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Nikon D800 for astrophotography ?

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5: Off-axis guide camera

In the beginning I plan to use the ASI120mm as guide camera for this system. Many details are 3D-printed and made off plastic. It flex more than metal brackets. Then it's good to have a off-axis guider that look through the same lens as the photos is taken from, no internal flex.

Guide camera:

I bought this AIS120mm camera used. It's the older model with USB 2 ports. It caused some trouble and I had to flash the camera with an upgrade software, it's only in Linux system you have to do this: Upgrade ASI120mm camera.

The camera has a back focus of 12.5 mm. It's a monochrome 1.2 M pixel sensor, the pixel size is only 3.75 my and the sensor size is 4.8x3.6 mm (diagonal 6.5 mm). The off axis adapter I use now in the beginning has a hole diameter of 9x5.5 mm which work with this small sensor. In the future I plan for a bigger sensor to get a wider fov and then I must built my own off-axis adapter with a bigger hole, something like 10x8 mm.

Off-axis adapter:

This is the part I plan to use, a prism from my T48 off-axis guider in the beginning. It has a tiny hole of 9x5.5 mm that connect to the guide camera. Later I will design a new one with much bigger opening.

Something like this it will look, from the camera side.

From the lens side it look like this. On top of the tube is the prism placed, not drown here. Behind the off-axis tube is space to install a 2 inch IR/UV block filter.

Future design:

The thin unmounted filter give more free space and I can use a bigger off-axis adapter. It's designed to take a rectangular hole up to 10x8 mm to the guide camera. I can then have a bigger sensor to get a bigger fov. The off-axis adapter point in direction to the cameras bottom side. The only side where there is space for this guide camera. The mount for the guide camera is the T2 (42 mm) standard. Must find where I have to focus point and fit the dimension to that. Still there must be a focuser for fine tuning, the normal drawtube isn't very appealing, maybe a helical focuser if it's enough space for it. In this drawing I have a 45 degree tilted first surface mirror, but could be a prism also.

Back focus:

Red = Nikon D800 camera body
Light Blue = Camera lens adapter
Dark Blue = Pentax 645 bayonet
Yellow = Off-Axis adapter
Light Green = Guide Camera (ASI120mm)

After some calculations I found where I have the focus point for the guide camera, 20.5 mm below the yellow device. The ASI120 camera has a back focus of 12.5 mm. Now I need some focus mechanism between the yellow and green device that doesn't rotate during focusing, 8 mm left to built this focus device on.

Ray trace:

The blue line represent the light ray coming in from the lens to the main camera and the guide camera. A bigger opening to the guide camera had let in more photons. With a medium format lens this is much easier to solve with its bigger image circle.

Guide camera attached on the main lens:

If my calculations is correct the guide camera sit +/- 2 mm from correct focus. No other items block the camera's move when focusing.

This system is very compact compare to my earlier setup with a separate guide telescope. Earlier setup: 300 mm lens. This camera and lens is heavier but in total I save a lot of weight.

A deeper look at the guide camera's focus:

When I did my first test I couldn't reach focus with the guide camera. More, the adapter body hit the Nikon body (lower arrow). I tested with a 11 mm Pentax M42 extender, this has another thread M42x1.0mm and the threads fitted bad, of course. But from this I can do a calculation.

Some dimensions:

Pentax 645 back focus = 70.87 mm
Nikon DSLR back focus = 46.5
ASI120 back focus = 12.5 mm (guide camera)
Prism center position = 15 mm behind Pentax flange
Prism center position = 15 mm from optical center

Correct distance between the upper arrows:

70.87-15-12.5 = 43.37 mm

One of the items of the Off-Axis adapter has to be redesigned.

New design, total height 20 mm, 10 more than the original. I couldn't design a M42x0.75 mm thread in FreeCAD. I made the outer diameter to 41.6 mm and hope it will self thread when I force it on. If it doesn't work I have to go deeper in Helical design in FreeCAD to master how to do it. Anyway, the 3D-printer doesn't have that high precision.

The new 20 mm long adapter attached to the guide camera.

The main camera and the guide camera in place. Now the guide camera has a lot of free space to travel when focusing. The focus system is terrible but it will be a better design later when I find a bigger prism and can have a bigger opening up to the guide camera. Now I must do a new test if I can reach focus for the guide camera, it's promised a clear sky tonight.

Early auto guiding test:

I was able to get the guide camera in focus and could do a first test.

This mount is special built and have a custom gear ratio. The driver EQMOD isn't compensated for this yet. Still I can see a big improvement in the guiding. The 50% longer focal length helps but the use of the same optic as the main camera is important. Off-axis is the way to go. The ASI120 camera works much better in the new 64-bit software.

New attachement of guide camera:

I started with parts of my T48 off-axis adapter. With that I could do the first tests and get experience. Now I want something much better. More stability, bigger opening, easier focusing. This is the first item, that attach to the barrel where the lens and camera attach to. It has a light barrier around the opening up to the guide camera. Two bars guide it to a movement along the Z-axis.

The moveable guide camera bracket. It's designed to have a focus travel of +/- 4 mm. Two M3 thread screws used to fine adjust the focus. It doesn't use the T2 thread to connect to the camera, it squeeze around it.

With the guide camera attached it look like this. Much bigger opening to the guide camera allow for bigger sensors. A mirror or 90 degree prism can be mounted to the brackets that protrude from top. I haven't find any yet so its dimension are not known yet. They have a fixed position, only for full frame sensors.

Prism bracket:

Last item that missing, a first surface mirror or a prism. I have not found any mirror or prism of correct size yet. But now very eager to test the system. I can move the prism from my old off-axis adapter to my new. I did a device where I can attach the prism and then my off-axis adapter.

It fits direct on the tabs where I plan to glue the mirror.

Looking closer how to remove the tiny prism. An even more tiny screw hold it in place. The screw has an 1.3 mm allen head.

Looking among my tools and find one that fit. But after I loosen the screw the prism still was stuck there. They have glued it. I dare not break it off, it might break.

Prism to optical axis distance:

The easiest way to see if the is a free path down to the prism is to looking into the lens from the front. Here is the prism edge adjusted to 13 mm distance from the optical axis. Plenty of space around it, I will move it out to 14 mm later to clear the sensor which sit behind the mirror in the background.

Prism or mirror:

The prism I use here has the dimension: 10 x 6 x 6 mm, but need something like 12x8x8 mm. I can buy a new prism as spare part, but they are too small, like: 8 x 8 x 8 mm. If I place two of them side by side I get 16x8x8 mm, but will be difficult I think. A mirror of the dimension: 12 x 13 mm will do the same.

First surface mirror:

I have ordered a couple of mirrors with the dimension of 20x13 mm, maybe I have to cut the mirror down, it's very limited space around it. I must also grind two of the edges to 45 degree angle.

The mirror must be of First surface design: First-Surface.

To the mirror it must be a bracket. Complicated little thing to design.

Mirror installed in the bracket. This is the backside of the mirror.

This is where the light beam from the telescope goes in. The silver surface of the mirror is protected by a blue plastic film. The edge of the mirror that protrude out must be grinded down later.