14 replies to this topic

[minhlead]

So Tom Polakis just dropped a wild animation that I have to share. Not just because I’m the guy behind the SHG 700 (okay fine—maybe that’s like… 63% of the reason), but mostly because it’s just plain cool and puts things in perspective.



Contrast and uniformity are where spectroheliographs truly shine—often leaving traditional etalon filters in the dust.

This blink compares an H-alpha image from the $880 MLAstro SHG 700 to one from a GONG station at Cerro Tololo (one of the better one at GONG network). That GONG setup? An 80mm f/12.5 scope paired with a $28,000 “research-grade” 0.4Å FWHM etalon. Yes, you read that right. Twenty. Eight. Thousand. Dollars.

And yet, the humble SHG 700 holds its ground—delivering punchy prominences, filaments with attitude, and chromospheric networks that look like someone set the contrast to “overachieve.” The whole disk even carries a subtle 3D vibe the GONG image kind of lacks, it just looks flat.

Now, before the etalon crowd comes at me with pitchforks: yes, etalons have their strengths too. This isn’t a dunk. Just a fun, for-the-heck-of-it comparison. Different tools, different tradeoffs.

But if you like results without the etalon lottery, sweet spot drama, or organ auction… well, you know where to look.

Huge thanks to Tom Polakis for the stellar work.

Edited by minhlead, 20 April 2025 - 11:55 AM.

[bigdob24]

GONG seems to be the gold standard for comparison , and it works.

I like the SHG images and all there contrast and detail , to me some almost look on the verge of over processed.

Im that way because I’m a visual guy and I judge photos by what I see on an excellent day.

If the SHG was a visual instrument I would already have one to try out 

Always interesting check out your photos and write up

Dan

[thesmiths]

I think it's also fair to say that an SHG of a particular aperture gives an H-alpha image with resolution comparable to "one size smaller" etalon image. As indicated here, a 100mm SHG gives about the same spatial resolution as an 80mm etalon. Similarly, an 80mm aperture SHG gives about the same spatial resolution as a 60mm etalon. I've seen some evidence that a 127mm aperture SHG gives comparable resolution to a 100mm etalon.

 

The reason for this is likely the fact that SHG stacking does not really use "lucky imaging". The acquisition time for a single SHG frame is on the order of 10 sec, compared to perhaps 10ms for a single etalon frame. By stacking SHG frames, you can toss out truly bad frames and "average out" some turbulence, but you are not able to select for the moments of truly excellent seeing which maximises the resolution of the telescope.

 

On the other hand, at the Ca-K/H wavelengths, aperture is not really the determining factor (due to the shorter wavelength). An 80mm aperture is diffraction limited to around 1.25 arcsec at 400nm, which is likely to be seeing limited under almost all circumstances.

[bigdob24]

I forgot to ask in my previous post 

I was under the impression that the GONG scopes were all considerably smaller in aperture that 80mm?

Have to look into that

Dan

[minhlead]

GONG seems to be the gold standard for comparison , and it works.
I like the SHG images and all there contrast and detail , to me some almost look on the verge of over processed.
Im that way because I’m a visual guy and I judge photos by what I see on an excellent day.
If the SHG was a visual instrument I would already have one to try out 
Always interesting check out your photos and write up
Dan

The SHG does look unfamiliar if you are used to etalon filter visually. But I can assure you that very little processing gone into that SHG image (definitely less than the GONG image is).
The SHG images naturally have better contrast.
All GONG are on 80mm/1000mm scopes and a Daystar Quantum PE 0.4A filter.

I think it's also fair to say that an SHG of a particular aperture gives an H-alpha image with resolution comparable to "one size smaller" etalon image. As indicated here, a 100mm SHG gives about the same spatial resolution as an 80mm etalon. Similarly, an 80mm aperture SHG gives about the same spatial resolution as a 60mm etalon. I've seen some evidence that a 127mm aperture SHG gives comparable resolution to a 100mm etalon.
 
The reason for this is likely the fact that SHG stacking does not really use "lucky imaging". The acquisition time for a single SHG frame is on the order of 10 sec, compared to perhaps 10ms for a single etalon frame. By stacking SHG frames, you can toss out truly bad frames and "average out" some turbulence, but you are not able to select for the moments of truly excellent seeing which maximises the resolution of the telescope.
 
