Lens in front of secondary mirror on Celestron Newtonian?

[Shed9]

As the title says, I have an older Celestron Newtonian, a C-8N which like most C8N's is a 200mm / 1000mm f5. This one however is considerably shorter than a conventional C8N and it measures around 50cm (20") in length. I do happen to have a standard C8N to compare it to which measures around 92cm (3') in length. This short tube version has been sat on a shelf for about a year, I bought it with some other astro kit which was connected to an ebay auction winning bid. I just assumed that someone had cut it down in a failed experiment but today whilst sorting out some old scopes to sell on, I tried looking through it (albeit terrestrially) for the first time with a 24mm EP and the image was perfectly sharp, in fact it was incredibly good. Anyhow, when I looked at it further I noticed there is a small lens in front of the secondary mirror, obviously doing some form of optical correction for the shorter length.

The scope is a David Hines delivery so I know it had a little more love on its dispatch than the usual scope vendors of that era.

I've not seen anything like this before and can't find anything online about it. Any ideas what this is? I've taken a picture of it as best I can, you can make out the lens holder and the glass lens in the middle of the image.

 

Edited December 16, 2022 by Shed9

[Cornelius Varley]

Its a Jones-Bird reflector, possibly a G-8N Celestron 8″ G-8N Reflector (telescopeinfo.com) . The cheaper Jones-Birds normally place the corrector lens in the focuser.  

[Shed9]

Of course it is, I knew they do this design for the smaller newtonian's, not seen it on an 8" before. Why was this not obvious to me?

I don't do much with newt's but have to say, despite the usual negative comments on bird-jones, this is actually quite nice.

Off to Ebay it is though.

Edited December 16, 2022 by Shed9

[Peter Drew]

The Jones-Bird design utilises a "fast" spherical primary mirror, a corrector lens system prior to the secondary mirror is intended to correct the aberrations of the primary and also increase the effective focal length by around 2x.  The design offers an inexpensive system and a reduced physical length.  The success of this arrangement relies on the quality of the optics and their relative alignment.  Many reports suggest that this falls short of ideal in the mass produced models.     🙂