Astrograph Doctor Services

[Stefan Buda]

Hello astrophotographers!

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.

[Andy01]

I'll vouch for Stefan's services and workmanship. :D

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! :thumbsup:

[strongmanmike]

Stefan Michelson :P

[Startrek]

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

Martin

[Stefan Buda]

Thanks, guys, for the supporting replies.

Andy, if you read this, you left behind a tool on your last visit, so feel free to drop in any time to collect it.

Stefan Morley?

[Atmos]

Stefan gets a :thumbsup: 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.

[strongmanmike]

:question: this is cool (https://www.zazzle.com.au/retro_worlds_best_optician_ever_t_s hirt-256496949186483264) :D

Mike

[Stefan Buda]

Thanks Colin!

If I wear one of those, everyone will want to buy spectacles from me.:lol:

[Stefan Buda]

Another optical train cured of Tilt.

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.

[Joshua Bunn]

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.

[Stefan Buda]

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.

[By.Jove]

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.

[Stefan Buda]

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.

[Stefan Buda]

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.

[Stefan Buda]

I forgot to attach some pictures.

[AstroViking]

Wow, that's an interesting tale of misadventure.

Would this be a manufacturing issue, or a previous owner??



Cheers,
V.

[Leo.G]

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.

[Stefan Buda]

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.

[Stefan Buda]

Leo,
My lathe is a cheap Taiwanese one, not a precision machine, but I improved it a bit.

[DMillward]

Stefan,

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

Cheers

Dave

[Stefan Buda]

Hi Dave,

No I haven't dealt with many SCTs in the past as I never considered them as serious instruments, the C14 used as a planetary imager being an exception.
Wedge error refers to edge thickness variation of a lens, mirror or any round part and it means that the two sides, or more precisely the rotational axes of the two sides are not parallel.
For runout you can have a look here: https://en.wikipedia.org/wiki/Run-out#:~:text=Run%2Dout%20or%20runout %20is,line%20with%20the%20main%20ax is.

[DMillward]

Thank you very much for the explanations Stefan.:)

[Stefan Buda]

Well, after sleeping on it, I came up with two possible ways to fix the OTA.
I thought I could make a tool that would allow me to grab the offending baffle/guide tube without damaging its surface and hopefully be able to unscrew it. Once out, I would probably just need to recut the shoddy thread.
The second option, which I went with, was to introduce a tilt error in the mounting of the primary in such a way as to cancel out the error of the baffle/guide tube.
So the first thing I did was to measure the exact runout of the front end of the problem tube. I was able to accomplish that using the cross slide of the lathe and a dial indicator. Total runout measured 2.3mm. The length of the tube, 200mm, giving a tilt angle of 22 arc minutes, or a bit over a third of a degree.
Next I set up the slider tube that holds the primary and, using the dial indicator and the cross slide, I banged it around with a soft hammer until the end was showing a runout of 1.6mm, corresponding to the same amount of tilt as the other longer tube.
Once I was happy with the amount of tilt, I machined the surfaces that clamp the primary. I was careful to mark the radial direction of the error on both tubes so that during assembly I was able to align the two errors in a way that they would cancel out.
And they did! I set up the OTA again on the lathe, gave it the spin test after assembly and I was very pleased to see that only a very small amount of wobble remained.
Tomorrow I'll set up the artificial star and get the secondary collimated.

[DMillward]

Stefan,

Excellent thought process and options for the solution. Would you say from your experience with this specific case, that the most likely explanation for the error that is was introduced during manufacturer?

I will look for the outcome from your star test with interest.

I am sure the owner will be grateful if this is successful.

Cheers

Dave

[Stefan Buda]

Thanks Dave!

Although the multitude of texter markings on the back of the primary mirror indicate that somebody spent a lot of time trying to correct the tilt by trying many different orientations, I have not found any evidence to suggest that the error was introduced after the scope left the factory.

This morning I completed the reassembly of the OTA and placed it back on my EQ mount. After adjusting the secondary mirror to get a centralized retroreflection of my laser, I had a look down the tube from a distance and I was pleased to see concentric reflections.
I can't set up the artificial star due to the miserable weather we are having, but I'm confident that the scope is fixed.

[Stefan Buda]

This morning I was able to set up the artificial star, between two downpours, and collimate the secondary.
First I used a 10mm eyepiece and then I plugged in a ZWO ASI120MM, which is a better way because the eye's astigmatism is less involved.
Then I decided to go the extra bit and captured 10sec AVIs on both sides of focus. After stacking, I put the images through the Roddier software to see just how good this scope is.
The conclusion is that it is not great, but not too bad for a mass produced instrument. The main mirror seems to have a bit of a trefoil error, with one of the "lobes", near the edge, having quite a bit of a departure from the required tolerance. Although it is a small area, it increases the peak to valley error to 1/2 wave. The Strehl ratio of 0.77 is a bit short of the diffraction limit but not bad for long exposure imaging where the seeing would dominate anyway.

[gb44]

Very interesting Stephan.
The interferogram is pretty bad. Surely that mirror wouldnt have passed at the QC stage in the factory. Maybe the marks show it went back and forth but scraped through in the end. Could have resulted from bad annealing.

GlennB

[Stefan Buda]

I don't know what the cutoff point for the factory QC was, but the RMS value of the wavefront error seems better than one tenth of a wave, or 45nm, which may have been deemed acceptable.
If you open up the image and cover the bottom right of the interferogram, where the maximum departure is, the rest is not too bad. The wiggles don't seem to be more than one quarter of a fringe.
The anomaly will produce flares on brighter stars but it should not produce elongated stars because it represents a small portion of the total area of the aperture.