My fork mounted 200mm telescope

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  • Diameter 200mm
  • Focal length 920mm  f/d=4.6
  • diagonal mirror 60mm
  • Motorized through a threaded rod and a circular sector (343mm radius)

  • For transport the telescope comes in 3 parts:

    The base:
    This part of the telescope includes the RA axle, th motor drive and of course the three feet which are 16mm threaded rods. For polar alignment, the south rod can move to adjust the latitude. There is no adjustment in azimuth and I need to push the whole telescope. This can be done when the feet are on a wooden board but it is not easy and it's difficult to be precise. I will need to improve this part.
    The axle is the rear axle of a 2 CV (a popular car here in France which is not build any more). It is a hollow steel tube with has 65mm outside diameter and 5mm walls. The old bearings (from the car) were replaced with new ones.
    On this view we can see the RA drive but I will give more details at the bottom of this page.
    Under the polar axis there is some place where I will be able to put a small battery.
    This part weights about 15 kg.

    The fork:
    The fork fits on the RA drive and can be tightened with a bolt. The black disk on the side is a counterweight. On the picture we can see the openings where the declination axis comes and the brake on the side. The fork is made out of 15 mm plywood (used in double thickness in some places). Cutting the wood has been difficult because everything needs to fit perfectly. The final assembly required a lot of hand work with a file. The boards are screwed and glued together.
    The fork weights about 12 kg.

    The OTA:
    The walls are 10mm thick. A door at the bottom of the tube  gives access to the mirror. The inside diameter is 270mm. The helical focuser can accept 2" or 1"1/4 eyepieces as well as a camera at prime focus. There are a lot of small holes in the OTA to attach piggybacked cameras, counterweigths guide scope, finder, ... Since I didn't know exactly where I wanted them to be, I drilled a lot of holes nearly everywhere on the tube. The finder scope has a 44mm diameter. It is made of PVC pipe and an old achromat (probably from binoculars).
    There are 2 ball bearings in declination (50mm outside diameter).

    At this time, the movement in declination has no motor (but I will add one later). When the telescope is in use the brake is not tightened strongly. When th OTA is well balanced the brake adds only some friction to the movement and the telsecope is moved like a dobsonian. You push the tube where you want and it doesn't move any more when you stop pushing. There is no need to use the brake knob in the whole night. The manual control is used when guiding astrophotos or for recentering at high magnification. This is made possible because the brake disk turns around the declination axis (there is some teflon to alllow a smooth movement).
    The OTA weights about 12kg.

    The RA drive
    A stepper motor is used to make a stainless steel threaded rod turn. A nut which cannot turn translates on the rod and tracts a ribbon which is attached to a sector of a circle (fixed on the RA axis except when doing some manual moves for finding objects).
    Every 2 hours, I need to make the nut come back to its starting position. This is done with the motor running at full speed in the reverse direction. This takes some time and I need to improve this.

    The stepper has 200steps/revolution. It is used with a 25:1 reducer. The threaded rod (14mm in diameter) rests on 2 bearings. The nut (2 nuts in fact) cannot turn. The arm has a ball bearing at its end which rolls on a flat piece of aluminium. The weight prevents the nut turning in the other direction.

    The nut tracts a steel ribbon which is fixed to the wooden sector of a circle. On the back of the sector there is spring which applies a force opposite to the traction of the ribbon. 

    The sector can be tightened to the RA axis. It is used in the same way as the declination brake. It is possible to move the telescope manualy and when I stop pushing the telescope there is enough friction between the sector and the axis for the motor to make the telescope move at sidereal speed. All in all for visual use, the telescope is used in the same way as a dobsonian with the added advantage of the motor drive. For photography however, the brakes are tightened so that there is no accidental move during the exposures.
    Finally, this telescope is stable and easy to use. The only drawback is its weight (more than 40 kg). It is already satisfying for visual use but I need to make some improvements for long exposure astrophotgraphy.

     The first pictures taken with this instrument