www.astrocafe.hk

This is an improved version being inspired by 社長’s expensive artificial star. I like the name “Virtual Star” instead. The main components are high intensity LEDs. I used R, G, B and W high intensity LEDs. They can be bought easily from Ap Liu street. However, one needs to use laser cutter to drill four 50um holes on the acrylic plate. The circuit is simple. Just connect the 4 LEDs to the battery in parallel via a switch. One important point is that as the LEDs are solid state device, their upper operating voltages are not arbitrary. Common high intensity LEDs are operated at 3V except Red which is operating at 1.8V. So a 50 ohm resistor in series is needed for standardize all the applied voltage to be 3V. With this Artificial one can test the performance of a telescope at different colors. Most telescopes are optimized for green light. Their red & blue ends may not be good. By doing a star test at different colors, one can understand the telescope better. One practical application is that to test a telescope’s performance in Ha.
If you don’t have a laser cutter, you can still use total internal reflection and lens formula to make a virtual image of about 50um. A shot focal length concave lens and thin fishing line are needed. One can use a concave lens of focal length about 1cm and a fishing line of 0.3mm. Direct the LED to one end of the fishing line and the other end of the fishing line then becomes a spot size of about 0.3mm or 300um. Place the concave lens at about 5cm in front of the spot. A virtual image of spot size 50um is then generated.

willis 寫:If you don’t have a laser cutter, you can still use total internal reflection and lens formula to make a virtual image of about 50um. A shot focal length concave lens and thin fishing line are needed. One can use a concave lens of focal length about 1cm and a fishing line of 0.3mm. Direct the LED to one end of the fishing line and the other end of the fishing line then becomes a spot size of about 0.3mm or 300um. Place the concave lens at about 5cm in front of the spot. A virtual image of spot size 50um is then generated.

Willis, 以前的愛好者是用小鋼珠去產生亮點的，你提到用魚絲產生0.3mm的亮點，這個很有簡單實用，謝謝。另外是否用Convex lens已可以?

這裡有一篇以人工星點測試望遠鏡的全理距離，可以參考一下：
http://www.telescope-optics.net/star_te ... escope.htm

社長 寫:Willis, 以前的愛好者是用小鋼珠去產生亮點的，你提到用魚絲產生0.3mm的亮點，這個很有簡單實用，謝謝。另外是否用Convex lens已可以?

這裡有一篇以人工星點測試望遠鏡的全理距離，可以參考一下：
http://www.telescope-optics.net/star_te ... escope.htm

Yes! One can use steel ball to do it. But my design is that I can pack everything easily in a small box. The fishing line, optical fibre, can be bent inside the box. This means the locations of the components are more flexible. Both convex & concave lens are ok. I used concave lens because I have one on hand. In fact, one can easily find a short focal length convex lens in web cams.