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Captain Magenta

Televue Paracorr 2 & Panoptic 35mm

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If you follow the thread What Did the Postman Bring? (esp @PeterW who requested a report), you may have noticed that I received a new Televue Paracorr Type 2 Coma Corrector yesterday, for use with my SW 300p f/4.9 newt. And what's more, the very day it arrived was forecast to be a perfectly clear night.

Amongst my eyepiece collection I have a Televue Panoptic 35mm, which in theory was for use as a widefield eyepiece but whenever I have used it, such as to show the Pleiades to my neighbours last weekend, I've been disappointed by the severe coma-stars starting not far from centre-field. I've even been considering selling the Pan 35 for that reason, and have rarely used it. So I was especially curious to see how much of a difference the Paracorr would make.

I set up last night as usual, without the Paracorr to begin with to have a direct front-of-mind point of reference, and after alignment I slewed straight to M45. Sure enough, the stars were spangly and the view was annoying.

I put in the Paracorr, rotated the slider to setting "G" as per instructions for the Pan 35, inserted the eyepiece itself, and with a sense of trepidation I took a look.

I no longer plan to sell the eyepiece.

The view was suddenly wonderful. A field of extremely bright, pinpoint stars all the way to the edge, just as M45 should be. Sure some diffraction spikes on the very bright ones but I don't mind those. I went on a tour of open clusters, M37 with its central red bright star, M36, and a host of fainter but no less more-ish clusters in the vicinity, using my Nexus DSC and its brilliant "Tour" function set to "within-10-degrees".

It's expensive but it has completely transformed wide-field viewing with my medium-sized newt.

Cheers, Magnus

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Edited by Captain Magenta
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Did it require any additional in-focus relative to the eyepiece alone?  My GSO CC requires 11mm of in-focus.  As a result, neither my 25mm Paradigm (Starguider) or S-W 5-8mm zoom will come to focus with it (I have a very low profile focuser optimized for an undersized secondary).  I've read it is anywhere from 10mm to 19mm.

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Nice report. I've had a paracorr 2 for a few years and always use it with my f4.6 dobsonian, it does exactly what it says on the tin. 

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Very interesting report - thanks for posting it :thumbright:

It is interesting to note the differences in coma between an F/4.9 newtonian and an F/5.3 (which I have). I guess there will be some present at F/5.3 but even with my 100 degree AFoV eyepieces I don't find that it is noticeable.

Good to know that the Paracorr will do the job if I do start to notice it :smiley:

Edited by John
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Nice report and glad it works so well.
A paracorr is on my want list, so interesting.

Having just bought a Nexus DSC for my dobson, also of interest too,
is that running with a SW AZEQ6?

 

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2 hours ago, Louis D said:

Did it require any additional in-focus relative to the eyepiece alone?  My GSO CC requires 11mm of in-focus.  As a result, neither my 25mm Paradigm (Starguider) or S-W 5-8mm zoom will come to focus with it (I have a very low profile focuser optimized for an undersized secondary).  I've read it is anywhere from 10mm to 19mm.

It did, I think. I thought initially it could simply replace the Baader 58mm extension on my Steeltrak focuser, but by doing that and sliding the Paracorr all the way in I couldn't reach focus. Luckily it has a very long nosepiece, so I just drew it out a bit and achieved it. Looking closely at one of my pictures, it does seem like about 10mm. I'll measure and compare properly in due course.

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33 minutes ago, John said:

Very interesting report - thanks for posting it :thumbright:

It is interesting to note the differences in coma between an F/4.9 newtonian and an F/5.3 (which I have). I guess there will be some present at F/5.3 but even with my 100 degree AFoV eyepieces I don't find that it is noticeable.

Good to know that the Paracorr will do the job if I do start to notice it :smiley:

Funnily enough the OO mirror I ordered during Lockdown has just been completed and is about to be delivered. It is 1590mm apparently and is intended to replace my SW 1500mm, turning my f/4.9 into a f/5.2 - the comparison will be interesting. Though I'll need to do a bit of tube-surgery to install it. In time I'm going to order a custom carbon tube for it.

