Last winter, the Bavarian weather was terrible. From November to January, we haven’t had a lot of clear nights. So as soon as the sky cleared up, I was very excited to photograph the Orion Nebula!

The last time I photograph Orion was in December 2017. Back then, I just had a tripod and the fabulous Samyang 135mm with me. The picture I made from the Swiss mountains wasn’t bad at all, considering I had just begun my astro-journey!

Since then, I’ve been improving a lot, mostly in the post-processing department (even though I still have a lot to learn!). So I was eager to had another try at Orion!

Last January, I finally had time and a clear night to photograph the most famous nebula. This time, I used my Canon 300mm f/4L lens and its 1.4x teleconverter, in order to fill the frame as much as possible.

Here is there result!

Mount: iOptron SkyGuider Pro
Camera: Fujifilm X-T1
Lens: Canon 300mm f/4L USM + TC 1.4x
Aperture: f/5.6
ISO: 800
Filter: IDAS LPS D1
Lights: 40 x 30 seconds
Darks: yes
Flats: no
Bias: yes
Post-processing: DeepSkyStacker, Photoshop
Location: Englischer Garten in Munich (Bortle 7)

The acquisition

It was a nightmare! But as always, it was also an excellent lesson. Orion was quite low in the sky (about 30-35°), and I was using a 420mm lens on an APS-C sensor, unguided. So, for the first time, I experienced the astrophotographer’s nightmare: periodic error.

A few words about periodic error. Motorised equatorial mounts, like the SkyGuider Pro, use motors to track the stars. But these aren’t 100% perfect. All mounts have slight mechanical irregularities in their gears, which results in very small unwanted movements when tracking the stars.

Furthermore, periodic error (PE) becomes worse as you use longer focal lengths, which was the case. Also, due to the Earth’s rotation, PE is stronger at lower altitudes. Finally, the longer your exposures, the more chance you have to experience PE.

More specifically, periodic error messes with your pictures as if someone was poking your lens during an exposure! An equatorial mount is supposed to give your round stars, as the mount tracks the stars. But with PE, this is what you get:

Example of periodic error on 4 consecutive 30s exposures

So what’s happening? Well, as the picture shows above, the mount is tracking the stars, but at some point during the exposure, there’s a slight movement that transforms round stars into ugly dashes.

How to prevent this? Well, these irregularities are certainly annoying, but luckily for us, they are also periodic, which means they occur every few minutes. So, if you could “study” the mount’s accuracy long enough, you could anticipate when the PE will occur, and counteract the unwanted movements by slightly adjusting the mount’s movement as a preventive measure.

This is basically what autoguiding is. To eliminate PE, you can use another camera with another lens, linked to a computer. Using a dedicated software program, like PHD2, this guide camera will lock on a star and study how this star moves while the mount is experiencing periodic error. After studying the mount, the program will know when PE occurs, and will adjust the mount’s movement accordingly.

Autoguiding is used by most astrophotographers, because it is so efficient! Remember: PE is stronger when the exposure and the focal length increase. So, without autoguiding, you’ll be limited to short exposures and small focal lengths…

However, if you have an autoguider that is carefully setup, however, you’ll be able to go from 30 seconds to several minutes long exposures! Night and day! :-)

Getting into autoguiding

Luckily for me, I can now use my little ZWO ASI224-MC planetary camera as a guider for my telescope. I bought a small guide scope, the Artesky 32mm f/4. It’s made of carbon fiber, and is very lightweight: perfect match for my portable setup!

I can’t wait to try it out! And a big thank you to the folks over at Stargazers Lounge for their support!

The Orion Nebula (January 2019)
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