Astrograph Doctor Services

[Stefan Buda]

Originally Posted by 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

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-ou...%20main%20axis.

[DMillward (David)]

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 (David)]

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 (Glenn)]

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]

Originally Posted by 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

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.