Can I ask what tolerances you are working to with the machining?
When you say the backplate was a challenge to get 'flat', what are you aiming for?
After dabbling with some machine work I have developed a much greater appreciation for tolerancing and metrology.
Flat for me is a multi faceted challenge of getting two sides flat, and parallel, and a certain distance apart. And then being able to actually measure the result with any degree of accuracy.
I've not found any specific advice for astro so very interested to find out how tight this work needs to be.
Can I ask what tolerances you are working to with the machining?
When you say the backplate was a challenge to get 'flat', what are you aiming for?
After dabbling with some machine work I have developed a much greater appreciation for tolerancing and metrology.
Flat for me is a multi faceted challenge of getting two sides flat, and parallel, and a certain distance apart. And then being able to actually measure the result with any degree of accuracy.
I've not found any specific advice for astro so very interested to find out how tight this work needs to be.
Hi Tim,
My OTAs are made to tight tolerances in order to eliminate the need for collimation of the primary mirror.
The back plate was held, on the lathe, against a sturdy steel ring, with three screws. The steel ring was about half the diameter of the aluminium plate.I used washers as spacers between the steel ring and the aluminium plate so that the plate was free to change flatness as internal stresses were released during machining. Flatness was tested with a dial indicator that has 2 micron divisions.
The finished disc was reading deviations of no more than 10 microns near the periphery on both sides.
If the primary mirror is adjustable for collimation then such accuracy is not needed.
I finished figuring the primary today. Unfortunately I got a bit over confident about airborne dust and ignored the north wind as I did some of the figuring.
As a result I ended up with a couple of scratches, which is very annoying although it won't affect the performance.
It was interesting to see how much easier it is to figure an f/3.7 mirror versus an f/2.7, as in the CDK. The interferograms are much easier to capture as the interferometer is not pushed to it's limit, and the processing of the interferograms is also easier.
The attached screenshot is a bit of a cheat because it shows only the spherical terms of the Zernike polynomials. If I enable all terms the Strehl ratio drops to 0.95 but it is way past the diffraction limit anyway.
You sir are amazing!
Will the scratches polish out?
When I win the lotto I'm going to have a clean room for various things I do ( I have been known to strip an occasional hard drive to resuscitate it and a lot of server gear and I strip down way too may camera lenses multiple times when I realise one cat hair made it's way in.). Owning (or being owned by) 5 cats means no matter how I clean or what I do I always find at least one stray cat hair. It will be a cat free room.
They always leave their mark somehow.
The front end is now finished, minus the polishing and coating of the secondary mirror.
I used the same collimation arrangement as I did on the CDK but a bit simplified by using a single central spring instead of 3 distributed ones.
Looking very impressive indeed!
It's a pity there isn't an applicable coating which could be smeared on and fill the scratches. I guess they would only be very light then it would have to be something suitable for the mirror coating to go over.
I've broken thousands of mirrors (that may explain something) working in the glazing industry early 80s but not sure on what would be suitable if anything.
If using for visual they wouldn't be such a problem would they?
They could be edited out with darks and flats for imaging I'd guess so still a win, win situation.
If using for visual they wouldn't be such a problem would they?
This is only a cosmetic issue. Scratched products don't sell well even when the scratch has no effect on the intended use.
I think I mentioned earlier that I made a 10" f/16 Dall-Kirkham before, a long time ago. Well, fortunately, I still have the test plate I made back then for figuring the secondary mirror. This test plate was manually polished and figured with a Foucault test apparatus as I did not have an interferometer at the time.
Today I had a go at testing it with the Bath interferometer and the summary of the test is on the attached screenshot image.
The test plate was made much larger than the secondary in order to avoid possible edge figure problems. As the test results show, the inner 70mm of the test plate is quite adequate for the purpose.
That looks like a great result. Am I reading the graph correctly - it looks like the largest error at 35 mm radius is about 1/16 wave on the wavefront? If so, that’s fantastic. You were obviously very careful with your original testing.
I’m new to interferometry but now have all the parts to complete a bath. Can you analyse the interference fringes for the secondary with DFTfringe?
The test does contain artefacts as it is based on a small number of interferograms. If we take that into account then yes, most likely the central area up to 35m radius is about 1/16 of a wave on the wave front. If we include all the "noise" then 1/10 wave peak to valey should be read.
This was not an easy testplate to make, even though it is spherical, simply because it is f/2.6 and hard to see the full surface with a classical test aparatus. Something like an 80mm diameter would had been a better size, much easier to make.
For the secondary, if you can take good on axis images of the fringes then DFTF has no problem analizing them. You will need a good Fizeau interferometer so that the fringes are produced and seen in parallel light.
I started work on the carbon fibre sandwich tube and I'm very happy that I managed to get good quality 3mm Coremat. When I made the last two CDK tubes it was impossible to get the good stuff and I had to use whatever was available at the time. The main problem was the uneven thickness which required extra work and the result was still not as good as I would've liked.
I designed the DK so that one CDK tube can be cut in half to make two DK tubes.
Also I started work on a second primary mirror, this time using good quality glass from Schott.
That looks amazing Stefan!
Do you use a vacuum device when doing the carbon fibre?
I've not done anything with it (done a reasonable amount of fibreglass over the years) but I have been told you need vacuum formers for the CF?
That's a very nice job and I will re-visit my thought of using it for something if I remember what that something was. I was assured without the vacuum forming I'd have extreme issues. I just don't remember what I wanted it for back then.
