Part 2 : Your Station setup is done, time for taking shots
Let’s start shooting the full package of pictures
The picture of this post is what we call a dark shot (I updated the picture, exposure 120s). What is that ? Let’s start with the basics. Vulgarization is the most difficult thing to deliver, but I’ll try my best.
Here you are with your new telescope, the type of it (Newtons, Cassegrains, APO …) is irrelevant to the topic. You have then an optical chain : a tube acting like kind of a camera lens and a camera attached to it (with some adaptor such as T2 ring for a DLSR Camera, or some M48 adaptor or something like that for a dedicated astro-camera, or even an Iphone camera on top of your ocular in the worst case).
To keep it simple, I will not go into the camera sensor and pixels sizes in this post.
So, your station setup is done (Part 1, remember ?) and now you can start shooting. You have an object in mind and you ask your GoTo mount to slew to it. Just be sure that your focus is ok (try again a bathinov mask on a near star), be sure your object is rightly centered, you’re ready ?
Your shooting will involve four types of shots : lights, darks, biases, and flats.
lights : these are the shots you’re taking with your optical chain. Meaning the object you shoot. If you have a cooled camera it involves camera temperature, and then exposure time and Gain (Gain/ISO, I will not go there). Time of exposure is a topic I will elaborate in another post. But longer your expose on one shot, more details you’ll get from the objects (nebulae, galaxies, stars, clusters, supernova …). BUT, the longest exposure you could take will involve the same exposure on your darks …. careful. You’ll probably take tens of shots, and the addition of all these shots will give you the addition of total exposure (a way to say that 100 shots of 10 seconds will give quite the same result than 10 shots of 100 seconds … quite quite …)
darks : these are the shots at same exposure, camera temperature and gain than the lights BUT with the cap on the tube. It will be used to delete some noise or artefacts from your lights shots. For instance, your can see on the post dark picture some starry light on the right side of it and it is a phenomena from long exposure with my camera ZWO 183MC Pro. This phenomena is also on the lights shots, and darks are needed to subtract this Amp-Glow (that’s the name of it) from the lights. They are important shots for final stacking your shots
biases : pretty much darks shots (cap on the tube) but with short exposure. It will get readout noise from pixels, that again will be deleted from your lights shots during stacking
flats : oh my gosh, they are the worst … they are shots of daylight with histogram peak around 1/3 or 1/2, same Gain/ISO/temperature/focus than lights. But how can I get daylight during my nighty sessions ?? Well, I do them during dusk, just before the night. But there are other solutions such as Flat screens or the T-Shirt trick to take them during the night. They are important as they catch the air perturbation in the tube, the eventual vignetting of your lights and also the stains or dust on your telescope lens (and even dust from your camera sensor). These defaults will then be deleted from the lights shots during the stacking process.
A correct ratio of shots would be 100 lights, 25 darks, 25 biases and 25 flats. Though, biases and flats are at short exposure so it will not cost you that much time to shoot more of them.
So, now, when you’ll see the “Exif” details on a post showing a target shot, it is about the details of the setup used, and the numbers of pictures lights/dark/biases/flats (sometimes there are no flats because I didn’t take any) that I will stack for the final picture.