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35 minutes ago, Alan White said:

... Having just bought a Nexus DSC for my dobson, also of interest too,
is that running with a SW AZEQ6?

Yes I drive my AZ-EQ6 with it. It generally works very well, but the menu logic for it takes some learning, specifically the functions of some buttons. There's one particular sequence of button-presses during alignment that is intuitive obvious and logical, but WRONG and fatal (to the alignment). Once you make that mistake, you need to reset the scope position and start again. I do it almost every time. But once you've got through that, it's very good. You can use everything you observe as an extra alignment point, so it gets more and more accurate during the session. The "Tour" feature, the catalogue-selection, the sheer number of catalogues, and the notes on each object, are all brilliant.

I have ordered an encoder unit for my AYOii mount which is due soon, so I'll be able to use the Nexus DSC in "push-to" mode as well, as originally designed. And in due course I intend to get a largeish Dob.

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Great piece of kit in a fast scope.

IMG_0109.JPG

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Good to hear it delivers... easier than setting the Explore.

Peter

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3 hours ago, Captain Magenta said:

Funnily enough the OO mirror I ordered during Lockdown has just been completed and is about to be delivered. It is 1590mm apparently and is intended to replace my SW 1500mm, turning my f/4.9 into a f/5.2 - the comparison will be interesting. Though I'll need to do a bit of tube-surgery to install it. In time I'm going to order a custom carbon tube for it.

Mine is an OO 300mm with a focal length of 1590mm so F/5.3.

I'm sure there is still coma present of course but at lower levels than at F/4.9.

That said, I used to own an OO 250mm F/4.8 and happily used Naglers without a coma corrector in that scope so perhaps I'm not particularly sensitive to coma ?. I don't like astigmatism though.

 

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2 minutes ago, John said:

Mine is an OO 300mm with a focal length of 1590mm so 

Ah yes mine will be 300 not 305 so 5.3 of course. Given both Orion, obviously the same mirror ☺️
 

I have had no problem with any of my other eyepieces but then I wouldn’t, seeing as the next up was my 18.2 so cutting out much of the coma’d area.

As you say perhaps you are just not sensitive to it. I hope I haven’t caused you to “once you look for it you can’t again miss it”.

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10 minutes ago, Captain Magenta said:

..... I hope I haven’t caused you to “once you look for it you can’t again miss it”.

Well it might be there but at a level which does not intrude, for me at least.

I've often let targets like Saturn and double stars drift right to the field stop and the detail seems to stay sharp right until the object disappears behind the field stop edge.

 

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At f/5.3, the size of the field that hides the coma inside the Airy disc (i.e. does not reduce the Strehl of the scope below 0.80) is 2.65mm wide.

Think about the field stop diameters on your eyepieces.  I would bet none of them has a field stop that small.

So some, if not most, of the field in an f/5.3 scope will suffer from coma to the point the image is degraded.

And in a low power eyepiece like Magnus' 35mm Panoptic (38.7mm field stop), the difference is huge!

Because the Paracorr essentially makes the coma in the scope the equivalent of an f/13.3 scope, so a 40mm field has zero coma

(I think it's a lot larger than that, but that's as far as the charts go).

Field sizes without coma larger than the Airy Disc:

f/4--1.14mm

f/5--2.22mm

f/6--3.84mm

f/7--6.10mm

f/8--9.1mm

f/9--13mm

So, assuming your low power eyepiece will have a field stop somewhere in the 38-46mm range, it really looks like if you want a coma free low power field, it kind of doesn't matter what the f/ratio of the scope is--

you need a coma corrector.  I tend to view a newtonian as a catadioptric scope: 2 mirrors and a coma-correcting lens system.  My scope is f/5 and I view it as essential.

 

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Generally, the lowest power eyepiece that I use with my 300mm F/5.3 Orion Optics dob is my 21mm Ethos. The field stop on that is 36.2mm.

 

Edited by John

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9 hours ago, Louis D said:

Did it require any additional in-focus relative to the eyepiece alone?  My GSO CC requires 11mm of in-focus.  As a result, neither my 25mm Paradigm (Starguider) or S-W 5-8mm zoom will come to focus with it (I have a very low profile focuser optimized for an undersized secondary).  I've read it is anywhere from 10mm to 19mm.

