Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 844
Loc: Guilford, Connecticut
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Quote:
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.
I think this is true of the Ross type coma correctors. I don't know that the Paracorr under-corrects.
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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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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I am curious..... If the paracorr causes undercorrection was would the ideal conic constant be for the primary? i.e. how do you calculate this?
Say you wanted to make something ridiculous like a f/2 to f/3 scope. You are pretty much resigned to using a coma corrector at that point, why not figure your mirror to best conic to match the corrector?
Quote:
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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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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Quote:
Quote:
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.
I think this is true of the Ross type coma correctors. I don't know that the Paracorr under-corrects.
If you go to the Televue site http://www.televue.com/engine/page.asp?ID=56 and then click on the Performance graphs for the Paracorr you will see that the line for the image spot size with the Paracorr never goes to zero in the center of the field. This indicates that when using the Paracorr that it introduces some spherical abberation. The faster the system, the more spherical introduced. While at or below 1/4 wave it is still present and this will add to any additional undercorrected spherical abberation that the primary has. If one wanted to use the telescope for planetary viewing, it would be better to be able remove the Paracorr so the center of the field has the best correction.
If the Paracorr is to be permanently installed, both coma and total spherical abberation would be better corrected if the primary was 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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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Tests by a few people have shown the current Paracorr interacts with the steep light cone of very fast mirrors (note that it's still converging while passing through) in such a way as to introduce differing amounts of SA with different EPs. With some it's none. Al offered to share the design SA of the Paracorr with me if I wanted to make mirrors corrected to a specific conic that would counteract that amount, but to me that limits the primary, no matter how well corrected, to one optical solution and I don't see that as viable.
In any case you can't get good correction below f/3 (or really much below f/3.5) with the current model - that's the reason for the new one. Mike L. told me Al said they expected to have it in production this next fall, in time for the Winter Star Party. 
In time for my 20" f/3.3 as well. 
Best,
Mark
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Lynnblac
member
Reged: 04/06/09
Posts: 29
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"If you go to the Televue site http://www.televue.com/engine/page.asp?ID=56 and then click on the Performance graphs for the Paracorr you will see that the line for the image spot size with the Paracorr never goes to zero in the center of the field. This indicates that when using the Paracorr that it introduces some spherical abberation. The faster the system, the more spherical introduced. While at or below 1/4 wave it is still present and this will add to any additional undercorrected spherical abberation that the primary has. If one wanted to use the telescope for planetary viewing, it would be better to be able remove the Paracorr so the center of the field has the best correction.
If the Paracorr is to be permanently installed, both coma and total spherical abberation would be better corrected if the primary was figured to a mild hyberbola."
- Dave
No optical system has a zero spot size. In fact newtonians have one of the smallest on axis spot sizes of any telescope.
The Paracorr is aspheric so the on axis light passes through the Paracorr unchanged.
Lynn
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2928
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Quote:
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.
Such a device would also make Jones-Bird designs more feasible.
-drl
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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No optical system has a zero spot size. In fact newtonians have one of the smallest on axis spot sizes of any telescope.
The Paracorr is aspheric so the on axis light passes through the Paracorr unchanged.
Lynn
The Televue graphs shows that the Paracorr does introduce spherical abberation and the graphs show that it is greater in the center of the field of view then the system without the Paracorr, so any on axis image would be effected. Televue has stated this as discussed in the above message.
- 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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GlennLeDrew
Pooh-Bah
Reged: 06/18/08
Posts: 1304
Loc: Ottawa, Ontario, Canada
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I, too am in disagreement with the statement that the aspheric figure of the Paracorr passes axial light unchanged. No matter what the figure, a refractive optic never passes light 'unchanged.' Even a completely planar element, such as a filter substrate, exerts some optical effect.
One could theoretically design a sufficiently complex corrector which both reduces off-axis aberration and field curvature *and* preserve axial definition. But who would be willing to pay for one which could hold its own for the most demanding *visual* application? (Imaging requirements are generally a bit more relaxed, at least for deep-sky work.)
