1) They are optimized for a given distance from the focal plane. This wasn't much of a problem back in the film days because each camera manufacturer had a fixed distance from the T-ring to the film. However, today, you could end up with any distance given the myriad combination of camera, filter wheel, and off axis guider combinations. Then you have to hope that your threads between all these components match. If they don't, then you have to introduce adapters that also complicate your backfocus.
2) They are optimized for a given field of view. This also wasn't a problem in the film days because all cameras had the same field of view, 35mm. Today, 35mm is much larger than the size of most CCD chips. Since the Ross style correctors actually degrade image quality on axis by introducing spherical aberation, you actually end up with a reduction in image quality and don't really reap the benefits of the coma correction which is taking place far off axis beyond the edge of your CCD chip.
The Lurie-Houghton places absolutely no constraints on your imaging setup beyond the normal back focus requirements.
I will be interested to see how well the scope performs in actual use. I have yet to see how well it maintains its collimation from night to night and as it tracks across the sky. I'll also be interested to see how well it maintains focus during the night and how well I can square the optical axis to the CCD chip. All of these factors must work for the scope to deliver the performance I've posted with that image.
So far, all I have demonstrated is that it is possible to attain the flat field that the design advertises. I have yet to see if it is practical. If everything turns out ok, I think it will be worth it. I don't know of any telescope that you can buy which delivers a flat field, at fast f-ratios, without diffraction spikes.
All I have to do now is find an optical coater that won't charge me more than the cost of an entirely new telescope just to coat the corrector. If I can't, I may just leave the corrector uncoated for now. The effect is barely worse than introducing another mirror in the optical path: 4% reflection at each glass surface surface, 96% transmission, 4 glass surfaces (96%)^4 = 85% transmission.
From: Jeff Rowe <jeffrowe@austin.rr.com> To: Gil McFarlane <gilboastro@yahoo.com> CC: Chris Dalla Piazza <dalchri@hotmail.com>, ATM <atm@atmlist.net> Subject: Re: [ATM] Lurie-Houghton real word picture Date: Sat, 18 Aug 2007 13:00:02 -0500 Chis ;Thank you for your good work and images! A minor heresy, has anyone compared an LHN to the results of a standard Newtonian with a Parracor or Ross corrector?Regards Jeff Rowe Gil McFarlane wrote:Do you feel that the extra weight and effort is worth it to get the performance you have with the LH? - Gil --- Chris Dalla Piazza <dalchri@hotmail.com> wrote:Hopefully this will help anyone who comes after me to make their decision by going beyond spot diagrams. I would still love to see something through a Wright Newtonian._________________________________________________________________
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