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Forum: Troubleshooting and Beginner Help
09-10-2014, 09:06 PM
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Yes you are correct, I made an error when entering data to that site and have yet to change it.
It's easy to forget which settings were used on the fully manual lenses after a long night of imaging, don't have the luxury of exif data to check ;)
When I inspect the raw image(5184 x 3456px) there are no diffraction spikes around the bright stars, though perhaps they are hidden by the softening from the cloud.
Yes, shot from the southern hemi, polar aligning the Astrotrac is relatively easy when using the optional wedge and polar scope. We don't have a nice bright polar star like Polaris in the north, but we do have a star named Sigma Octantis that is very close to the southern celestial pole.
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Forum: Troubleshooting and Beginner Help
09-10-2014, 11:07 AM
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John hit on something I forgot to mention, primes being better than zoom lenses. This is absolutely true. Zoom lens design is a compromise and I was never happy with any I tried.
For astro I only use prime lenses.
The image at the following link was a single 7.5 main exposure taken with a 105mm wide open (f2.8).
Coma is not too bad but there is mild axial CA evident (red halo's around the orange stars) This also means the detail in the red nebula's is not as sharp as it could have been had I stopped down to f4 or f5.6
---------- Post added 09-10-14 at 11:08 AM ----------
Sorry, forgot the link and I can't edit my previous post...
Eta Carina Nebula (Simon Williams) | AstroBin |
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Forum: Troubleshooting and Beginner Help
09-10-2014, 10:50 AM
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What type of astrophotography do you want to do? Nightscapes and starscapes are not too demanding on optics, most short focal length lenses will give decent results tho you may need to stop down some.
If you want to do widefield DSO's (Deep Sky Objects) like nebulas and open star clusters, you will need moderate focal lengths (~100 to 400mm) and for planetary and/or galaxies you will need very long focal lengths (into the metres) and the optics will need to be superb for use at low f ratios.
Astrophotography is the harshest test for any lens.
While faster lenses make sense for shorter exposure times, quite often they exhibit severe aberrations that typicaly don't affect daytime photo's.
The most common are coma, where the stars at the edge of the frame are stretched, and axial and lateral chromatic aberrations where the stars will not focus correctly and have blue or red halo's. Coma and axial chromatic aberration can be reduced by stopping down, lateral chromatic aberration cannot be fixed by stopping down.
Some very expensive ED glass can be used with decent results wide-open, but more often than not you will see better results with the lens stopped down.
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