Louis,

It will require about 14mm of in-travel compared to an eyepiece that focuses at the focal plane of the scope.

[It could be more or less if compared with eyepieces that do not focus at the focal plane.]

As for achieving focus, the focuser doesn't move more than a millimeter once the Paracorr's position is optimized--all eyepieces will become focused by the movement of the Paracorr's top in and out.

You take an eyepiece with a pre-determined setting of the Paracorr's top, set the top to that setting and insert the eyepiece, focus the scope.

All other eyepieces can find their optimum Paracorr setting by focusing using the tunable top.

In practice, you pre-set the top to the setting for that eyepiece before you insert it and it will be in focus.  For fine focus, you still will move the focuser a millimeter or so.

In essence, the top parfocalizes all your eyepieces from all brands (though there are a couple that cannot be optimized).

A near-sighted person would focus the tunable top to the correct position for his eye.  This might cause issues if the eyepiece in question has a 20/20 vision setting of setting A.

In that case, the A setting would be selected and the Telescope focused with the focuser as it would without the Paracorr.  Though not optimum, removing 98% of coma

is still better than removing zero.

 

In your current CC, try using a drop-in adapter, where the eyepiece's shoulder actually sits below the shoulder of the CC.  Such adapters can be obtained that give you a half inch of additional in-travel just with the adapter alone.

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5 minutes ago, John said:

Generally, the lowest power eyepiece that I use with my 300mm F/5.3 Orion Optics dob is my 21mm Ethos. The field stop on that is 36.2mm.

 

Two things:

1. your coma free field is only 7% as wide as the field of the 21 Ethos.  Almost all the field has coma.

and

2. the wider the apparent field, the more visible coma is.  I'll explain:

Coma is a linear aberration--double the distance from the center (where coma = 0), and the linear size of the comatic star doubles.

So coma is twice as large at the edge of a 20mm 100° eyepiece as it is at the edge of a 50° 20mm eyepiece.  Makes sense, right--twice as far out, you see twice the coma in linear size.

Now, use a 10mm 100° eyepiece.  it has the same true field as the 20mm 50° eyepiece, so coma at the edge has a linear size equal to the 20mm 50° eyepiece, or 1/2 as large

as the 20mm 100° eyepiece.

But, alas, the magnification is doubled.  So the linear size of the comatic star at the edge appears twice as large as the 50° 20mm eyepiece, in fact, the same size as the 20mm 100° eyepiece.  2 x 1/2 = 1

So the appearance of coma at the edge of the field is the same at all powers in 100° eyepieces.  And the same at the edge in all 50° eyepieces.  Etc.  Coma's apparent severity is tied to the apparent field of the eyepiece, not magnification

or even field stop.

One way of looking at it is that coma correctors made 100° eyepieces possible for use in newtonians.

 

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So why am I not noticing coma ?

I've been using Ethos eyepieces with this scope for over 5 years now. Without coma correction.

I guess I thought I was a reasonably good observer. I'm beginning to wonder now ....... :undecided:

 

 

Edited by John

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3 hours ago, Don Pensack said:

At f/5.3, the size of the field that hides the coma inside the Airy disc (i.e. does not reduce the Strehl of the scope below 0.80) is 2.65mm wide.

Think about the field stop diameters on your eyepieces.  I would bet none of them has a field stop that small.

So some, if not most, of the field in an f/5.3 scope will suffer from coma to the point the image is degraded.

And in a low power eyepiece like Magnus' 35mm Panoptic (38.7mm field stop), the difference is huge!

Because the Paracorr essentially makes the coma in the scope the equivalent of an f/13.3 scope, so a 40mm field has zero coma

(I think it's a lot larger than that, but that's as far as the charts go).

Field sizes without coma larger than the Airy Disc:

f/4--1.14mm

f/5--2.22mm

f/6--3.84mm

f/7--6.10mm

f/8--9.1mm

f/9--13mm

So, assuming your low power eyepiece will have a field stop somewhere in the 38-46mm range, it really looks like if you want a coma free low power field, it kind of doesn't matter what the f/ratio of the scope is--

you need a coma corrector.  I tend to view a newtonian as a catadioptric scope: 2 mirrors and a coma-correcting lens system.  My scope is f/5 and I view it as essential.

