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PostPosted: Mon Nov 26, 2018 10:54 am 
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We know that the optical quality of float glass has improved considerably over the years. Has anyone attempted to use a piece of float as an optical window to keep the elements out of their Newtonian? Purists might balk at the idea, but it's worth a try.
Even through domestic double-glazing, we can get sharp images of the Moon on low and moderate magnifications; I've yet to try higher powers. It's stellar targets that attract my interest most, so here we are only dealing with point sources, which should give more tolerable results than trying to see details on planetary surfaces and other extended objects.
BAADER sells its ultra-thin "turbo-film" for placing over the front of open-tubed 'scopes ( the late Peter Grego made use of it ), but the material is expensive and hardly robust.

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PostPosted: Mon Nov 26, 2018 12:43 pm 
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brian livesey wrote:
We know that the optical quality of float glass has improved considerably over the years. Has anyone attempted to use a piece of float as an optical window to keep the elements out of their Newtonian? Purists might balk at the idea, but it's worth a try.
Even through domestic double-glazing, we can get sharp images of the Moon on low and moderate magnifications; I've yet to try higher powers. It's stellar targets that attract my interest most, so here we are only dealing with point sources, which should give more tolerable results than trying to see details on planetary surfaces and other extended objects.
BAADER sells its ultra-thin "turbo-film" for placing over the front of open-tubed 'scopes ( the late Peter Grego made use of it ), but the material is expensive and hardly robust.



Actually it has been used in observatories over the world where glass seals the observatory (I remember on 2 8 and 10" refractors, and the scope looks through the OPTICAL glass. All you need to be aware of is reflections so when using it on a reflecting scope there should e no problem except for the purist !


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PostPosted: Mon Nov 26, 2018 4:16 pm 
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Why do we not see Newtonians being sold with float glass optical windows?

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PostPosted: Mon Nov 26, 2018 4:53 pm 
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brian livesey wrote:
Why do we not see Newtonians being sold with float glass optical windows?



lower quality more air to glass obviously, weight, cool down, think about it

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PostPosted: Mon Nov 26, 2018 7:16 pm 
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I was told some time ago that an amateur in Worthing sealed the slot of his small observatory with some kind of PVC sheeting. Also I did once come across the mention on the Internet concerning a public(!) Observatory in the U S A had set up an viewing observatory with for telescopes though a glass window of some sort? These ideas were not to protect the telescopes but rather to protect the observers from the cold etc. Viewing the sky through double glazing with small instruments and low magnifications and when viewing bright objects like the Moon etc certainly beats dragging a tripod out on a cold and windy clear winters night.

In a similar vein, the Camera Obscura at the Foredown Tower views through a glass window, presumably double glazed as are all others in the viewing gallery. This instrument,. with lts 12" achromatic-doublet f18 lens by David Sinden with its 18" optically flat, front-aluminised mirror, produced excellent images on the 5ft diameter, concave fibreglass screen below, if conditions are good. However the magnificaton is low. The four supporting pillars for the top of the cupola are so close to the mirror & lens that they are completely out of focus on the screen below viewing the scenery.
Using spotting scopes and binoculars through the double glazed windows of the viewing gallery work fine, however, as these windows on all four sides, one tends to suffer from s multitude of reflections from the opposite window. This is verry obvious in the evening. The viewing gallery is the adapted tank of the water tower and the windowns are all vertical...windows in the control towers of airport are , I believe, set at an angle to stop this problem. When the Camera Obscura is being demonstrated the room is plunged into darkness by electrically operated blackout blinds , unfortunately all close together otherwise it might be possible to only have one side open and the rest closed.
That is enough about that as I am about to leave and walk up the the Tower... unfortunately Mike Oak and Foredown Hill are presently completely lost in fog! Regards maf


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PostPosted: Tue Nov 27, 2018 9:56 am 
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I suppose the obvious thing to do would be to experiment with a suitable piece of float-glass over the front of the 8-inch Newt. Plate glass might not be suitable because of its green tint. As for cool-down time, this shouldn't be any different from the length of time it takes for an SCT to cool down.

