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bang-for-buck coma correction in f/5 Newtonian

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#1 Jay_Bird

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Posted 12 December 2012 - 02:46 PM

The good news is I'm quite pleased with a 'clearance' Meade LXD 75 6" Newtonian OTA. With the extra aperture and 2-inch focuser, it's pretty much all the "more" I hoped for when I tried one of the PortaMount packages some years back and kept the Porta for my short tube refractor, and sold the 130mm reflector OTA after a few months use with 1.25" eyepieces.

The bad news is that it's such a nice blend of aperture and Porta-Mount size, I'm now tempted to get a coma corrector accessory.

With 27 Panoptic and especially with 17 T4 Nagler, I'm pretty sure I'm seeing coma well in from the edges of 2° - 2.5° field of view.

I never studied up on Paracorrs while using a 1990's 10-inch f/6 Dob. Now I only see a Type 2 Paracorr 'tunable top' selling new for almost $500.

There are some other coma correctors: one by Vixen that is listed for use with 150 and 200mm fast Vixen reflectors for under $200, and the Baader MPCC for about $200.

The Baader MPCC reviews list spacer-ring adapters (with ring combinations specifically for the 27P and 17N) sold as a kit for about $80. The Baader + spacers kit look like a direct fit into 2-inch focusers with the MPCC first, then rings and eyepiece all as one unit threaded together. The rings seem to offer an equivalent to the 'tunable' idea.

For a wider sharp visual field with the 6" f/5:

is the Vixen lowest-cost choice threaded for the 'generic' Chinese 2" focuser of the Meade OTA to fit as a thread-on under the eyepiece holder, and will it work well at f/5?

is the Baader MPCC + adapter rings the best value, and is it really a plug-in fit to 2" eyepiece holders?

Does TV make a 2" paracorr that is less modular (and expensive) than the current Type 2?

Thanks in advance!

#2 star drop

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Posted 12 December 2012 - 02:56 PM

Currently Televue only makes the type 2 Paracorr. The type 1 Paracorr does come up on the used market every now and then.

#3 Starman1

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Posted 12 December 2012 - 03:28 PM

Simple, 2-element, coma correctors like the Baader MPCC are intended for photography. Visually, they add spherical aberration to the center of the image, and you wouldn't find that acceptable, especially at high power.

More complex, 4 or 5 element coma correctors correct better but cost more to make.
And if you want the unit to correct all your eyepieces equally well, then complications set in. The focal plane of the eyepiece has to be a set distance from the coma correcting lens to provide the best correction.
The 5-lens TeleVue Paracorr has a helical top that can easily be adjusted for each eyepiece.
The 4-lens Astrotech coma corrector doesn't have the helical top, so setting the correct distance for each eyepiece is a matter of spacers, parfocalizing rings, o-rings, or whatever.
The method for determining the correct setting for an eyepiece is the same in both, just a lot easier on the Paracorr.

You dial one eyepiece in by moving it in and out relative to the lens, then refocusing, until the coma correction is at its best. You set every other eyepiece relative to that corrected eyepiece (without moving the focuser) by finding the position where the 2nd eyepiece comes to focus by sliding it in and out of the coma corrector. The in-focus position for the 2nd eyepiece is its correct distance from the coma correcting lens. It's easy on the Paracorr by simply dialing the top in and out. On the Astrotech, you'll have to slide the eyepiece in and out until it focuses. Note the distance from the eyepiece "shoulder" to the coma corrector and install a spacer on the eyepiece to hold the eyepiece at that distance.

In essence, the coma corrector parfocalizes all your eyepieces. The Paracorr does it with a tunable top; the AT does it with user-selected spacers, rings, etc.

You could look for a used TeleVue Paracorr I, or get the AT and play with spacers (parfocalizing rings like those from FarPoint or ScopeStuff work fine). Either way, coma will be a thing of the past. At f/5, coma is completely banished.

#4 cjc

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Posted 12 December 2012 - 05:43 PM

Availability of the AT, Astro-Tech has been intermittent. The GSO is the same product.

#5 Jim Romanski

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Posted 12 December 2012 - 06:19 PM

Don has done a very good job of explaining the differences in coma correctors.

You could look for a used TeleVue Paracorr I

I think this is what I'd do if I were in your position. There have been several different iterations of the Paracorr (Type I) over the years. Prices for used units would probably be between $125 to $250 depending on the condition and vintage. The earliest units did not have a tunable top (though they made/make an add on tunable top). The next version worked fine until the 31mm Nagler came along and needed a larger aperture Paracorr so they made one with larger lenses. The last version of the Type I had nicer hardware.

A used Type I Paracorr would certainly be the simplest low cost solution.

#6 Jon Isaacs

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Posted 12 December 2012 - 06:55 PM

A used Type I Paracorr would certainly be the simplest low cost solution.



:waytogo:

I believe I saw a couple on Astromart yesterday.

Jon

#7 Jay_Bird

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Posted 12 December 2012 - 07:20 PM

Thanks all - Don especially. A Paracorr is in my future!

The 'parfocal' aspect makes sense - thanks for explaining.

