I just want to know if my idea breaks any rules of physics!
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.
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.
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.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,344
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.
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!
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.
I think that "no photo" comment was in regards to OxEclipse's post:
Quote:
Originally Posted by OxEclipse
The photo below is of my friend Glenn Schneider, veteran of 35 total solar eclipses.
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!
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
Thanks Philip, bit over the top for me, especially as I'm just testing a concept at the moment!
Nice job on your Meade 8” Schmidt Newtonian dobsonian by the way.
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!
OK, the concept/idea was good, the optics were not!
Both my lens and first surface mirrors are obviously not up to it.
However I have enjoyed the exercise, now to find another one!
I do it all the and love it, you might be onto something, 
but you will never know if you don't have a go.
