Connecting a camera to your telescope is obviously the main step in converting your set up for use in astrophotography, but what is the best way to do it? What about adding a filter wheel, and how does your guide camera send corrections to the mount? The following video covers all the basics of creating a system that will have you taking photographs of the night sky in no time.
Hello. I’d like to talk today about how we take a telescope and convert it for use for astrophotography. So what I’ve got here is a simple 80mm refractor, we have the eyepiece here, it’s sitting on a mount and the mount itself has a GoTo Sytem on it, so it’s able to track the stars and it’s able to find different objects in the sky.
Attaching a Camera
This is one of our Atik 4-series cameras. This is how it comes out the box and we see it’s got a little end cap on here so we’ll remove the end cap and that comes on a 1 ¼” drawtube adaptor. If we look down there we can see the sensor down the bottom of the camera, and what we need to do is basically replace the eyepiece with a camera.
But what we’re going to do is, rather than just replace the eyepiece on its own, this telescope has a star diagonal and we don’t need that for imaging. So, what I’m going to do is remove the additional optical element that we don’t really need. I’ll just put that down there.
And then we come up with the next problem we’re going to need a solution for. If we put this camera into this focuser tube, obviously we need some kind of adaptor there. The focuser in this case is 2” and we’ve got a 1 ¼” drawtube on this side. So what we need is a 2” to 1 ¼” adaptor. What we’re going to do is just slide that into there, do the grub screw up and this is now a 2” and can slide into our drawtube.
Okay, now this obviously is physically connected to the telescope. We have to bear one more thing in mind, and that is, when we had this diagonal in place, it placed the eyepiece relatively far away from the focuser tube. When we’re looking to gauge roughly where the camera’s going to need to sit to come into focus, what we need to consider is the front of the camera should be roughly where the front of the eyepiece would normally sit.
So you may find that the focuser itself can just be racked out far enough that that’ll place the camera in roughly the right position, and here this is pretty close. There are some downsides. If you pull some focuser tubes out too far they can start to get a bit wobbly.
But what we’re going to do, just to demonstrate this piece, is we can introduce some adaptors. In the world of astronomy, we can always add some adapters, there are always adapters to be added, and in this case, what I’ve got is a 2” and it comes through to a t-thread on this side. So it just slides in here, and then in this case, we could use this one with a 2” and we’ve got the grub screws here to do it, but what I’m going to show you here is how to connect this up to a t-thread.
So if we undo the 1 ¼” adaptor, what we’ll reveal under here is an astronomy standard t-thread. Wherever possible, try using t-threads in preference to drawtubes because they tend to be more secure.
So then this is screwed into the focuser tube, it’s really quite solid and has very little flexure at that point and it’s in roughly the right place so we should be able to gain focus.
Adding a Filter Wheel
So we could look at guiding next, but what I’m going to do is first of all is talk about adding filters, or a filter wheel. So if we have a monochrome camera it’s quite useful to get colour images by using red, green and blue filters, maybe use narrowband to get round light pollution, these kinds of things. We’re going to want to use a USB controlled filter wheel to do this. So this is the Atik EFW2. It has a USB controller in there so we can use a computer to select which filter is going to be in place next.
I’ve connected this up using the t-thread on the back of the EFW2 connecting directly to the camera. The connection here has a very short backfocus, it’s a flush fitting which means we get the camera as close to the front of this optical train, which helps with a number of systems where backfocus is limited.
Now what we tend to do when we astroimage is to align one of the axis of the CCD up with the RA axis and one up with DEC axis, and to do this simply you can kind of see where the axis is on this camera. So up is in this direction and this is side to side, and we can align the camera such that that corresponds to the two axis we have on our telescope.
The Atik One with Integrated Filter Wheel
What I just want to mention is the Atik One. This is an Atik One Camera. We called it the One because it has an internal integrated filter wheel, so it’s just the one system to do your RGB and your narrowband imaging. So the filter wheel’s actually just inside and we’ve got a standard t-thread on the outside again. These cameras need just a single 12V input and a USB connection so it makes things more integrated and more simple than adding an external wheel on.
So I’m just going to put a 2” adaptor in here. Then this camera itself, just align the axis roughly, you can see what up and sidewards is, slide into the focuser, and then that’ll be all the camera, all the equipment we need to do narrowband or RGB imaging.
What we should mention now is guiding. Basically, to take longer duration pictures – so if you want to get into the pictures with exposure times over a minute, up to 20 minutes, and up to several hours if you really want to push it – what you need to do is to actually guide the telescope. So there you have one camera taking pictures every seconds or so and sending corrections to the mount to make sure that the main camera remains steady and focused on the object that you’re looking at.
There’s a couple of ways of doing this. We could mount a second telescope parallel to the first one and put a camera on the back of that. It’s a very simple way of doing it but what I really want to mention now is using off-axis guiding.
If I pick up an off-axis guider here, what this consists of is a tiny little pick-off prism so some of the light that’s going to miss the main imaging sensor – which would just be in the middle here – so some of the off-axis light will be bounced up, up at a different angle and onto a camera, a guide camera, that’s at 90 degrees to the main imaging camera.
Loads of benefits from doing it this way, mainly related to the fact that you’ve got a single scope, so you’ve got a single imaging train, you’re not worried about any differential flexure between two different tubes.
Sending Guiding Corrections to the Mount
What we also need is a way of telling the mount what corrections it needs to make. So the best way of doing this is to have a connection from the mount itself back to the computer. If we have this, normally if it’s a GoTo system, we can then control the mount completely by computer to get it to go to different objects, and also when the images come back from the guide camera, the computer software will work out what corrections are needed, and these can come back through the USB lead, and the mount will then interpret how much correction it needs to do.
That’s not the only we can send corrections to a mount. So there’s a slightly older standard called the ST4 standard, and this is one of our 3-series cameras and here we have an ST4 port. This was originally designed to work with relays and slightly more basic electronic drives. What this will do is this will link directly into a guider port, if the mount supports guiding, using the ST4 standard. The computer will take images from the guide camera, will then interpret them, work out what corrections are needed, then send them via the USB link into the camera, the camera will then send it to the mount using the ST4 link.
Okay, I think the last thing I’d like to talk to you about is adding a focal reducer into the imaging train. This is actually going to change the focal ratio we’re imaging at, so for example, an f7 scope could go down to f3 and that’ll make it a lot faster and more suited to astrophotography.
Now the thing we need to take most care of when we’re looking at focal reducers is their backfocus requirements. So when we’re using a simple camera, we can just use the focuser tube to basically space the CCD the camera the right distance from the front optical assembly.
When we’ve got the focal reducer in place, these will have a specification for the distance that the back element of the focal reducer needs to be from the CCD. So that’ll be that distance there to the distance where the CCD is inside the camera itself. We, on our website, will describe how far the CCD is from the front of the camera you may then need to have some different spacer tubes made or bought to space the focal reducer the correct distance away from the CCD element itself.
Okay we’ve mentioned adaptors an awful lot. So in astrophotography, you have many different types of adaptors to adapt from different thread types and different draw tubes. The right people to be talking to about which adaptors you might need are Atik Dealers. They’ll know all about our cameras, they’ll know about many of the different telescope types that are out there, and they may well even have the right parts in stock to be able to allow you to connect cameras directly up to your telescope.
Okay then, we’ve covered an awful lot, but I hope that’s been helpful and thank you for watching.