I can see why money spent for a Paracorr with tunable top, even if used with only two or three 2" eyepieces, would save observing time vs. fiddling time with spacers or rings on a different corrector system.

#8 Tim Gilliland

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Posted 12 December 2012 - 09:56 PM

I got one with the tuneable top right here on Cloudy nights in the Swap and Shop for Ep's a few months back.

#9 cjc

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Posted 13 December 2012 - 08:48 AM

For the AT/GSO coma corrector, you do not need to fiddle with rings in the field. Roger Ceragioli, the optical designer, has written that the spots look good within +/-10mm of the nominal 75mm back focus. I have a compromise position which covers all my eyepieces. There is a review and guide here.

#10 Starman1

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Posted 13 December 2012 - 10:28 AM

Chris,
If you use the technique of focusing the eyepiece by pulling it out of the coma corrector once you have it set perfectly for one eyepiece (using parfocalizing rings to fix the eyepiece at the distance where focus is obtained), you won't have to fiddle with spacers at all from that point on. Each eyepiece will automatically be at the optimum distance when inserted.
And you will be +/- 2mm from the perfect setting instead of +/- 10mm.

#11 cjc

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Posted 13 December 2012 - 10:35 AM

Don,

Noted and thanks.

#12 tezster

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Posted 13 December 2012 - 11:33 AM

I keep hearing rumors about ES working on a coma corrector - if this is true, does anyone have any details regarding this product?

#13 Jim Romanski

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Posted 13 December 2012 - 11:33 AM

While above I recommend a used Paracorr I should mention that for my big scope I have fixed coma correctors in focuser draw tubes and use parfoclizing rings to achieve the proper eyepiece placement. This isn't the lowest cost method and it requires some work and measurement. Once it's setup it's the simplest. But I don't recommend it for everyone.

I have a scope with two focusers. In one (a Feathertouch) I have a Universal Paracorr that I machined off the lip in a lathe. In the other (a Moonlite) I have a Lumicon coma corrector. Both are held in place with set screws. The 31mm Nagler and the 21mm Ethos have focal planes up above the point where the 2" barrel meets the eyepiece. So I have the coma correctors setup to have the proper spacing between them and the focal plane of these two eyepieces. All other eyepieces have parfocalizing rings and sit out from the focuser top at different precise distances. To do this I setup the 31mm Nagler at focus then one at a time put each of my eyepieces in and marked the how much above the top of the focuser they sat to come to focus.

You can now buy the Feathertouch SIPS that incorporates the Paracorr and makes doing all the things that I did unnecessary but it costs about $900. Not to mention that I like using the Lumicon in my Moonlite focuser with the 31mm Nagler. Yes it will induce spherical abberations that you can see at high power. But I wanted to get as much FOV out of my 31mm as I could without the 15% Paracorr hit on it (Don and I have debated this point before).


The beauty of the ole Paracorr tuneable top it that you can take it and use it in any Newtonian that you have and adjust it for just about any eyepiece. So I still think it's the low cost simplest solution for the OP.

#14 dan_h

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Posted 13 December 2012 - 01:11 PM

For the AT/GSO coma corrector, you do not need to fiddle with rings in the field. Roger Ceragioli, the optical designer, has written that the spots look good within +/-10mm of the nominal 75mm back focus. I have a compromise position which covers all my eyepieces. There is a review and guide here.


How does the coma corrector fit with respect to the focal plane of the scope without it? The reason I ask is because it would seem you have to have adequate focuser travel to properly place the corrector optics once any spacer rings are added to it. If a 28mm spacer is added, does it require that the focuser has to come back the 28mm?

dan

#15 cjc

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Posted 13 December 2012 - 01:45 PM

No. You sink the whole assembly with spacers (70mm or so) into the focuser tube, however it still does use 10mm or so of in focus. I cover this in my review, referenced above.

#16 Starman1

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Posted 13 December 2012 - 02:46 PM

I keep hearing rumors about ES working on a coma corrector - if this is true, does anyone have any details regarding this product?

The other thread running on coma correctors mentions that ES currently has no production scheduled for a coma corrector, though they've thought about it.
That means it's a long ways out, IF they decide to do it.

#17 Starman1

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Posted 13 December 2012 - 02:59 PM

For the AT/GSO coma corrector, you do not need to fiddle with rings in the field. Roger Ceragioli, the optical designer, has written that the spots look good within +/-10mm of the nominal 75mm back focus. I have a compromise position which covers all my eyepieces. There is a review and guide here.


How does the coma corrector fit with respect to the focal plane of the scope without it? The reason I ask is because it would seem you have to have adequate focuser travel to properly place the corrector optics once any spacer rings are added to it. If a 28mm spacer is added, does it require that the focuser has to come back the 28mm?

dan


In practice, the coma corrector will require some in-travel of the focuser to put it in the right place in the optical path.
Since large, long focal length, 2" eyepieces often have their focal planes farther up inside the barrel than small eyepieces, it's probably best to "set" the coma corrector with one of them. One caveat: the Paracorr I doesn't perfectly correct coma in a Nagler 31 or an Ethos 21, so I wouldn't use either of those to determine the optimum setting of the coma corrector.

