Having a hard time finding a particular spacer...

[UlteriorModem]

Just got a new ZWO OAG and have it all put together. But it's overall spacing is 55.5mm. Just a tad too long.

What I need is an M42 male thread to M48 female thread less thickness than the 16.5mm ones I have. 12-15mm would be perfect!

Any leads?

Thanks!

[KathyNS]

Have you tried Precise Parts? They can be pricy, but they can whip up any adapter.

[UlteriorModem]

I don't know if it will come to that but thanks Kathy. I would hate to pay 50 bucks just to shave off a few mm.

Besides I looked it up and evidently I am within tolerance for the focal reducer of 51.5mm +/- 5mm. So I am 'inside' by 1mm

I saw some adaptors that were 8mm but that would be too thin as the Camera in the OAG would hit the imaging camera.

[JimMinCT]

Why not just sand off the 5? It will come off so quickly, you won't believe it.
I ground off some of my adapter that was too long. As long as the threads go in far enough to be there once it's to length, you should be all set.

[UlteriorModem]

That's not a bad idea! and I have 2 of the 16.5mm adaptors so if I botch it somehow I sill have another.

[JimMinCT]

Just use a block of wood to keep it square while you're doing it and take your time.
You may want to do some, check for improvement and then continue.

[UlteriorModem]

I have access to a belt and or disc sander. Also a large collection of sanding blocks in all kinds of grades. Should be easy!

[JayTee]

I have access to a belt and or disc sander. Also a large collection of sanding blocks in all kinds of grades. Should be easy!

Let us know how it turns out, please.

Thanks,
JT

[UlteriorModem]

Well I can report it worked just fine!

I did it manually with a sanding block and 220 grit wet/dry sandpaper. It probably took about 1/2 hour of sanding to remove 1 mm. I just sanded in a circular motion, rotating the ring occasionally. Came out nice and square and flat. Lots of breaks and checking with the calipers. In real time I spread it over two days of work.

After that I washed it with warm soapy water to get rid of the aluminum dust. Then taped it all up to protect the threads, and painted the raw metal with satin black, just so it would look pretty.

My spacing now calculates out to 53.5mm which is well within the 51.5 + / - 5mm. The camera itself has 17.5mm of back focus which makes it a bit of a challenge.

The ZWO OAG works out great with the M48 threads allows me to put the filter behind the prism. No more issues with low signal on the guide camera with the narrow band filter It then threads directly onto the field flattener for a nice solid setup. Now when I go to store it I remove the FF and the rest of the assembly in one piece. Very easy to get it indexed between sessions!

20200126_131916 by Tom Whit, on Flickr

20200126_131932 by Tom Whit, on Flickr

Previously I had some stars way out on the corners / edges that were showing some coma. Now... well I don't know, wont have a chance to check it until Wednesday with any luck.

[JimMinCT]

Glad it worked out for you!

[yobbo89]

get a lathe tom

[UlteriorModem]

get a lathe tom

I have access to one of those too. I just did not feel like driving over to the shop. I just sanded away while watching Netflix

[UlteriorModem]

Well had a chance to test it and I need to shave off more or find some shorter rings.

Here is a star field I shot. Click it a couple of times to zoom in and look at the corners.

Field 1 by Tom Whit, on Flickr

Here is what ccd inspector had to say about it.

Curve 4 by Tom Whit, on Flickr

Here is a measurement before the shortening of about 2mm. There was some improvement!

Curve 1 by Tom Whit, on Flickr

[Bigzmey]

Well done Tom!

[STEVE333]

Well had a chance to test it and I need to shave off more or find some shorter rings.

Here is a star field I shot. Click it a couple of times to zoom in and look at the corners.

Field 1 by Tom Whit, on Flickr

Here is what ccd inspector had to say about it.

Curve 4 by Tom Whit, on Flickr

Here is a measurement before the shortening of about 2mm. There was some improvement!

Curve 1 by Tom Whit, on Flickr

If 1mm got the curvature from 41mm to 31mm maybe you need another 3mm to get the curvature to zero? Just a guess.

Congratulations on your approach.

Cheers,
Steve

[UlteriorModem]

Well done Tom!

Thanks, I plan on taking one of the rings to the shop and shaving it down to 12mm, another 3 mm. Then test that.

I cant shorten it much more than that because the OAG body would hit the camera body.

The back focus of the ASI071mc has proven to be a real challenge at 17.5mm. That's a rough starting point! Add to that the OAG at 16.5mm, and the filter width of 5mm.

Anyhow plan to go shave one down to 12mm and 'might' get a chance to test things out on Sunday. Stay tuned

[UlteriorModem]

I took the ring over to the shop and chucked it into the lathe. Only took a few minutes to turn it down to 12mm.

Once installed I found I overshot the reduction spacing which is now at 50.5 mm with is inside the 51.5 stated requirement for the Field Flattener. But hopefully it will be close enough.

Unfortunately it is not supposed to clear up until after midnight tonight. So maybe tomorrow (Sunday) I might get a chance to test it.

[STEVE333]

Hope it works Tom. You have put a fair amount of work into this.

Fortunately you have the spare spacer if you need it.

Steve

[JimMinCT]

I took the ring over to the shop and chucked it into the lathe. Only took a few minutes to turn it down to 12mm.

Once installed I found I overshot the reduction spacing which is now at 50.5 mm with is inside the 51.5 stated requirement for the Field Flattener. But hopefully it will be close enough.

Unfortunately it is not supposed to clear up until after midnight tonight. So maybe tomorrow (Sunday) I might get a chance to test it.

It's super easy to overshoot the mark if you don't use the lathe every day.
Hard to believe just how little it takes to remove the Al.
Good luck on the final figuring.

[UlteriorModem]

Thanks, the over shoot was more of a mistake in my math That and the final finishing took a little more off.

If I have to I will purchase another ring and turn it down. But I think the 40.5mm spacing should be okay.