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Last edited by brian livesey on Sat Dec 08, 2018 3:25 pm, edited 1 time in total.

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PostPosted: Tue Nov 27, 2018 5:51 pm 
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brian livesey wrote:
I suppose the obvious thing to do Skyhawk is for me to experiment with a suitable piece of float-glass over the front of the 8-inch Newt. Plate glass wouldn't be suitable because of its green tint. As for cool-down time, this shouldn't be any different from the length of time it takes for an SCT to cool down.



No harm can be done

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PostPosted: Tue Nov 27, 2018 6:40 pm 
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Hmmm... given that one can buy glass solar filters to fit a range of telescope diameters, it must be possible to obtain the same item but uncoated, but I can't find any :?

Someone must manufacture the blanks after all, and they would make great windows for Newtonian telescopes off the shelf,

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PostPosted: Wed Nov 28, 2018 11:46 am 
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The price of an uncoated, optically-worked, 8-inch diameter glass solar filter would surely be "astronomical".

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PostPosted: Wed Nov 28, 2018 5:45 pm 
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brian livesey wrote:
The price of an uncoated, optically-worked, 8-inch diameter glass solar filter would surely be "astronomical".


Well, not necessarily? :wink: :

https://www.rothervalleyoptics.co.uk/se ... ilter.html

Then less the cost of aluminising of course?

regards,

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PostPosted: Thu Nov 29, 2018 11:19 am 
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The optical window of my 12.5 inch Dall Kirkham cost more than the main mirror.

Horace Dall's optically worked selected attic window, I think plate glass, gave incredible views of Mars with his 8 inch Maksutov at 400x at a big angle.
I didn't note a green cast, but wasn't looking for it. The detail on Mars was amazing.

I observe terrestrially through double glazing at 125x with the Skywatcher 90mm Maksutov with very good views.
I used up to 190x, but not so good. The Sun was very bright behind me.
In poorer light the contrast of the Maksutov is not so good.

I have seen 4 Jupiter belts with the Acuter 80mm spotter at 95x through double glazing but the image was soft.

My double glazing varies in quality and there are sweet spots.

The PST has no degradation through double glazing at 60x at up to 25 degree elevation.
The detail is sometimes amazing.

The larger the scope the poorer the image through double glazing in my experience.
So an 8 inch Newtonian may present problems,
But try it and if it is a large bit of glass try to find the best part.

At lower powers it may be O.K., but 200x and above for planets may show quite a lot of degradation. But again, one may find a sweet bit of glass.

The lenses for Broadhurst Clarkson's scopes in WW2 were made from shattered window glass that littered the streets. Maybe eyepieces and relay lenses. Not sure about objectives.

Broadhurst Clarkson large magnifying glasses, 4 inch or so diameter are very green when looked at side on, but O.K. in use.

Regards,
David


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PostPosted: Thu Nov 29, 2018 2:54 pm 
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Concerning the PST- type solar telescope, I have often wondered why it requires a achromatic OG at all as only a very narrow wavelength of incident light produces the final image. ?? Regards maf.


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PostPosted: Thu Nov 29, 2018 3:34 pm 
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If I remember correctly, the original Lyot-design coronagraph had only a single element objective lens,
Bob


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PostPosted: Thu Nov 29, 2018 7:47 pm 
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In his original post, Brian was wondering about the use of a piece of float glass. I don't think that optically-worked glass was part of his plan. Those glass solar filters, at under £100 each for an 8-inch instrument would seem to be much superior in optical quality to float glass, if(?) they could be sourced without the metallic coating,

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PostPosted: Fri Nov 30, 2018 12:09 pm 
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Thanks for the interesting info' chaps. Yes, those solar filters look good and are definitely affordable. As you said Brian, the problem is with the orange coating.
There might be a suitable chemical that can remove it. I've had no problem removing old aluminium from parabolic mirrors using caustic soda.

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