My astronomy project:
Sigma 150mm f2.8 vignetting and use on Star Adventurer mount
1: Introduction, why use this lens?
Lately I bought a used transportable mount, the Star Adventurer, it was broken but after two weeks of work it's repaired. Now I need some high quality lenses to it. I can't have my 5" 680mm APO refractor, it's too far heavy for this lightweight mount.
Among all things I collected under all years I have a Sigma APO 150mm f2.8. I liked it very much and used it to ordinary photo, I feel this lens is of high quality. But will it work for astronomy photography?
Is it sharp enough? Not too heavy? I have a full frame Canon EOS 6D DSLR camera to it, will that give me vignetting problem? Here I have done a very simple test of the vignetting of the lens. It's taken as jpg photos so it's not linear but will give some ideas about how it vignettes. I also did a simple test if corners are sharp enough.Back to contents
2: Vignetting test
I have taken photos through a white paper as a diffuser. I used ISO400 and the apertures 2.8, 4.0, 5.6, 8.0, 11.0, 16.0 too see the difference, I compensated the exposures as the aperture shrinks. The software I used for this is, Fitswork for the vignetting graphs and APT for camera control.
Sigma APO 150mm set to ISO 400 aperture 2.8
Sigma APO 150mm set to ISO 400 aperture 4.0
Sigma APO 150mm set to ISO 400 aperture 5.6
Sigma APO 150mm set to ISO 400 aperture 8.0
Sigma APO 150mm set to ISO 400 aperture 11.0
Sigma APO 150mm set to ISO 400 aperture 16.0
My plane is to use the aperture set to 4.0 or maybe 5.6. The corners sharpness will determine which too prefer.Back to contents
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3: Sharpness in corners with star test
Here I have done a simple star test to see how well the lens focus at the edges. When I did my this first star test I couldn't get the liveview focus to work. Later when I did a indoor test it worked. I don't think I got the focus where it's best. Anyway, here is my first photo. Canon EOS 6D, 60x15 second at iso800, aperture set to f4.0, position close to M102. This time I used the EQ6 mount, no guiding. To this I used my EQ6 mount is controlled by CdC and EQMOD through ASCOM. No auto guiding this time.
Zoom in, center 1:1 scale
Zoom in, lower left 1:1 scale
Zoom in, lower right 1:1 scale
Zoom in, upper left 1:1 scale
Zoom in, upper right 1:1 scale
For me it looks very promising, other lenses I has deliver much worse result. Remember it's a full frame camera. And I think later I can bring it in better focus. With this small lens I don't have the vignetting problem as I get with the refractor because of the DSLR too narrow body.Back to contents
4: Canon 6D and Sigma 150mm f2.8 mounted on Star Adventurer
My idea to use this lens on the Star Adventurer mount, how will it look? Here it's mounted together with camera and balance weight.
Compare to my EQ6 mount and peripherals this is a dream of lightweight. But I must have a heftier tripod to it with a 3/8" bolt.
At last I maybe can do astrophotography at dark places, and also bring this equipment with me on vacation. I have to modify the setup a bit to get rid of the balance weight, have to center the camera over the RA axis. The future looks great!
Later test will show if I can polar align at the precision I need to get it track good enough. But I have also read about others who use 300mm lenses in success.Back to contents
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5: Photo on M31 and M45
Now I have started to collect astronomy photos with my Sigma APO 150 mm lens, my firsts photos are on the objects M45 Pleiades and M31 Andromeda galaxy.
You can examine them in detail here:
Look and see what you think of the Sigma lens. Note, the polar align wasn't perfect.
There isn't much written about this lens used to astrophotography. For me it feels that it deliver quality without too much costs.Back to contents