@ChrisPBacon wrote:
@TCampbell wrote:I'm wondering what camera you were using when you said you needed to cut the exposure time to 17 seconds.
Canon EOS 6D Mk II — and a highly modified Canon EOS 6D — with a Canon EF 24mm f/1.4L II USM lens.
Hi Chris,
As long as the G11 has a precise polar alignment (I typicaally use a "PoleMaster" system by QHY-CCD) then unguided exposure duration shouldn't be a problem. I've done 8-minute un-guided exposures with mine with a 540mm APO on the mount (I use a TeleVue NP101is refractor). The shorter the focal length, the more forgiving it is about tracking accuracy. If your G11 mount has the Gemini 2 controller then there is a built-in polar alignment assistant you can use. Losmandy has a YouTube channel and a tutorial video on how to use that feature.
But lenses also have optical issues which can causes the brighter stars to no longer be "round" when shooting long exposures. The lower the focal ratio, the more you would notice these effects.
Lenses don't create a perfectly "flat" field (even very expensive lenses). If the lens is focused to tack-sharp in the center, objects in the corners will not be tack-sharp. But you can think of field flatness applying to more than just focus... there's also flatness with regard to coma and astigmatism.
When you shoot a lens wide open for long periods of time, it allows the photons to collect and can reveal issues such as astigmatism or coma that wont be easily spotted in traditional photography.
Coma can cause stars to take on a tear-drop appearance. Astigmatism can cause the stars to appear to grow tiny "wings". If you check the orienation of these effects on various stars you should notice a pattern where they are oriented with respect to the center of the frame.
If you stop down a bit, you can cut down on the issues. Of course you are also collecting less light so now you need a longer exposure to compensate. But even with the longer exposure times, the images are usually better.
The G11 mount will *easily* handle very long exposures (I've done 8-minute un-guided exposures with a 540mm APO on the mount) ... as long as you have a precise polar alignment.
If your non-round stars are oriented all pretty much in the same direction then this usually means you are experiencing some sort of tracking issue (e.g. mount did not have an accurate polar alignment, mount was bumped or moved, incorrect tracking rate, etc.) If the non-round stars have an optical effect with an orientation relative to the center of the frame, then it is probably related to an optical effect of the lens (and stopping down will usually reduce or eliminate the issue).
Here's an example...
This first image is shot at f/2 (Canon EF 135mm f/2L USM) on a Canon 60Da. The camera was on a tracking mount. This was a 60-second frame.
To test tracking accuracy, I usually also shoot a long exposure. Here's the longer exposure.
This was shot at f/10 (same lens, camera, etc.) This was an 8 minute exposure (unguided)
Full disclosure... the first image is the converted CR2 into a JPEG with no adjustments other than Lightroom default profiles. The second image was darker so I boosted exposure by .85 in Lightroom ... but no other adjustments.
Mainly look at the three stars on the left (this is the lower region of Orion turned on it's side).
Notice in the 2nd image at f/10 that the stars are diffraction spikes caused by the lens aperture blades but otherwise everything is tack sharp.
But also notice that the first image shows some little bat-wings coming away from the stars ... in the meridonal direction. (much easier to notice if viewing the image in a larger size). If you imaging concentric rings around the center of the image, these bat-wings are extending along the direction of those rings.
The more the lens is stopped down, the lens of the overall glass is used. Mostly the center of the glass elements is used and that region tends to be optically better.
Some guidance that an accomplished astrophotographer gave me when I started was to orient the camera so the top of the frame is "north up" (the viewfinder bump on the top of the camera should be pointed in the direction of the north star). This makes it much easier to determine if elongation caused by tracking issues is a result of poor right-ascension tracking vs. declination drift. If the issue is caused by alignment or mechanical issues with the mount, this makes it easier to diagnose.
Hopefully this info is helpful and lets you get much longer exposure durations.
