Compact Gregorian anyone?

[billdan]

Came across this on CN today, so I thought I would share it here.

The Gregorian telescope uses a doubled mirror primary and a flat secondary making it possible for the OTA length to be 1/7th of its focal length. Imagine a 2100mm focal length scope the size of a 12 inch ruler.

I note the article was written in 2004 so I have no idea if these ever went into full production.

https://www.photonics.com/Articles/Compact_Telescope_Design_Suited_to_ Military_and/a19919

Cheers
Bill

[Wavytone]

Hi Bill,

That it appeared in Photonics virtually confirms that example is for something like a military tracking or targeting system, either infrared, or a non-imaging application (laser cannon or photometry) where extremely compact size was required but diffraction-limited performance is not require, so aberrations are pretty much irrelevant. Could also serve as the beam-expander for a high-powered laser cannon where refractive elements would explode as a result of the thermal stress from the beam.

If it had been for diffraction-limited imaging applications it would have appeared in Applied Optics. However I'm dead certain that configuration isn't new - there are a number of papers in Applied Optics concerning 2 and 3-mirror and double-bounce designs that appeared circa 1978 - 1981 that went through all the possible configurations. I can probably find the reference. The motive at the time was that NASA funded several studies to investigate alternative designs for the Hubble optics.

This configuration appears in one of those papers but no-one would have attempted to make one at the time for a simple reason - while it can be corrected for spherical aberration the off-axis aberrations will be atrocious - notably coma (3rd and 5th order) and astigmatism.

As with all Gregorian designs - the aberrations of the concave secondary mirror (in this case the inner soupbowl ground into the centre of the primary) add to those of the primary mirror - instead of more-or-less cancelling them - which is what happens in a cassegrain thanks to the convex secondary.

That the aberrations of a convex secondary mirror can cancel those of a concave primary was the motive for the Loveday folded Newtonian which appeared in the June 1981 copy of S&T, it employed a double bounce off a Newtonian parabolic primary (no central hole). While it was novel - and a couple were made - the reality is that a 2 mirror classical Cass is easier and performs just as well. The point at that time is that the central zone of the primary is the best polished, and should have the best figure.

If you have access to the computer-controlled precision grinding machines used in China these days, I'm sure you could make a variant of this by regrinding and polishing the central area to a different radius to achieve pretty much any focal ratio you like, and ideally fold the beam out sideways to make it a Nasmyth configuration - starting with a 20" mirror you could easily design an altaz scope (BTW cannibalise a Skywatcher AZEQ6 for the electronics) with a seat you could ride on - with you as the counterweight hanging off the altitude axis.

Another much easier idea if you have a big primary mirror is to put a circular flat of diameter say 33% central obstruction above it and bring the newtonian focus down towards the primary as in the sketch in the second post in this thread https://www.cloudynights.com/topic/562024-folded-newtonian-worth-the-trouble-issues/

[billdan]

That's interesting Nick, I never heard of the Loveday folded Newt before, interesting concept. I agree there is no free lunch when it comes to telescope designs, they all need an optical corrector somewhere.

The company FSONA who wrote that folded Gregorian article, produces high speed point to point private data networks. They use a 350mW, 1550nM Laser beam as the carrier wave for the data to be added on top. It's Full Duplex so there is a Laser/Receiver at each end of the network.

There is no mention of this Gregorian on their web site though.

Bill