8.08.2024 And then it was folded. [Updated 28.10.24]

 ~*~

 

 Thursday 8th. I am expecting a parcel with the Bresser 2" star diagonal today. Now I can start playing with optically folding the 150mm/ 6" f/10 H-alpha refractor. I hope I have enough 2" extensions to lift the filter stack clear of the main tube. I need some of my modest stock for spacing the stack components correctly. Each star diagonal has its own minimum light path distance. Due to the physical dimensions of the body which supports the 45º mirror. This is unavoidable.

 Some of the Lunt stack Modules are very bulky. The pressure tuned etalon is the largest. With the FT focuser a close second. Which will mean the optical axis of the modified filter stack. Has to be lifted even further away from the 6" objective's optical axis. Further increasing the demand for a shorter main tube. Just to ensure the correct optical spacing. Though with the added benefit of an even shorter instrument overall. 

 I would like to retain the primary FT focuser with its Baader Twist-lock clamp. This will provide stability and fine adjustability of etalon distance. The second Lunt-FT focuser is part of the original MT filter stack. Providing fine focus adjustment for the ZWO camera. It cannot easily be removed because of the unique interfacing with the etalon module. Will I run into vignetting problems? We shall soon see!

 Image left: First trial at optical folding using 2 x 2in star diagonals. No real attempt at getting the spacing precisely correct. Just ensuring it all fits together. There is a non-standard diameter on the front, Lunt etalon stub. I had to use my lathe to turn the inside of a 2" extension. Then groove it for the bronze clamping ring. This is the 2" Omegon extension.

 There is a Lunt 12mm, straight-through, blocking filter. Hidden inside the long extension just behind the second focuser. Most blocking filters are housed in solar diagonals.

NOTE EXTREME DANGER OF THE HEAVY LUNT COMPONENTS COMING APART AT THE PUSH FIT JOINTS!! The filter stack MUST be firmly bracketed to a main tube ring for safety. [Simulated with a spacer but not properly implemented in the image alongside.]

 Folding has saved roughly 44cm/17". On the length of the previously cantilevered filter stack. The folded OTA length is now 132cm. The straight version was roughly 167cm. All provisional measurements prior to exact spacing. Quite a saving in length. Making the OTA easier to house and to mount.

 I found the folded arrangement VERY unstable. Provided it is firmly attached to a main tube ring it should be manageable. Without adequate support there is extreme danger of thousands of pounds/dollars worth of equipment. Literally dropping like a stone onto the floor or the ground! You have been warned! 

 My original no-name 2" star diagonal has only one compression screw. Three screws would be much safer. To stop the whole assembly from rotating or falling out! The offset of so much weight 3.32kg or 7lbs makes the OTA want to rotate about it own axis. I shall probably invest in another Bresser 2" star diagonal if I can test the new optical layout somehow. I currently have no useful mounting. 

 Note that shortening the telescope by folding means the main tube must be shortened. Otherwise the focus will be within the H-alpha filtration components. Or even before them! It is difficult to judge how much main tube shortening is required with a simple tape measure. The exact positioning of these components is vital to a sharp, monochromatic image. 

 The spacing between the reflecting surfaces of the  two star diagonals  "X" is the vital thing to measure. This is the method of shortening and is unavoidable. Or the later sections of the filtration stack will not physically lie outside the main tube.

There is also the matter of the main tube baffles and possible blocking of the full diameter of the light cone from the objective. Fortunately the primary focuser is oversized. The choice of 2" star diagonals was also deliberate for the same reason. To reduce the risk of vignetting.

 Taken to extremes I could fold the entire 150mm/6" f/10 telescope. Using the same optical flats I used for my 7" folded refractor. Then the main tube becomes redundant. I could use Porsa aluminium profiles. Like I did with the 7"/180mm refractor. While the weight would not be very different from the straight filter stack telescope the reduction in length is very important for stable mounting. Moment = Mass x Distance from the pivot point. Adding more optical surfaces increases the risk of dewing and light loss at each reflection. The latter is not usually a problem with large aperture, solar telescopes.

 

  ~*~