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124mm f/7.2  Inexpensive Dob
Refigure and Rebuild

Built by Salvatore Antonio Spampinato of Vicenza, Italy

First of all, I was seeking for a 125 mm ordinary glass blank. Then I preferred another way: buying a second hand very low priced OTA with a horrible primary mirror inside. I needed a lightweight dobsonian for ease of use and didactical purposes also.

Soon I decided to refigure the optical parts, before building the mount. Now it's a 124 mm f/7.2, after having beveled the primary mirror.

The secondary mirror was about 1/2 wavelength off. I improved it by flash polishing the back which had seemed to me too coarse (Twyman effect helped?).

The tube is longer than the original, infact I added 100 mm so that only the inner core of the diagonal mirror is met by the primary mirror optical cone, trimming out the worst edge. Then I had to make a very simple and cheap low profile focuser (3€). Unexpectedly, the rest of the drain tube can act as a good collimation eyepiece.

Four screws in the bottom of the cradle lock  the OTA in the desired position. Screws and tube are separated by thin PE spacers which prevent (further) scratches on the tube. I never meant to repaint the old factory blue.

I'm really satisfied with the optical performance of this telescope, it delivers enjoyable views of the Moon, I often push it to 225x with no loss of quality, despite the obstruction is 0.31 (before one friend's little help, it was even more than 0.36 - quite a strange factory design). Besides, the FigureXP response of the primary mirror had been encouraging. This is the mirror on which I spent less time than ever.

The solar film filter (homemade frame) proved to be very practical also.

More infos and updates in my personal website (work perpetually in progress): or Facebook: Aldo Baràn

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