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Options for small solar Newt?

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#1 JoeInMN

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Posted 09 August 2020 - 11:36 AM

Edit: I had some questions regarding the building of a dedicated solar Newt using uncoated optics; I see this may violate restrictions on discussions of DIY solar equipment, so I'm retracting it for now... if the topic is OK, I can re-post; otherwise, my apologies.

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Edited by JoeInMN, 09 August 2020 - 12:59 PM.


#2 sarastro

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Posted 09 August 2020 - 01:40 PM

I would recommend posting in the ATM, Optics and DIY Forum.

There have been several past discussions of solar reflectors.

I've thought about making one as well.



#3 BYoesle

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Posted 09 August 2020 - 01:50 PM

There appears to be no TOS violation that I can see as you are not otherwise modifying a commercial solar telescope.

 

Use an uncoated primary, and a fully coated secondary. See Christian Viladrich's 300 mm solar Newtonian here.



#4 JoeInMN

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Posted 09 August 2020 - 02:31 PM

Ah, OK re: the TOS. Thanks! It is kind of an ATM topic too, but most of the info I'm seeking relates specifically to making a scope safe for solar viewing, so I figured I'd put it here.

 

Original post was this:

 

A 4.5" f/8 Meade Newt OTA that I picked up at a nearby thrift store a while back had badly degraded coatings on its mirrors (pic here), so a few days ago I decided to strip the coatings completely and use the clear glass to make a little dedicated solar Dob. A couple of questions for those who know more than I do (so, pretty much everyone here):

 

Googled-up opinions seem to vary as to whether the flat back of the primary should be clear (a diverging beam is reflected back up the tube) or frosted (light is diffused off the back surface). Mine is clear. I'm kind of leaning toward roughing it up with sandpaper under running water, but I will not be able to reverse that if I change my mind after doing it.

 

What's the best option for a secondary mirror? I have both coated and uncoated secondaries. I plan to mount the required ND filter into the focuser. Would a coated secondary call for the same ND3 filter as a Herschel wedge? What density would be needed if the secondary were uncoated? The original secondary is now clear, but will give a double image off of its rear surface. I could use the long face of a right-angle prism from an old porro set, in the manner of a Herschel wedge variant, though I'd rather have something elliptical. An uncoated elliptical flat with a wedged profile to deflect the ghost image would be my preference, but I don't have the means to make something like that.

 

I figured an uncoated secondary would cut the primary's 5% reflection down by another 95%, but I'm guessing the resulting .0025 would still be too much without further filtration. I assume that I also need a UV/IR filter, as a Newt's image path doesn't pass through any glass as it does with a refractor.

 

I have both front-end AstroSolar filters and a Lunt wedge for my refractors; this project is just to scratch the gotta-make-something itch, and avoid having a decent piece of glass go to waste...



#5 vincentv

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Posted 10 August 2020 - 10:26 AM

My 2c,

Use a beloptik uv/ir on kg3 filter to make sure you only get the visual spectrum. Add the ND or double polarizer afterwards 'till you get a comfortable brightness. The beloptik filter would mean you're only taking into account light that can be seen or captured by a camera, so no surprises from long IR.

A trick to dial the brightness is using a camera with the astro solar or wedge filters and another scope. It takes some basic math and playing with exposure settings to figure out if you're within the same brightness range or way out. 


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#6 DAVIDG

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Posted 11 August 2020 - 01:44 PM

 I made one a number of years ago with excellent results. It includes a rotatable Herschel wedge that allows one to tune the brightness of the image by taking advantage of the fact the light being reflected off the uncoated diagonal is polarized. So by rotating the Herschel wedge you cross polarize the light and can adjust the brightness of the image to match the sky conditions and magnification used.

 Here is a link to my write up http://www.considine...dgroski/wlnewt/

 

             - Dave 

whitelight solar newtonian.jpg



#7 JoeInMN

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Posted 11 August 2020 - 09:00 PM

My 2c,

Use a beloptik uv/ir on kg3 filter to make sure you only get the visual spectrum. Add the ND or double polarizer afterwards 'till you get a comfortable brightness. The beloptik filter would mean you're only taking into account light that can be seen or captured by a camera, so no surprises from long IR.

A trick to dial the brightness is using a camera with the astro solar or wedge filters and another scope. It takes some basic math and playing with exposure settings to figure out if you're within the same brightness range or way out. 

Yeah, the UV/IR is pretty much my only concern; I always worry about it blazing through invisibly even though the visible image might be completely comfortable. With a coated secondary, I'm sure I'll need a ND filter similar to what is in my Herschel wedge, in addition to the UV/IR filter. Even with my filtered or Herscheled refractors, I still feel a bit of instinctive anxiety about pointing a telescope at the Sun and looking into it, even though I know it's totally safe.

 

 I made one a number of years ago with excellent results. It includes a rotatable Herschel wedge that allows one to tune the brightness of the image by taking advantage of the fact the light being reflected off the uncoated diagonal is polarized. So by rotating the Herschel wedge you cross polarize the light and can adjust the brightness of the image to match the sky conditions and magnification used.

 Here is a link to my write up http://www.considine...dgroski/wlnewt/

 

             - Dave 

attachicon.gifwhitelight solar newtonian.jpg

That is interesting. My mirror is f/8 rather than /10, but I could use a low-profile focuser that I have, and probably be OK with the secondary that I have. The difficulty would be getting ahold of another Herschel wedge, which isn't compatible with my budget at the moment, heh... Nice instrument. I like the fork mount, too... I do keep wanting to put a balancing weight on the other end of the RA shaft, though.


Edited by JoeInMN, 11 August 2020 - 09:05 PM.


#8 trainsktg

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Posted 13 August 2020 - 06:34 PM

Here is a very old photo of a 4.25” f13 solar newt still in my family’s possession. I haven’t looked through it for some time because it is at my Dad’s house some States away, but I remember being very impressed with the images at the time. If I can talk my Dad out of it I’ll try to bring it back home with me when I visit again in a few months. 

 

IIRC the builder’s description correctly (he died many years ago) it utilizes  an uncoated primary, a Herschel wedge used as the secondary and a neutral density filter in the drawtube. 

 

Sam Brown’s “All About Telescopes” has a page dedicated to the amount of attenuation necessary per inch of aperture  to achieve a safe level of viewing. 

 

Keith

 

 

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#9 JoeInMN

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Posted 19 August 2020 - 09:56 PM

Here is a very old photo of a 4.25” f13 solar newt still in my family’s possession...


Keith

Nice... Mine would most likely use a somewhat oversized sturdy cardboard tube that I have on hand, or maybe some lengths of oak that were originally intended for tripod legs.

 

This is going on the back burner for a while, though. I could build the OTA now, but I need to get a clearer understanding of how the UV/IR rays behave in such a system, so I can be a bit more confident that I've laid out an optical train that's safe for staring at the Sun.




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