SolarChat!

There has been some interesting discussions on the performance of the commercial filters and what the digital SHG can offer.
http://solarchat.natca.net/viewtopic.php?f=9&t=17095

Let me say a few words about the digital SHG....

A digital spectroheliograph (SHG) is no more than a combination of a telescope with a high dispersion spectrograph and a fast frame mono camera. There are about ten digital SHG active around the world at the present time.

(A spectrohelioscope (SHS) is a visual adaptation of the SHG - See Fred Veio's documents etc. There are only a handful of SHS still in existence and maybe two or three still in active use.)

The spectroheliogram image is produced by obtaining a high resolution spectral image, as an AVI file of the target wavelength as the slit in the SHG scans across the solar surface and then extracting a strip, a few pixels wide from the AVI and combining the strips into a mosaic image - a spectroheliogram.
The final solar surface (spatial) resolution obtained is exactly the same as would be obtained from the same telescope with any commercial filter (white light or narrow band).

However, the filter images obtained with a similar camera can be enhanced by selecting the best quality images, stacking and wavelet processing. This allows the frame rate to freeze (or at least improve) the momentary seeing conditions. The spectroheliogram doesn't (at the moment) have this luxury, each individual strip is an individual image.

The imaging bandwidth used is fixed in the commercial filters (we'll come back to the issue of leakage later) and for narrowband filter usually around 0.7 to 0.3A. With a SHG, any bandwidth can be selected for the imaging process.

The spectral resolution depends on four key factors:
The width of the entrance slit gap
The spectrograph optics (collimator/ imaging lenses)
The grating groove density (l/mm)
and the pixel size of the camera.
To get a final bandwidth comparable with the filters, say 0.3A, the spectrograph spectral resolution needs to be 0.3A or slightly better. This resolution is controlled by the dispersion (A/pixel) and Nyquist sampling. Dispersion (A/pixel) is NOT the same as resolution (A). The 0.3A bandwidth would normally require a dispersion of at least 0.1A/pixel.
This can easily be obtained with a slit gap close to 2-3 times the pixel size - a 5micron pixel needs a 10 to 15micron effective slit combined with a long focal length optical spectrograph system and a large l/mm grating (>1200 l/mm)
It's normal to use a reference lamp to verify resolution. The emission lines (say neon) are measured and the FWHM noted. Also the spectrograph is calibrated in wavelength based on the dispersion. We can then accurately determine the wavelength position of the target central wavelength - it's a very easy task then to nominate say "on band" or 0.2A or 0.7A "off band" to access the red/ blue wings (This is difficult with the filters)
In summary:
- The surface resolution of the SHG is exactly the same as would be achieved with the same telescope in a single frame exposure.
- The imaging bandwidth of the SHG can be accurately selected in width and wavelength.

Any questions/ comments?

For those large solar telescope with adaptive optics, the details of SHG can be much better?

Hi Ken.

If you're making a quality comparison of spectroheliograms and filtergrams then you bring up a key problem with the SHG; the seeing limitation. I don't know if this problem would ever be surmountable! In the time it takes to scan the solar surface with the SHG, atmospheric turbulence will affect each scan "strip" independently and randomly. As you say, each individual strip is an individual image. Maybe someone can come up with an extremely clever (but, no doubt, extremely computationally intensive) method to stack, as in conventional filter imaging, but I can't see this happening soon. I'd envision having to stack hundreds of videos, as opposed to hundreds of video frames! Wah's point about adaptive optics is well taken. In principle, this could provide the solution but I don't think it's been done by any of the professional observatories. I think this scheme would take the SHG technique out of the realm of possibility for an amateur astronomer. By the way, the seeing problem obviously doesn't apply to space-borne instruments and I believe there are several currently operational.

As far as the discussion about continuum "leakage", this is purely a function of the spectral passband lineshape of the wavelength selective element (etalon / block filter for a filtergram or grating for a spectroheliogram). Gratings have a nice property in that the lineshape wings drop off extremely rapidly for large groove density. Bob Yoesle, Christian Vladrich and others have brought up the problem of significant transmission in the wings of the filter bandpass leading to loss of surface contrast in H-alpha and the "double limb" effect. In principle, this would not be a problem encountered with the SHG.

A final comment is that the SHG may not be competitive with conventional (etalon) schemes for high quality imaging, but it offers more possibilities for looking at the detailed spectral properties of solar features. Studies of the appearance of the solar disk in a wide diversity of chromospheric lines is easy and detailed studies of magnetic fields and velocity distributions, for example, become straightforward to carry out.

Other than that, I think you've summed up the SHG story quite nicely!