 

So, would putting a 3x Barlow in that f/4 system make the coma appear no worse than in an f/12 scope?  That is, do Barlows (telecentric or not) have any effect on coma?

I ask because my higher powered eyepieces with strong negative sections (10mm Delos, 9mm Morpheus, 7mm XW, 5.2mm XL, 3.5mm XW) tend to look roughly the same with or without a CC in my f/6 Newt.  The same can't be said of my longer focal length premium eyepieces, even accounting for the stronger field curvature without a CC.

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15 hours ago, John said:

So why am I not noticing coma ?

I've been using Ethos eyepieces with this scope for over 5 years now. Without coma correction.

I guess I thought I was a reasonably good observer. I'm beginning to wonder now ....... :undecided:

 

 

Some reasons:

--you haven't looked for it and/or you don't look at the edge.

--your eyes  have too much astigmatism to see coma.  Comparatively, coma is a lesser issue than astigmatism.

--the outer part of a comatic star is fainter than the inner part.  If observing in light pollution, the outer part of the comatic star might not be visible.

--your optical instrument doesn't deliver sharp enough star images to notice it, either because of poor seeing or lesser optical quality.

--you eyepieces don't deliver sharp enough star images to notice it (not in your case, but often the case with many observers using low-end widefield eyepieces)

--You haven't trained your eye to see it  (this might be a blessing)

--You see it, but you ignore it (this is often the case, I find, with observers who aren't very fussy with image quality--probably not your case).

 

I can't really know why you don't notice it--When I look for it, I can see it in f/8 newtonians.

I suppose the obvious question is: "Do you expect the stars at the edge to be in as tight a focus and as sharp as the very center of the field?"

If no, then coma may not be an aberration you need to correct.

If, like me, the answer is yes, then you will do whatever it takes to accomplish that.

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13 hours ago, Louis D said:

So, would putting a 3x Barlow in that f/4 system make the coma appear no worse than in an f/12 scope?  That is, do Barlows (telecentric or not) have any effect on coma?

I ask because my higher powered eyepieces with strong negative sections (10mm Delos, 9mm Morpheus, 7mm XW, 5.2mm XL, 3.5mm XW) tend to look roughly the same with or without a CC in my f/6 Newt.  The same can't be said of my longer focal length premium eyepieces, even accounting for the stronger field curvature without a CC.

Unfortunately, no, because the magnification comes after the image produced by the primary mirror, which has coma.  All you get is a magnified comatic star image.

I have noticed that many people notice coma less at higher powers.  It isn't related to the eyepiece, but probably more the size of the objects viewed at high power.

Because if you try viewing a star cluster that is larger than the field of view, you will see coma at high powers.  I can see it in my 3.7mm Ethos without the CC.

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50 minutes ago, Don Pensack said:

Some reasons:

--you haven't looked for it and/or you don't look at the edge.

--your eyes  have too much astigmatism to see coma.  Comparatively, coma is a lesser issue than astigmatism.

--the outer part of a comatic star is fainter than the inner part.  If observing in light pollution, the outer part of the comatic star might not be visible.

--your optical instrument doesn't deliver sharp enough star images to notice it, either because of poor seeing or lesser optical quality.

--you eyepieces don't deliver sharp enough star images to notice it (not in your case, but often the case with many observers using low-end widefield eyepieces)

--You haven't trained your eye to see it  (this might be a blessing)

--You see it, but you ignore it (this is often the case, I find, with observers who aren't very fussy with image quality--probably not your case).

 

 

Thanks Don.

Rather depressing though :undecided:

 

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28 minutes ago, runway77 said:

God bless < 70 degree eyepieces and f/6 and slower scopes ;)

Fair point but up until now I was really delighted with the 100 degree Ethos's in my F/5.3 scope.

Come to think of it, nothing has actually changed with either the scope, the eyepieces or me so I guess I'll go on being happy with them :smiley:

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Apologies to @Captain Magenta / Magnus for diverting this thread onto my particular issues.

Thanks again for your interesting report :thumbright:

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