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In your set of 4 images above there are obvious indications of turned edge and marked surface roughness.

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milk bone - dog biscuits I'd say. I always got that when I cold pressed.

I don't cold press any more.

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thanks for your encouragements.

I still remember that early days when i wan 11 or 12 yr old, put together a Foucault testers with light bulb, battery, shaver knife.... it was terrible

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Slitless testers tend to make very good Foucaultgrams - IF the edge itself is clean and sharp.

Best wishes in your ATM endeavors - sounds very promising to me!

Best,
Mark

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Mine is slit-less, and I use a good and very hard steel knife edge, so the contrast and brightness is good I think.

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The edge should be straight and smooth but it needs not be sharp or hard to cut into a bundle of light!
It might be a good idea to add a second knife edge to form a kind of slit - the extra edge will not affect measurement but make the fully illuminated parts of the mirror less confusingly bright. Easy to experiment...
And the brightness is easy to control, by adding an extra resistor or potentiometer (1 kohm would likely work) in series with the LED.

While you gan get a good idea of general smoothness and reveal a number of possible problems, you can not readily decide the correction (conic constant) without measuring it, with a Couder mask.

Nils Olof

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it needs not be sharp or hard to cut into a bundle of light!

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yes, I am doing measure with Couder mask, and based on my experience, I found for medium focal ratio, like f/4 to f/6 mirrors, 1/10PV is more or less the limit, any beyond that is very subjective judgement, is that true?

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you mean my last image? yes, it's hand polished by one of the 17yr old student. I do have the figure and just for the 3 zones (30, 70, 90%), the measurement is within 1/8 PV or so, but removing the mask, the shadowgram shows that the paraboloid is not perfect of course. Allowing certain margin for error, I would say this mirror is good being a first handmade mirror, and I encourage them to use it more under real sky. They can always strip the coating and refigure it some time later if they really want to.

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There may be some minor defects without the zonal mask, but that mirror looks darned good to me. If it is an f/5 like yours, I'd say that mirror is sure good to go.

You can tell that 17 year old ATM for me that he has done a very nice job and that he should get that mirror into a telescope and some starlight as soon as possible.

1/8 wave PV is a nice standard; the Foucault Test, in spite of what others may say, can show defects that have little or no consequence under the stars.

Trust me

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The edge should be straight and smooth but it needs not be sharp or hard to cut into a bundle of light!

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A regular straight-edge razor blade, under high magnification, isn't terribly smooth, although it is sharp.

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Put it in a tube and see how well it works. That's more educational.

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Put it in a tube and see how well it works. That's more educational.

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yes, that's what I normally do, e.g, this is a mirror from a school in Macau (a city near Hong Kong), we assemble it and imaged the M8 in heavily light polluted Hong Kong. The student is amazed with what they can achieve with bare hands, and forget whether it's 1/4 or 1/20 wave.

there is no harm for perfect, but helping most students get it done is #1 priority.

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another image from this telscope, I can say the image is solid at 200x or so, not comparable to real high quality telescope, but students are very happy already. At least this can keep them going in this hobby.

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you can always make your students construct the masks thru geometric construction instead of using the computer, it works everytime.

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yes, this is the one we made, like our self-made Bahtinov Focusing Mask, even though it's ugly, but works....

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You can tell that 17 year old ATM for me that he has done a very nice job and that he should get that mirror into a telescope and some starlight as soon as possible.

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Thanks Ed for your encouragements, I also send them this thread and let them know. I am sure they are happy to know that.

I agree putting together everything and use it under real sky is very important - we need to finish the work started.

Side track a little bit, this is a 4" f7 Newtonian, which I grind and polish the mirror when I was 11 year old, but only finish parabolizing in recent years, then I want to order a curve base for my Feather Touch focuser and use on this telescope. Wayne of Starlight instruments puzzled with what I am doing, so he reconfirm with me, whether I am ordering a curve base for 5.5" diameter tube or 5.5" radius tube, apparently no one is using Feather Touch on 4" Newtonian in US....

I told him I am finishing my 30year long, cross-century project, I am mounting this telescope on Paramount ME, guidescope is a 8" APO refractor, so get me the base quick..

And this is the M42 taken with this 4" Newtonian, under heavily light polluted Hong Kong - brighter than white zone.

Edited by Savio Fong (08/07/12 10:53 PM)

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