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Tutorial News

My astronomy project:
Replacing a 2.5" field flattener with a 3"


  1. Introduction and unpacking
  2. Planning for needed adapters
  3. Disassemble adapters and assemble them again
  4. Replacing off-axis adapter with a temporary guide telescope
  5. First Light and test
  6. Ultra thin M48 adapter
  7. Test with ultra thin adapter
  8. Guide telescope holder distances

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

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.

M97 with 3" field flattener and fullframe (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.

Line graph diagonal

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

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:

Adapter Length comments
What I have today   focal length 910 mm and f/7
M80 to M48 Cone adapter 62 mm  
M48 to Canon EOS adapter 11 mm  
Canon EOS back focus 44 mm  
Above gives a total back focus of 117 mm  
Recommendations from Altair Astro    
M80 to M48 Cone adapter 62 mm  
Add extra distance 51.5 mm  
Above gives a total back focus of 113.5 mm to what focal length is this calculated?
Recommendations from Telescope Express    
Total distance 111 mm focal length 610-700 mm
Total distance 106 mm focal length 710-800 mm
Total distance 103 mm focal length > 800 mm
With ultra short adapter    
M80 to M48 Cone adapter 62 mm  
M48 to Canon EOS adapter 1 mm ultra short design
Canon EOS back focus 44 mm  
Extra distances x mm if needed
Total distance 107 mm  

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.

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