3 coma corrector options, what difference will I see?

[Cumbria Jon]

I would greatly appreciate peoples advice and experience on this.

I am not very experienced, and upgrading from a 6" Newtonian to a 16" Dobsonian (Skywatcher), visual but no astrophotography, and lucky enough to have dark skies.  I want to start with one or two wide FOV high quality eyepieces (Nagler or even Ethos if I can afford it) and then maybe add more later.

So I understand I may need a coma corrector, given the wide FOV and fast-ish scope.  Some obvious options are:

Paracorr, £450, increases f-ratio by 1.15, adjustable - (but I do not understand what the adjustment is doing).

Baader MkIII, £150, no effect on f-ratio

Skywatcher coma corrector, £98, f-ratio reduced (by 0.9x)

Big differences in price, significant differences in effects on f-ratio and FOV. but what difference might I actually see with my eyes?  Do coma correctors differ in how well they do their job?

 

 

 

[Scooot]

Hi Jon, I see this is your first post so welcome to SGL.

i just googled 16" Skywatcher and the one that came up was an f4.4. At this f ratio you will notice a difference using a coma corrector but more so with the longer focal length and widefield eyepieces. With the higher powered eyepieces the difference in the quality of the view will be less noticeable but there all the same.

With regard to the coma correctors I can only comment on the Paracorr as that's the limit of my experience. For coma correctors to work properly, ie remove coma, the spacing between the corrector and the focal plane has to be correct. The focal plane tends to vary with different eyepieces, so the spacing needs to be adjusted. So by adjustable I think your referring to the paracorrs tuneable top. This rotates to adjust the spacing and comes with instructions telling you the correct setting for each Televue eyepiece. The corrector works just as well with non Televue eyepieces but you'd need to work out the settings yourself.

The Paracorr also increases the focal length and therefore the magnification by 15% . This is not always desirable when using low magnification, and therefore wider field eyepieces, if trying to frame big targets, but what can be seen will show round stars across the whole fov instead of just in the centre.

My advice would be to wait until you get the scope and decide whether the coma bothers you, but be prepared for more expense if it does.

Hope this helps a bit.

[Louis D]

Technically, you need the coma corrector to remain at the same spacing from the primary for all eyepieces, thus the FeatherTouch/TV SIPS system where the coma corrector remains in a fixed position below the focuser.  Without this, you need to either parfocalize your eyepieces or keep adjusting the spacing for each eyepiece that focuses at a different distance from the primary.

Also keep in mind you'll need about 30mm or more of infocus to use most non-Baader MPCC coma correctors.  Then there is the whole issue of induced spherical aberration seen at higher powers with some CCs.

[Cumbria Jon]

Thank you Scooot / Louis D.

There is much to learn!  So for a coma corrector to work as designed, it must be positioned at a fixed position in relation to the primary, AND at the correct position in relation to the focal plane as determined by the eyepiece (s).  If the eyepieces are parfocal, then they will all need the same spacing from the corrector.  If not, then changing eyepiece, means changing the spacing.  The Paracorr makes this latter adjustment easy.

And thank you for the point about infocus, which I had missed too.

"induced spherical aberration" - looks like more reading up for me!

 

[Scooot]

Yes that sums it up nicely. With my paracorr, once I've focussed with one eyepiece, I can find the correct spacing for another eyepiece by refocusing using the tuneable top adjuster instead of the focuser.

There is also an explore scientific coma corrector with which you can adjust the spacing per eyepiece which is cheaper than the paracorr . I haven't tried it but there's a recent thread about them on here somewhere.

[Don Pensack]

On 4/10/2016 at 00:44, Cumbria Jon said:

I would greatly appreciate peoples advice and experience on this.

I am not very experienced, and upgrading from a 6" Newtonian to a 16" Dobsonian (Skywatcher), visual but no astrophotography, and lucky enough to have dark skies.  I want to start with one or two wide FOV high quality eyepieces (Nagler or even Ethos if I can afford it) and then maybe add more later.

So I understand I may need a coma corrector, given the wide FOV and fast-ish scope.  Some obvious options are:

Paracorr, £450, increases f-ratio by 1.15, adjustable - (but I do not understand what the adjustment is doing).

Baader MkIII, £150, no effect on f-ratio

Skywatcher coma corrector, £98, f-ratio reduced (by 0.9x)

Big differences in price, significant differences in effects on f-ratio and FOV. but what difference might I actually see with my eyes?  Do coma correctors differ in how well they do their job?

 

 

 

Yes, It turns out that there are only 2 that are really easy to adjust for the individual eyepiece: TeleVue Paracorr and Explore Scientific HRCC.

The Baader MPCC-III and the Astrotech/GSO units require different spacers be added to each eyepiece to make them optimized for each eyepiece.

and it turns out that slight amounts of magnification are necessary to tame spherical aberration added by the coma corrector.  That's why the TeleVue is 1.15X, the ES HRCC is 1.06X

and the Astrotech/GSO one is 1.10X.  I'm dubious about the 0.9x of the Skywatcher Coma corrector.  This may be optimized, like the Baader, primarily for astrophotography.

 

You see, different eyepieces have their focal planes at different points inside the eyepieces--some above the "shoulder" and some below.  EAch of these eyepieces will need a different spacing from the lens to

optimize the coma correction (after all, you get a coma corrector to eliminate coma, not just reduce it, so why wouldn't you adjust the coma corrector to optimize each eyepiece).

