My astronomy project:
Replacing a 2.5" field flattener with a 3"
5: First Light and test
After one year delay because of a lot of other projects I could take my first photo with the new field flattener. This was very exciting to see what the new 3" field flattener deliver. Do I have solved the vignetting problem with this big 3" field flattener?
When doing this test I have the telescope mounted on my balcony. Very high light pollution here but it doesn't matter for this purpose. The object I chose for this first light was M97, a planetary nebula.(click on the image to get a full resolution photo in a new window)
This are just 4 x 60 seconds images of M97 stacked. But as you see already here on the background light pollution that the vignetting is much lower compare to my earlier field flattener, the 2.5" Riccardi wasn't designed for full frame sensors but otherwise very good with field correction less then radius 17 mm with my setup.
With the tool Fitswork I did a simple analyze.
Here is a line graph diagonal from one corner to the other over the image. The back ground pollution is very flat, very different compare to what I'm used to, the edges normally goes down very step. That is very promising for further use. The little bent to the right comes from the main camera house and its mirror, it block the edge rays, DSLR camera. Later with flat calibration reference image the vignetting will be more clear.
How about the field curvature correction?
Upper right corner, 1:1 pixel scale and full frame. This was not good, at the radius 15mm and above you see that the field curvature correction isn't correct. It must have something to do with the distance from field flattener and camera sensor. I thought that this shouldn't be a problem. I have the original adapter for Canon EOS and according to the information I had then it should be the correct distance. I have either to reduce or lengthen the distance.
Telescope Express also sell a field flattener that looks to be exactly the same, from there I got more information about the distances.
Table over what I have and recommendations:
My TS 130 refractor telescope have a focal length of 910 mm at f/7.
According to Altair Astro I shall shorten the distance by 3.5 mm and according to Telescope Express I shall shorten it by 14 mm.
If I replace my M48 to EOS adapter that has a built length of 11 mm with this:
This adapter only take 1 mm and then I shorten the distance by 10 mm.
With this short adapter I can reach the distance that Altair Astro recommend but not the shorter length that Telescope Express recommended. To that I need a shorter M80 to M48 adapter.
I shall write to Altair Astro and ask what they believe will be the correct distance. Later I will not use the conical adapter because I will implement an off axis guider adapter, but I want to test the field flatter before I buy a lot of expensive parts.