Dennis G
24-04-2011, 11:46 AM
A DIGITAL FOCUS FOR LX90 TELESCOPE
Couple a DSLR camera to the telescope and lap top computer and an average bright star in focus is about one millimetre in diameter. A fraction off focus either way and it will disappear! The focus system requires an indicator to show a close approximation of focus over the focus range of recording devises used: e.g. DSLR camera both in prime focus and with added magnification, three eye pieces and the DMK imager.
Williams optics (May/June issue Sky and Telescope) has produced a rack and pinion focuser incorporating the digital calliper principal in their digital focuser. I investigated the use of a digital calliper, which uses a lineal capacitive sensor to trigger the digital read out, but I needed a rotary sensor with an up and down counter that could be incorporated with my existing remote focuser.
Most digital counters require a base line to be reset. Switch off the power and the system requires returning to a zero point and resetting. I’m well aware this is the design’s failing, but it takes only a few seconds to return the focus either to minimum or maximum depending which base line is the closest from the last setting used after powering off.
When the focus rotation is changed with the slide switch from CW to CCW or visa versa, the counter is changed from count down to count up etc. Before the count is made for the recording device being used; camera, web cam etc., the focus (counter) requires to be set at minimum 0000 or maximum. 9999 and the count made up or down to the required number for the device in question.
Construction .
My LX90 focus knob requires 38 full rotations CCW to move the mirror from maximum to minimum (closest to the corrector lens) where it is positively stopped one mm short of the split ring in the end of the baffle tube. (see article Positive Focus Stop, LX90 Yahoo Groups) The second slide switch on the remote focus controller changes the rotation pulse movement. Course = 8 pulses per revolution and Fine = 48 pulses per revolution. I used a small Interrupter, which incorporates a LED and a Photo Diode as the counter sensor over a rotating perforated disc attached to the focus knob. No gears and no frictional load on the focus system.
The sensor disc has an o/a diameter of 40 mm with 48- 0.794 mm holes at 35 mm dia centres. The rotational speed for the course pulse is far below the counter and it counts the 6 holes without error while the fine setting provides a count of one per pulse.
The LX90 focus knob is in reality an aluminium tube with a short knurled section as a finger grip. The plain portion is 0.840inch dia and the knurled portion approximately 0.920 inch dia. This increase was a hindrance to my design, so I removed the knob and turn the knurled portion off allowing the sensor disc to be loaded without pulling the focus assembly down. The motor drive was simplified to permit it to slip if driven to either extreme, thus avoid loading the focus system and it can be pulled back leaving the focus free for manual adjustment.
www.ozitronics.com (http://www.ozitronics.com/) provided the counter kit. A professionally constructed kit, easy built and Ozitronics had it in the mail the following day.
Combining the original remote focus circuit with the counter, made the over all package large, but it is comfortable to hold and could be reduced in size with careful redesign. THREE PICS TO THIS ???
Dennis Greeve 2011
Couple a DSLR camera to the telescope and lap top computer and an average bright star in focus is about one millimetre in diameter. A fraction off focus either way and it will disappear! The focus system requires an indicator to show a close approximation of focus over the focus range of recording devises used: e.g. DSLR camera both in prime focus and with added magnification, three eye pieces and the DMK imager.
Williams optics (May/June issue Sky and Telescope) has produced a rack and pinion focuser incorporating the digital calliper principal in their digital focuser. I investigated the use of a digital calliper, which uses a lineal capacitive sensor to trigger the digital read out, but I needed a rotary sensor with an up and down counter that could be incorporated with my existing remote focuser.
Most digital counters require a base line to be reset. Switch off the power and the system requires returning to a zero point and resetting. I’m well aware this is the design’s failing, but it takes only a few seconds to return the focus either to minimum or maximum depending which base line is the closest from the last setting used after powering off.
When the focus rotation is changed with the slide switch from CW to CCW or visa versa, the counter is changed from count down to count up etc. Before the count is made for the recording device being used; camera, web cam etc., the focus (counter) requires to be set at minimum 0000 or maximum. 9999 and the count made up or down to the required number for the device in question.
Construction .
My LX90 focus knob requires 38 full rotations CCW to move the mirror from maximum to minimum (closest to the corrector lens) where it is positively stopped one mm short of the split ring in the end of the baffle tube. (see article Positive Focus Stop, LX90 Yahoo Groups) The second slide switch on the remote focus controller changes the rotation pulse movement. Course = 8 pulses per revolution and Fine = 48 pulses per revolution. I used a small Interrupter, which incorporates a LED and a Photo Diode as the counter sensor over a rotating perforated disc attached to the focus knob. No gears and no frictional load on the focus system.
The sensor disc has an o/a diameter of 40 mm with 48- 0.794 mm holes at 35 mm dia centres. The rotational speed for the course pulse is far below the counter and it counts the 6 holes without error while the fine setting provides a count of one per pulse.
The LX90 focus knob is in reality an aluminium tube with a short knurled section as a finger grip. The plain portion is 0.840inch dia and the knurled portion approximately 0.920 inch dia. This increase was a hindrance to my design, so I removed the knob and turn the knurled portion off allowing the sensor disc to be loaded without pulling the focus assembly down. The motor drive was simplified to permit it to slip if driven to either extreme, thus avoid loading the focus system and it can be pulled back leaving the focus free for manual adjustment.
www.ozitronics.com (http://www.ozitronics.com/) provided the counter kit. A professionally constructed kit, easy built and Ozitronics had it in the mail the following day.
Combining the original remote focus circuit with the counter, made the over all package large, but it is comfortable to hold and could be reduced in size with careful redesign. THREE PICS TO THIS ???
Dennis Greeve 2011