There Are No Shortcuts

Recently, a friend was looking at my astrophotography and said “You’ll have to show me how to do that!” I wish it was that simple. It took me four years to go from a complete noob to getting decent results. There were long nights of practice, frustration and perseverance. Sometimes I had to buy my way out of problems. It takes the correct hardware/software mix and the will to learn how it all works together.

This article covers the things involved in taking an image. I’m not going to talk too much about how to use the software in this article. That’s an encyclopedia in itself.

Optics

Novices tend to think you need a big telescope for astrophotography. That isn’t necessarily the case. While larger telescopes are good for planetary photography or very distant galaxies, they’re terrible for nebulae, which are light years in size. If you want to photograph nebulae, you’ll need a good, doublet or triplet refractor with a focal distance between 250 and 600mm. On the higher end, a reflector like the Celestron RASA will give you exceptional images.

The Rowe-Ackermann Schmidt Astrograph is Made Specifically for Astrophotography

William Optics GT-81 Refractor on a Celestron Advanced VX German Equatorial Mount

The Mount

You can’t expect to slap a telescope on a tripod and get deep space photographs. The problem is that things in the sky move as the earth rotates. To compensate for this movement, you’ll need a German Equatorial Mount to track the night sky.

Focus!

Let’s face it: if your image isn’t in focus, you’re not going to be happy with it.

Placement of the image sensor is critical if there are any lenses in the optical train. For example, a Schmidt Cassegrain telescope (SCT) only uses mirrors for focus and the sensor can go almost anywhere (within reason) behind the visual back. However, place a focal reducer lens on your SCT and you’ll need to know how far back to place the sensor. The Celestron SCT focal reducer/flattener has a sweet spot at about 105mm. For my William Optics refractor’s focal reducer/flattener, it’s 58 to 64mm, depending on the telescope it’s on. You’ll need to dig in to the specifications of your equipment for these numbers.

Next, is, well, getting those stars in perfect focus. If you’re using software like Sequence Generator Pro and have a focus motor, SGP has the ability to do automatic focusing. Other image capture programs (e.g., Sharpcap Pro) have focus assistants to get pinpoint stars. All other mortals need to use a Bahtinov mask. When you place the Bahtinov mask in front of the telescope, it creates diffraction spikes on bright stars. Simply adjust your focus until the spikes are centered in the star and you’re in focus. If you’re using an SCT, you will need to adjust focus throughout the night as temperatures change and on the meridian flip to compensate for “mirror flop.”

Don’t Skip Your Calibration Frames

You need calibration frames to compensate for hot pixels, amp glow, sensor noise, dust motes on the optics and vignetting. Stacking software, like the free DeepSkyStacker or the commercial Astro Pixel Processor will use calibration frames to remove defects from your images before stacking. How many and what type do you need? It depends. I only take dark frames and flat frames with my dedicated, cooled astrophotography cameras. If you’re using a DSLR, you’ll also want to throw in bias frames. You’ll need somewhere between 20 and 100 frames of each. I get good results with 25, but some say you should do around 50 each.

Dark Frames: Darks compensate for camera noise. You can re-use dark frames, but I limit my dark library to one season and take new darks every year. With a cooled camera, you can take these indoors at your leisure. These are taken at the same exposure length and gain/ISO settings as your light frames.

Flat Frames: These short exposures compensate for dust motes and vignetting. You can’t really create a library of flats to re-use. Flats are taken with the camera attached to the telescope. I use a light box to create my flats after my imaging session. Others will cover the telescope with a white t-shirt and point it at the sky. Oh, and you’ll need a set of flats for each filter you’re using.

Bias Frames: If you’re using a DSLR, you’ll want to do bias frames, which compensate for the inherent read noise of the sensor. These are similar to dark frames, except you take the fastest exposure possible with the lens covered.

Guiding

Unless you’re using a very high-end equatorial mount to track the stars, you’ll need a guide scope and camera to provide correction pulses to the mount if you want exposures longer than 30 seconds. Otherwise, you’ll end up with elongated stars. The other advantage to guiding is to keep your target in frame. PHD2 is the go-to software for this function.

Dither Like Your Life Depends On It

Dithering is a function of guiding. Simply put, the guiding software is jogging the telescope a bit between frames, which has the effect of moving the image slightly on the sensor. When stacked, this tends to average out defects like hot or dead pixels. Dithering is especially important for one-shot color cameras and DSLRs due to the bayer filter on the sensor, since each pixel can only “see” a limited light spectrum. For mono cameras, I’ve gotten good results by dithering every other frame.

The Right Camera for the Job

Let’s face it. If you want to shoot anything but galaxies and planets, your stock DSLR isn’t going to cut it. The problem is that these cameras are made for general photography. They have an IR cut filter over the sensor that limits the light spectrum reaching the sensor. Those rich, red hydrogen emissions in nebulae? Pretty much gone. You can either have your DSLR modified (have the IR cut filter removed) or you can buy a camera made for astrophotography.

One-shot color cameras work great for planetary and galaxy photography. You’ll want to use filters specifically for color cameras to enhance nebulae.

If you don’t mind putting more work into it, mono cameras give exceptional results. The sensors are more sensitive than one-shot color cameras, but it’ll take four imaging sessions with LRGB filters to create a single color image. If you plan to do narrowband Hubble palette images, mono is your only real choice.

ZWO ASI533MC Pro One-Shot Color Cooled Astronomy Camera

How Many Light Frames (Exposures) Should I Take?

Light frames are your long exposures. The exposure length will vary depending on the object you’re shooting and your equipment. Now the question is how many of these light frames does it take to get a decent image? The magic number is at least 40. So for a one-shot color, that’s 40 light frames total. For a mono camera using LRGB filters, that 4 x 40, or 160 frames (40 for each filter). Personally, I like to have 60 frames if I can manage it. Sometimes, I’ll end up shooting an object over two or more nights to get the frames that I need.

This Image of the Orion and Running Man Nebulas Was Composed From 160 Individual Light Frames

Conclusion

Astrophotography has a lot of moving parts. It takes the right mix of hardware, software and technique to get results.