There is subdued interest in my CDK250 astrographs at the moment - probably a post Covid effect as people are spending more on travel, rather than premium instruments.
However, I'm getting a bit of a procession of local astro people with equipment problems, and I thought that maybe I should let the IIS community to know about it.
I'm happy to track down and fix misalignment and collimating issues in catadioptric and refractive optical trains - not Newtonians though. Newtonians are more difficult and may also suffer from lack of rigidity, in which case there is no point in doing any benchtop alignment work on them.
A recent example is the C14 EdgeHD below, which needed the optics cleaned, the focuser reconstructed, the screws that attach the back end to the tube, tightened, and the secondary collimated. For the screws tightening I had to improvise a very long spanner, to reach to the bottom of the tube to stop the nuts rotating while tightening the screws. For the collimation I had to set up the OTA on my milling machine table and use an artificial star across the road, about 50 meters away.
I may also accept optical testing and aluminizing, depending on the specifics of each case.
Over the years, he's successfully helped me with several jobs, including collimating my Tak refractor, making many perfect adapters and re-machining focusers, etc., to better than original factory tolerances.
There's commercial quality, then premium quality, and somewhere way above that is Stefan quality!
I’ve only dealt with Stefan on two occasions for a small order of custom spacers for my coma corrector but they were precision made to spec , high quality finish and delivered in a timely manner.
A pleasure to deal with him
Stefan gets a from me. Fixed my RH200 that wasn’t put together correctly in the factory and own one of his CDK250 (SN002). Great to work with and excellent results.
Stefan gets a from me. Fixed my RH200 that wasn’t put together correctly in the factory and own one of his CDK250 (SN002). Great to work with and excellent results.
Astrophotographers please note that the largest tilt errors I'm finding in this type of optical trains is coming from the filter wheel assembly.
I think the reason is that the housing of these large FW's is machined out of solid plate and the remaining "skin" on both sides is only 3mm thick. So during the CNC machining residual stresses in the material are released and when the part comes off the machining fixture it changes shape.
No easy way to overcome this (machining) problem.
Very true, Stephan. One way to fix this during the machining process is to do all the roughing, then release the part from the fixture, even flip it to machine the underside face, re-clamp then run the finish passes. But like you say, this is not easy, and it adds time and cost.
Yes, that is what I do when turning the backplate for the CDK250.
Four setups, two for roughing and two for finishing. Also, the plate is held at 3 points so that it can move as the stresses are released during machining.
If the filter wheel cases are just 3mm thick they will warp or flex under the weight of a camera cantilevered off the back of them. I had good look at one at a club night and thought it really needed a couple of stiffening ribs across the front and back faces.
Yes, flexing is also a problem with the large filter wheel housings. Fortunately most cameras are not as heavy as they used to be back in the CCD years.
An interesting new case: The 8" Meade Lx200 ACF that can't be collimated.
The owner showed me a TriBahtinov image with the best possible collimation and it clearly didn't look good enough.
To start, I placed the OTA on my EQ mount, in a horizontal position, and I examined the multiple reflections for concentricity. From a distance of about 7 or 8 meters I could clearly see a good amount of misalignment.
Next I used a well collimated laser to check the retroreflection from the secondary mirror and the beam came back with an offset of about 12mm.
That is a huge error and would not be consistent with the reported performance if it was simply due to a misaligned secondary mirror.
Before going any further I unscrewed the Baader 2" clicklock adapter and measured it to make sure it was not introducing a tilt. The result was less than 10 microns tilt across 52mm - very good.
So I proceeded to set up an artificial star about 30m away to have a look at the collimation. I found that the defocused donut was not too bad - consistent with the reported performance.
Then I reinstalled my laser into the Baader clicklock and adjusted the secondary collimation to get the secondary collimated and then I replaced the laser with an eyepiece to have a look at the artificial star. This time the star donut looked completely eccentric - telling me that the OTA must have a severely tilted primary mirror.
This is where the problems started. When I tried to separate the back assembly from the tube, it turned out that the tube is bonded to the back casing.
Not being able to get to the primary I decided to put up the whole OTA on the lathe and give it a spin to confirm my reasoning about the location of the problem.
