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Go Back   IceInSpace > Equipment > ATM and DIY Projects

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  #61  
Old 02-05-2017, 09:56 PM
Stefan Buda
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I've done some ray tracing of my own and, not surprisingly, got the same results.
I also explored the possibility of a null test, for back reflection from R2 through R1 and for autocollimation with a flat. I could not get a good enough null for either setup with a simple null lens.
Looks like I have to make the test plate for R2 and leave the figuring of R1 last, as a complete instrument in autocollimation.
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  #62  
Old 18-06-2017, 01:54 PM
Stefan Buda
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Still extremely slow progress with this project. One reason is that I took on the job of salvaging a brand new 10" astrograph that came with unusable optics and for which the manufacturer failed to take the responsibility of replacing it with a usable one. Well, it turned out to be a much bigger job that I thought it would be and it's been using up much of my spare time. The primary mirror needed complete regrinding/figuring to get rid of major astigmatism, plus a very long list of other mechanical and optical problems that needed fixing.

Anyway, I purchased 3 slices of aluminium alloy 6061, 178 diameter, for the main structural parts, and I cast an Ultracal30 disc for the R2 test plate lap. I prefer alloy 7075 for my projects, for its hardness, but the 6061 alloy also anodizes very well and it is much easier to obtain.
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  #63  
Old 09-07-2017, 09:21 AM
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SimmoW (SIMON)
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Amazing work as ever Stefan.

Can you at least pm me the manufacturer of the 10 inch so I avoid them in future? Publication of their negligence would be preferred though.

Hey, I'm thinking of getting some aluminium tube rings made for my new 10 inch made by Alistair Sam, 314mm external diameter, do you know of a good cnc guy who charges reasonably? And will want them red anodized :-)

Oh, good luck with your Malin award shortlisted pics.
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  #64  
Old 09-07-2017, 07:11 PM
Stefan Buda
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Hi Simon,
I bumped into you today before I saw your post.
On this link you can read about the scope, but the problems listed are only minor compared to what I found when I started pulling it apart and began testing and measuring all components: http://myastroshop.com.au/ssmassey/r...-10-review.htm

Getting back to the Honders, I did put into it only a few hours since my last post, but not so my 3D printer. The second set of 3D prints for the sacrificial OTA mandrel is done. Another marathon, 52 hours of printing.
This mandrel is 10mm smaller in diameter than the first one.
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  #65  
Old 09-08-2017, 07:59 PM
Stefan Buda
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I took advantage of the heatwave today (17C) to cast the lap for the R2 test plate.
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  #66  
Old 10-08-2017, 07:35 PM
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Quote:
Originally Posted by Stefan Buda View Post
I finished the small lenses, except for the AR coatings.
For the coatings I can think of three possible options:
1. Find someone that can BBAR coat them for me.
2. Just do my own single layer magnesium fluoride AR coating.
3. Upgrade my vacuum system and do my own BBAR coating.

Problem with the first option is that I'm not aware of anyone doing commercial BBAR coatings in Australia, except for spectacle makers perhaps.
Problem with the second option is that single layer AR coating on low index crown glass is not as good as BBAR - 1% vs 0.25% reflectivity per surface across the spectrum.
Problem with number three: Big learning curve.

hi stefan i am a spectacle maker and i may be able to get the latest multi coatings done for you / glass is rarely used in opthalmics these days and a different mixture, from memory is used, but i have a lot of contacts of independant grind laboratories around australia
pat
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  #67  
Old 10-08-2017, 09:03 PM
Stefan Buda
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Hi Pat,

That sounds very promising. Please let me know if you find someone willing to do it. The refractive index of BK7 is not all that different from the ophthalmic plastics, so the coatings could be very similar I guess.

Stefan
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  #68  
Old 10-08-2017, 10:23 PM
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blank size may be the limiting factor but i will make some enquiries
pat
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  #69  
Old 11-08-2017, 09:45 AM
Stefan Buda
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Pat,

At this stage I would prefer to have only the two small lenses coated and they are only 50.8 diameter.
I would wait with the three large surfaces until I have the complete OTA tested under the stars.

