Hello!! It's Joseph

my understanding of the Bird-Jones design is that it utilises a spherical primary/modified secondary morror and a corrector lens in the focuser tube to effectively squash a 1000mm focal lengthinto a short 500mm tube. So you get what looks like for example a 500/F4.4 scope but is actually a 1000mm/F8.8. Some people seem to get on OK with them -give or take some interesting colimation issues - but to my mind they seem to add a layer of added complexity with no worthwhile pay-off. If you want a slowish 114/130 reflector you can get decent one without the bird-jones compromise . OK - not quite so compact - but that's the choice. from astronomyforum the Bird-Jones design has some downsides: 1. The primary mirror is much shorter focus than it normally would be in a reflector of that focal length. This means the secondary had to be bigger, reducing light grasp marginally and contrast significantly. The shorter tube should however tend to make the mount more stable - unfo...

so far, method 1 and 4 is most popular. method 1: using adapter method 4: using barlow lens

comparing these 2 CMOS sensors: ZWO ASI1600 SBIG ST-8300 sensor panasonic Kodak KAF-8300 sensor tech 4/3" CMOS full frame CCD pixel count 4656 x 3520 3326 x 2504 FOV 21.9mm diagnoal 22.5 mm diagonal read noise 3.5e- 9e- measured read noise 7~8 e- 9~10 e- total noise 12.25e- 81e- pixel size 3.8 micron 5.4 micron FWC(full well capacity) 20000 e- 25500 e- ADC 12 bit electricsheep

From Texereau's inspiration: metal marble:

interpretation   treatments

Work continues with emery 1m whose grains are comparable to those carbo 100 or 120 , but that leaves less deep fractures because of its lower hardness . Now we will adopt general way for the rest of the work, a normal stroke on which it must attract the attention specially: they consist a back and forth roughly centers (Fig 22 and 23), the total amplitude is about 1/3 of the disk diameters (therefore it is necessary to exceed one sixth of each side , 3 to 4 centimeters with a mirror of 20 centimeters), with a lateral departure constantly variable limit 1/8 at the each side.   the shape of the strokes may well affect that of a V or W or of a loop more complicated ∞ as an example ( G. W. Ritchey ) . Every five or six strokes it turns a little the upper disk in his hands and one turns oneself as in  roughing(coarse grinding) . What is essential in all this is only to respect roughly 1/3 the average amplitude and to vary as most p...