Beloptik to stack a Quark

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Spectral Joe
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Re: Beloptik to stack a Quark

Post by Spectral Joe » Sun Aug 16, 2015 7:11 pm

Merlin66 wrote:Bob,
I re-read your CN posting but still have problems with your 'double edge' = continuum leakage.
The spicule edge region is visible on band and off band....at 0.3 FWHM even with the "tails" you are still working within the wings of the Ha....
OK
My real issue is when you use a spectroheliograph (SHG) to generate a solar spectrum and then extract a strip from the absorption line the bandwidth of which will depend on the dispersion of the spectrograph and the PSF of the instrument. I'd like to think, that at least in theory, the extraction say of a 0.3A wide strip (based on a dispersion of >0.05 A/pixel and a measured resolution (using a neon reference line) of <0.1A) would NOT give the same "leakage" results of the etalons etc. but an almost "clean" 0.3A image with NO side band interference.
I have had images submitted for the new book on the digital SHG which don't show a double limb (but plenty of spicules!!).
Any comment?
Ken

Bob's explanation is quite complete, but one detail is missing. Ironically, the image that started the whole double limb controversy in 1963 came from a spectroheliograph, the one at the Mount Wilson 60 foot tower telescope. This instrument was considered to have extremely low stray light, but the test used to determine this was flawed in an important way. The Mount Wilson astronomers used a mercury arc and looked for Rowland ghosts and other features. The mercury arc was primarily a line emitter, and the level of stray light from the lines was too low to see. On the other hand, the solar continuum provided plenty of off band light, and what was considered a high quality grating in 1915 doesn't compare with what was available even a short time later. Thus, the scattered continuum light formed a ghost image on top of the Ha image. The double limb seen in etalon filters and Lyot filters have mixed sources (sidebands and wing effects) but the effect is the same. Modern gratings are much better than the early ones, and much cheaper. With a modern grating and care in keeping the post-slit optics clean a spectroheliograph today will provide the spectrally purest images available without going to a six figure investment.
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Re: Beloptik to stack a Quark

Post by Valery » Mon Aug 17, 2015 10:58 am

Spectral Joe wrote:
Merlin66 wrote: With a modern grating and care in keeping the post-slit optics clean a spectroheliograph today will provide the spectrally purest images available without going to a six figure investment.
Hi Joe,

Thanks for your input. However I never have seen ANY sun pictures taken on a SHG which has same good resolution and contrast as
the pictures taken with Lyot and air-spaced filters.


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Re: Beloptik to stack a Quark

Post by Bob Yoesle » Mon Aug 17, 2015 1:22 pm

It's a question of application. The SHG is more ideally suited for spectral line resolution, not optical resolution, where a filter is superior. I think this is what Joe is referring to.

I'm sure Joe will correct me if I'm wrong, but I believe SHG resolution is a function of the slit width and the image scale at the slit itself. The result is that to get the resolution of a typical etalon filter of over 50 to 60 mm, the SHG would need and extremely narrow slit equivalent to the effective resolution of the filter, which would be very difficult to achieve (for amateur size applications at least).

But for spectral line purity at multiple wavelengths, the SHG is the way to go.

B-)
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Re: Beloptik to stack a Quark

Post by Valery » Mon Aug 17, 2015 8:21 pm

Bob Yoesle wrote:It's a question of application. The SHG is more ideally suited for spectral line resolution, not optical resolution, where a filter is superior. I think this is what Joe is referring to.

I'm sure Joe will correct me if I'm wrong, but I believe SHG resolution is a function of the slit width and the image scale at the slit itself. The result is that to get the resolution of a typical etalon filter of over 50 to 60 mm, the SHG would need and extremely narrow slit equivalent to the effective resolution of the filter, which would be very difficult to achieve (for amateur size applications at least).

But for spectral line purity at multiple wavelengths, the SHG is the way to go.

B-)
Hi Bob,

A SGH is not for spectral lines resolution. For this astronomers do use spectrographs. SGH is a device for imaging Sun disk or it's part in
a given wave length (normally at the same wave length as some given spectral lines). So, both Lyot (or air spaced) filter and SGH are
the sun imaging devices and their comparison is correct.

Valery
Last edited by Valery on Tue Aug 18, 2015 12:52 am, edited 1 time in total.
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Re: Beloptik to stack a Quark

Post by Bob Yoesle » Mon Aug 17, 2015 9:54 pm

Hi Valley,

I think this might be an apples and oranges thing, or I might be confused (as usual), as this is not an area I delve into with much frequency, and there is some overlapping terminology when the terms are (frequently) used interchangeably.

I thought there is a slight difference in meaning between a SHG - spectroheliograph - used to study the sun's line spectra (a solar spectrograph):
SpectrohelioscopeFirstLight_Sunspot.jpg
SpectrohelioscopeFirstLight_Sunspot.jpg (58.82 KiB) Viewed 1539 times
Image credit to Solar Chat member Wah.

Verses a SHS - spectrohelioscope - which adds an image synthesizer to the SHG in order to create an image for visual inspection or imaging as with traditional filters:
Ha5protuJPo.jpg
Ha5protuJPo.jpg (129.01 KiB) Viewed 1539 times
Regardless of this triva, you are of course correct in the fundamental issue of optical resolution, whether the instrument used is called a SHG or SHS, and images spectral lines or synthesizes images at a particular wavelength.

Again, I think Joe's point is that an SHG - or SHS - can have much greater spectral specificity or spectral resolution - as implied by the use of the term "purity" - when used for imaging. The optical resolution of a traditional FP etalon or filter is usually greater, but a different point altogether. These filters usually can get nowhere near what a high quality SHS or SHG can attain in terms of spectral resolution.

Boberino ;-)
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Re: Beloptik to stack a Quark

Post by Merlin66 » Mon Aug 17, 2015 10:16 pm

Bob, Valery et al,
Having just finished 6 months detailed research on the SHS and the current digital SHG it may be more appropriate to start a new thread on the comparison of the digital SHG to the current commercial solar filters - give it it's own thread.
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Re: Beloptik to stack a Quark

Post by Bob Yoesle » Tue Aug 18, 2015 3:24 pm

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