Original+TRO+Making+the+secondary+flat

This was to be made using an old photocopying mirror (kindly donated by Peter Drew), cast in a box with plaster prefaced with 4mm plate glass either side to provide protection and then trepanned using a stainless pipe. This was later to be a mistake.

I began by scrounging round local fabricators for a 6.25 inch diameter stainless steel tube about 12 inches long. This was purchased for £5.00 and then cuts 25mm long where put in the end every 20mm around the circumference. The other end was plugged with two discs of 3/4 inch ply spaced about 3 inches apart. One at the top and the other 3 inches down. The circumference of the tube was drilled to fix the tube to the ply. Stainless is extremely hard and it took 6 drills to make all the holes and 4x4.5 inch wheels for the angle grinder were used to make the slots. The wooden discs had a 16mm central hole through which a 16mm-threaded rod 48 inches long was passed. This was then passed through two phosphor bronze pillow block bearings attached vertically to a wooden jig. The weight of the assembly would be enough to cut through the glass. At the end of the threaded rod was fixed a large tommy bar made of 2x2. The idea being that the operator walked round the jig to cut the glass or if he had long arms to pull it round from one spot.

This was motorised using a washing machine motor to speed up the process. The speed of cutting was increased 20 fold and was down to the end in a few hours. The final bit was completed by hand but proved to be fatal. The last 1 inch section broke while grinding possible due to a small piece of the 4mm plate breaking off and going down between the tool and mirror. Jamming and stressing the 9mm glass. The majority was saved including the aluminium surface and was used for setting up and initial tests within the tube assembly.



The optics where first fully tested within the tube assembly on Wednesday the 11/03/98. The tube was assembled by myself Hazel (my wife) and her mother (Jean) before Geoff and Dave arrived. The moon was nearly full and rising in the east. Focus was found and the moon looked excellent. Bright with much fine detail. The centre of the filed had a shadow of the secondary but this was probably due to the eyepiece being too close. The tube had to be shortened by 4 inches to bring the eyepiece to the correct position within the drawtube. Geoff, Dave and myself proceeded in shortening the truss assembly by 4 inches and the system was recolimated. The moon was looked at again and looked even better. A slight wobbling of the image was noticed as you moved your eye across the image in front of the eyepiece. Stars where then tested. These looked distorted with an aberration that could not be easily defined. Double stars and the trapezium looked great with lots of resolution but the stars had lots of flare around them inside and outside of focus. 'S' shaped and 'triangular' shapes could be seen in the star images and stars would 'jump' near the centre of the field as you moved your eye from left to right. Star images looked a little disappointing and we decide to try the unaluminised oval flat cut by Specialised glass instead of the damaged one hoping this was causing some of the aberrations. This proved not to be so but it was difficult to say if the image was improved due to the lower light grasp now being received (approximate equal to a 1 inch!). We decided to end the session and grind the oval flat and try again.

Over Saturday the 14/03/98 we ground the secondary flat and polished it enough do tests. The flat was done by using 240 grade carberundum against the 14x14 inch square plate, face to face. The surface was showing high areas after only a few minutes work. This was continued until both surfaces where opaque and then finer grits down to 12t where used before polishing. A polishing lap was made of 12x12 inch ply 3x 3/4 thick and then covered in pitch, pressed flat using the 14x14 plate, and then faceted. A polish was visible after only 15 to 20 minutes' work. The polish starting from the middle and working towards the edge. We carried on for another hour or so until the whole surface had a polish, but we did not take out all the haze.

The next testing session was on Wednesday the 18/03/98. We made flat and collimation adjustments before testing, (moving the flat 10mm away from focus. See formula below). The results where not much better. Stars at certain points of the field where in focus but in other parts had massive flare. Stopping down improved the situation a little but only just. Images straight up where better and Mizar/Alcor showed good detail with less flare. The next step is to remake the primary support. This was a 3-point edge support with the back direct onto foam carpet. This will be made into a 50% edge strap support.

On the 23/03/98 and we used my 3.5 inch minor axis flat from the 18 inch. This was mounted on the secondary holder for the 30 inch and collimated. The results where much more encouraging. Castor could be spilt at only 78X magnification. Above image shows the cutting of the secondary flat. Unfortunatley the flat broke near the end of the cut. We are in the process of making another.

Original TRO Introduction and concept Original TRO Purchasing the blank and hoging out Original TRO Testing the curve for accuracy Original TRO Making the tools for grinding by hand Original TRO Evening out the irregularities left by rough grinding Original TRO Polishing and testing Original TRO Making the mount and tube assembly Original TRO Making the secondary flat Original TRO Aluminising the primary mirror Original TRO First light