question on SHG

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highfnum
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question on SHG

Post by highfnum »

I notice most use 1200lpmm grating
can you use 2400lpmm? and what changes in lens fl and software would be needed

do you shg users use ruled or holographic gratings?

if ruled its about 500nm blazed?


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Re: question on SHG

Post by Merlin66 »

You can use almost any grating, if you can get the right size to suit the collimator.
The dispersion is obviously related to the l/mm, and going to a 2400 l/mm from a 1200 l/mm will double the dispersion, increase the resolution and half the intensity of the spectrum.
Drop me a PM for a copy of the latest SimSpecV4 which will allow you to calculate these variables.
Ruled, blazed gratings are the way to go. For Ha the 500nm blaze is better than the 1000nm blaze.

I use Optometrics gratings in all my spectroscopes....
http://www.optometrics.com//App_Themes/ ... _Blaze.pdf


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Re: question on SHG

Post by highfnum »

thx

im getting up there in age
Ive done a lot of solar experiments
SHG is one that I missed im hoping to complete this project
its probably last visual solar scope ill do
any help will be welcome


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Re: question on SHG

Post by DSobserver »

i'm using a 2400l/mm and it works fine.

The 2 critical parts for a SHG are the grating and the slit (not easy to find...)


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Re: question on SHG

Post by thesmiths »

In general, the higher the grating count the better. If you want to capture a whole spectrum at once, there is some logic in using a lower grating count . But for SHG, you are zooming in on a narrow portion of the spectrum at a time. You can then rotate the grating to look at a different part of the spectrum. It's not really practical to try to capture widely spaced SHG images at the same time (ie H-alpha and Ca-K).

I would also highly recommend using precision made laser slits. Using variable "razor blade" type slits for SHG leads to all kinds of problems since the slit will be non-uniform across the diameter of the Sun. This will require a huge amount of work to remove the artefacts later.

For H-alpha, I would use the smallest slit available (for example 5 micrometer). For Ca-K, you can use a wider slit ) thereby getting more light and fewer artefacts (for example 10 micrometer).

The only problem with the laser slits is that they are not extremely long, typically only 3mm. So you then need to make sure the focal length of the optics does not lead to an image of the Sun greater than 3mm in diameter (if you want a full disk image).

I've bought both the gratings and slits from Thorlabs. They have good stuff.


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Re: question on SHG

Post by highfnum »

to "thesmiths"
thx

do you use fixed slit (if so what are parametric) or variable slit device ?


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Re: question on SHG

Post by Merlin66 »

I can supply a reflective slit plate (etched chrome on glass) with multiple slit gaps (from 20 to 90 micron).
The slits are 6mm long.


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Re: question on SHG

Post by thesmiths »

To highfnum: yes, I have found fixed slits to be better. Perhaps other SHG builders can give their opinion. As I mentioned, I have found 5 micron to be useful for H-alpha and 10 micron useful for Ca-K. But as Merlin suggests, you need to look at the whole system together, in particular the size of the pixels and the size of the CCD. Actually, for SHG, you want small pixels and ideally a rather one dimensional CCD (ie very long and narrow). This is not easy to find so the next best thing is to make sure the CCD can do ROI (region of interest). This will increase the data throughput and therefore decrease the image capture time, which is a key problem with SHG. I have found the ZWO ASI 120MM to be cost effective and useful.

The reflective slit plate that Merlin refers to may not be so useful for SHG. It is designed to use with stellar spectroscopy so although the 6mm length is very good, the slits, which I believe are radial, may not be able to accommodate the diameter of the Sun. The 20 micrometer slit may be useful but the larger ones will likely not provide sufficient resolution for SHG. I would use a single fixed non-variable slit.

It is useful to recall that the Sun's diameter in mm projected at the focal plane of the telescope will be the focal length in mm divided by 107. So a 500mm FL telescope will give a projected diameter of 4.67mm. This means for a full disc, the slit will need to be at least 5mm long (realistically more like 6mm). With a 3mm fixed slit, I limit myself to using a 200mm f4 (ie 50mm diameter) camera lens.

With all the other problems in making SHG work, I would definitely not get "aperture fever" right away. If you can get images even close to a 50mm etalon type solar telescope, you will feel like you've accomplished something great.


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Re: question on SHG

Post by Merlin66 »

The reflective slit plate has 9 slit widths each 6mm long.
The plate can be orientated and a cover plate fitted ( I use a 10mm x 6mm aperture) to isolate the entrance slit in use.
The solar image can then be imaged across the nominated slit.....


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Re: question on SHG

Post by highfnum »

i was away im back
thx for replies
my one fear is on collimation lens and how far to place behind slit and before grating
so it will focus properly

ok so fixed slits is way to go - first info I need
I have both 1200 and 2400 grating
im thinking 1200 would be a bit easier?


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Re: question on SHG

Post by highfnum »

is reflective or transmission grating better ?


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Re: question on SHG

Post by thesmiths »

To answer your questions:

For SHG, the higher the number of grating lines, the better.

Reflective gratings are in general better. For one thing, transmission gratings are typically no higher than 600 l/mm or so. Transmission gratings are good for low resolution work but SHG demands high resolution.

The collimator should be placed reasonably close to the slit but the exact distance is typically not important. Since the output of the collimator is collimated (parallel) it makes no difference how far away it is from the grating. What is more important is the focal length of the collimator. It should chosen to optimally illuminate the grating.


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