Intro

I have poor conditions and equipment for astrophotography:

high light pollution

shaky manual telescope mount

old Canon 650d camera with high noise level on ISO6400

The 500 Rule

The “500 rule” states that longest exposure to avoid star trails is calculated:

Exposure Time [sec] = 500 / (Focal Length [mm] * Crop Factor)

Actually this time depends on declination of the object and sensor resolution, so this is a rough equation.

Let’s calculate maximum exposure time for my parameters:

ET = 500 / (900 * 1.6) = 0.347

This means that I have to use high ISO values for objects that aren’t bright, that brings noise to the image (but in accordance to this comment this is not quite correct).

Some of objects that are actually persist in the image are lost in the noise, especially when exposure compensation is used for underexposed images.

The Proper Solution

As far as I understand things now, astrophotograpy is about stacking images (correct me if I am wrong).

There are programs like “DeepSkyStacker” that are specialized for this case, but it requires a specific number of stars to use them as anchors for stacking.

Here again we have noise issue because if star isn’t bright enough to be clearly extracted from noise, it can’t be used as anchor.

Star Trails

Now let’s talk about workaround of this issue, but it is important to understand a general limitation: we are talking about point objects without details.

The main idea is use long exposures to get trail as regular signal on the random background.

Synthetic Demonstration

In the image editor I created an image with “stars” of a different size and applied Gaussian noise of different intensity (between 0 and 1) to the image.

Animation:

Individual frames:























As you can see, on the last frame only a “star” with size 4 can be barely seen, while even short trail of smallest “star” can be seen.

Real Life Example

Below you can take a look and compare short and long exposure in Mirach area.







Psi Aquarii and Neptune (both photos are normalized to have close histograms):







On the extremely processed image below it is relatively easy to spot stars with magnitude 11+ (my visual limit is 10) near Neptune in the middle of the frame.

Extremely Processed Image

Due to the fact that star trails have bigger scale than point star images, it makes them easier to find.

Conclusion

Even if you don’t have proper equipment to get great images with lots of details and hundred hour exposure, that shouldn’t stop you from enjoying your hobby.

Clear Sky!