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Polishing an optical window - Advice needed from mirror makers

angusb1

By angusb1
in DIY Astronomer

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angusb1

angusb1

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Hoping someone with experience of polishing optics can help with some questions I have about polishing an optical window.

I guess it would help if I explained what I want to do first. I want to make a newtonian with a piece of glass that holds the secondary like a maksutov newtonian does but instead of using a meniscus corrector to remove coma I want to use a piece of glass that is flat on both sides, an optical window. I am interested to see how much of a difference to contrast a closed tube and the removal of spider vanes makes, and I have always wanted to make my own telescope so I thought it would be fun to build and then do some experiments with. I have a coma corrector anyway so don't need the lens to be a meniscus.

The prices I have been quoted for an N-BK7 blank are quite low. For the same thing polished to 1/4 wave on both sides the price is 6 times higher so I would like to find out how difficult and time-consuming it is to polish the blank myself.

So what I am hoping someone can tell me are:

1. How good will the quality of the polishing need to be? From what I have read on the Oldham Optical site, "A mirror needs an surface accuracy of better than 1/8λ, but a Corrector is a refractive device and only needs to be within "a few waves" at any point on the surface to give a matching performance" so what kind of surface accuracy will I need? My primary and secondary are only 1/4 wave PV anyway so I need to know the minimum surface accuracy I will need so I don't detract from the quality of the view these will provide.

2. How do I go about measuring the surface accuracy? Would I do the same as for a mirror?

3. Will I need to ensure the faces are parallel? If so how would I do this? Could I use a tool like the one described in Roger Ceragioli's refractor construction page

4. How would I go about the process of lapping? Would I need to get two blanks and use one as a tool to lap the other or could I use a much cheaper type of glass, or even a different type of material?

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angusb1

angusb1

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Hi John, I was hoping you'd be one of the people on here who could advise me ;).

I'm planning on using a Skywatcher 1145P mirror set. I haven't been able to find out the diameter of the secondary but the primary has a diameter of 114.5mm and a focal length of 500mm. I did post a question on here about the size of the secondary and the best guess was that it would be around the 34mm mark.

I'm getting the Texereau book for christmas and have been hoping it would help with these sort of questions.

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Glasspusher

Glasspusher

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I will try to answer your questions..

1. The optical window will need to be made as accurately as possible if you are not to compromise the whole system. You will need to ensure that the window as no wedge, ie difference in thickness round its diameter. You do not want the window to act like a prism!!

2. Texereau’s method of testing the window involves using a spherical mirror at least as big as the window, so you might need to make this before you start.

3. Your window will need to be parallel, in other words have uniform thickness.

4. My preferred method would be to use three disks and grind them on each other in the order 1 on 2, 2 on 3, and 3 on 1. This would need to be done for both surfaces of course. This is the traditional engineers’ methods for producing a flat surface. One of the disks would be your window, the others could be plate glass of the same diameter. After grinding the window could be polished on a lap of the same diameter. The surface not being polished would need to be protected to avoid scratching, and of course the polished surface would need protecting whilst working the other side.

The process is quite involved and represents quite a lot of work. The optical system that you are working with has a large secondary obstruction which would limit the performance to no better than ¼ wave. This size secondary would contribute to a significant loss of contrast in the system without the optical window. An optical window would be better suited to a system with minimal secondary obstruction, a system designed with a window as an integral part of it.

With regard to a sealed tube, this can be problematical if the air in the tube is at a different temperature to the air outside of it. Some SCT’s are now being marketed with ventilated tubes to address this problem.

Just some thought’s hope they help.

John

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FraserClarke

FraserClarke

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Another thought... Wouldn't the optical window need to be coated just like all the other glass elements in the optical train, such as eyepiece lenses, filters..... ?

Ideally yes, otherwise you'll lose ~4% of the light at each surface. It will also increase the risk of ghost images from bright objects in/near the field.

It's certainly a feasible project, but it's at least as hard, if not more so, than making a mirror. As John suggests, if you want to go down this route, it might be better to think about designing the whole system from scratch.

An off-axis mirror of this size would be a pretty nice system. Making a ~250mm paraboloid yourself and only using an outer 115mm would be a lot easier to build/test, and would probably give better results than a windowed newt...

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Peter Drew

Peter Drew

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We made a small number of 10" and 12" Newtonians with optical windows in the 1980's. These were customer driven rather than our own enthusiasm for such things. Pros:- no secondary spider vanes, mirrors protected, tube currents reduced. Cons:- slight loss of light if uncoated, dew magnets, heavy, EXPENSIVE!. I have a spare 12" diameter optical window with a 1" central hole made by David Hinds if anyone has a large project in mind. Angus, I'm sure that I have a suitable optical window with 1" central hole also by David Hinds which you can have provided I can find it! ;)

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angusb1

angusb1

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We made a small number of 10" and 12" Newtonians with optical windows in the 1980's. These were customer driven rather than our own enthusiasm for such things. Pros:- no secondary spider vanes, mirrors protected, tube currents reduced. Cons:- slight loss of light if uncoated, dew magnets, heavy, EXPENSIVE!. I have a spare 12" diameter optical window with a 1" central hole made by David Hinds if anyone has a large project in mind. Angus, I'm sure that I have a suitable optical window with 1" central hole also by David Hinds which you can have provided I can find it! ;)

Thankyou Peter, if you can find that optical window I would be very grateful. Please let me know if you can.

John, thankyou for your replies to my questions. If Peter is not able to find the optical window I will probably try to make one so this is very valuable advice to me. I found some youtube videos this afternoon showing just the 3 blank procedure you describe here which were very interesting but it is the testing I have not worked out yet and which could make the whole thing impossible for me.

Your advice re the size of the secondary makes me think I should also consider replacing the secondary mirror from the set I just bought this afternoon with something smaller. The scope I want to make is primarily for planetary use and as a portable scope to take on holiday so it doesn't necessarily need a large secondary. I will have to go back to the secondary calculations I made last week and see if I can use a smaller one.

Chris, yes, as Peter says, the window should ideally be AR coated. I haven't got as far as finding out how much this would cost though so I'm not sure whether or not I could do it.

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angusb1

angusb1

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I might try that too, the idea behind the experiment was to find out how much of a difference spider vanes made to contrast. If I have a window to support the secondary I can add different spider vanes in front of it to see what a difference various types make.

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