Folded refactor plan

[chrisp9au]

Please don’t laugh.

I just want to know if my idea breaks any rules of physics! :D
There's no such think as a silly question...

They say if you want to know how something works, take it apart.
I’m trying the other way, putting some bits together to see if they work as a unit.

Plan is to make a folded refractor for moon and planetary viewing, as per the attached pic.

I have various bits sitting around, and I feel like tinkering!

1. An 83mm 900mm F/10.8 achromat objective*
2. A ZWO ASI224MC camera
3. A Baader Hyperion Mark IV 8-24mm zoom eyepiece
4. A couple of first surface mirrors*
5. A wooden box, 350mm x 250mm x 170mm

* = quality to be determined!

The eyepiece/camera will be fixed.
Focus will be achieved by moving the objective on a linear guide rail.

Using Astronomy Tools (https://astronomy.tools/calculators/field_of_view/)
suggests that it will give me a reasonable view, especially with Barlow lens.

Total weight about 5kg, which should be OK on my AZ-GTi mount running Synscan Pro.

[leon]

Hey Chris, tinkering is good, ;) I do it all the and love it, you might be onto something, :shrug: but you will never know if you don't have a go.:)

Leon :thumbsup:

[doug mc]

Japanese company Unitron had folded refractors many years ago. Not in the style your planning though.

[Leo.G]

Interesting design but it's not going to be a fun task aligning both mirrors and lens assemblies I would imagine?

[chrisp9au]

I'm thinking a laser collimator will do the trick Leo.

[Tinderboxsky]

A very doable project, Chris. I have had similar ideas a couple of times in the past when interesting lenses have come up for sale. The large IStar lens in the classifieds is an ideal starting point for a folded optics solution!!

Will the unit be for terrestrial or Astronomical observing? I know my body will not let me observe using straight through refractors any more.!

If it was me I would fix a 90 degree diagonal to the box to allow for positioning the eyepiece for comfortable use. The alternative would be to insert a third mirror in the box to achieve the same outcome. The eyepiece would be attached to the top of the box.

I think the best first surface mirrors reflect about 95% of incident light - that is, there is about 5% light loss. Your two mirror design will have light loss of just under 10%. This level of loss is not easily detected in a comparison with a straight through refractor of the same aperture. If the test refractor has a diagonal, then the difference is only 5%, which would not be detectable at all. With my suggestion of a third mirror, the light loss climbs to 14.3%. However if the comparison is again with a refractor with a diagonal, the relative light loss is again just under 10% and not easily detected.

Shall be interested to see this project unfold.

[chrisp9au]

Thanks Steve, astronomical use only, I've got my Orion ST80 if I want to do terrestrial.
I'll be imaging with the ASI224MC, so I'm not concerned about eyepiece location for visual.
If the concept works out, then I may look around for some quality glass.

[RB]

:lol:

[chrisp9au]

Thanks Andrew, missed the pun completely! :P

[RB]

:thumbsup:

Very interesting project Chris.
:)

[OzEclipse]

Hi Chris,

No problem doing that.

Just remember that the errors on the optical surfaces compound. Hopefully your first surface mirrors are good quality. The photo below is of my friend Glenn Schneider, veteran of 35 total solar eclipses. The white box contains a Nikon 5" f12 ED objective. Glenn only used a single flat mirror inside the box so the camera is "pointing the wrong way." From memory, the flat was 3.5"-4" and he ordered a high precision flat from the now closed D&G Optical.

The box was pointed at another high precision 6" flat mounted on a helio stat that kept the suns' image directed down the optic axis. Using two flats, you probably want 1/20 wave flats. But if you have two already, try them and see?

They each need to be a bit bigger than the diameter of the optical cone to avoid vignetting.

Interesting project.

Good luck.

Joe

[Nikolas]

There is no photo?

[chrisp9au]

No photos yet, I'm still in the design stage!