The large 2" eyepiece will probably sit all the way into the coma corrector (we're talking the GSO/AT one, here) when coma is best-corrected. That means most, if not all, other eyepieces will need to have somewhat more distant settings for best correction.

But, as I said, once you have one eyepiece in the best setting for coma correction, every other eyepiece can simply be slid into and out of the coma corrector until it's in focus, and that will be the best setting for that eyepiece. You can do this without adjusting the focuser on the scope at all. That's why parfocalizing rings on the eyepieces make sense. Perhaps the perfect setting for an eyepiece is pulled 3/8" out of the coma corrector. Well, you could add a 3/8" spacer to the barrel, or simply move a parfocalizing ring down until it touches the coma corrector. The next time the eyepiece is inserted, it will stop at the ring, which means it will be in focus and it will be at the right setting to best correct coma for that eyepiece.

In the field, you might still have to tweak the focuser a tiny bit to achieve the best focus, but the total amount of in and out on the focuser to focus all eyepieces won't be more than a couple millimeters.

And, if you get a new eyepiece, you can simply repeat the process to find the best setting for that eyepiece.

If using a Paracorr, you focus using the tunable top and write down the setting for that eyepiece. Doing that, you could quickly find the proper settings for a big box full of eyepieces (and there are many here on CN that have a big box full of eyepieces!)

#18 Jim Romanski

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Posted 13 December 2012 - 05:20 PM

One caveat: the Paracorr I doesn't perfectly correct coma in a Nagler 31 or an Ethos 21, so I wouldn't use either of those to determine the optimum setting of the coma corrector.

I think what your are saying is that the Paracorr Type I tuneable top does not have a low enough setting to position either of these two eyepieces with the proper spacing between the corrector lenses and the eyepiece focal plane.

It certainly has enough apperture and will properly correct coma for them if you can achieve the correct spacing. But you won't be able to do it with the tuneable top. I wonder how close it gets.

Televue states that the Paracorr requires 55mm spacing +/- 4mm between the closest lens surface of the corrector and the focal plane in order to obtain optimal coma correction. I haven't experimented myself but I wonder how much coma you'd see if you were +/- 10mm of optimal spacing on a Paracorr.

#19 Starman1

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Posted 13 December 2012 - 05:49 PM

One caveat: the Paracorr I doesn't perfectly correct coma in a Nagler 31 or an Ethos 21, so I wouldn't use either of those to determine the optimum setting of the coma corrector.

I think what your are saying is that the Paracorr Type I tuneable top does not have a low enough setting to position either of these two eyepieces with the proper spacing between the corrector lenses and the eyepiece focal plane.

It certainly has enough apperture and will properly correct coma for them if you can achieve the correct spacing. But you won't be able to do it with the tuneable top. I wonder how close it gets.

Televue states that the Paracorr requires 55mm spacing +/- 4mm between the closest lens surface of the corrector and the focal plane in order to obtain optimal coma correction. I haven't experimented myself but I wonder how much coma you'd see if you were +/- 10mm of optimal spacing on a Paracorr.


It gets within 1 click of perfect, i.e. 1/8" (0.125") for the 31N and 21E. And the coma correction, while not being perfect, is very very good. So being 1/8" off is certainly not a killer of quality. The amount of residual coma is still less than an f/7 scope (to my eye) without coma correction, so it's pretty darned good.
But, if you are trying to dial in the perfect setting for eyepiece X, you want to start with an eyepiece whose perfect correction is within the range of the Paracorr's tunable top. That way, the only eyepieces that won't come to perfect focus by merely rotating the tunable top will be those few whose perfect coma correction settings are outside the range of variation. If it's too far out, no big deal--just add a barrel extender to the eyepiece and maybe add a parfocalizing ring.
If it's too far in, set the Paracorr at the inmost setting and just enjoy the views.

The Paracorr II goes in a little farther, and the 31 Nagler and 21 Ethos do get to the fully-corrected position.

When an anastigmatic eyepiece is at its perfect setting to eliminate coma, the view is better than a refractor--no chromatic aberration, remember, larger aperture, and tight pinpoint star images from edge to edge.

As a point of language, is a newtonian reflector with a coma corrector really a catadioptric scope?

#20 Jim Romanski

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Posted 13 December 2012 - 06:05 PM

As a point of language, is a newtonian reflector with a coma corrector really a catadioptric scope?

Hmmm...you're probably right about that. Better be careful or the moderators will come along and move this thread over to the land of C8s. :Uh:

#21 Scanning4Comets

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Posted 13 December 2012 - 07:00 PM

In practice, the coma corrector will require some in-travel of the focuser to put it in the right place in the optical path.


Is this the case with most eyepieces in a Paracorr? If so, that would fit the bill for me because my focuser drawtube is almost ALL OF THE WAY OUT for all of my eyepieces!

#22 Starman1

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Posted 13 December 2012 - 07:56 PM

Wherever the Paracorr comes to focus, and it's usually about 1/2" in, all your eyepieces will focus there because differential focuser position is taken up by adjusting the tunable top.
The Paracorr, in a way, parfocalizes your eyepieces.






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