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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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Lynnblac
member
Reged: 04/06/09
Posts: 29
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This is from the Paracorr Tech page in the Televue site. "Life with Paracorr
Paracorr is a universal corrector that tightens and intensifies star images [see performance graphs here] on all f-ratios from f/3.5 to f/8 without adding any false color or spherical aberration! You no longer have to constantly shift a Dobsonian to keep objects centered for sharp viewing. Using a Tele Vue eyepiece, put M-13 at the edge of the field and enjoy resolving its beauty as it drifts across your view."
Again, the on axis image is at least as equal to the uncorrected on axis image.(adding no spherical abberation" We know the Paracorr changes the f/ratio but this is not the issue. It adds no S.A. or C.A .
Edited by Lynnblac (10/25/09 01:47 AM)
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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The Paracorr is designed as a removable accessory for paraboloid Newts. If you decide to commit to an catadioptric system where the sub-aperture corrector is a permanent part of the optical train (a Jones-Bird is one), the designer would be free to vary all parameters; primary figure, number of elements in the corrector, radii of curvature, glasses, thicknesses, and spacing to optimize performance, manufacturability, and cost. Nowadays, this would be done using optical CAD. What you'd be doing is trading off a ladder for increased cost and complexity of the scope. Many would consider this a worthwhile trade.
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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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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 640
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How much spherical aberration is "no spherical aberration"? In this case, as can be read from the curves Al Nagler shows, there is indeed some spherical aberration at low f/ ratios. Even if not much. The same is true for any eyepiece, even if it seems noone ever measures it. Anyway, both with EPs and the Paracorr, spherical aberrarion increases quite rapidly with decreasing focal ratio. So, who knows what spherical aberration you might get when combining a Paracorr and any particular EP? At any rate, the sph abb is possibly insignificant at realistic f/ rates, and never of any interest at low or medium magnifications.
Nils Olof
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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If you look at the graph for the Paracorr used with a f/4 parabola, it shows that at the center of the field of view that with the Paracorr, you are just diffraction limited or at 1/4 wave of spherical abberation. The yellow band at the bottom of each graph is the size of the Airy Disk or the 1/4 wave limit, when the image size is greater then this you are no longer diffraction limited. For low power work which is what the Paracorr is designed for it makes little difference but, if you were to permanently mount the device, you would reduce the performance of the telescope when observing the planets. I understand that this is most likely not what the purpose of a telescope designed with non-removeable Paracorr would be but it something to consider. Both Nagler's published data shows this to be the case and also his statements to Mr Cowan shows that the Paracorr does introduce spherical abberation and by figuring the primary to a mild hyberbola one can correct this.
- 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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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 844
Loc: Guilford, Connecticut
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Quote:
Quote:
Quote:
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.
I think this is true of the Ross type coma correctors. I don't know that the Paracorr under-corrects.
If you go to the Televue site http://www.televue.com/engine/page.asp?ID=56 and then click on the Performance graphs for the Paracorr you will see that the line for the image spot size with the Paracorr never goes to zero in the center of the field. This indicates that when using the Paracorr that it introduces some spherical abberation. The faster the system, the more spherical introduced. While at or below 1/4 wave it is still present and this will add to any additional undercorrected spherical abberation that the primary has. If one wanted to use the telescope for planetary viewing, it would be better to be able remove the Paracorr so the center of the field has the best correction.
If the Paracorr is to be permanently installed, both coma and total spherical abberation would be better corrected if the primary was figured to a mild hyberbola.
Well you're correct. I've looked at those hand drawn graphs by Al many times over the years and never took notice that they don't quite go to zero till you get to F6.
I will say however that at F4.5 they seem to end less then halfway up the airy disk region. So what does that mean 1/8 or 1/16 wave? At F4 one might one might want to consider this in the design but at F4.5 or F5 it's almost insignificant.