On the other hand, at the Ca-K/H wavelengths, aperture is not really the determining factor (due to the shorter wavelength). An 80mm aperture is diffraction limited to around 1.25 arcsec at 400nm, which is likely to be seeing limited under almost all circumstances.

I'd go even lower and say that a 100mm scope on SHG will give detail similar to that of a 60mm scope with etalon and lucky imaging. Lucky imaging make HUGE difference, especially when the seeing is not ideal

Edited by minhlead, 20 April 2025 - 11:31 AM.

[minhlead]

I think it's also fair to say that an SHG of a particular aperture gives an H-alpha image with resolution comparable to "one size smaller" etalon image. As indicated here, a 100mm SHG gives about the same spatial resolution as an 80mm etalon. Similarly, an 80mm aperture SHG gives about the same spatial resolution as a 60mm etalon. I've seen some evidence that a 127mm aperture SHG gives comparable resolution to a 100mm etalon.
 
The reason for this is likely the fact that SHG stacking does not really use "lucky imaging". The acquisition time for a single SHG frame is on the order of 10 sec, compared to perhaps 10ms for a single etalon frame. By stacking SHG frames, you can toss out truly bad frames and "average out" some turbulence, but you are not able to select for the moments of truly excellent seeing which maximises the resolution of the telescope.
 
On the other hand, at the Ca-K/H wavelengths, aperture is not really the determining factor (due to the shorter wavelength). An 80mm aperture is diffraction limited to around 1.25 arcsec at 400nm, which is likely to be seeing limited under almost all circumstances.

IIRC, GONG images are single sub, not "stacked" image so what we are seeing here, the etalon is actually at a handicap since the best tool in the shed (lucky imaging and stacking) wasn't used

Edited by minhlead, 20 April 2025 - 11:36 AM.

[gstrumol]

I forgot to ask in my previous post 

I was under the impression that the GONG scopes were all considerably smaller in aperture that 80mm?

Have to look into that

Dan

I believe you're correct, Dan.

 

My research into GONG revealed that the instrument has an aperture of 28mm with a FL of 1000mm. Its filter is at a 1 A bandpass on the NiI line @ 6768 A. Its image detector is a CCD camera with 2.5 arcsec square pixels, giving a resolution of 5 arcsec.

 

And while I generally use Cerro Tololo in my comparisons (I also think they are often the best) all GONG installations have exactly the same equipment.

[minhlead]

I believe you're correct, Dan.

 

My research into GONG revealed that the instrument has an aperture of 28mm with a FL of 1000mm. Its filter is at a 1 A bandpass on the NiI line @ 6768 A. Its image detector is a CCD camera with 2.5 arcsec square pixels, giving a resolution of 5 arcsec.

 

And while I generally use Cerro Tololo in my comparisons (I also think they are often the best) all GONG installations have exactly the same equipment.

For some reason, this PDF, also off from GONG website said the diameter of the scope was 8cm

https://drive.google...ew?usp=drivesdk

I also got another file on the design and consideration of the Ha system. Ironically, they find the 28.000$ etalon from DS a good compromise between affordability and performance. (Solar Spectrum and Coronado etalon also considered)

https://drive.google...ew?usp=drivesdk

[BYoesle]

The SHG definitely is the winner by virtue of the very good much higher spectral resolution.

A couple of caveats to consider though in the SHG & GONG filter comparison.

1) Cost - Although they are what GONG uses, DayStar filter costs are not a very valid comparison methodology, The DayStar price you cited ($28K) is very expensive under the most recent ownership's price scheduled.

For example the 2.0 A DayStar CaK line filter is ~ $18,000. Just few years back they were half that price. You can get a hard-coated 25 mm Alluxa 1.5 A CaK filter for less than $2500, and the needed ancillary filters will be even less to employ it successfully. A Lunt 2.4 A CaK module can be had for less than $2k, and when double stacked will have a narrower bandpass, and even better CaK selectivity.

The GONG network was established in 1995, when DayStar was the only game in town for more affordable consumer narrow-band H alpha filters. Solar Spectrum was established in 2002 using the identical mica etalon technology used by and learned from Del Woods (the original DayStar owner), and it products are significantly less expensive and just as well-made. Additionally they have a superior TEC temperature control methodology. A Solar Spectrum 32 mm 0.3 A Research Grade filter (employing superior hard-coated ancillary filters) cost about $6000 USD. DayStar now appears to be wanting to price it etalons similarly to what Light Machinery and other high-end research and government filter suppliers would. DayStar employs a sophisticated testing apparatus for PE filters - but in my and others experience this will not necessarily result meeting the specified bandpass or good performance.

