This video demonstrates the detailed process of machining with a mill the spider vanes (secondary holder) of a VIXEN VISAC VC200L Telescope.
After carefully centring the spider vanes on the milling table, the procedure is straightforward if done with care and produces excellent results.
Read on for more astronomical information 🙂
The Vixen VC200L telescope features VISAC (Vixen Sixth-Order Aspheric Cassegrain) optical system which is free of coma, field curvature, spherical and chromatic aberration.
The telescope has an enviable reputation for its flat field for astrophotography, producing exceptionally sharp images with no chromatic aberration due to the VISAC design.
The most significant complaint people have about the Vixen VC200L is the thickness of the spider vanes. They are 6mm thick, probably to achieve good results in the casting aluminium process.
This leads to, fat rhomboid – square core for brighter stars instead of the actual – desired centre/rounded. At 2mm, the vanes are still very strong, and no detrimental effect can be seen regards to holding the secondary mirror in place, being still solid as a rock at 2mm thickness.
The obstruction of the 6mm spider vanes is also significantly more in astronomical terms. By milling out 71mm(length) X 4mm(width) X sides, we are gaining back 1136sq/mm, which is roughly the size of the secondary mirror!
We are counting photons with these instruments, after all, so every bit of light/aperture helps 😛
Milling a large radius on a milling machine does not have to be limited by the size of the rotary table.
When tilting the head of the mill with a large cutter, be it a boring bar, fly cutter etc, the resulting cut is an ellipsis. This technique has been used for a long time by machinists before PC’s, CNC’s and other technological aids made their appearance in the industry.
The problem (or not 🙂 depending on the specifications tolerances) with the existing literature (such as the Machinery’s Handbook and others) is that the formula being used assumes that the desired width of said large radius is 0!
The following calculator averages the angle required to accommodate the width as well to provide a better approximation. By using the following calculator you can approximate a true circle radius with an accuracy of few microns.
The goal of this project was for the automated focusing system to have great holding torque (80Ncm) for heavy image trains (4-5kg) and very fine steps to accommodate the super tight FSQ106 critical focus zone.
The formula to calculate the critical focus zone on a telescope is : CFZ = Focal Ratio * Focal Ratio * 2.2 For the FSQ106ED we have : CFZ = 5 * 5 * 2.2 = 55microns So to be able to have perfect focus achieved we need every step on the motor to be 55microns or less for even better resolution.
The motor used on this build is geared and has a reduction of 250:3 which gives us 4000 steps per revolution.
One full revolution on the focuser coarse knob (where we are coupling our motor) makes the focuser move 29.8mm. Now we have all the data we can do the maths 😛 (29.8mm * 1000) / 4000 = 7.45 microns per step.
The actual resolution achieved with this build is 7.45 microns per motor step!
Date : 26/04/2020 Telescope: Vixen VC200L Camera: ZWO ASI120MM-S
6 part mosaic comprised of 800 frames for each panel. Stacked on registax and then assembled in Pixinsight.
The Moon today is in a Waxing Crescent Phase. A Waxing Crescent is the first Phase after the New Moon and is a great time to see the features of the moon’s surface. During this phase, the Moon can be seen in the western sky after the sun dips below the horizon at sunset. The moon is close to the sun in the sky and mostly dark except for the right edge of the moon which becomes brighter as the days get closer to the next phase which is the First Quarter with a 50% illumination.