My first post - SHG build/Lenox Slits

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NM_mike
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My first post - SHG build/Lenox Slits

Post by NM_mike »

Hi Solarchat,

My first post!

I've been around the Cosmos for awhile now but this is my first serious venture into solar astronomy. Based in the American Southwest, I hope to gain a deeper understanding of our star and an exciting experience building new instrumentation. So the construction of my first SHG has begun!

It seems that the limiting component in the design is the slit. Does anyone have an opinion on these fixed slits by Lenox?

https://lenoxlaser.com/shop/optical-ape ... ical-slit/

Thanks,

Mike


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Re: My first post - SHG build/Lenox Slits

Post by thesmiths »

Those slits look pretty good. I had not heard of them before. Go for the ones with 10 micron width. You might want to think about a way to hold it as they are fairly delicate. I notice that they are made from soda lime glass. That kind of glass is quite susceptible to thermal shock (see the link on the webpage for Soda Lime Glass and compare to Borosilicate glass and Fused Silica). Therefore don't expose it to direct focused sunlight -- use a Herschel wedge, ND filter or dielectric narrowband filter.

What's the design for your SHG?


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Re: My first post - SHG build/Lenox Slits

Post by NM_mike »

This is good to know about the soda glass... I have ordered two slits for testing that I hope will be successful. I anticipate building two SHGs to start.
I have a pair of 70/72mm f/6 ED scopes, one FPL 53/Lanthanum and the other is the FK 61 glass. I would like to implement a Littrow setup with these similar to Fulvio Mete's design. I think the shorter focal length would be great with the blue end of the spectrum and I will try a stopped down 127mm f/9 refractor for Ha. My local seeing can be quite good at night. I have little experience with solar observing but I think living at 2300 meters should help with daytime seeing. The sun appears white to me here and atmospheric dispersion is greatly reduced as compared to low elevation sites.

I also have a good selection of fixed focal length lenses and am intrigued by your design and excellent results.

Mike


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Re: My first post - SHG build/Lenox Slits

Post by thesmiths »

NM_mike wrote: Sat Jul 30, 2022 1:33 am I anticipate building two SHGs to start. I have a pair of 70/72mm f/6 ED scopes, one FPL 53/Lanthanum and the other is the FK 61 glass. I would like to implement a Littrow setup with these similar to Fulvio Mete's design. I think the shorter focal length would be great with the blue end of the spectrum and I will try a stopped down 127mm f/9 refractor for Ha. My local seeing can be quite good at night. I have little experience with solar observing but I think living at 2300 meters should help with daytime seeing. The sun appears white to me here and atmospheric dispersion is greatly reduced as compared to low elevation sites.
I think the Littrow design is somewhat harder to start with. I would suggest starting with the Solex type of design. Also, just build one at a time.

By the way, Fulvio is now using some of my chrome-on-quartz slits on his devices and says they are better than the air gap slits he was using. These are similar to the ones you ordered but on fused quartz instead of soda lime glass.

You probably have excellent seeing early in the morning. I've discovered that my best results are just as the sun has risen above my trees, and then it deteriorates quite quickly (within an hour) after that. If you also have a clear horizon, which you probably do, that would also be an advantage.


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Re: My first post - SHG build/Lenox Slits

Post by fulvio.mete »

Mike, in my experience, the true, important thing in a SHG is the slit: it will decide the quality of the spectroheliograms,together with the conversion software. Then, if you use a littrow design (in my opinion the easier to handle) you must make a choice between an adjustable air slit (such as those of Surplushed, eventually refurbishing them) or a lithographic slit on a glass support.I tested a few days ago the quartz-chrome silts of 9 micron that Douglas Smith kindly sent to me, and I find them a very good choice, considering the low cost.


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Re: My first post - SHG build/Lenox Slits

Post by thesmiths »

With regards to the choice of diffraction grating: the minimum size I would choose is 30mm x 30mm (available from Edmund Optics). The 2400 l/mm holographic grating is generally the most useful (choose the one for use in the visible range). If you have money to spend, I would probably go with 50mm x 50mm (available from many suppliers), In case you decide to use bigger optics, having a larger grating is useful. But also, because of the angle at H-alpha, you actually need a very wide grating to capture all the light.

It's possible to find 25mm x 50mm, but I would just go with 50mm x 50mm. The size of the grating then determines some of the other physical parameters of the instrument. If you go with 30mm x 30mm, you can make everything correspondingly more compact.

Here's a piece of advice: either make everything extremely precisely so that there are no tilts in the optical system; or more practically, think about building in a way to compensate for the inevitable tilt misalignment of the lenses and grating.

Choice of camera: the best price / performance is certainly the ASI 178MM. Make sure it's the USB 3.0 version. The choice of the camera sensor (primarily the width of the chip, but also the size of the pixels) can determine some of the other design choices. If you have money to spend, the ASI 183MM gives you more sensor real estate to play with.

What's important in a computer: I actually don't think having a super fast CPU or a huge amount of DRAM is that important. FireCapture will by default reserve 50% of the DRAM as a memory buffer. It would be unusual to need more than 4GB so having 8GB on the computer is probably sufficient (I've actually monitored DRAM usage to test this assumption). The most important thing is actually the type of disk storage. First of all it should have very fast write speeds. Ideally, NVMe interface, which is faster than SATA. Obviously, it needs to be SSD. And finally, 500 GB is the minimum size. I would highly recommend 1 TB. The USB-C interface is actually also quite fast, so the other way to arrange things is to write to an external USB-C SSD.


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Re: My first post - SHG build/Lenox Slits

Post by NM_mike »

Thanks for the advices!

I currently have an ASI174MM and also ASI290MM from my earlier projects. Perhaps I will upgrade sensors later.
My slits from Lenox have been dispatched. Soon I will be able to start experimenting. I very much enjoy building instrumentation and have a small lathe and mill to fabricate most anything. With the monsoon rains here in full force, workshop activities are a better used of astronomy time for me right now.


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Re: My first post - SHG build/Lenox Slits

Post by marktownley »

Welcome to the forum!


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http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
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