Review of the New SHG700 spectroheliograph

Review of the New SHG700 spectroheliograph

By Larry Alvarez

Background:

The history of the design of the SpectroHelioGraph (SHG) goes all the way back to the 1890’s.  George Ellery Hale created it to image the Sun by isolating a single absorption line and taking multiple images of it as he scanned the solar disk.  Each image would be one small slice of the whole disk and when painstakingly exposed on a photographic plate side by side, it would form a full disk image in whatever absorption line was isolated.  The beauty of an SHG is that it is capable of imaging just about any visible spectrum absorption line created by the Sun.  This capability sets it apart from most other narrow band imaging systems.  Its filtration, depending on the build, can be very tight and if directly likened to a dedicated solar telescope it would be in the 0.3 angstrom range.  The device itself was ground breaking upon its invention but the weak link to make it a consumer ready product was the lack of photographic technology and automated processing.  124 years since the SHG invention, finds that photography, programming, and computerized processing have finally caught up.  Photographs are now captured easily with CMOS sensors at hundreds of frames a second and the computer processing power required to reconstitute the solar disk image from the scanned slices is a relatively easy thing to do with a modern laptop with the right program.  All the pieces are now in place to take the technology to the next level.

Within the past decade there have been a few amateur level homemade entries that coupled the SHG hardware with modern telescopes, computers, and software with pretty good success but lately, more mainstream entries have appeared.  First came the Sol’Ex and now more recently the SHG700.   The Sol’Ex is an SHG designed and created by Christian Buil.  The unit was designed to allow the amateur solar enthusiast the ability to build it themselves at very low costs over time.  The main body can be 3D printed and it’s STL patterns can be found online for free.  There are also places selling the optical parts needed for it like the lenses and grating.  When everything comes together you have an instrument that can image the Sun with a very tight tolerance.  The Sol’Ex works well but has a few limitations and it is meant as a starter SHG for anyone interested.  Don’t let the word “starter” fool you, this system packs a powerful 0.3 angstrom punch but for those wanting a more polished, turn-key, next level system, the SHG700 is just that.  It features an aluminum build that is milled from a solid block of aluminum and has next level design features both internal and external that allow it to be tuned much easier than the Sol’Ex. 

The SHG700 was developed by PhD. Nguyen T. Minh.  He runs a website called MLAstro and sells not only his SHG700 but also lens kits for the Sol’Ex and accessories too.  He created the SHG700 to provide a robust, precise, and reliable SHG that was focused on imaging Sun in various wavelengths. To that end he employed several features that you only find on high end SHG’s.  His first set of SHG700’s sold out quickly after their announcement and there have been more and more images showing up on various chatrooms and forums that were taken with the SHG700.  I was lucky to get one from the first batch.  My SHG700 arrived and I noticed some of the screws were loose in the box.  The unit came from half way around the world and the temperature difference between the places it visited along the way could account for the loose screws.  I did inform the MLAstro and Minh was very nice to deal with.  He said he would work on the packing and shipping of the next set based on the feedback.  It was nice to get a quick response from him. Other than the loose external screws the unit was fully functional and I tested it on the next clear day.

Design Specifics:

The SHG700 is a professional grade SHG that is built to exacting standards.  Even though the SHG700 and Sol’Ex do the same thing, you will find that the design, precision build, and support for larger apertures are much different.  Below is a quick lookup table comparison between the Sol’Ex and the SHG700:

Let’s back up a bit and try to understand all this.  Yes, the overall prices between the two are only $200 apart, but if you were to 3D print the Sol’Ex yourself it knocks off over $184.00.  That’s a substantial savings if you are just a casual enthusiast.  So why would one consider the SHG700?  The Devil’s is in the details.  Putting things together yourself is not as easy as it sounds and when I got my 3D printed Sol’Ex I found I had to sand, shim, and tune just about every part on it.  It works, and I love doing that kind of stuff, but it took a lot of fine tuning to get it right.  The SHG700 comes ready to go, with no sanding or shimming necessary.  It is also built out of metal and has higher grade parts that give it an edge for imaging and repeatability.  The biggest benefit of the SHG700 is its support for larger focal length scopes. 

Looking down the front of the SHG700, you will see the slit.  It is a small metalize quartz plate that has a small slit on it in the middle.  The build material for the slit was “by design” to allow for the use of the SHG700 without the need for an ERF on the telescope front. 

Quartz 7um slit on adjustable cell

It is rated to be used on scopes up to 102mm aperture without an ERF.  This is possible because quartz has a low expansion coefficient so it is not affected by direct sunlight like other glass types.  It can take the heat and still stay in the kitchen.  I still use a Herschel wedge with my setup to keep the heat build up to a minimum. 

Taking a peek on the inside of the SHG700 we can see some of the design features.

Internal view of SHG700

In the Field

First things first, you have to make sure you put the SHG700 on a scope that has a 700mm focal length or less.  For my testing I used an old Orion 100mm F6 telescope.  I’m definitely still a novice when it comes to using an SHG but they all seem to have 4 locations required for a proper calibration prior to and during shooting, the front slit rotation angle, collimator lens, grating dial, and camera focus.  You have to tune these on the Sol’Ex to get the best view but the SHG700 comes pre-calibrated for the slit rotation angle and the other locations are close to their needed spots but still need to be checked.  The tuning of the collimator is one of the more critical areas for an SHG.  On the SHG700 it’s done by first moving the grating dial to the #0 location.  To move its dial by hand you have to loosen the lock screw and then rotate the dial as seen below.  The micrometer does not have the ability to move the dial all the way.  It is meant for a limited range but loosening the set screw will allow you to reset its location at that spot.

