29 replies to this topic

[Terra Nova]

I think the reason for the smaller rear element is because the front element is very steeply curved and the exiting light tapers very quickly from 110mm to 100mm in just 12mm.

I’ve seen some copy lenses like that. Maybe it was a prototype using some surplus lens system designed for something else? That is also not unlike some wide-angle camera lenses like some used in aerial photography. The early Zeiss Tessar design also has a wide spacing and a smaller rear element affording a very fast system.

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[Jon Isaacs]

Bear in mind his Orion was an ED scope -- FPL-51, and a 110mm doublet at F/6, so not exactly color-free, but ED just the same.  It would have less CA than any achromat.

 

I believe the Orion 110mm ED was F/7 rather than F/6.  William Optics had a 110mm F/6 with FPL-51 class glass that many used minus-violet type filters with.

 

Jon

[Jon Isaacs]

I thought that measurements had shown it to be closer to 114mm.

 

 I never measured one but my recollection is that most measured them at around 120mm.  I should have measured the one I had but didn't.  

 

Jon

[Terra Nova]

 I never measured one but my recollection is that most measured them at around 120mm.  I should have measured the one I had but didn't.  

 

Jon

Jon, here’s the thread that I was remembering. Based on the results of tests by several posters including yourself, it looks like you’re a bit closer to right than I am. The consensus seems to be 118mm. It’s a shame that after all these years, they still haven’t corrected the baffle and primary size to yield an effective aperture that is near equal to the stated aperture.

[sunrag]

Still working on the collimation but with little progress.

First, I shimmed the gap between the cell and the lens elements with thin pieces of paper, so that the elements are better centered. They were not bad to begin with as the gap was not very much.

 

Then I put the OTA in a mount and focused on an artificial star. The defocused image looks like this, with a large asymmetrical flare.

 

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Then I slowly rotated the OTA within its rings, and noticed that the flare moved in the same direction with the rotation. So this tells me (i think) that this a tilt issue of the cell.

 

Since the cell is a screw-in type it is not collimatable. But to get some way to adjust the tilt, I loosened the cell from the OTA (a couple of turns) so that it droops a little bit due to gravity. I then rotated the OTA within the rings as before hoping that at some point the star image will propitiously get better as gravity changes the tilt. But that did not happen. I also tried to tilt the objective by hand carefully, but still the flare refused to go away. 

Edited by sunrag, 03 May 2025 - 06:44 PM.