NHRob
Post Laureate
Reged: 08/27/04
Posts: 3159
Loc: New Hampshire
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OK you optical gurus ....
I was thinking that, for a fast newt, why not design the ParaCorr elements in as an integrated part of the optical system? Why have it removeable?
The coma is strictly dependent on the primary f-ratio. To correct for coma at the focal plane, the ParaCorr optics do not even "know" what eyepiece is going to be used and I would think it shouldn't matter. It's job is to correct the wavefront, delivered to the focal plane, from comatic aberration.
What's wrong with this thinking?
Why is their a tuneable top for the ParaCorr?
Thanks,
rob
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Rob
**********************
Canon 12x36 IS binos
TEC-140: #433 on CGEM
Faworski orthos (7,10,16.7mm)
stuff ...
no free time ... lots of clouds !
" Life is what happens to you while you're busy making other plans" !!
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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An excellent idea, and one I discussed with Al Nagler. If the Paracorr lens is mounted in a fixed position with respect to the focal plane, simply focusing the EP puts it at the right place with respect to the Paracorr. Tunability is required to accommodate the differing location of the focal planes between EPs. With a fixed paracorr, no eyepiece dependent adjustment is needed as focusing accommodates the different EP focal plane locations. I also spoke the new owner of Feathertouch focusers suggesting he collaborate with TV to make a focuser with a Paracorr element supported in it and fixed with respect to the OTA. They both agreed that it was a sound idea and worthy of consideration. That was about 3 or 4 months ago and I have not followed up since. Perhaps they are working on it.
A down side, of course, is it'd be more difficult to remove the Paracorr element and then compensate for the resulting shift of the focal plane.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3463
Loc: Doylestown, PA
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Its a great idea but has a second down side.
That is the Paracorr elements need to be removed for collimation with a sight tube, cheshire or autocollimator.
I'm sure it could be arraigned so the Paracorr elements could be easily removed from the inside of the focuser and accurately returned.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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Larry Geary
professor emeritus
Reged: 09/24/06
Posts: 732
Loc: NJ, USA
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If the Paracorr is attached to the focuser draw tube, then it moves with respect to the original focal plane but is fixed with respect to the eyepiece, so you still need additional adjustment. Don't you want it in a fixed position before the focuser? In that case, it needs to be larger than 2" to allow the full light cone to pass. It could be attached to a filter slide or a swing arm. There is a custom corrector out there - Wynn I think it's called - that does this, but I don't know anything about it, except that it's expensive.
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The future is not set. There is no fate but what we make for ourselves.
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HandyAndy
professor emeritus
Reged: 01/11/08
Posts: 652
Loc: West Midlands and around
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Hi,
For planets at high powers coma is not an issue and the four elements in the Paracorr will reduce transmission and contrast.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
I was thinking that, for a fast newt, why not design the ParaCorr elements in as an integrated part of the optical system? Why have it removeable?
Sure, that's option A Uncle Nagler brought up when we first chatted about sub f/4 optics late last year. It doesn't have to be a Paracorr then, it can be a dedicated larger system delivering some advantages. But it'll be low production and cost buckets to make, as well as locking the OTA into one particular app.
It needs to be bigger if it's going to be fixed in place, SFAIK. And of course you need to design the OTA to accommodate it. Which means it can't be used without it - and you don't need the Paracorr on a tracking scope for use at center of field (ie, planetary) because the center of the field in any Newt is aberration free.  Although if you get down to around f/3 the disc of Jupiter will show a bit of coma...
Best,
Mark
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 844
Loc: Guilford, Connecticut
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Quote:
An excellent idea, and one I discussed with Al Nagler. If the Paracorr lens is mounted in a fixed position with respect to the focal plane, simply focusing the EP puts it at the right place with respect to the Paracorr. Tunability is required to accommodate the differing location of the focal planes between EPs. With a fixed paracorr, no eyepiece dependent adjustment is needed as focusing accommodates the different EP focal plane locations.
This would work if the Paracorr is fixed in regard to the primary mirror. It would not work if you are going to put it inside the focuser drawtube and have it move in and out.
I'm currently using a scope with a Paracorr and a Lumicon each mounted in the drawtubes. I use parfocalizing rings on my eyepieces to "tune" them to the correct position.
