7" f/12 iStar folded refractor 20: Porsa OTA framework completion.

*

Assembling the Porsa framework held some hidden hurdles. There is a minimum length of square tube before the joints butt up against each other leaving a visible gap between the tube and the joint's cubic center.

I wanted to reduce the height of the step [or shoulder] on the framework but was unable to go beyond a certain point. The unexpected arrival of a gap at the joint meant I needed to saw the ends off the joints. I found I could take the joints apart again, if necessary, but it was quite a struggle.

Before sawing the OTA to length I tried a number of full size, paper, light cone, folding options with the frame on the floor and the mirror shells in position. [Image above] Finally I decided that an overall length of 95cm would do the job. If the 1st mirror cell proves to be too deep, in practice, I can shorten it.

Cutting the partially assembled frame to length was rather unkind to the black finish. It was also difficult not to scratch the adjoining members with the saw blade in the miter frame. I propped the framework on one folding workbench while the miter saw was clamped to the other.

The images show the nearly completed Porsa framework. It seems rather bulky but would be difficult to make any smaller given the dimensions of the lens, its cell and the focal length. I needed 183cm from the cell mounting plate to the surface of the backplate where the focuser will sit. This was the dimension taken from the former straight-tubed design.

The frame is remarkably stiff now it has been closed at the rear. Any thoughts of dropping an upright tube from the shoulder have been set aside for the moment. Though such a tube might have added useful structure just where the Dobsonian altitude bearings will be attached. The advantage of the Porsa system is that such changes are still possible. Albeit with some effort required to separate the framework at the joints. The flanges on the tube can provide a strong attachment to a plate of aluminium or plywood. Even a sandwich between inside and outside of the flanged tubing. There is no shortage of room within the structure.  

The third image shows the objective's 'bayonet' plate resting on top of the fixed OTA plate sitting in the front recess. The OTA is still easily manageable without the objective lens in place. The next job is to fix the front OTA plate firmly to the framework with CSK screws. The backplate can also be given the same treatment. I have chosen to use firm, furniture protection pads between the two objective mounting plates to allow room for the protruding T-nuts for the objective 'pull' screws.

I had no small CSK screws to hold the front and back plates onto the Porsa framework. So I have used round, cross-head screws for the moment. I made the screw holes 8mm from the edge of the plate to avoid them getting too near the edge of the 15mm wide tube flanges. I shall try to obtain a small metalworking countersink while I am shopping for screws.

The OTA, as seen in the image, now weighs exactly 6kg or 13lbs. To which must be added the focuser, folding mirrors, their cells and baffles. The objective will only be added once the OTA is safely mounted for each observing session. This will save having to lug another 10lbs around outside. The present balance point is about 8" behind the 'shoulder.' This tendency will still be well to the rear once the folding mirrors and focuser are added. The 10lb objective should bring the balance closer to the middle again. There is a clear advantage with the moment arm of such a short OTA. The tube is much less sensitive to changes in eyepiece weight compared with a 'classical' straight tube. Hopefully no balance weight will be necessary for the much shorter, optically folded 'tube.'

*