DIY finder/guider

[angryowl]

Just thought I’d share a little project I’ve been working on the past few days. 
Combining a finder and a guider in one single unit using a 50/50 beam splitter is an idea I have had since I decided to add guiding to my setup as didn't really want to have two separate scopes for this.

Setup is simple; use a 50/50 non-polarising beam splitter cube to split incoming light into two portions, 50% gets reflected at a 45 degree angle off to a finder eyepiece, the other is refracted through to the guiding camera.

The beam splitter cube came from SurplusShed It’s AR coated and 30mm. Although any non-polarising beam splitter would do. Enclosure for it all was obtained by removing the mirrors from two diagonals I had laying around and using the remaining plastic case. Some screws to hold the cube in place and allow fine adjustment of it (screws have a bit of rubber glued on the end so as not to crack the cube as a result of screwing them in too far)

Here’s some images of it all
The diagonals sawn off and holes for mounting drilled

The beam splitter itself

 

[angryowl]

An improvised cap for the side of the splitter which is unused

The eyepiece attached

[angryowl]

View through the eyepiece side

View through the camera side

To achieve focus the finder tube will need to be considerably shortened which will involve hacking a portion of the back off
The finder/guider will slot into a 2” to 1.25” adapter and the adapter will be glued inside the tube using two part epoxy
The eyepiece and the camera need to be correctly spaced from the cube mount so that both come into focus at the same time

So far I have done a rough test and PHD seemed to be able to pick out stars in the FOV despite the aperture decrease of what I assume is about an f stop but have not tried PHD calibration as of yet 
The tricky bit here will be to correctly align the camera and the finder eyepiece but I’m hoping the 8 screws supporting the beam splitter will allow for a precise enough alignment. At the moment they are relatively well aligned with a small offset between them but I’m hoping after I get everything setup small enough adjustments will still be possible.

I will update the thread when I saw off the finder tube to the correct size and fit the 1.25” adapter in and try a PHD calibration.

[angryowl]

An update to wrap this up...

Aligning the cube inside was relatively difficult and wanting to maintain this alignment I filled the spaces in the case with mirror adhesive (this adhesive is apparently low expansion and bonds very well to glass). So now the cube is mostly held by the adhesive yet fine alignment using the screws is still possible.

Had two imaging sessions with this over the last couple of days and on both occasions PHD was able to find plenty of stars and perform calibration.

The only thing I dislike is focusing, as the supporting plastic screws now have to grab hold of the front lens and so it can't be rotated whilst mounted, the only way to focus is to loosen the front screws a bit so the lens can rotate, no biggie really just a slight inconvenience.

Here's the tube 

And here it is all assembled