sreesha How can I reduce the space?
You cannot. But you can increase the optical path.
To compensate for a mechanical distance that is too long, you can increase the optical distance by adding a filter, as long as it does not increase your mechanical distance any farther.
When you add a piece of glass in between the lens and the sensor, the optical focal length will increase by 1/3 the thickness of the glass --e.g., a Chroma or Radian filter that is 3mm thick will increase the optical distance by 1mm. Filters from other brands are usually thinner; often as thin as 2mm, so will not increase the optical path by as much.
But, you can also stack filters, if the thickness of a single filter is not enough -- but you will need very high quality filters when you stack them.
Unfortunately, if your stars are really 2-3mm in diameter, and assuming that the lens is f/2, then the focal distance is actually off by 4 to 6mm (!), and it will be impossible to just add filters to solve your problem.
However, I suspect your 2-3mm measurement is completely wrong. With the ASI071's 4.78 µm pixel, 2.5mm corresponds to 523 pixels! Another way of looking at it is the ASI071 sensor is only 23.6mm wide -- a star that is 2.5mm in diameter would be 1/10 the width of the sensor!
So, I suspect that your star size is really not 2.5mm, and adding a filter might indeed work.
Chen