I turned a solid 2" internal thread to fit the back end of the OTA and I was able to get the scope attached and rotating nicely on the lathe.
Everything looked reasonably concentric but the primary mirror showed a very serious wobble. I wish I could post the video.
Now it looks like I will have to try and extract the primary through the front end and find out what is giving it the tilt.
Last edited by Stefan Buda; 28-06-2024 at 06:36 AM.
Just curious what type/size of lathe would you be using for your not so pleasant adventures?
I've considered using the lathe to check some lasers I have considered using for collimation. I tried with an old kit lathe (originally I think) with a bent headstock shaft (threaded water pipe the chuck screwed onto). I keep meaning to machine a new headstock with the better lathe I got my son but haven't gotten to it (like a lot of things). I haven't even welded up the stand for the milling machine and cross slide (compound) table I bought. One day, hopefully sooner than later.
Id like to pass the old kit thing off to a friend. It was always off from purchasing used on eBay with the seller showing me "the motor works", that was about all that did.
What do they say about a fool and their money?
I have the last laugh, I was broke before I bought it.
Would this be a manufacturing issue, or a previous owner??
Cheers,
V.
Once I got inside, it became clear that it was meant to be this way right from the factory. It's not a bad idea. The front ring is also bonded to the tube so nothing can come loose and shift like it did on the big SCT I dealt with recently.
The OTA turned out to be very easy to dismantle although the primary mirror is larger than the clear aperture. The trick is to turn the primary 90 degrees, after pulling it off the central supporting tube, and getting it out sideways through the two cutouts provided in the corrector plate seating.
I turned another fixture and set up the primary assembly on the lathe to see what was causing the tilt. When I started spinning it, I was dismayed to see just how bad that piece of glass looked like. There were serious runouts on every surface - I mean millimeters. The metal hub was running true and yet the outside diameter of the mirror had a major runout meaning that the hole is not in the center of the mirror.
I also measured a wedge error, at the rim of the mirror, of 0.7mm.
Also there are very rough and uneven (eccentric) chamfers on the front of the mirror and the one around the perforation is particularly troublesome because it produces an uneven contact area against the rubber washer that is supposed to define its squareness on the hub.
I ended up replacing the rubber washer with a cork one and after a lot of mucking around I managed to get the front of the mirror running reasonably true.
After that I reassembled the OTA and set it up again on the lathe only to find that it was only a bit better than before. The primary was still wobbling quite a bit and after closer inspection I saw why: The central tube that the primary assembly slides on, is not square to the optical axis.
I dismantled the OTA again but I can't unscrew the central tube to see what the problem is.
So I'm a bit stuck at this time and need to sleep on it.
The back of the primary is covered in texter marks indicating that someone else has tried hard to fix this problem before me.
I can't see how this scope ever passed the QC at the factory.
Last edited by Stefan Buda; 28-06-2024 at 06:33 PM.
Just curious what type/size of lathe would you be using for your not so pleasant adventures?
I've considered using the lathe to check some lasers I have considered using for collimation. I tried with an old kit lathe (originally I think) with a bent headstock shaft (threaded water pipe the chuck screwed onto). I keep meaning to machine a new headstock with the better lathe I got my son but haven't gotten to it (like a lot of things). I haven't even welded up the stand for the milling machine and cross slide (compound) table I bought. One day, hopefully sooner than later.
Id like to pass the old kit thing off to a friend. It was always off from purchasing used on eBay with the seller showing me "the motor works", that was about all that did.
What do they say about a fool and their money?
I have the last laugh, I was broke before I bought it.
Leo,
My lathe is a cheap Taiwanese one, not a precision machine, but I improved it a bit.
A very interesting issue.Have you come across many Meade SCT's with problems such as this? A couple of the terms you have used are foreign to be (not mechanically minded). Runouts on every surface, would you be so kind as to explain what a runout looks like on a mirror? Or a few more photo's to demonstrate the issue?
Wedge error? Is that an item that wedges the mirror on the Primary tube? Sorry if that is a dumb question.
It is very interesting to read your explanations as I have often wondered what is inside these big metal tubes and how the various parts interact with each other to provide us the opportunity to see the cosmos.
Thank you for this thread and the descriptions you are providing. Very interesting