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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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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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Jim,
Thanks for taking the time to view the graphs and validate what I have been saying.
- 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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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Hi Nils Olof,
Quote:
So, who knows what spherical aberration you might get when combining a Paracorr and any particular EP?
Both Mike Lockwood and Mel Bartels have tested the current Paracorr at f/3 against a variety of EPs, you could ask them for a summary of what was tried.
As I said Al himself says that Paracorr adds some SA, enough that correcting the primary to a conic slightly different than -1 would take it out completely, however I don't know what that correction is, as it's part of Tele Vues proprietary design. The long thread early this year on CN about coma corrector design covers all the questions asked here recently, as well. You could look at Mike's forum on Yahoo for much of the same stuff. It started out dealing with fast mirrors but now has been repurposed, I guess.
As I understand it currently, the prototype tested by Mike and Al at the Oakie-Tex star party on Mike's 14.5" f/2.55 was coma free for the Ethos 17mm but not for the 21mm. Mike however expressed to me that the performance of the f/2.55 (at 35.7" FL) with the 17mm (covering 1.87 degrees!) was outstanding with the newer Paracorr prototype. The click-positions (tunable top spacing) are critical though and this wasn't fully worked out at the time, so there's more to be wrung from it yet. Considering that it's working at f/2.55...well...
Coma correction really isn't needed on a tracking scope of fast f/ratio - I viewed Jupiter back in August at OSP for well over an hour off and on with a 5mm Nagler and one of Mike's 20" f/3.7 mirrors, with outstanding results. The tracking was off a bit though, after about 15 minutes the ball of the planet had moved enough off center (where there are zero aberrations) to warrant recentering it. What bedevils me is I never thought to look for detail on the moons, specifically. 
This is another reason why I don't see a huge appeal for fixed coma correction in scopes that will serve multiple uses.
Best,
Mark
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 844
Loc: Guilford, Connecticut
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Quote:
The long thread early this year on CN about coma corrector design covers all the questions asked here recently, as well.
That was a superb thread that started out about Mike's scope:
Mike Lockwood 20" f/3 in July 2009 S&T
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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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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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1) Only the owner of a non-driven scope would want to remove the Paracorr to view planets. Every place other than the center, the image would be better with the Paracorr.
2) The Paracorr could easily be fixed in the drawtube of the focuser. Since this would require adjusting the eyepiece-to-lens distance, the solution is simple: make the top of the focuser drawtube a tunable top.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 844
Loc: Guilford, Connecticut
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Quote:
2) The Paracorr could easily be fixed in the drawtube of the focuser. Since this would require adjusting the eyepiece-to-lens distance, the solution is simple: make the top of the focuser drawtube a tunable top.
Some of us have already done this. But rather than a tunable top I "tune" my eyepieces. I use parfocalizing rings on my eyepieces so that I can just slip them in the focuser and they're at the correct location for the Paracorr...and they're parfocal to boot.
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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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Nils Olof Carlin
professor emeritus
Reged: 07/26/04
Posts: 640
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Hi Mark,
Quote:
As I understand it currently, the prototype tested by Mike and Al at the Oakie-Tex star party on Mike's 14.5" f/2.55 was coma free for the Ethos 17mm but not for the 21mm. Mike however expressed to me that the performance of the f/2.55 (at 35.7" FL) with the 17mm (covering 1.87 degrees!) was outstanding with the newer Paracorr prototype. The click-positions (tunable top spacing) are critical though and this wasn't fully worked out at the time, so there's more to be wrung from it yet. Considering that it's working at f/2.55...
The original Ross corrector at Palomar Mt worked at f/3.3.
But I wonder about those Ethos EP-s - coma originates in the primary and will appear (or be suppressed) at the focal plane independently of an EP.
There are marketed other correctors (see Wynne) with different complexities and properties. The Paracorr has no doubt done faithful service, but a new addition from Uncle Al will be welcomed, I am sure.
Nils Olof
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