Mewlon 250 Collimation

[cwjohn (Chris)]

Hi

I have dug out my old Mewlon 250 after many years and much travel. It is approximately 15 years old and unfortunately it has had a rough time after many moves.

The problem I have is that the primary is significantly out of alignment and all of my attempts to adjust it have come to nothing. The problem is that in the older Mewlons there was no proper adjustment and they relied on correct mechanical machining in the factory to get the primary adjustment correct. Every time I attempt to use the locking screws (3 off) to adjust the primary, when I tighten them the primary mechanism simply slips back via the bevel on the base to its natural position which unfortunately, due to various drops by freight movers over the years, is now seriously out of alignment. I think the newer Mewlons have incorporated a proper primary adjustment mechanism.

Does anybody know whether there is anybody in Australia who can possibly look at this, or does it have to go back to Japan for rectification. Also, given the potential cost of rectification, and the prices I have seen here for a second hand units does it make sense to even consider fixing this.

Thanks in advance.

Chris

[multiweb (Marc)]

In the base of the mirror cell there should be 3 small grab screws that are usually set and sealed with varnish or other. If you haven't touched these then take the primary assembly out of the telescope. You might have to unscrew the top of the baffle tube to slide it out. Then put in on a flat level table pointing up. Release the three locking screws on the sides of the mirror cell. It should fall back into place on its own with the weight. Then secure it and put the assembly back in the scope. If you have a tak collimation scope recenter the secondary then finish off with the secondary on a star.

[cwjohn (Chris)]

This procedure has been done several times and the primary is out of alignment. You can determine this easily on the bench by simply sighting down the tube and comparing the secondary fixture with the baffle tube. It is evidenced by a significantly distorted airy disk (not the rings but the total disk) when sighting on off focus star.

As I said I have tried to adjust this out by moving the cell and adjusting the screws but on tightening they simply move the cell back into its natural position by virtue of the bevel they tighten onto.

Secondary collimation is easy. I have done it many times in the past, but without a correct primary alignment there is no way secondary collimation can adjust it out.

Clearly, there is a mechanical distortion that has developed over time on the primary alignment.

[multiweb (Marc)]

The screws on the periphery are to lock the mirror in position. The ones on the back plate are for adjusting its tilt so once set it shouldn't move. Do you have photos?

[cwjohn (Chris)]

Yes, the phillips head screws on the periphery (4 in total) are no problem and the backplate sets in place uniformly all around. The three internal adjusting hex crews are used to set the position of the mirror, and there is a phillips screw for final set. But the hex screws are not really adjusting screws like on the secondary. They are simply designed to hold the bevel behind the mirror in place once it is set in its natural position i.e. by gravity as you describe. There is what appears to be a cork base which the mirror bevel settles to.

When I last used the scope which was many years ago, the natural set of the mirror base as you describe was perfectly aligned. Over time with rough handling it appears that this alignment has been compromised.

The problem is that if you use these three screws to attempt to adjust the position of the mirror when tightening the bevel simply reverts back to its natural position.

I understand that in later Mewlons Takahashi incorporated proper adjusting screws for the primary.

[multiweb (Marc)]

Here's a couple of pics of what the cell should look like. Is this what you're seeing?

[cwjohn (Chris)]

This is what I am seeing.