You mentioned you anodize small parts at home, I've yet to try it at home but my extender/adapter isn't exactly small, primary thing I want to do.
If you're using black are you using a specific dye?
I've only seen mention of black food colouring online (other than very expensive dyes) being suitable but haven't tried it yet.
It's different to working in the industry over 40 years ago where they had all of the precise chemicals and I no longer remember what was what. Plus they brought a chemist to get chemical mixtures right, I was happy not having to play with cyanide (I ran the zinc tanks but also did anodizing in another tank next to the one with cyanide) for a yellow/gold finish on aluminium products.
If you're using black are you using a specific dye?
Yes, I use Sanodal Deep Black MLW from Clariant. I paid a couple of hundred dollars for a few hundred grams, over 20 years ago, and I still have enough left for a few more years.
The DK250 won't have a mounting rail for the dovetail plate, in order to minimize weight, and that means that the dovetail plate wont be adjustable for balancing the DEC axis. The DK won't need to cope with the wide variety of possible gear like the CDK, so the center of gravity won't shift as much.
To accommodate CG shifts I made the dovetail plate longer. The problem is that I'm only guessing where the CG will fall.
Finished the back plate. This is the most time consuming structural part of the OTA.
I decided not to sandblast the parts because I'm worried about distortions caused by the induced stress. It was difficult getting the back plate flat as every time I skimmed one surface on the lathe, the released internal stress distorted it. I was holding it at 3 points and had to alternate machining both sides, taking smaller and smaller cuts.
The back plate is very similar to the CDK250 version, the main difference being the addition of a pocket that will house the speed controller for the fans, the power jack and a red LED.
Very much looking forward to seeing the finished product, the design is a bit more complicated than I envisaged, with the cutouts for fans and switches etc. How much stress was there in the plate from when it was rolled, that it would distort when skimmed, would have been frustrating to have to re-machine each side of the plate.
Do you still have to do the mirrors for it or are they done already.
Very much looking forward to seeing the finished product, the design is a bit more complicated than I envisaged, with the cutouts for fans and switches etc. How much stress was there in the plate from when it was rolled, that it would distort when skimmed, would have been frustrating to have to re-machine each side of the plate.
Do you still have to do the mirrors for it or are they done already.
The stress is always there when I make one of these plates.
The plate starts off at 16mm thickness and I machine it down to 14mm in stages so that I end up with no more than 12 microns runout near the edge.
I have two secondary mirrors ready for polishing and I'm modifying an old Pyrex test plate to become the first primary.
Quote:
Originally Posted by Leo.G
Lovely machining work, CNC or the old fashioned way?
The old fashioned way except for the CNC engraving.
How far away is the first trial assembly of the mechanical components? What is still to be made before you can do this?
Rod W
Hi Rod,
I had the large aluminium components "sand" blasted, after all, but using a gentle-on-the-surface process using fine glass particles.
Yesterday I dropped off the same parts at Collins Anodic Treatments. I could not let myself be ripped off by Electromold with their $450 minimum charge.
So, fingers crossed. I'll find out in about a week whether they made a mess of my expensive parts. After that I will be able to assemble the truss structure.
Meanwhile I ran into a bit of a problem with my glass blank for the primary. The glass shows a lot of internal structure when viewed between crossed polarizers. Doesn't really look like poor annealing, but more like nonuniform glass composition. Maybe I will still put it through the annealing kiln to see if anything changes.
If I can't polish it to a smooth sphere, I will just use it as a reference surface for setting the spherometer when making the next primary.
Last edited by Stefan Buda; 24-10-2024 at 09:10 PM.
Is that the glass blank that you got from me about 18 months ago or so. Hope the strain doesn't cause many problems when making the mirror though it sounds like it will.
Is that the glass blank that you got from me about 18 months ago or so. Hope the strain doesn't cause many problems when making the mirror though it sounds like it will.
No, this one came from a gentleman in Geelong. It was partially trepanned.
It will be good for getting all the grinding and polishing tools worked in ready for some good glass. I will be ordering some blanks from Germany soon.
I had a go at annealing the primary blank and, as I expected, it did not make any difference to the internal pattern seen in polarized light.
Exactly one week ago I did the casting of the disc for the polishing lap.
I covered the mirror blank with packaging tape to stop the plaster sticking to it. A few days later I was able to easily separate the two discs.
Today I picked up the 5 pieces of black anodized aluminium from Collins Anodic. They did a good job and the cost was reasonable at $99.
I'll be very busy the next 3 days, so I won't have the time to assemble the truss until early next week.
Great news about the anodising, that must be a relief.
Looking forward to seeing the first assembly of the components.
RodW
On closer examination I found a few spots where the the anodizing has been slightly damaged by careless handling. I think the next truss will have powder coated rings.
Anyway, the truss went together nicely and it feels very rigid.
It weighs 2050g.