Stefan
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  #70  
Old 15-08-2017, 10:28 AM
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R2 test plate finished.
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  #71  
Old 22-08-2017, 09:26 AM
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Some progress on the metal front. The rear reinforcement ring for the dew shield is done, as far as the turning is concerned.
First I skimmed the three aluminium discs on the lathe to make them flat and reduced the OD closer to the required size. Then I trepanned one of the discs and glued it onto another so that a can machine it distortion free.
The deep groove (2.5x2.5) in the middle is for a string of heating resistors. The shallow, angled groove is for engaging three spring loaded pins, when the dew shield is fully extended. There is also a 5mm wide undercut for a strip of self adhesive synthetic velvet, intended to prevent the scratching of the OTA when the shield is moved back and forth.
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  #72  
Old 13-09-2017, 10:18 PM
Stefan Buda
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Good progress on the metal front. All aluminium parts finished on the lathe, apart from the focuser components. After I drill all the required holes this lot will go to the anodizers.
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  #73  
Old 14-09-2017, 04:44 PM
Wavytone
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All very nice Stefan !
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  #74  
Old 24-09-2017, 09:47 PM
Stefan Buda
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Thanks Wavytone.

The weather has warmed up finally and I was able to proceed with some of the glass work.
The front lens is fully polished and R2 is figured against the test plate.
The R1 figuring will be done last, when the whole OTA is complete.
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  #75  
Old 01-11-2017, 04:12 PM
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Anodising done after a hiccup.
I've been getting my anodising done by Electomold, in Thomastown, for many years, because they work to military specs and are quite reliable. But their minimum charge has been going up way beyond de CPI and I had a bit of a shock when they told me that now it is $200 plus GST. So I took my bits to Collins Anodic and had them done for $70 plus GST - their minimum charge.

Even bigger hiccup with the front ring engraving.
I intended to have it laser engraved but for some obscure reason I got knocked back. Then I tried another laser engraver on Toorak road. They had a more primitive, CO2 laser based, engraving machine and wanted $85 for the job. I was not convinced that they were going to do it well, so I decided to mechanically engrave it myself. For that I had to cobble together a rotary platform and use my old CNC engraver. In the end I could not get rid of the backlash without making the axis too hard to turn for the stepper motor and the ancient software that runs it does not have backlash compensation. The end result is not much better than a hand engraving but it will have to do.
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Last edited by Stefan Buda; 01-11-2017 at 06:40 PM.
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  #76  
Old 20-11-2017, 10:13 PM
Stefan Buda
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Second CF tube done.
Back plate and front lens housing glued in.
Concentricity and parallelism held to better than 0.006mm on both the front and the back elements.
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  #77  
Old 23-11-2017, 12:47 PM
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Just stumbled across this thread - wow

Stefan this is really taking the diy projects to the next level

looking forward to first light - you will have to do some astrophotography also?
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  #78  
Old 24-11-2017, 08:28 AM
Stefan Buda
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Thanks Troy, and yes, I will use it for AP just like all my previous DIY instruments.
Crazy, but for me an astrophoto starts with designing the instrument with which to capture it.

As an example, I made a dedicated instrument for capturing the Aureole effect during the transit of Venus in 2012. http://asv.org.au/images/gallery/Ste...nusTransit.png
..and I haven't used it since, because we haven't had any more Venus transits.
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  #79  
Old 25-11-2017, 03:19 PM
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Sharp images of Venus and it shows the rare phenomenon well (aureole effect).
I know from your planetary images the 16" D-K is a fine scope also.

Stefan is optics/design - is that your background or is it self taught?

⌐■_■

Cheers
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  #80  
Old 26-11-2017, 09:53 AM
Stefan Buda
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Troy,
Thanks for the compliments.
My background is mechanical engineering. Optical design is self thought, based on an early interest in optics going back to my primary school years.

Talking of optics, I have nearly finished grinding the mangin mirror for this project. I have been scratching my head for a long time trying to come up with a procedure that is not too difficult and will result in a usable optic.
The main problem is the big hole in the middle, as it causes all sorts of difficulties.
I decided to polish and figure the front surface with the "plug" still attached by a couple of millimetres of glass. That should help with the testing, and the polishing can be done with a normal full lap. After that I will finish the trepanning and I will do a precision cylindrical grinding operation on the internal diameter. Hopefully not too much residual stress will be released and the front (finished) surface will cope better with a bit of distortion, being a refractive surface, even though it works in a double pass mode. The second surface will be done with a full diameter polisher that will have its centre dug out to match the hole in the mirror. Testing of the second surface will be difficult.
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