Ethos EPs usage with a Paracorr
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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Steve W
super member
Reged: 09/27/04
Posts: 148
Loc: Dundee, OR
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Quote:
I also spoke the new owner of Feathertouch focusers suggesting he collaborate with TV to make a focuser with a Paracorr element supported in it and fixed with respect to the OTA. They both agreed that it was a sound idea and worthy of consideration. That was about 3 or 4 months ago and I have not followed up since. Perhaps they are working on it.
I've been told by a well known mirror maker that something like this is underway already between TV and Feathertouch. Beyond that I don't know any details about what they are doing, when it will be available or how much.
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Steve
18" f/4.5 Obsession #354 with AN/SC
12.5" f/5 Plettstone ultralight dob
Working on a 22" f/3.3 dob
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johnnyha
Pooh-Bah
Reged: 11/12/06
Posts: 1139
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Wouldn't the permanent Paracorr be potentially sticking out into the light plane?
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Johnny
Spicewood, TX
Sherman Oaks, CA
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
I've been told by a well known mirror maker that something like this is underway already between TV and Feathertouch. Beyond that I don't know any details about what they are doing, when it will be available or how much.
That'll be interesting!
Mark
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cdndob
professor emeritus
Reged: 07/28/06
Posts: 660
Loc: The Great White North
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I gather there'd be no way of mounting it just below the secondary right?
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NHRob
Post Laureate
Reged: 08/27/04
Posts: 3159
Loc: New Hampshire
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Yes, it would be fixed relative to the mirror surface. Thus, not in the drawtube.
Didn't know it was under development. Neat.
There must be a good way to collimate with the elements in place.
Hmmmm ....
Rob
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Rob
**********************
Canon 12x36 IS binos
TEC-140: #433 on CGEM
Faworski orthos (7,10,16.7mm)
stuff ...
no free time ... lots of clouds !
" Life is what happens to you while you're busy making other plans" !!
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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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Yeah, the assertion that in order to collimate, a Parracor would first have to be removed does make me wonder. If any optic is part of the system and properly centered, collimation must be achieveble via some means. But some methods may not be as amenable due to the distortions introduced by aspheric corrector, I should think.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Home-made Mk II RA bino, using interchangeable objectives and eyepieces
My Gallery
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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I think the assertion is wrong, but I don't think it's worth arguing over. 
Best,
Mark
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1165
Loc: Gateshead.NSW Nth Coast,Austra...
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Probably a dumb idea, but it seems to me that ideal location for a fixed paracorr lens is between the primary and secondary mirrors , in fact attached to spider in front of the secondary ....
Only, for this to work there would have to be a range paracorr sizes to accomodate without obstructing the secondary , the sizes of secondarys employed ie 3" , 3.5" , 4" and the like.
I would imaging big paracorrs like that will be expensive beasts and would need special combined secondary+paracorr holders to get the spacing right.
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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Yes, fixed not to the draw tube but as said in my post, "a fixed position with respect to the focal plane."
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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Gene Hunter
sage
Reged: 12/29/07
Posts: 288
Loc: SC
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I am working on an 8" F 5.6 Folded reflector just for fun that has a perforated primary and a large secondary round flat 4", the paracorr will be located inside the primary perforation to extend the focal plane and correct coma, and the camera will be attached to the back of the primary. The setup will be focused by moving the primary and camera and paracorr all as one unit.
Carbon Fiber tube also.
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Clear Skies
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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>>>The setup will be focused by moving the primary and camera and paracorr all as one unit. <<<
Per the previous discussion, the Paracorr is best left fixed at the appropriate distance from the primary or focal plane.
Moving the camera and primary together (optically equivalent to moving the secondary) will make focusing twice as fast as moving either alone. This is usually not considered desirable.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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NHRob
Post Laureate
Reged: 08/27/04
Posts: 3159
Loc: New Hampshire
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The farther you move the corrector elements from the focal plane, the larger they need to be to handle the incoming light beam. I would think that moving them in front of the diagonal adds a lot of complexity and difficulty.
I was thinking that if you could mount the elements just below the focuser, but outside the light path, that would be a great place. One could perhaps build a mechanism, much like a filter slide, for putting it in and out. Of course I think such a slide mechanism would have to be well engineered in order to maintain accurate positioning of the corrector.
Rob
Rob
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Rob
**********************
Canon 12x36 IS binos
TEC-140: #433 on CGEM
Faworski orthos (7,10,16.7mm)
stuff ...
no free time ... lots of clouds !
" Life is what happens to you while you're busy making other plans" !!
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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One other thing to consider is a that a ParaCorr causes undercorrection. This one reason why if you want the best possible image that the primary should be figured to a mild hyberbola.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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