RASA 8 backfocus is 29mm, RASA 11 is 72.8, but these vary depending on filters in your optics, even your camera manufacturer may state that backfocus is +- up to. Get your optics and light nice and parallel to your optical train this way. Many guides exist out there, and on SCT and RASA’s it’s the 3 screws on the front of your scope. This is easily done with a defocused star in the ‘center’ of your image. Collimation simply centers all of your mechanical components in your scope so they are centered when exiting your scope. Most RASA’s have good factory collimation, but sometimes you need to tweak it, and they suggest to in their own documentation if it’s been shipped or travelled much.
We designed our own ASG Photon Cage Here to deal with filter sliding, tilting, and backfocus all in one device.
While some cameras come with tilt plates, they are often impossible to reach. For tilt and backfocus, you need a special tilt device. SCT’s for example provide collimation screws in the front secondary assembly, as do the RASA. In order to collimate, your scope will have collimation screws built in. Refractors for example will still require good backfocus and tilt, while an SCT or RASA require more attention to collimation and scope speed can affect sensitivity to these items. In my case, I utilize the RASA fast optics system and it becomes even more important to achieve these three items. SCT’s or Refractors or RASA’s all require the same good flat images, but achieve it slightly different. Good collimation, good backfocus, and good tilt! We can do those things in this order, but there’s no rules! This process can hold true regardless of your overall equipment. To get those really flat images requires 3 things.
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