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!

7: Adjust tilt and back focus

Now with the shorter adapter and other things I have changed I will try to adjust the tilt and back focus. I use the software Aberration Inspector as I did earlier on page 5. I have setup the camera, Nikon DSLR D800 to 5 seconds exposure, with a 2 seconds mirror delay I get three seconds exposure. I get best result with the ISO set to 100.

My main problem earlier was that I didn't get enough usable stars to do this measure. But now I know a little bit better how to set it up. I have reduced the movement of the focus so I don't lose any stars because too out of focus. And most of all, a clear sky, not perfect but much better relative previous night. Here I have got all 5 measurements correct. With the M3 tilt screws I start to adjust, 1/4 turn each time, that's 0.125 mm at a radius of 42 mm. I don't know where the Delta figures are measured, maybe 2/3 distance relative the outer corner of the sensor. It compare to a radius of 15 mm (full frame diagonal 44 mm). The mosaic panels could be setup smaller and more separated, but then fever stars to collect.

When adjusting you must know where the corners are. Top left in the image is bottom left on the camera, the lens in a refractor mirror left and right, up and down. But there are more.

Here is how it behave with my equipment, it's normal for a refractor. The Left, TR and BR tilt is the tilt adjusting screws placement on the camera adaptor.

Here I have reduced the focus movement even more.

But a bit confusing, the error increases. One reason, KStars/Ekos works internally with the FITS format, FITS has its origin at top left, not bottom left. It's a heritage from the old CRT scan technology used in earlier days.

Now when I know how to adjust the screws it goes faster. The back focus is still too far away from the field flattener.

All the corners are know almost symmetric. The tilt is okay they are all collected together but far away from the center focus.

Now the clouds moved in and I had to stop here.

Still the back focus isn't okay. But now I have turned the tilt screws all the way in, the adapter is too long. -87 my focus delta correspond maybe to a couple of mm movement of the back focus. The CFZ for this setup is 43 my so it isn't very much compare to that. I must make a new shorter adapter, but how much shorter ? And another question, how good is this old field flattener, can the curvature be adjusted better ? I will try with a 3 mm shorter adapter, 103 mm. When it was 116 mm (10 mm longer) the stars at the edges looked terrible, now it's just a fine adjustment left to do.

I had to wait two weeks until I got a clear sky. I could adjust the adapter another 0.5 mm shorter and got it a little bit better. If a 0.5 mm makes the back focus differ move from -87my to -73 my it indicate that the adapter is still 5.2 mm too long.

Is 73 my a big difference, no look at the CFZ, the smallest move that can make any difference. I wait with 3D-printing a new shorter adapter until I have some other problem with it to solve. It has never been as good as it is now.

New adapter, it's now 99 mm long, compare to the field flatter data sheet that say it should have a backfocus of 106 mm at 900 mm focal length. I can come closer, 11 my backfocus at best but then I had bigger tilt. Now I want to see how stable this is over time and when the telescope point in different directions.

I repeated this one month later to get an idea how stable the tilt setup is. When taking the telescope in and outdoors with different temperatures and touch different sensitive parts it's still okay.

I had to adjust the focus for the guide camera after I installed the shorter adapter. The free travel wasn't long enough, I had to design a new that's 3 mm shorter. A bit strange because the guide camera is par focal with the main camera and shouldn't need that extra adjustment. Notice the fine 0.75 pitch threads, for T2 adapter, works perfect.