NHRob
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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
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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
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Reged: 02/13/07
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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
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 8" VISAC: (GP2)
Car: 6" F5 MPCC: SP, 5" 127mm F7.5 (GP2)
TV 55mm, Paragon 40mm, UO Pretoria 28mm
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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: 1300
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
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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
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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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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: 1300
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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