A lot of doing successful astrophotography is having the right tools for the right job. When you’re swapping views, through a telescope, between your eyepiece and your DSLR, you’ll notice a huge loss in magnification. When you attach your camera to your telescope, you’re using the telescope as a camera lens. You will get some magnification from the camera’s image sensor chip, but never enough to compensate for the magnification that comes from using a medium or high power eyepiece. This is especially important when you’re imaging a planet or globular star cluster. When you’re using a telescope as a lens for your DSLR there are three ways to regain your magnification of the target.
Cropping the picture in post production is the easiest and the worst solution. Every time you crop a picture you’re sacrificing resolution for the sake of making the subject bigger in the frame. Your pictures will be less sharp and less satisfying. If you find yourself constantly doing lots of cropping to make the subjects the size you want, you’re going to have to consider getting a more expensive camera so you have more pixels to sacrifice.
Tele-extenders, like the Celestron Deluxe Tele-Extender, give you a way to connect your camera and look through the eyepiece at the same time. To attach your DSLR to the tele-extender you’ll need a brand specific Camera T-Ring, either Canon or Nikon. When it works, it’s a great solution. Problems can arise though when the length of the tele-extenders’s metal tube is too long to clear some mounts, when observing objects that are high in the sky. Tele-extenders work fine with most Plossl eyepieces, but many of the wider eyepiece body designs won’t fit inside the tele-extender sleeve. There can also be issues with the screws holding the tele-extender in place. They can get loosened during use and are all that is holding your expensive DSLR camera from hitting the ground. Some telescopes will only allow you to use one of the two screws provided! A lot of users will add additional straps to secure their camera. Some also put a thick elastic around the tele-extender tube, to help keep the screws underneath from moving. When things work well, a tele-extender is great way to get up close to your subject when doing astrophotography. You just need to make sure you have eyepieces that will fit inside the metal sleeve of the tele-extender, you’re not fighting the mount, and are mindful of how well secured your camera and tele-extender are to the telescope.
What I find myself using most for astrophotography is the Celestron Barlow T-Adapter. It’s a Barlow lens and T-Adapter in one! The Barlow lens doubles your magnification, offsetting a lot of the loss of magnification from removing the eyepiece. To attach your DSLR to the Barlow T-Adapter you’ll need a brand specific Camera T-Ring, either Canon or Nikon. Unless you’re using a solar telescope, remove the diagonal and attach the Barlow T-Adapter to the visual back of the telescope. Depending on the telescope that you’re using, you may have problems reaching focus. At that point you’ll need something like Sky-Watcher’s Adjustable Camera Adapter or similar extension tube. What makes me rely on the the Barlow T-Adapter is that it’s very easy to use and requires a lot less fussing around than a tele-extender.
Another feature I really like about the Barlow T-Adapter is the same threads that attach the Barlow to the T-Adapter can be used for eyepiece filters. That means I can take any of my eyepiece filters, designed to attach to 1.25″ (31.7 mm) barreled eyepieces, and use them with this T-Adapter.
A few months ago, I used this feature to take a series of pictures of the Orion Nebula, using different filters. You just unscrew the Barlow on the end of the T-Adapter and screw on filters (stacking them if I want to).
I can also screw on a filter and then re-attach the Barlow to the end of the filter, but this means you’ll need to re-adjust the telescope’s focus. Unfortunately, you can’t attach filters to the end of the Barlow, because there are no threads available.
The most important thing to remember, when you’re doing astrophotography with a telescope, is to get the focus right. The focus of your telescope, when you’re looking through an eyepiece, will always be different from how your telescope is focused for the camera. To focus my camera I aim the telescope at the brightest star I can find, adjust the ISO setting on my camera to maximum, and the exposure set to BULB. I use the LCD screen and maximize the image to get the star as big in the screen as I can to get the best focusing results. After that I don’t touch the focus. This means if you’re swapping between observing and astrophotography, you’ll also have to change the telescope focus each time you do so. Also, always use a remote shutter trigger for taking pictures to minimize camera shake.