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Re: A float-glass optical window

Posted: Thu Dec 13, 2018 3:45 pm
by brian livesey
I purchased a round wall clock in ASDA's yesterday for £6. The clock per se didn't interest me, but the circular glass front and its aluminum surround did.
I removed the innards of the clock until I was left with just three items: metal rim, glass ( 230 mm diameter and approximately 2 mm thick ) and internal circular plastic frame that supports the glass. I checked the glass for quality and could detect no ripples in it when tilted under lights. The glass has high transparency, too.
The whole assembly fits snuggly over the front of the 8-inch Newt as a potential optical window. All that remains now is to send off the primary mirror for a recoat.

Re: A float-glass optical window

Posted: Thu Dec 13, 2018 4:46 pm
by Brian
That is thinking outside the box Brian. :) Look forward to hearing how the glass performs on your 'scope,

regards,

Re: A float-glass optical window

Posted: Thu Dec 13, 2018 8:33 pm
by David Frydman
Is the clock accurate. perhaps radio controlled?

Horace Dall said that one should tilt optical windows one degree if there were any ghost images.

I was once asked for a very large iris to fit a telescope.
I said that large lens irises were not big enough.
I suggested getting one from a Vent Axia fan.
Possibly by Dudley Fuller, who also commented on lateral thinking.

Regards,
David

Re: A float-glass optical window

Posted: Fri Dec 14, 2018 9:07 am
by skyhawk
we have gone from a glass optical window to a clock face

Re: A float-glass optical window

Posted: Fri Dec 14, 2018 1:06 pm
by brian livesey
When tilted to extremes, there are no discernible ripples or ghost images on the glass.

Re: A float-glass optical window

Posted: Wed Jan 02, 2019 11:20 am
by brian livesey
We've had lots of persistent cloud in the North West over the holiday period, but yesterday evening gave an opportunity, at last, to try out the ASDA clock "optical window".
Within the constraints of the magnifications used, it worked!
The MEADE zoom eyepiece ( 24mm-8mm ) was used, as it was a quick and convenient way to change powers because of the possibility of freezing fog cutting the session short. Zoom magnifications with the 8-inch/f.6 Newtonian ranged from 50X to 150X.
Stars of varying magnitudes, including in the Pleiades, Hyades and Orion, were observed. At the intra and extra-focal positions, star images showed no sign of astigmatism or ghost images; the stars maintained their pin-point images. Astigmatism would have indicated that the float glass window was unevenly annealed.
It will be interesting to make further tests with higher magnifications, and also to see the results on lunar and planetary detail. There's satisfaction in knowing that the Newtonian optics are now protected from the elements: no grime-laden damp and turbulent air entering the instrument. To afford some protection to the window itself, a dew shield was made from foam camping mat, lined with matt-black art card.

Re: A float-glass optical window

Posted: Wed Jan 02, 2019 11:43 am
by skyhawk
Why again have you wanted to add glass to a scope that was not designed that way ?

Re: A float-glass optical window

Posted: Thu Jan 03, 2019 3:40 pm
by brian livesey
Nothing ventured, nothing gained. :D

Re: A float-glass optical window

Posted: Thu Jan 03, 2019 7:12 pm
by skyhawk
brian livesey wrote:Nothing ventured, nothing gained. :D

Doesn't answer the question does it,

so it was for no reason then

Re: A float-glass optical window

Posted: Fri Jan 04, 2019 9:55 am
by brian livesey
Your Cassegrain reflector wasn't meant to have a Schmidt plate ( or Maksutov shell ) on the front Skyhawk. The reason for wanting to have an optical window on the Newt was explained in the fifth post back and in the final paragraph.

Re: A float-glass optical window

Posted: Fri Jan 11, 2019 1:45 pm
by brian livesey
Further brief ( thanks to creeping cloud ) observations were made a couple of evenings ago with the DIY optical window, and good results were obtained using a TELEVUE 32mm Plossl and a VIXEN 9mm eyepiece.
Will there be a mysterious run on ASDA's black-faced, £6 clock? We'll have to wait for their quarterly results to know. :lol:
The Moon will be the next target once this persistent, seasonal, cloud clears.

Re: A float-glass optical window

Posted: Fri Feb 15, 2019 4:14 pm
by brian livesey
Yesterday evening was the first opportunity to try out the 8-inch reflector with its float glass optical window on the Moon. It didn't disappoint. There was some atmospheric scintillation, but it was early evening with the atmosphere still cooling to its night-time temperature. After leaving the 'scope outdoors for ninety minutes to cool down, the first quarter Moon was nice and sharp at X200.
Opinions vary about the merits or otherwise of attaching an optical window ( let alone an unworked one! ) to a Newtonian reflector. Some say that the sealed off tube generates tube currents, but the same would surely apply to refractors and SCTs, but we don't hear many complaints about tube currents from their users. The opposite camp, of which Yours Truly is a member, says that an optical window gives internal air stability. One thing we can be absolutely certain about is that a window keeps atmospheric muck and damp out of the tube.

Re: A float-glass optical window

Posted: Fri Feb 15, 2019 5:44 pm
by Brian
Will you need a dew-heater band around the window I wonder? :?
regards,

Re: A float-glass optical window

Posted: Sat Feb 16, 2019 3:44 pm
by brian livesey
I wasn't out in the cold for long enough for dew to form on the window. I made a dew shield from camping mat to help keep dew at bay.