2) The processing used by GONG generally improves the contrast appearance to be that of a much narrower or double stacked filter system. So it is not really a valid "apples-to-apples" comparison.

Edited by BYoesle, 20 April 2025 - 01:45 PM.

[gstrumol]

For some reason, this PDF, also off from GONG website said the diameter of the scope was 8cm

https://drive.google...ew?usp=drivesdk

I also got another file on the design and consideration of the Ha system. Ironically, they find the 28.000$ etalon from DS a good compromise between affordability and performance. (Solar Spectrum and Coronado etalon also considered)

https://drive.google...ew?usp=drivesdk

Crazy, man! My info was from here: https://nso.edu/telescopes/nisp/gong/

 

where they say: 

 

The instrument consists of two mirrors tracking the Sun in elevation and cross- elevation axes that feed light horizontally into a cargo container housing the rest of the equipment. The optical system is sealed by a filtered window and has an effective aperture of 2.8cm. Near the focus of the 1-m focal length objective lens is a box that contains various optics that can be moved in and out of the beam.

 

And yet in your link, they say:

 

The instrument consists of two mirrors tracking the Sun in elevation and cross elevation axes that feed light horizontally into a cargo container housing the rest of the equipment. The optical system is sealed by a filtered window with an aperture of 8 cm. ... Near the focus of the 1-m focal length objective lens of aperture 8 cm is a box that contains various optics that can be moved in and out of the beam.

 

So the difference it seems, if I'm to take their English as precise, is that while the window has an aperture of 8 cm, it only (?) has an effective aperture of 2.8 cm.

 

Would anyone else care to weigh in on this seeming contradiction GONG has published?

[ch-viladrich]

Some additional details about GONG.

 

The objective was to have one Ha image/min using the full 7 cm apertue of GONG instrument, and to have rapid transfer to the Internet.

 

The Ha filter requirements were:

- Bandpass narrow enough to see flares, plages, filaments and prominences.

- affordable and readily available.

- Robust for unattended field use.

- Useable with existing optical system.

 

Back then a SHG would not have met these criteria. And even right not, I am not sure it would.

 

Several Ha filters were tested : Coronado, Daystar and Solar Spectrum.

Based on overall quality, Daystar Quantum PE 0.4 Å was selected. (Plan B was  Solar Spectrum).

Ten filters were ordered.

 

Source: GONG Hα Instrument. J.Harvey and GONG Team, Sept 2009, Gong Ha review.

 

And don't forget that the main use of GONG is to get Dopplergram to study the oscilllation of the solar surface. GONG : Global Oscillation Network Group.

 

"The GONG Instrument: Is based on a Michelson interferometer called a Fourier
Tachometer and is supported by a highly automated, portable installation,
reminiscent of a spacecraft experiment in its design philosophy. The instrument
consists of two mirrors tracking the Sun in elevation and cross elevation axes that
feed light horizontally into a cargo container housing the rest of the equipment.
The optical system is sealed by a filtered window with an aperture of 8 cm....

 

Near the focus of the 1-m focal length objective lens of aperture 8 cm is a box that
contains various optics that can be moved in and out of the beam".

 

The Ha channel was a "bonus" that used part of the "main instrument" through a beam splitter."

 

Source : https://www.prl.res....G-Telescope.pdf

 

And as said by Bob, back then the price of DayStar were much lower.

 

The guys who designed GONG knew what they were doing ;-)

Edited by ch-viladrich, 20 April 2025 - 02:54 PM.

[gstrumol]

Good Lord, now we have a 7cm claim? 

 

Ok, let's try this: if you attach an LS50 DS etalon on the front of an LS100, the effective aperture is just 50mm. Doesn't matter that the scope is 100mm in aperture (yes, I know the FL is still what it is for the 100).

 

So what is it really on the GONG instruments? 2.8cm? 8cm? 7cm? Pi cm? 

 

It shouldn't be this difficult to get a proper answer ... 

[BYoesle]

The H alpha GONG images certainly don't look to have anywhere near a 7 cm aperture resolution. 2.8 cm seems more plausible.

[bigdob24]

Who’s going to go on a road trip and check one out

Dan

[BYoesle]

Took a trip... to Wikipedia: