Matching your astro camera to your main telescope focal length

Calculate your image scale in arc-seconds per pixel and use that to see if you are over- or under-sampling.

Regardless of the physical astro camera you use, the size of its pixels in microns, which is published, plays a key role in how well it matches up to your main telescope's focal length. There is no one right answer here, but there are wrong answers.

In other words, there are enough permutations of how you set parameters that multiple configurations are possible, but certain parameters will produce bad results.

You want a well-focused star, under your seeing conditions, to occupy 2-3 pixels on your camera. That is the mathematical goal.

To do this, you want to know how many arc-seconds of sky are in each pixel, converting each pixel width to arc-seconds. It is not hard to calculate.

Each pixel in your camera covers this many arc-seconds:

[camera pixel size in microns x 206]

divided by

[Focal Length including reducers or Barlow]

= arc-seconds in each pixel

My set-up is 3.76 microns x 206 / 1955 = 0.396 arc-seconds of sky in each pixel.

With 2.5" (arc-second) seeing, that 2.5" star blob will sit over 6.3 pixels. And that's too many. To get that number to 2-3 pixels, I have three choices:

1. Get a different camera with 9.6 micron pixels;

2. Use this camera with focal lengths not exceeding 775mm;

3. Use Binning to change the 1 x 1 (3.76 microns) to 2 x 2 (7.52 microns).

I use #3. At Bin 2, my math updates to 7.52 x 206 / 1955 = 0.792 arc-seconds in my new larger virtual pixel. That spreads a 2.5" star over 2.7 pixels, and a 2" star over 2.5 pixels. This works well for my sky. I never go "out of bounds" on image scale. Some nights ASIAIR Plus even reports me down to the low 2s in pixel size.

The only deadly sin is to cover less than one pixel. In this case, an enlarged image will show square stars.

Here is a link to a calculator that does the above graphically, allowing you to enter in your telescope and camera and Barlow or reducer into drop-down lists and then get an answer about your match. It also lets you toggle the binning assumption.