So, I have an older Nikon D5700 and bought a t-mount (no lens, just an attachment ring and tube) to take pictures with it through my Celestron NexStar 130SLT telescope. I’d be happy with photos of recognizable celestial objects, rather than award-winning 4K pictures. Last night I attached a 2.5x Barlow lens to the t-mount and inserted it into the telescope objective tube. I couldn’t get the image in the camera focused, even with the telescope focus racked all the way down. Am I correct in interpreting this to mean that the Barlow lens focal length is too short? What telescope objective would I need to make this work?
@Spiderman probably has relevant expertise. I sure don’t.
A Barlow lens is used to extend the focal length of the telescope. Usually for planets.
In general you don’t need an additional lens when using the telescope. The telescope by itself is the objective optical element, and already focuses a real image onto the sensor.
You use an eyepiece lens to create a virtual image that your eye can focus on, and thence create a real image on your retina. Cameras don’t need this.
If anything you may find the focal length of your telescope too long. Focal length reducers aka telecompressors are popular for this reason. But just using the camera naked on the scope should be a start.
Like Francis_Vaughan said.
So, Vlad_Igor, if you’re struggling to get any image in the first place, there are some additional things you can try.
Aim the telescope at an easy target, such as the Moon, or some very distant terrestrial target like a big building on the horizon (which will stay still in your field of view even though the Earth is spinning, unlike the Moon). Put an eyepiece in, and focus the telescope. This will establish where the telescope focuses a real image. Now pull the eyepiece out and look inside it (from the open end not the eye end). There’s a darkened ring in there, an aperture stop whose edge creates the sharp edge of field when you’re using it. What’s the position of that ring with respect to the end of the telescope’s eyepiece tube? That is, what’s its position with respect to the step on the outside of the eyepiece that stops it from sliding all the way into the telescope? This gives you the distance from the end of the telescope eyepiece tube to the location of the telescope’s focus, probably a small positive distance. Look at your camera with its t-mount installed. There’s probably a marker on the camera body, a circle with a line crossing it, for where the sensor plane is. This is where you need to place the telescope’s focus. What is its distance from the step on the t-mount that goes on the end of the telescope’s eyepiece tube? You have to be able to adjust the focus of the telescope to place that circle-line mark at the telescope’s focus. I’d bet you will need to adjust the telescope’s eyepiece tube into the telescope, but it might be either way.
A cheat you might use to get just any image at all, if you can’t send the eyepiece tube far enough in, is to try aiming the telescope at closer targets. A closer target will create an image further out of the end of the eyepiece tube. My house is surrounded by trees, and sweeping my telescope horizontally while aiming into the woods lets me create a great number of different focus locations quickly. This doesn’t get you to astrophotography yet, but it lets you see something, and try the camera. Distant astronomical targets will come to focus closer to the telescope than anything else you can look at (though things on a distant horizon may be practically the same distance).
Thank you for this perspective. In a previous career in medical labs, I did a fair amount of microscope work and am used to a lot of lenses between me and what I am looking at. The idea of the telescope being the lens is counterintuitive.
You have to be able to adjust the focus of the telescope to place that circle-line mark at the telescope’s focus. I’d bet you will need to adjust the telescope’s eyepiece tube into the telescope, but it might be either way.
Thank you, Napier. I discovered something about my telescope today: I can unscrew the eyepiece tube and the camera attachment ring screws into the eyepiece tube attachment point. So, this means I can get the focal plane a lot closer to the tube diagonal mirror. Yesterday, I actually was able to see tree tops through the camera viewfinder with the Barlow lens in place, so I was disappointed when I couldn’t focus on the moon last night.
We may have cloudy skies for the next 3-4 days, so I’ll try the new configuration when the skies clear. That leads me to another question: how can I optically enlarge the image going into the camera? If I get a good view of Jupiter and moons, how can I turn it from a speck to a larger size with some details?
You have two different options when doing astrophotography, the first, using a t-ring adapter, turns the objective (lens for a refractor, mirrors(s) for a reflector) into a telephoto lens, this is often called prime focus. I have a Celstron C8, with a focal length of 12000 mm. When I attach my camera with a t-ring adapter the telescope becomes a 1200 mm telephoto lens. With my digital camera, this results in a field of view just slightly less than 2 degrees, which I know because I can’t quite get the full disk of either the sun or moon, both are slightly clipped. I’ve not tried this setup for planets, as the OP mentions, their image would be pretty small.
The second option needs a different kind of adapter, one that lets you place an eyepiece in the telescope and the adapter fits over the eyepiece at one end and the camera mounts on the other, this is usually called eyepiece projection. Because you use your telescope eyepieces, you can control the magnification. I’ve not tried this method, but here’s a link to an adapter that appears to let you do both types.
The linked camera adapter above covers a lot of useful stuff.
In particular it talks of eyepiece projection. This is closer to what using a microscope is like. The eyepiece is used to focus a real image onto the camera sensor, with much greater magnification than the raw telescope. This technique usually needs a bit of distance between the eyepiece and the focal plane. So the adapter provides the extra space.
Actually photographing a planet and its moons isn’t trivial, even with modern camera sensors. Atmospheric distortion is a big limitation, and the telescope mount can also be a significant limitation. The NextStar appears to provide tracking, which is essentially mandatory. There are ways to ameliorate these problems as well. Planetary moons are hard as they are much dimmer than the planet and getting both in a satisfactory image is challenging.
There is a rabbit hole of huge depth to follow if you get enthusiastic. Digital sensors have revolutionised what can be achieved. Software for both camera control and image processing further revolutionised things. It has become ridiculous what a modest device can accomplish.
Thanks, all. A digital sensor or an eyepiece sleeve is an option. We had clear skies here last night, so I took my telescope and camera out, hoping to take a few pictures. It was for naught as I still couldn’t get the telescope focused through the camera viewfinder. Some research afterward revealed that my particular scope has a focal point within the focusing barrel and can’t reach the camera focal plane. So, a Barlow lens or an eyepiece sleeve is needed. A third option requires minor surgery on the mirror back, essentially adding spacers to move the mirror forward in the tube and thus moving the scope focal point out of the focusing barrel and within reach of the camera sensor plane. This is the video I found on modifying the mirror: https://m.youtube.com/watch?v=jDpSXPNCnS0. I’m considering my options. I don’t mind rabbit holes as long as they are not too expensive and I don’t end up with a lot of surplus equipment I’ll never use.