[AlexN]

There was a phenomenal folded refractor on this forum back in the late 2000's... I can't remember who it was... Dietmar perhaps? had a Thomas M. Bach 9" F/9 triplet APO that he built a folded body for... it was an absolute galaxy hunting weapon!


Edit: Found it - it was Dietmar Hager...
https://www.iceinspace.com.au/41-465-0-0-1-0.html

[chrisp9au]

Thanks Alex, there's some good advice and detail in that post.
I'm thinking hard about how I position collimation screws as we speak.
I hope to start tinkering in the next few days. :D

[Leo.G]

I think that "no photo" comment was in regards to OxEclipse's post:





My mention of alignment with mirrors should have mentioned errors compounding as was mentioned by someone else.
It still looks like a very interesting idea and it will be great to see how the build progresses.
Good luck mate!

[chrisp9au]

Latest design graphic, prepared in "SketchUp for Web"
Fabulous resource, and it's free! :)

[5ash]

Why not go the whole hog and make a coude refractor . I remember Frank Hythe having such an instrument in the 1960’s in his Martello tower observatory in the 1960’ ‘s. Perhaps a little
Ambitious. Will be nice to see your progress ,
Regards philip

[chrisp9au]

Thanks Philip, bit over the top for me, especially as I'm just testing a concept at the moment! :P
Nice job on your Meade 8” Schmidt Newtonian dobsonian by the way. :thumbsup:

Did a bench test on the lens and the ASI224MC yesterday to check focal length, as a result I'm adjusting my plan to fit the facts! :D

[chrisp9au]

OK, the concept/idea was good, the optics were not! :P
Both my lens and first surface mirrors are obviously not up to it.
However I have enjoyed the exercise, now to find another one!

[OzEclipse]

Hi Chris,

Sorry, I have not checked back into this thread for a while and didn't see the post to say I forgot the photos.

Here's a couple of photo's of Glenn and his folded Nikon 5" f12 ED folded reflector. The scope only contained one high precision D&G optical 4 inch flat and so the camera looked in the opposite direction to the objective. The box was a little less than half the 60 inch focal length thanks to the camera and the objective sitting outside the box. The objective was removed and secured in a padded holder inside the box for transport.

In the second photo, Michael Gill is helping Glenn align the 400mm telephoto lens & camera on top of the box, with the optics in the box. This served as a finder and a wide field camera.

We positioned ourselves 160km from the end of the eclipse path so the sun was only 2 degrees above the horizon and the box horizontal. Totality lasted 18s and so he didn't worry about tracking. For longer overhead eclipses, he had another 6" high precision flat built into a heliostat that would keep the image of the sun in field of the refractor for the entire duration of the eclipse. In those cases, he did not have the wide field camera & lens mounted on the box.

cheers

Joe

[chrisp9au]

I'm thinking of having another go with this project. :D

Firstly I'm going to acquire some quality first surface mirrors.
I understand 1/20th wave is the best quality, least error, to aim for? :question:

I need a couple, one 65mm diameter, one 35mm diameter.

Any suggestions on the best source for these would be appreciated. :question:

Then, if they show that the objective is poor quality, I'll acquire a better one.

Thanks for looking...:thumbsup:

[Leo.G]

I thought this project was becoming a sun thingie?
Chris perhaps you could discuss mirrors with Stefan Buda, I very much get the impression he's an optical and engineering genius.

[chrisp9au]

It was going to be a sun 'thingie' Leo, but I am so easily distracted!

I get into my box(s) of bits and pieces, find something I've forgotten about, and a new journey starts!
I found a 2" extension piece with a different thread, then found that it fits a helical focuser I also had in the box, so my brain takes off... :P

My sun 'thingie' seemed like a lot of effort to just look at the sun, why not set it up to do more?
Who knows what I'll finish up with!

In the meantime I found a piece of anodized aluminum that could make a decent handle for my refractor...:D

Happy days