Old Celestron (and Tasco) true Bird Jones scopes

[benzomobile]

Hi to all,

Bird Jones scopes have  three advantages respect Newt ones:

1) their size is compact, so they are lighter;

2) their primary mirrors have spherical shape (very easy to figure out);

2) they show neither axial nor extra-axial distorsions as Newt ones do, having same aperture ratio.

I purchased back in 1985 a Comet Catcher Jr (Cometron). The instrument, as you may know, is a TRUE ONE  Bird-Jones 125mm diameter and 1000mm equivalent focal length.
I had noticed early on that the trouble with this OTA depended on a huge collimation difficulty, NOT because bad optics!Its mechanical arrangement has been designed in a very rough way   :
a cylindric 'mug' housed the small elliptical secondary. It also supported the corrector at its end, and what's more, the entire block was precariously supported by only three rods attached to the tube. In addition, the primary mirror apparently had no possibility of being collimated.However, the instrument by some stroke of ... luck (!) could occasionally give me planetary images of better quality than a classic very good Newtonian, having the same optical-geometric characteristics (5" f/8).

I noticed that on the primary cell edge, there were three threaded holes (M3). It was evident they were used to collimate the mirror ONCE FOR ALL,  before the scope being placed on the market (only tightening thoroughly the three screws that secure the cell to the tube). Once the cell was removed - or even accidentally moved, it was impossible to reposition it properly.

At that point I set to work to make its collimation more  precise and stable possible.

This was my intervention:

1) I fastened three short M4 hook screws 120° apart on the inside edge of the cell. I then put three more screws from the outside of the tube - secured on the inside with nuts, about 90mm from the end of the tube and in line with the screws on the cell.

Once I extracted the mirror (it slides out from the outside, after  removed the round plate that holds it in place, I then applied three coil springs of appropriate strength between each of the two pairs of screws.

I put the mirror back in and reposition the plate in its place.

I buy short M3 knobs to screw them into the 3  threaded holes of the cell.

Now the primary mirror coluld be easily and precisely collimated!

2) A well trusted glazier drilled a hole of 35 mm exactly in the center of the glass plate.  The precision was pinpoint and the work was perfect.

With the lathe I make a round plaque + an extension ring - both  aluminum,  that I will use to secure at the glass plate  the cylindrical block housing the corrector + the secondary.

The extension ring has adequate length to ensure the original distance corrector/primary mirror.

In this way, the block cannot longer move by itself.

I replaced the original secondary pivot with a piece of M4 bar, I put  a coil spring on it and I placed  three adjustment knobs - which were not originally designed to be there (!).

Knobs of course go through the central round plaque.

Now, the secondary position could  thus be either adjusted along the optical axis, or tilted from it, and it was fairly locked in a suitable position with two nuts screwed on its the axial bar.

Here you can also very well collimate the secondary mirror, too!

 

Beppe

Edited September 7, 2021 by benzomobile

[RobertI]

Interesting sounding upgrade. Would be interesting to see some pictures?

[benzomobile]

10 hours ago, RobertI said:

Interesting sounding upgrade. Would be interesting to see some pictures?

Of course. 

I'm actually doing a tutorial with photo documentation. I haven't used that tool in many years, so I need a few days for this.

Beppe