Focuser for OAG?

[kbrown]

Before inventing the wheel again, I thought I'd ask whether anyone has come across a good DIY focuser for an OAG camera? I could just get something like this but the inner maker in me wants to 3D print something useful

Edited April 6, 2023 by kbrown

[michael8554]

That would be a useful project.

Because the helical focuser that comes with many OAG's is often too long to match prism-to-sensor distances.

So I had to buy that low-profile Baader focuser.

Michael

 

[kbrown]

Not quite the helical focuser I first had in mind but I had a rummage in my (one of many) random bits and bobs box and found these small bearings. I thought why not run a screw through them and have the drawtube ride along it. A couple of days later I had designed and printed this:

 

It's a little wobbly when unlocked due to the one sided support design but once locked it doesn't move. Not perfect but I think it'll do just fine for this purpose:

 

[Chriske]

Looking good..!
Around the inner perimeter you could add a few teflon strips to make the action smoother. To get rid of the wobbly action a few springs or even small bolts pushing against these teflon strips.

[michael8554]

Good choice of design, the linear focuser is easier to drive than helical .

Michael

[kbrown]

3 hours ago, Chriske said:

Looking good..!
Around the inner perimeter you could add a few teflon strips to make the action smoother. To get rid of the wobbly action a few springs or even small bolts pushing against these teflon strips.

That's a good idea. I will first have to test this to see whether I can achieve focus with this. I'm only just converting my system to an OAG. Haven't had the chance to try it out yet.

[kbrown]

Didn't have any teflon strips but I remembered I had a roll of kapton tape which has a smooth and fairly slippy surface. Two layers of it seems to get rid of all the play and it's still smooth to operate. I'm happy with this

[Chriske]

Nicely done.!
I was more thinking of vertical strips. That should do the job even better imo.

[kbrown]

2 minutes ago, Chriske said:

Nicely done.!
I was more thinking of vertical strips. That should do the job even better imo.

Probably would have to glue the strips in place as the tape doesn't stick very well on the printed surface. Might try that at some point. Thanks.

[Chriske]

One more possibility to consider.
Why not redesign the focuser just a little bit adding two bevel gears so the focuser knob is at a more 'natural' position. iow at  a 90° angle to the optical axis.

Do not misunderstand me, I think it's a genius concept, but some users are not prone to use a vertical focusing knob.

[kbrown]

3 hours ago, Chriske said:

One more possibility to consider.
Why not redesign the focuser just a little bit adding two bevel gears so the focuser knob is at a more 'natural' position. iow at  a 90° angle to the optical axis.

Do not misunderstand me, I think it's a genius concept, but some users are not prone to use a vertical focusing knob.

That's a great idea. The way it is now is a bit fiddly to operate. Could even print those gears I suppose...

[MG01]

Love this...one day my prin(ce)ter will come!

[kbrown]

Argh... Looks like I'm having to change my design as I'm not able to achieve focus with my OAG camera (QHY5L-II mono). But before I do so I want to make sure my setup is correct as I've never had and OAG before...

Currently the OAG prism is right after my Baader MPCC Mk3 Coma Corrector. Behind the prism is my fitlerwheel and then the imaging CCD at the correct distance (55mm + 1/3 * 2mm filters) from the MPCC. Does this sound right?

I supposed my OAG sensor needs to be at the same distance away from the centre of the OAG prism? It's practically impossible to measure this the way things are connected mechanically but it feels like I'm not able to get close enough to the prism. My focuser adds about 4.2mm to the distance the way it is at the moment. There might be some room to shorten this distance with a bit of re-design but I just wanted to confirm my setup and assumptions are correct first?

[kbrown]

A little update. I removed the front glass along with its retaining ring/adapter in front of the QHY5L-II mono in order to get it 5mm closer to the prism. That seems to do the trick. I can focus now. Not sure whether that glass is just AR coated or whether it's also UV/IR block?

Anyhoo... Back to the drawing board with the focuser then...

Edited April 3, 2023 by kbrown

[michael8554]

"but it feels like I'm not able to get close enough to the prism."

That's why way back I explained why you should make a focuser like the low-profile Baader you posted an image of ...... :-<

"It's practically impossible to measure this the way things are connected mechanically"

Fairly easy.

With a ruler measure from the middle of the OAG body to where the imaging camera chip is.

Then use the ruler to show the same distance from a guestimate of how low the prism is, then locate the QHY5L-II sensor there.

The OAG focuser will take up the inaccuracy.

Michael

Edited April 4, 2023 by michael8554

[kbrown]

Thanks Michael. I did try to get it low profile but obviously not quite low enough. However all is not lost. I just realised I can slide the T2 thread down a bit along the OAG prism assembly thanks to the TS Optics TSOAG9 genius design. That probably already gives me the millimeters I need but I'm still going to re-design the focuser to make it as low profile I can. Gives me a chance to change a couple of other minor things too.

Edited April 4, 2023 by kbrown

[kbrown]

Now printing this:

Idea is still the same. Modifications include:

• Only adds 0.7mm to the optical path (vs 4.2mm before)
• Actuator screw further away from the body
• 2x Bigger knob for easier operation
• Overall height reduced by about 6mm

I played around with @Chriske's idea of beveled gears to turn the knob 90 degrees to the side but simply just run out of patience to get it done Don't get me wrong, I still like the idea but the bigger knob will do for me. I'm hoping that once I get this in focus I can just lock it in place and forget about it...

 

Edited April 4, 2023 by kbrown

[kbrown]

Attaching the CAD files (FreeCAD + step) here in case anyone else wants to play with them. Enjoy

OAGFocuser_v004.zip

[Chriske]

[Chriske]

Only now that I come to think of it, why do you call this a Helical focuser..? imo it is not a Helical at all. Helical focuser all rotate the eyepiece barrel with a eyepiece in it. There is one exception, and that is the one @Vlad made. It is a helical focuser indeed but Vlad's eyepieces do not rotate.
But the good news is that you just made a completely new type of focuser...😃 ...for which you need to find a new name...😉

[kbrown]

10 minutes ago, Chriske said:

Only now that I come to think of it, why do you call this a Helical focuser..? imo it is not a Helical at all. Helical focuser all rotate the eyepiece barrel with a eyepiece in it. There is one exception, and that is the one @Vlad made. It is a helical focuser indeed but Vlad's eyepieces do not rotate.
But the good news is that you just made a completely new type of focuser...😃 ...for which you need to find a new name...😉

You're right. Helical was my original plan but as I said earlier in the thread I came up with this idea instead after I found the little bearings in one of my surplus parts box. Don't think I can change the subject of this thread any more?

Btw, I just put the new version together yesterday and it works perfect. I omitted the end stop ring so I can actually push the camera right against the T2 thread so nothing is lost in terms of optical path length.

Edited April 6, 2023 by kbrown

[Chriske]

you can, just ask one of the mod's to change the title.

[kbrown]

Ah. Turns out I just had to edit the original post.

[kbrown]

Pretty happy with it now. I put the kapton tape in as three vertical strips this time as suggested by @Chriske. Two layers of it per strip. One layer is on the draw tube and the other one is on the inside of the focuser body so that the smooth and slippy tape surfaces are touching. The bigger knob also makes it a lot less fiddly to operate.