7" f/12 iStar refractor 17: Success and failure!

*

Having left the plywood packing rings glued to the metal tube rings for an hour to dry/set I unclamped them from the tube. Whereupon they promptly fell apart! There was no adhesion at all. I shall have to buy some Araldite epoxy resin if I intend to continue with the plywood packing idea. The Liquid Metal did not respond to sanding or filing so I used a ball burr in the drill to get back to bare metal. 

With another clear evening ahead I was forced to make a decision about mounting the OTA. I still had some black foam left from a thick kneeling pad. Since it was so difficult to cut neatly even with a new blade in a knife I decided to try my bandsaw instead. With the fence set to 30mm I found the saw cut neatly and accurately but left regular saw [kerf] marks. Though I could have smoothed them with the sander I decided to use them as they were. It was still a huge improvement on the horribly ragged edges made by the knife! I think they use a heated tool to cut this stuff.

With some difficulty I was able to mount the main tube on the MkIV with the foam strips merely sandwiched against the main tube. Though as a precaution I tied Prussic loops of strong cord around the main tube and lashed it tightly to the saddle. With the OTA secure I found I had to add an extra but smaller weight to the tip of the Declination axis to achieve balance.

These images were taken at dusk as I prepared for an evening of observation with my "new" telescope. Dragging the instrument [slightly uphill] over to a suitable spot where I could work all around it was still hard work but a vast improvement on the former casters.

I balanced the OTA itself by sliding it through the foam lined rings then tightened them via the clamping knobs. Then I had a last peak at distant Ash trees as the light failed and was still happy with the contrast and colour correction. The image shows the extension tubes needed for "straight through" distant focus with the focuser at about half way extended.

Here the focus point is shown but using a 2" star diagonal and one short 2" extender instead. The star diagonal uses up quite a lot of focal length compared with the straight through arrangement. For most purposes the star diagonal is best left in place to avoid fiddling with extension tubes in the dark. 

Arcturus was the first bright star to make itself visible to the West. It was certainly very bright indeed but small and intense through the telescope. Outside of focus there was a magenta fringe but this changed to green inside of focus. At focus there was just a subtle hint of blue. Focusing was remarkably precise for an f/12. A slow motion knob would really have helped achieve best focus.

It was when I turned the telescope on Vega, high overhead, I was suddenly glad for the cord security loops. The foam strips started to fall out as the telescope slid slowly down through the rings! Of course I was completely unaware this as I was literally in the dark as the reason for the sudden descent. Fortunately the OTA remained under safe control thanks to the cords and the retention of the closed rings.

I now had to continue with a very badly balanced OTA. Enjoying spectacular views of the Milky Way as I swept slowly across the star studded sky. It was so dark that very little was visible around me. I sat down on a crate to be able to reach the very low eyepiece in the star diagonal.

Stars were all nicely small and tight through the telescope using a 25mm Meade 4000 Plossl at 86x. [2160/25] With many stars considerable variations in their individual colours. Sadly there were no planets or the Moon to stare at tonight. So I packed everything away again just before 10pm as the sky grew slightly milky. Getting the OTA down almost vertically proved to be easier than arranging the dec axis horizontally. The imbalance was far too much to risk acrobatics on the stepladder in the dark.

The next plan is to securely fix the foam strips into the tube rings with epoxy adhesive. I shall have a third, security ring just above the top mounting ring to avoid transverse loads on the foam liners. This top ring will stop the OTA from sliding down through the rings if adhesion should fail at high altitude pointing angles. The foam lined tube rings will still hold the telescope laterally but no longer be relied on to keep the tube in its exact place. The third ring will probably need a plywood ring liner for a secure grip.

The foam strip, donor kneeling pad seems to be some sort of closed cell, thermoplastic foam. It has a memory for being compressed. If I could lay my hands on some quality 18mm thick neoprene foam it would probably be a considerable improvement. Perhaps I should really persevere with the plywood ring packing with or without a thin liner of foam?

I decided to go with the plywood and this time I am using Dana Lim 299 clear construction adhesive. Hopefully I will get a strong fix of the plywood rings to the, now mostly bare, inner surfaces of the alloy tube rings. This glue is is not unlike clear silicone but less sticky and supposed to be highly resistant to damp.

After clamping the rings onto a stub of telescope tube for several hours I shall allow overnight curing. I am hoping to ensure full strength before subjecting the modified rings to the full weight of my 17kg OTA at high pointing angles. For now, the early clear skies are already becoming covered in cloud by 19.30pm.[CET] I really want to test the OTA on the Moon or even a planet. This was the real purpose of building the thing in the first place. Perhaps I should buy a sheet of Baader Solar Film and make a full aperture solar filter?

Click on any image for an enlargement.

*