Diy oag-onag

[bojan]

Being in lock-down again, I was thinking to use the time and try to design OAG over weekend(s)...

Something made of machined (on lathe) 2-3mm aluminium plates and tubes, held together with 3 screws and spacers.. Overall thickness could be 14 mm.

There could be a lot of freedom for mirror and camera adjustment between spacers.
Threads should be dimensioned for mounting on C11.

Just an idea...

[Merlin66]

Bojan,

Ideally the prism/mirror should be adjustable in tilt relative to the optical axis, this gives a better guide image.

[bojan]

Thanks for suggestion, this is definitely possible..

BTW, I have couple of mirrors from dismantled scanners, have to find the way to cut them to size.

Also couple of small prisms (from various military optical gadgets and binos) that could be used here.

[Sunfish]

Looks like a great project with a good drawing. It will interesting to see the complete design.

[bojan]

Slightly modified design, using off-the-shelf box: less machining...

[Merlin66]

Bojan,
I assume you meant to say T thread - M42 x 0.75??

[bojan]

Ken,

no, in my case M42x1 is better because I have a number of those extenders (Practica-Zenith-Pentax legacy).. and there is a low profile converter from 1 mm to 0.75 on ebay for couple of $.

[bojan]

First step: machined fitting between SCT and box.
It is actually M39x1 mm adapter (accessory from my MTO 1000A, I modified it long time ago, now it is reused again).
On this goes 39 => 42 adapter ring (2$ from ebay.. for that price not worth the effort to machine a new one).
All this will go into M42 x 1 mm thread on Jaycar box lid (I need to go and buy one now).

Threads are done, now I can attach the spacers for camera.

63.15 mm may not be enough (for FR) but previously mentioned 1 => 0.75 mm converter may do the job.


Also, I have to think how to attach guiding camera (Logitech 4000 Pro), this will require a bit of tinkering.

[Sunfish]

Looks good. Handy to find some off the shelf parts to start.

[bojan]

Yes.. It looks bit like improvisation but it doesn't bother me as long as it performs intended function.

I bought two boxes, another one was supposed to be used for guiding camera enclosure, but that PCB (Logitech 4000) doesn't fit unfortunately.

[multiweb]

Pretty cool project. What's a good source hollow of alu tubes?

[bojan]

I am using Aluminium Trade Centre (https://www.aluminiumtc.com.au/), they have a good product range, and sometimes off-cuts as well

[multiweb]

Thanks mate. :thumbsup:

[bojan]

Now, about back-focus distance.. and absolute confusion around Meade/Celestron 4000 Series FR/FF.

In data sheet that came with reducer, nothing was mentioned about that, they supplied only drawing of un-specified adapters and spacers and cameras..

Standard SCT - 2" adapter + EOS camera adapter (ebay, Alibaba etc.) is 69 mm long.. which means the back-focus for EOS camera is 114.5 mm, measuring from SCT flange.

I reduced this distance to widely accepted/recommended 105 mm value by using low profile EOS adapter.

BUT.. here (post #21) (https://www.cloudynights.com/topic/614745-measured-focal-length-of-63-meade-reducer-210mm/) Meade says backfocus distance should be:

"The backfocus distance on this 6.3 reducer is pretty forgiving. The customer should use a 85mm back focus when measured from the last lens."

I am pretty sure here the intended information was lost in translation.. and "Last lens" actually means "last side" of the FR.. because measuring from "last lens" means measuring from glass surface.. which can result in glass surface damage. And, optical distances are usually referenced to "optical centre" which is a point usually (but not necessarily) located in the space between glass elements.

Anyway.. 85 or 105 mm, that needs to be determined experimentally, and criteria for optimal distance value is not f/0.63 but shape of the star images.
Does anybody have some recommendation about this?

[Merlin66]

Bojan,
The Meade/ Celestron reducer is always an area of contention.
They have been used and abused for years. The "ideal" spacing doesn't seem to exist. 105mm or 115mm or what?
The reducer effect x0.63, x0.6 etc is independent of the optical corrections which may (or may not) be beneficial to the image.
I regularly use the x0.63 reducer for the spectrograph on the C11, but I'm more interested in the effective focal ratio/ length than any "corrections" the lens may give. (Working on the optical axis with a 20 micron slit)


I developed a spreadsheet to give the effective reduction based on spacings for both the x0.63 and the x0.33 reducers. (Measure the focal length of the reducer using the sun's image, just to verify!)


I also found the Baader website had a couple of images showing the effects of generic reducers at different spacings.

[xelasnave]

Great project.
Alex

[bojan]

Thank Alex, I hope for the best :-)

Ken,
My concern at the moment are corrections..
On images of M42 I took some time ago (https://www.iceinspace.com.au/forum/showthread.php?t=173640&highlight=focal+reducer), stars in the centre of the frame are acceptable for both with or without FR.
However, in corners (APS) without FR, there is a bit of coma (which is expected) but with FR they are quite distorted, almost as if field curvature is bad. And that distortion somehow doesn't look anything like Baader images...

I haven't done much about this problem since then (mainly because the investigation of this could be quite tedious and my observatory was still in plans..), but now I would like to sort it properly.

[bojan]

Last night I was looking for other DIY projects, found this webpage..
https://stargazerslounge.com/topic/124645-diy-oag/page/2/
Additional tilt and rotation of the prism may not be needed (PH2 will compensate for field rotation during calibration).
Hm.. need to think about it some more.

[bojan]

Mirror/prism adjustment - concept.

[Merlin66]

The tilt of the prism/ mirror is minimal but important.
Celestron on one of their early OAG just used a tapered pin and a flexible support.

[bojan]

That's what I am thinking of...