A. T. wrote:
Once again I come to my UHH family for advice. My wife and I will be traveling to Wyoming for three weeks starting September 7th to enjoy that wonderful place once again. I am an avid wildlife and landscape photographer but would like to photograph the night skies. I have been reading and watching videos on the subject and I came across a few who were talking about the use of a star tracker for better image quality of the stars and possibly the milky way. I know absolutely nothing about a star tracker and would like to get input from you guys and gals about the subject as well as the need, or not, for a star tracker. I have professional equipment and an assortment of fast lenses. What is your opinion and or advice on the subject? Thanks again for my very valuable family in this forum.
Once again I come to my UHH family for advice. My... (
show quote)
Some examples are useful to see how star trails are formed if the shutter time is too long.
First image is Comet Neowise in July 2020. The FL was 360mm and was done on an APS-C camera. Shutter time was 2 sec and the ISO was 25600 @ f6. 38 images were stacked to help reduce the noise. And with stacking, there was the need to do some cropping too, and it was cropped fairly heavy. You can see the stars are a bit elongated towards the upper right.
Second image is a FL of 100mm @ f4. Shutter time was 6 sec at ISO 3200 using a m4/3 camera for an effective FL of 200mm. Thirty images were stacked. It was cropped, but not as heavy as the first image. Again, you can see star trails.
Should point out that the comet was to the north and the closer one gets to Polaris, the shorter the trails will be. As you move farther away from Polaris, the longer the trails will be.
The best advice is to do test shots and zoom in and look at your results.
Also be aware that some lenses are much better at doing stars than are other lenses. And the wider you open the aperture, the worse the lens faults become. The price of the lens and the manufacture of the lens are not necessarily a good indicator of how good or how bad a lens it. You can do tests on your lenses before you go.
See:
https://www.lonelyspeck.com/lenses-for-milky-way-photography/Stars are point sources of light and can really create distortions which cause stars to absolutely look terrible. Some lenses are far better than others at this. Types of distortions include CA, Astigmatism and Coma distortion plus others. And stopping down a bit can certainly minimize the problem. You want wide open, but if the stars look bad, you have to stop down a bit. And stacking can certainly help.
For more info on distortions you may encounter, see:
https://www.handprint.com/ASTRO/ae4.htmlOne more piece of advice is focusing. Stars only look good at perfect focus. If you try to split the distance to have something close in focus as well as the stars, it won't work. The stars have to be the ones in focus. As a star goes out of focus, it turns into a disc that gets wider and dimmer. The light that you have from a star is then spread across that wider disc and the result is that it gets dimmer real fast.
And I don't like marking on the lens to show where best focus is. Just not good enough. I find on a mirrorless that I need to use focus magnifier for focus. Not peaking. And I start with a brighter star and get the best focus I can. Then I turn my attention to dimmer stars. And with these stars, I focus for brightness. At best focus, the star is as small a point of light as the lens will allow and all that light in one tiny spot makes it appear brighter. As you rock the focus back and forth an almost imperceptible amount, the star can dim to completely out of sight back to as bright as possible. And as your dim star becomes as bright as possible, look for an even dimmer star and repeat. This process produces the best possible focus.