7" f/12 iStar folded refractor 25: Pretend first light.

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A cool but dry morning offered a chance to temporarily rig up my folded refractor OTA. I brought out the 180mm objective but left the precision optical flats safely indoors. The 'bayonet' objective fitting works unbelievably well and has proved effortless and secure. I have ordered a pair of stainless steel handles to aid handling the heavy objective in its cell.

All I needed to know for the moment is the average, eyepiece, focus point with any two flat mirrors in place. The Porsa framework was mounted on the B&D workbench with props under the front legs. This gave me a potential view of a leafless tree about 150 yards away silhouetted against the sky. I needed a high contrast object to ensure it was easy to find the image in the eyepiece. 

The image shows first setup was with the bare, flat, shaving/makeup mirror dangling by a cord from the tilt strip. The equally pretend, second mirror was a compact, makeup mirror. Nothing more was needed at this early stage. 

I immediately discovered that I could not focus any image in the EP at any position of the focuser. Drawing back from the empty focuser with a 26mm eyepiece produced no image either. After hastily removing the focuser I found the focal plane just inside the backplate. Not quite what I was expecting despite the words of warning from my fellow ATMer with recent folded refractor experience still fresh.

The images show the crudeness of the lash-up required to find the focal point. It was relatively easy to prop, tie, dangle and tape the temporary mirrors to be able to roughly center the objective in their combined reflections. It wasn't as if I was about to make critical observations at high magnifications.

Due to the short focus I decided to move the 1st mirror forwards by propping it on its 3" deep shell. The focus was now pushed 6" away, well beyond the backplate. Hmm. So, any mirror movement, forwards or backwards, doubles the difference in the position of the focal plane. Or so it seemed in practice. Remember that we are dealing with a focused cone of light here. Not a parallel beam.

I quickly replaced the focuser and now I had some space between the EP and the extended focuser. What would happen if I cut down the 1st mirror shell? Let's assume for my present purposes that the mirror is 12mm or 1/2" in front of the shell. Removing this difference, with the proper mirror in its deep shell, would pull the focus 1" closer to the objective. Cutting an inch from the depth of the shell would move it 2" nearer the objective. 3" movement in total is not an insignificant distance!

The image shows the 1st mirror [roughly] pushed forwards by the 3" deep supporting shell. The pivot on the supporting frame will not allow the mirror to move any further backwards inside the shell.  Not that it matters much for this first trial.

The 2nd mirror is presently much nearer the framework than it would be in practice. It will be pushed forwards by at least 1.5" even with a cut down shell depth. Resulting in the 3" gained [from moving the 1st mirror backward] being cancelled out. Using the full depth of the second shell would bring the focus to the required point for permanent, star diagonal use. Or so I hope!

Ideally I want the infinity focus to lie somewhere around the half way point of focuser movement when the 2" star diagonal is in place. So really, I need the focus to be pushed even further beyond the focuser and should need the focal extender tubes to reach focus when not using the star diagonal. Just as everything was with the 'classical' straight steel tube in place. 

Initially, I shall use the full depth, 3" cells for both mirrors. Exactly as I had measured from the paper light cone. If the focal plane proves to be pushed out too far beyond the focuser I can always trim the mirror supporting shell to taste. In theory I could cut both shells down and shorten the OTA framework. Not an option I currently favour unduly until proven with both optical flats properly secured.

Here is the second cheapo mirror crudely supported on its shell with twine and tape. With both mirrors mounted on their respective 3" deep shells I now found I could focus on objects 150 yard distant  using the star diagonal and the focuser pushed fully inwards. Any more distant object would need much more inward focus movement which I just don't have. So the OTA framework really is too long even with both mirrors fully forwards. [Forward extension pushes the focus further out.]

Here is an image taken through the 7" achromatic objective of the eyepiece in the focuser [very roughly] centered in the folding mirrors. [arrowed] Even with these very crude support arrangements it is possible to adjust the collimation well enough to get a recognizable image of a distant birch tree. This extra experience in collimation will no doubt help the real thing when I have the proper mirrors in place.

It is just possible that one or both test mirrors is not very flat. This would tend to push the focus in or out depending whether either mirror was concave or convex. Mirror curvature moves the focal plane quite a long way.

So my next task is to exclude such errors. I must mount the precision flat mirrors securely in their full depth shells. The backing disks will be held to their respective tilt strips by a single central screw and nut. I can then start collimating more precisely and rule out focus changes due to the very cheap 'test' mirrors. It continues to rain and sleet so working outside is impossible.

I found throwing a dark towel over the top of the OTA excluded a lot of stray light and helped to improve contrast.

Click on any image for an enlargement.

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