lengths for spectroheliography, particularly if the goal is to image a full solar disk. A good width to start with would be 25 microns; reasonable from the point of view of spectral resolution yet not too small to diminish the available light excessively and not overly subject to the transversalium from accumulated dust. In my experience, the dust problem gets quite bad at 10 microns. That being said, the SOLEX spectroheliograph project mentioned by Christian in this thread
viewtopic.php?f=8&t=30551
uses a 3mm long slit of 10 microns width purchased from Shelyak Instruments. Full disk images are produced by mosaicing spectroheliograms.
Ealing Optics offers precision slits photo-etched on a chromium-coated optical glass substrate:
https://www.ealingcatalog.com/opto-mech ... slits.html
For the Ealing slits, the substrate opens up the problem of heating when used for solar spectroscopy, likely necessitating some attenuation (solar wedge?). As in stellar spectroscopy, it would be nice to have a reflective slit and some means of viewing the solar image on the slit in order to understand what feature is giving rise to your observed spectrum. The Ealing slits might offer this as a possibility if the chromium gives reasonable quality reflection.
As I mentioned above, my slits are used in older Jarrell-Ash spectrometers which do come up from time to time on eBay. I have bought an entire spectrometer simply for the slit holders! I have had new slits compatible with the Jarrell-Ash slit holder fabricated by the company Thermo Vision Colorado.

pictureposter: I've attached a montage of spectroheliograms from 2015 made using the Astro-Rubinar. It seems perfectly capable of imaging in the near uv. My goal is to squeeze as much light as possible from the new instrument in this spectral region.

Cheers.
Peter
8Lights_montage_19Oct_22Oct_rsz_lbls.png
8Lights_montage_19Oct_22Oct_rsz_lbls.png (1.81 MiB) Viewed 179 times



justapictureposter
Ohhhhhh My!
Ohhhhhh My!
Posts: 116
Joined: Thu Dec 24, 2020 3:29 pm
Has thanked: 102 times
Been thanked: 205 times

Re: A New Spectroheliograph

Post by justapictureposter » Sun Feb 21, 2021 2:01 am

Those are indeed great captures below 400nm! Well done and great accomplishment, looking forward to the upgrade and seeing your equipment vision come to life.

Beautiful!

What size slits are you guys looking for? I can probably get one of my guys to roll out a few m42 fixed slits or 1.25" slits using an EDM machine. It is usually less than $200 to have 10 custom aluminum disk shaped parts made totally anodized, and the threading on the lathe is only a few extra dollars. Seems like a great project.



User avatar
Carbon60
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 8382
Joined: Wed Mar 07, 2012 12:33 pm
Location: Lancashire, UK
Has thanked: 1388 times
Been thanked: 1222 times

Re: A New Spectroheliograph

Post by Carbon60 » Sun Feb 21, 2021 5:26 pm

Many thanks, Peter. I appreciate your detailed reply.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/

User avatar
MalVeauX
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1517
Joined: Tue May 09, 2017 7:58 pm
Location: Florida
Has thanked: 436 times
Been thanked: 406 times

Re: A New Spectroheliograph

Post by MalVeauX » Mon Feb 22, 2021 8:11 pm

This is awesome, I hope to build one, one day!

Very best,



Post Reply