Cheers.
Peter.

Thanks Peter!
https://lightmachinery.com/optical-desi ... -designer/
If you use the design tool and set up an Ha etalon, the transmission falls to 2% at +/- 0.5A and down to the "base" 1.3% at +/- 0.65A.
By my reckoning, that would mean a x40 difference in exposure to pull detail from the "leakage" area of an etalon.
Bray and Loughhead, "The Solar Chromosphere" p18-19 talk at some length about Parasitic light.
"The absence of the photospheric limb on narrowband photographs free from parasitic light shows...that in the CORE of Ha the chromosphere is optically thick along a tangential line-of-sight to the limb"
"High resolution observations show that the photospheric limb first begins to re-appear at Ha +/_ 0.65A....As one moves further out from the line centre, the photospheric limb rapidly becomes more prominent and appears as a sharp boundary crossed at irregular intervals by isolated chromospheric features."
This is well demonstrated in their accompanying plates 2.8 and 2.10.
They also note the use of a 1A prefilter on their 1/8 A Halle filter suppressed the bandwidth and parasitic photosphere.
Last comment: The actual wings of Ha extend 8A on either side of the core. It is obvious that ANY off-band (red/blue wing) data will be only be obtained close to the core "shoulder" - at less than +/-1.25A and not out further in the extended wings which will give, with any filter/ SHG bandwidth, access to the photospheric continuum light.

Interesting discussion, thanks!

Great info Peter and Ken, and nice to see quantitative analysis of "parasitic light" from the Bray & Loughead reference - good stuff!

only 10 SHG's?
did you include me in that count?
that seems low

SRS i can understand since that is a bit more complicated to build
but only 2-3 active left? wow that low

Yeah - definitely the "final frontier" for amateur solar observers - very few have gone before!!!

Here's an image that shows both sides of the resolution story:
The small (!) yellow square is 4 arcseconds on a side, details on that scale are visible in the penumbra. Above and below, however, are areas blurred by poorer seeing. This instrument scans at 350 lines per second, doing the full frame (2125 lines) in 6 seconds. This gives some advantage over taking 2 minutes to do a drift scan. I usually take images in groups of 5, the chances of getting a good image are improved. 5 images, an off-line flat and 5 more, repeat. All the while hoping for good seeing.

Can those 5 images be quality filtered + stacked as normal filter images?

Wah wrote:Can those 5 images be quality filtered + stacked as normal filter images?

That's what I thought...

Superb image, Joe.

Your SHS isn't as limited by seeing conditions as the digital SHG (especially a drift scan device, like mine).
Conceivably, you could take a large number of images and stack, as conventionally done with filtergrams.

What is the focal length and f# of the telescope you're using?
Also, presumably, you have calibrated grating settings to image in the wavelength of choice?

Cheers.
Peter.

Thanks Ken .Indeed a rare pursuit

p_zetner wrote:Superb image, Joe.

Your SHS isn't as limited by seeing conditions as the digital SHG (especially a drift scan device, like mine).
Conceivably, you could take a large number of images and stack, as conventionally done with filtergrams.

What is the focal length and f# of the telescope you're using?
Also, presumably, you have calibrated grating settings to image in the wavelength of choice?

Cheers.
Peter.

Peter

Actually, this one is digital, I just don't use drift scanning. The light path is 1) two mirror coelostat, 2) 81mm F/15 objective, 3) image relay combination that projects the solar image onto the entrance slit at a diameter of 22mm, as well as puts an image of the objective on a right angle scan prism, located 200mm before the entrance slit, 4) the spectrograph, a 1270mm FL Littrow type with a 1200 g/mm grating, 5) another magnifying image relay that produces a 50mm diameter image at the detector. This relay also includes a system for eliminating line curvature and a line shifter, and finally 5) the detector, which is a 1275 pixel linear CCD from a business card scanner. The scan prism is mounted on a galvanometer that is driven by the scan synchronization pulses from the detector. At 350 scans per second the 2125 pixel long image takes 6 seconds. Currently, the image occupies a 1275 pixel square area of the detector, in order to get full disk images. The raw image looks like this:
The dark line down the middle is an artifact of the butting of two chips to make one long one. The flat used to correct for this and other effects looks like this:
The result:
This is before any contrast stretch or sharpening.

The wavelength drive is calibrated and permits quick wavelength changes, and the line shifter (an 8.5mm thick glass plate) is mounted on a galvanometer which permits fast, controlled positioning of the shifter. It has a range of +/- 0.05nm, which I used to check whether I'm on line center, take doppler image pairs or do line profile scans.

Joe