Well, I’ve been busy. And rather tardy in writing things up. But here’s the construction in glorious technicolour!
The first step in the construction process is cutting the tubes to length. And the cuts needed to be square to the optical axis to ensure accurate collimation of the system. Had I a big enough lathe I would have popped them on and trued every thing up that way. But there is an old trick for cutting tubes square by wrapping a sheet a paper around and scribing a line. I find a piece of aluminum flashing and some Gorilla tape easier to work with and the metal edge is more reliable to scribe against.
Despite all my machine tools sometimes it does come down to hand work. Hacksaw first, then file and deburring tool.
Yeah, right… Filing is one tedious business. After about a half hour I dug out my portable grinder. Because I was grinding aluminum, it tends to clog the wheel, I used another old trick (it is nice being an old fart sometimes!) and loaded the wheel with beeswax before starting.
So the tubes are cut and square. Now on to the real machining!
Below is a solid model of the objective cell mount system. I designed it in two parts; one which screws onto the back of the lens cell and the second which attaches to the tube end. Since I had two parts I designed them to allow some collimation if needed. (More on that later) The main reason for doing it in two parts was because my stock wasn’t thick enough to do it in one piece.
I used 6061-T6 aluminum for most of this project. The first setup is below on the horizontal tombstone of my CNC machine.
Once I had the outside of the rings machined I mounted a three jaw chuck and machined the inside.
Checking the fit of the cell ring on the lens cell. Got it first time much to my amazement…
The next setup was to mill the internal baffles. I made them with a thick rim so I could screw them in but tapered the edges of the centre hole down to a knife edge. It worked great. I even managed to slice my finger open on one!
And here are the finished parts.
To make the junction between the dew hood and tube I machined a piece of black phenolic. I chose the plastic partly to act as a thermal break between the heated hood and the optical tube. While my focuser has temperature compensation I figured there was no use in confusing it with extra heat.
It fits! I will line the inside with self-adhesive felt so that it can extend and retract smoothly.
And now to make some mounting rings. Before I got started I decided to make a new fixture plate. The old tombstone was a bit challenging to clamp parts on securely. I broke a couple of expensive cutters because parts shifted. The new plate doesn’t rotate like the tombstone but it’s bigger, heavier and provides much stronger clamping.
And the new plate lets me setup vises and other fixtures quickly and very accurately. Life is good.
So I now have a bunch of shiny aluminum bits. But I want a bit of bling in the observatory! Enter anodization. I had a small anodizing line set up for doing aluminum and titanium as part of my violin fittings business so I had some idea what to do. The real challenge was scaling up and dealing with much larger amounts of the chemistry (especially the acid!). But it was worth it.
Bling!
But anodizing the tubes was impractical because of the size. So I had the outside powder coated and painted the insides and baffles flat black. To prepare the tubes inside I scuffed them up and sandblasted the baffles.
There has been ‘way too much written about black paint recently. I am not about to add to the furor. Suffice it to say the stuff below seems to be the blackest you can reasonably expect to get your hands on. Google it if you would like to see how the other half, artists, see the world.
You may have noticed that the original colour scheme was red tubes with black rings. Somehow, at the powder coater, it went through a phase reversal. But you know what? I think it looks goooood!
Putting it all together. Notice how dark the interior is even with the camera flash.
And it’s ready for first light. I rushed to get it on the mount as I had the first clear night in weeks. The mounts were not yet anodized and I didn’t have the camera ready so I had to actually LOOK through it! My wife and I had a nice view of Mars and other objects. The focus was right where it was supposed to be and the contrast was excellent. Bring on the camera!
A few days later another good night was in the offing so I did a mad scramble to get the camera mounted. I didn’t really expect much since scrambling equals cock-up in my experience. I shot 15 – 200s luminance frames and 10 – 100s each of red, green and blue. FocusMax converged on 1.9 pixels at best focus. I had to use my old guide scope (see below) and forgot to adjust the guide rate in EqMod to account for the now much shorter focal length, So I had to throw out two thirds of the frames. (I told you there would be cock-ups!) But even with the limited data and rushed processing… Damn!
I am a happy camper. It does exactly what I hoped for. Now to get the best out of it. Time to tweak and work out the remaining bugs.
As I could no longer use my OAG due to limited space between the focuser and filter wheel, I had to go back to my old guide scope setup. I was never happy with it so this was a good opportunity to improve things.
I ordered a new bar/plate from ADM and made a new set of mount rings in the style of the big scope’s. I added a third ring designed to clamp the draw tube extension near the camera. I never liked the way the camera was just hanging there in mid-air.
It looks pretty solid!
After much re-arranging, re-wiring and balancing it’s all installed in my very diminutive observatory. It’s an even tighter fit than the old scope. I think I must do something about that…
