My astronomy project:
TS130 refractor to Nikon D800 adapter

Content:

1. TS130 to Nikon adapter
2. Adapter prototype
3. Implement of a duo narrow filter
4. Preparing for first light
5. Aberration analyze
6. Shorten the adapter length
7. Adjust tilt and back focus
8. New bracket for guide camera
9. Baffles
10. Prism bracket
11. RayTrace advanced
12. To be continued

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

5: Aberration analyze

After I have rebuilt my adapter I must check that I got the correct distance from the field flatter to the camera sensor. It could be wrong because I have two figures on this distance, 106 mm and 116 mm. With some aberration analyze tool I can measure the curvature of the focus field (which should be flat). In the control software KStars/Ekos there is one tool built in, Aberration Inspector.

At Ekos Focus tab you find this Aberration Inspector tool. Normally it's grey and disabled, you have to setup some parameters first. There is a tutorial about this tool at YouTube: Aberration Inspector.

One of the parameters is the step size (length) of moving the focus one step. You find it at CFZ above. I calculated mine to be 1.25 my for each step and from this I get the CFZ to be 35 steps. This CFZ is the minimal move where that's traceable on the focus shift, in this case a 43 my move. I started with 35 steps for each focus move, but as can be seen at V-Curve above it was too small.

I have set the mosaic size to 22%, the maximum. The mosaic consist of a 3x3 square blocks. Lower than 22% separate them better but then even fewer stars. Read more here at KStars Ekos Focusdocument.

The algorithm must be set to "Linear 1 Pass".

Step size set to 100 step and the multiple to 6. When I only do auto focus I set the Step Size to 200 steps.

Notice, above position 5250 the HFR star analyze fail, the signal of an unfocused star is too weak and diffuse to be analyzed. The usable range is 1200 steps or 1.5 mm of focus change in my case.

The analyze fail because of too few stars in the Top Left and Bottom Right segment. What can be read from above is that the Top Right segment is 169 steps away from the center focus, or 0.211 mm, this is one part of the tilt. The back focus is -232 my, the sensor must be closer to the field flatter, this correspond the several mm I think. My tilt adjust screws move the adapters length by 0.5 mm for each turn (M3 screws).

I increased the number of steps to 50 but that wasn't enough either. Each analyze takes about 5 minutes, mostly because of the camera's long download time, a DSLR camera.

Increased the number of steps to 100 and now it looks okay. I also got to extend the range to 1900 steps.

Even now I have problem with too few stars in the Bottom Left segment. This must be because of the a high degree of curvature I have of the focus field. For next analyze I have increased the field flatter distance to the sensor by 1.5 mm. It's in the wrong direction but I get some knowledge how many microns this compare relative the curvature. Then I can make a new adapter with correct length and replace this prototype.