do SHG have newton rings like filters can have?

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do SHG have newton rings like filters can have?

Post by highfnum » Tue Jun 21, 2016 8:22 pm

tried shorter fl lens in h-alpha
im seeing rings like newton rings
is that possible?
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Re: do SHG have newton rings like filters can have?

Post by highfnum » Tue Jun 21, 2016 8:24 pm

i was using 2400 lpmm grating in both fl on new lens was 185 mm
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Re: do SHG have newton rings like filters can have?

Post by Montana » Thu Jun 23, 2016 7:00 am

It's a very good image :bow :bow :hamster: pity about the rings, is it vignetting?

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Re: do SHG have newton rings like filters can have?

Post by Merlin66 » Thu Jun 23, 2016 7:40 am

In theory, it could be possible to generate Newton rings with narrow band spectral images depending on the camera being used. Having said that I have never seen any examples in my years of spectroscopy.
I think what you are seeing results from the vignetting when using a long slit on a short focal length lens.
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Re: do SHG have newton rings like filters can have?

Post by Spectral Joe » Thu Jun 23, 2016 5:46 pm

Seeing the raw data would help in evaluating the problem, it could well be a vignetting issue. That said, interference fringes were noticed when the Kitt Peak magnetograph (using Reticon linear arrays) went into operation in 1975. It's discussed in Applied Optics Vol. 15 No. 1 Pg. 40, January 1976. Unfortunately, this publication is not in the public domain, so access to a technical library is needed. They had an issue with fringes, but it was not F number correlated, as the high index of silicon causes the internal F number to be high even with low external F numbers. So, could be interference or vignetting. I haven't had the problem.
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Re: do SHG have newton rings like filters can have?

Post by p_zetner » Thu Jun 23, 2016 7:04 pm

Could be vignetting but sure looks like a fringe pattern to me, although I've never seen the effect in my system. I've read papers describing the alignment of optics using the Newton's ring pattern generated by back reflection of laser light from a thin lens. Maybe your system allows such a fringe generating back reflection to take place. Obviously you're not using laser light but you're selecting very narrowband light to generate the image so such an effect would be visible. Can you post a diagram of your shg with dimensions? Generally there are three lenses; telescope, collimator and camera. What are their focal lengths and separations? Are there any glass windows in your system? Can you post a frame from one of your videos?

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Re: do SHG have newton rings like filters can have?

Post by highfnum » Thu Jun 23, 2016 9:09 pm

thx guys
Peter you gave me some homework
will get back asap

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Re: do SHG have newton rings like filters can have?

Post by highfnum » Thu Jun 23, 2016 9:12 pm

i only just saw this short fl lens other lens did not have this
ive posted pics before

would it be freq dependent?

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Re: do SHG have newton rings like filters can have?

Post by Spectral Joe » Fri Jun 24, 2016 1:29 am

"tried shorter fl lens in h-alpha"

Which lens? With what we have to evaluate, little definite can be said. While we're guessing, I'm guessing that the signal level (brightness) is lower at the beginning and end of the scan, and an extracted flat is being used to flatten out the intensity, along with some (possibly severe) contrast stretch. Taking weak signals and stretching them can result in visible level steps that look like fringes. If the signal level is low at the ends of the scan it's a vignetting problem, changing a focal length can aggravate the problem unless steps are taken to ensure that pupils are properly relay imaged in the system. Maybe something was overfilled in the previous setup and that has changed. All guesses, as there is no actual data.
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Re: do SHG have newton rings like filters can have?

Post by Merlin66 » Fri Jun 24, 2016 3:59 pm

remember the final spectroheliogram is made up from "strips" taken from exactly the same pixel column on the camera. This on it's own should minimise NR which are normally evidenced across the width of the CCD chip.
The variations seen (may be contrast artifacts?) infer that the "strip" at the edge of the solar disk has a different distribution from those seen in the "strip" from the central solar disk.
The only difference, other than the optical arrangement, is the movement of the solar disk across the entrance slit of the spectrograph......
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Re: do SHG have newton rings like filters can have?

Post by highfnum » Fri Jun 24, 2016 9:28 pm

ok here are numbers

1) telescope 50mm X 300mm FL
2) collminator 50mm X 180mm FL
3) slit 6mm long set at 25u (glass rotatory type)
imaging lens in this case was 50mm X 180mm FL (copy of colminator)
I also have 50mm X 300mm

here is image
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Re: do SHG have newton rings like filters can have?

Post by Spectral Joe » Fri Jun 24, 2016 10:54 pm

Almost certainly vignetting, especially if the grating is a 30mm square one (guessing again). Even with a 50mm grating there is likely an issue if there is no field lens. Try putting a simple lens (plano convex is best, double convex is ok) just after the slit (close, 5mm or so). 125mm focal length is best for this setup, but 100 would work. The idea is to have an image of the objective lens at the grating, that way no light is lost to vignetting. Refocussing of the collimator will be necessary.
Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!

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Re: do SHG have newton rings like filters can have?

Post by highfnum » Sat Jun 25, 2016 8:52 pm

yea 35mm 2400lpmm ( i think your right)
first time i tried Ha at this freq with this grating
I did not have problem like this with my 50mm 1800 lpmm
or 1200mm
even when I swapped out to 50mm X 300mm imaging lens (problem still there )
however higher freq do not show this problem

todays caH with 35mm 2400lpmm and 50mm by 300 FL
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Re: do SHG have newton rings like filters can have?

Post by highfnum » Sat Jun 25, 2016 8:55 pm

Joe -- Nice tip on lens - thx :bow
but how much will it change FL? I got about 3 inch to play with
without doing a whole redo

right now FL of collminator is 180 mm

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Re: do SHG have newton rings like filters can have?

Post by highfnum » Sat Jun 25, 2016 8:57 pm

oops 30 mm not 35mm

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Re: do SHG have newton rings like filters can have?

Post by highfnum » Sun Jun 26, 2016 4:09 pm

Joe I am convinced you are on track
i swapped out 2400 to 1800
problem went away
ha1800by300.jpg
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Re: do SHG have newton rings like filters can have?

Post by highfnum » Sun Jun 26, 2016 4:12 pm

1800 is 50mm by 50mm
2400 has problem in red area Ha
cah above was with 2400 - maybe red end is too shallow and beam is getting cut off?

here is with 180 lens -- some clouds drifted and screwed up image a bit but no rings
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Re: do SHG have newton rings like filters can have?

Post by highfnum » Sun Jun 26, 2016 4:15 pm

thx everyone - i like to document my SHG experience on forum
for those to come later
I started from scratch and many of you have help and watched be get better at this
and also see what pitfalls i have fallen into

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Re: do SHG have newton rings like filters can have?

Post by Spectral Joe » Sun Jun 26, 2016 6:34 pm

highfnum wrote:Joe -- Nice tip on lens - thx :bow
but how much will it change FL? I got about 3 inch to play with
without doing a whole redo

right now FL of collminator is 180 mm
The shift should be small, 5 percent or so. Depends on how close to the slit you can get, and the actual focal length of the field lens.
Observing the Sun with complex optical systems since 1966, and still haven't burned, melted or damaged anything.
Not blind yet, either!
Light pollution? I only observe the Sun, magnitude -26.74. Pollute that!

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