After the dial is set to #0 you start up the camera and proceed to examine the line.  Just a brief observation I had about the dial was that the indication arm for it covered up the line location on the body.  It was a little difficult to see exactly where I was but practically speaking, I didn’t really need it as I had the view on my laptop and could clearly see the lines I wanted on the screen.  The pointer is 3D printed so I may remake it and recess the point a little so that I can see where it’s at over the indication on the body.  For the SHG700 I chose to use a QHY585m camera.  It has 2.9um pixels and has a large 1” format sensor.  Small pixels and high resolution make the line look small on the screen so in order to focus it, I’ll usually crop into the end of it.  This will allow for fine focusing the location with the micrometer on the collimator lens.  When it looks good, I’ll use the micrometer on the camera lens to focus it more too.  I’ll go back and forth between the two until both are as sharp as possible.  You’ll find that the micrometers are really well suited for this purpose and make focusing a breeze.

Using the SHG700 is different than using a traditional narrowband solar telescope or filter made by the other mainstream solar vendors.  The SHG700 does not offer a visual view of the whole Sun at once.  All you see from the camera feed is the spectrum.  It looks like a large barcode with several thick and thin lines and it seems go on forever.

SHG700 Solar spectrum with Magnesium line at top

What it lacks in the visual experience, it makes up for with the high contrast image it provides after processing and the ability to image any solar absorption line in the spectrum at 0.3 angstrom.  Traditional solar telescopes top out at <0.5 angstrom filtration but that model has become the topic of several conversations lately when comparing brands.  You’ll find one company’s <0.24 angstrom is another’s <0.7.  It is probably due to how they measure it and how well the filters themselves are built.   Solar telescopes utilize Fabry Perot etalons to filter the light.  These are extremely difficult to produce accurately and require a bit of science and art to create and thusly costs thousands of dollars for the end product.  These kinds of telescopes are limited to imaging or viewing one wavelength unless you do a hard conversion on it and switch it to another wavelength by way of removing the filter and adding another.  Over the past 24 years I’ve personally searched for the best dedicated solar filters I could find.  The trick to getting the best views is finding filter pairs that work well together to create a doublestack configuration which puts the scope at <0.5 angstrom.  A lot of times you’ll find that some filters create artifacts in the images due to the tilting required to bring them “on-band” while others have artifacts due to no tilting used.  With the SHG700 and even the Sol’Ex there are few if any optically created artifacts.  The disks produced are very pure and evenly illuminated.   The SHG700 simply excells at full disk imaging.

Without removing the SHG700 from the telescope its possible to also shoot other wavelengths.  Taking an image after the calibrations are done is simple.  All you have to do is turn the dial to the area indicated on the side of the SHG700.  You then isolate the absorption line needed by drawing a box around the specific line.  After you crop to that absorption line you position the telescope slightly off one side of the disk.  You start the recording and use the telescope’s drive to scan across the disk while recording. 

When you run off the other side of the disk you stop the recording.  You import the video into a program called INTI.  I have to give a lot of credit to the people that created the INTI program.  It is an original program created for the Sol'Ex project.  The Sol'Ex and INTI program were created by Christian Buil & Valerie Desnoux.  This program makes it possible to reconstitute the disk from the scanned in data.

Summary

Build wise, the SHG700 is high end all the way with its aluminum body, 7mm quartz slit, and micrometer driven focus points.  I had no issues imaging the Sun with it but do have the experience of using a Sol’Ex prior to getting my SHG700 so I sort of knew what to expect.  It appears the designer took the best parts of the Sol’Ex and added updates that really make it easy to use and more durable.  The full disk images it delivers are exponentially more complex and richer than most dedicated Solar telescopes. 

Who is this system for?  The person that would get an SHG700 would probably be one that likes to take images of the Sun, has a solar telescope now but would like a more rich full disk image, and someone that is tech savvy in the ways of using a laptop to record images and doing processing.  On a solar telescope getting a 2^nd filter to double stack the one you have costs anywhere from $640 to over 5k.  You get an outstanding visual view at <0.5 angstrom but for $880 you could get an SHG and get 0.3 angstrom for several wavelengths.  Personally, I like having both.  I’m finding that the SHG is more forgiving in times of so so seeing conditions.  They also seem to be less dependent on aperture size for getting solid images.  On the flip side, Solar telescopes help you to connect visually to the Sun in ways the SHG just cannot.  Even though an SHG gives you the ability to see beyond what the scope can do for full disks there’s still that human element of visual observation that may crave.  Who is the system not for?  I guess it wouldn’t be a good fit for someone that is not technically savvy in the ways of processing images and probably not for someone who just likes to view the Sun occasionally.  Using the SHG700 does take some work to scan and process the data, figure out what speed to run the scan at, and figuring out what the best scope is for the purpose but after you do that, it’s a lot of fun.

Rating scores as follows:

***** = world class

 **** = better than average

   *** = may need some work

     ** = inadequate for the purpose

       * = Not acceptable

Build fit and finish – rating *****

                Function – rating ****

  Customer Service – rating *****

                  Packing – rating ***

                     Value – rating *****

What I liked about it: The SHG700 delivers 0.3 angstrom images for a ton of wavelengths.  The micrometers are perfect for the purpose and the lenses inside do a great job and deliver sharp images.  The price was a real value for the what it delivers.

What could be better: The packing could be better to prevent parts from coming loose.  The dial indication pointer could be made smaller so that it doesn’t cover most of the location on the line map on the body.