Has Anyone Made A Window?
Posted 03 January 2013 - 02:00 PM
The Astroscan seems to have a simple solution using some sort of plate glass.
How hard is it to make a window for a 6" or 10" Newtonian. How flat must it be and what about the wedge tolerance angle?
BK7 glass would be preferable. What is the cost of this material in 1/2" thickness?
Posted 03 January 2013 - 02:32 PM
It can be done by an ATMer, but probably not easily enough for many of us-- it's got to be VERY flat on both sides, and this is pretty tough to do, and tough to test, without, I believe, other flats of the same or larger size, for interference testing.
If it's not done well, you'd loose all the advantages. And, of course, it only removes a small component of the diffraction-- the rest of the obstruction, the secondary, is still there.
You'd have to really hate diffraction spikes to want to resort to this level of difficulty, instead of using, say, a curved spider and a sub %18 secondary.
I think Edmund did it more as a way to keep the scope maintenance free (i.e., control dust). Don't know how flat they are, but the 'Scan is not known for optical superiority, fun scope though it is...
Posted 03 January 2013 - 03:17 PM
Posted 03 January 2013 - 03:42 PM
Posted 03 January 2013 - 03:43 PM
As already explained by Joe in his reply above, putting the window in front of an accurate long radius sphere makes an excellent and sensitive test. The window doesn't need to be really flat, but it should at least be very smooth.
An experienced amateur could do the job very well. Be sure to use a high quality optical glass!
Posted 03 January 2013 - 03:49 PM
Texereau's test using a Foucault tester and spherical mirror with the window immediately in front of the mirror is the best non-interferometric test - very sensitive. You can do it as Spectral Joe describes above, but zones may or may not be as clearly visible if a paraboloid mirror is used. Using a truly spherical, zone-free mirror will show window zones the best.
I used a 23" sphere with 200" ROC figured at Zeiss (a very good sphere) to test aircraft sensor windows for transmitted wavefront error. The sensitivity was amazing. You could touch the window with your finger for one second, and clearly see the bump on the wavefront error.
Keeping the wedge to less than 0.001" is adequate to prevent prismatic effects, and is easy during end-stage fine grinding. Years ago I made a 12" window and for the heck of it, I got the wedge down to 0.0001" or less, as closely as I could measure, just to see if I could do it.
A critical thing often overlooked is to tilt the window a few degrees. This keeps inevitable ghost reflections from coming to focus anywhere within the focal plane. The purpose of AR coatings should be to improve transmission. They cannot eliminate ghost reflections, only reduce them.
Posted 03 January 2013 - 03:51 PM
In "How to Make a Telescope" by Texereau, there is a chapter on how to make a optical window and also a BASIC programming list to reduce it.
Posted 03 January 2013 - 04:01 PM
I'm aware that you can test the window in front of the primary during Foucault. The difficultly, at least for me, would be taking that information and using it to figure out what to do on what side. But then, my fabrication skills are very limited. But so are many, if not most, atmers.
I've noticed that my SCT (not to mention my Astroscan) has as much issue with tube currents as any scope, despite it's equivalent of an optical window (albeit figured for a different purpose). The trick would be to include other features, such as tube insulation, and fans, both of which work really well without a window. Without those things, it'd work just like any other closed tube design. Look at the efforts folks, including myself, have gone through to address thermals in our SCT's and Mak Newts.
I'm not saying it can't work superbly. I am saying, though, that with a lot less work you can get 'traditional' designs to work NEARLY as well. Depends on whether the teeny increase in performance offsets the difficulties of manufacture, or if the process is enjoyable enough that it's worth it to that end.
For my skill level, this would simply not be something I'd see as worth the effort, considering the other solutions at hand. I can certainly see, though, how it'd be worth it, and doable, for some folks.
Posted 03 January 2013 - 05:05 PM
Very keen observation, indeed, but something that should be blatantly obvious, so I am surprised no one else mentioned it. Without some sort of interference reference test using a test plate, it would amount to guessing which side needs to be corrected. But if you have a test plate then the mirror test seems superfluous. I'll have to read Texereau's chapter to see if he addresses the issue you mention.
Key point: "Experienced Amateur".
I'm aware that you can test the window in front of the primary during Foucault. The difficultly, at least for me, would be taking that information and using it to figure out what to do on what side.
Posted 03 January 2013 - 07:03 PM
sometimes used for Schmidts or Wrights. The 48" Schmidt is
plate glass. Could you use an optical flat along with the
Foucault test. Lay the window on the flat to see which side
has zones etc.Or use the three disk method of making a flat.
and you wouldn't need a test flat. I know these ideas
are a lot of work but if you want a flat window!Spikes don't
don't bother me all that much and with curved spiders you can reduce them quite a bit. as far as tube currents use a
trussed tube like Royce recommends.
Posted 03 January 2013 - 09:34 PM
Chapter 10 in "How to Make a Telescope" is "The Telescope Window". It will answer 98% of any ones questions on the material, fabrication and testing of an optical window for a Newtonian. Some of the topics covered are
Choice of Glass
Smoothing Tolerance and Parallelism
Rough Grinding Fine Grinding and Smoothing
Optical Testing of the Window
Polishing and Retouching
Quantitative Testing and Data Reduction
Posted 03 January 2013 - 10:24 PM
Why would you use the Foucault test if you can test the window with a reference flat?
However, if using Texereau's method of placing the window next to a spherical mirror, I am not sure how to determine which side of the window needs to be worked on, except with a test surface of a known profile.
Windows don't have to be flat. Unless you're using the three disc method, it's very unlikely you'll get it flat.
A 6-inch window can be made using a 4-inch spherometer reading to no better than 0.001" as long as both sides are parallel - in other words, you want a weak meniscus, but never a weak biconvex or biconcave lens.
Thus it would have to be 0.001 concave on one and 0.001 convex on the other side, wedge-free, and smooth. But you couldn't use an optical flat because there would be too many fringes (0.001" = 46 wave or 92 fringes!). You'd need a test plate of the same ROC and that's not suitable for a Foucault test, so you'd have the same problem as with the Texereau method of not knowing which side needs to be worked on.
That's why it's necessary to use the three-disc method and get it flat enough to be tested with a master flat.
Looking through Texereau in passing, I couldn't find anything on how to determine which side needs to be worked on. I'll go back to reading more.
Posted 03 January 2013 - 11:46 PM
Posted 04 January 2013 - 01:43 AM
The three-disk technique produces roughly 1/2 wave flatness. That's a sag no bigger than 0.00001" over the entire surface.
Good luck trying to polish out 5 waves of curvature. I use my hands, maybe with a turntable that's not so daunting.
Posted 06 January 2013 - 06:23 PM