Since every eyepiece would need to have its focal plane the same distance from the coma corrector lens, your entire set of eyepieces will become parfocal once all eyepieces are the correct distance from the lens.

And that is where the adjustable tops on the TeleVue and ES coma correctors come in.

Once any one eyepiece is set at exactly the right distance from the lens, any other eyepiece can be inserted and focused using the top of the coma corrector, and whatever setting of the top is derived is the exactly correct setting for that eyepiece.

You might think that eliminates having to focus with the focuser, but, in practice, you still need a mm or so to fine tune each eyepiece, even if you've "pre-dialed" the right setting of the top.

 

In the Baader MPCC or the GSO corrector, the adjustment for each eyepiece would be different thicknesses of spacers on the eyepieces (Baader) or parfocalizing rings added to the eyepiece where, once you've established

the correct position for the eyepiece, you move the parfocalizing ring down against the CC and tighten it down on the eyepiece barrel (GSO)--in that way, each eyepiece will automatically insert to the correct point for optimizing.

That's a lot more of a hassle than with the TeleVue and Explore Scientific, but it does have to be only done once.

 

You could parfocalize your eyepieces first with whichever of your eyepieces requires the most in-travel, and, that way, once you find the optimum setting for one eyepiece, you've solved it for all.

If you have your in-most travel eyepiece, you'd insert it in the coma corrector and pull the top back all the way, or pull the eyepiece 1/2 inch out of the coma corrector, then focus the scope.

Look at the edge.  Still coma?  Then move the eyepiece in toward the CC lens a couple millimeters and refocus the scope.  Keep repeating until coma at the edge of the field is gone.  When that happens, that is the proper setting of the coma corrector in the light cone from the primary.

Lock the focuser and insert each of your other eyepieces one at a time and slide the eyepiece in and out, or the top of the coma corrector in and out until that eyepiece is in focus.  Whatever setting that is is the setting for that eyepiece.

In short order, you can do a complete set of eyepieces of any brand.

The difficult one is the Baader MPCC, which, because it attaches to the eyepiece, requires a lot of experimentation with spacers to get the next eyepiece to focus once the focuser is locked down.  It's still the same principle--locking the coma corrector lens in the light cone at the proper place and adjusting the eyepieces in and out until they're in focus.  Baader makes a 1.25" adapter to thread onto the MPCC for 1.25" eyepieces to be similarly optimized.

What's happening by your being fussy about the adjustment on that one eyepiece is that you are finding the right place in the light cone to place the coma corrector.  Everything follows from that.

I really appreciate the parfocalizing of all my eyepieces that occurs.  Of course, the adjustable tops on the TeleVue and ES coma correctors make it possible to merely predial the tops to the right setting before the eyepiece is inserted, so you'll automatically find the eyepiece already in focus.  And if you have parfocalized your eyepieces anyway, only one setting of the coma corrector will be needed and there would be no in-the-field adjustments of the CC (except with the Baader).

 

The TeleVue requires about 3/4" of additional in-travel in your focuser be available to use it.  The ES HRCC requires 1-14" additional in-travel of the focuser to use it.  I am not familiar with the change in focus required for the others.

 

Hope my long-winded post helps explain what's done to optimize a coma corrector at first, and in the field.

[Louis D]

I've found that the GSO requires about 10mm of focuser in-travel when using the supplied eyepiece holder and an additional 25mm of spacing between that holder and the optics unit.  For every additional 4mm of spacing, you save about 1mm of in-travel required.  So I guess if you put 110mm of spacing in there, you wouldn't change the focus point at all.  Of course, the correction would be awful.

If you screw the same 70-75mm of spacing onto the back of your 2"-1.25" adapter, you'll need about 1.5 inches of out-travel to reach focus.  Since my focuser won't go that far, I just pull the whole assembly up and out of the focuser about and inch or so.  Thus, you'll always be able to reach focus with 1.25" eyepieces and possibly even with cameras setup for prime focus photography via the T-mount adapter that wouldn't be able to reach focus otherwise.

It definitely flattens the field and reduces coma to negligible amounts.  I can't find a spot that completely eliminates all edge coma or field curvature, but it is tamed down enough as to be nearly imperceptible when in focus and not looking directly at the edge.  I also find that spacing isn't all that critical.  I purposely shortened the focuser-shoulder to optics-barrel distance to about 65mm and began pulling various eyepieces out to look for changes in correction.  Anywhere from 65mm to 85mm seems to yield similar correction results.  It's not like there's a sudden snap in correction where you say "Aha, that's it!".  It's more like, "Yeah, that seems about as good as it's going to get".  I definitely don't fuss over getting that last 10% of edge correction.  As long as you're in that distance range, you'll get about 90% correction, which is a lot better than none at all.

[niallk]

Hi Jon,

Perhaps you've seen this: if you were to go for a Paracorr2, then at least for TV EPs, you can check per the table linked below how much adjusting you may have to do when observing.  My present eps (Pan24, 13T6, 7T6) all share the same tunable top setting, plus are all 1.25". Nice and handy to eliminate fuss in the dark when I use my Paracorr @ f/4.5 !  However, future EP aspirations will disrupt this nice convenience

Also the P2 adds 0.5kg at the focusser :/

http://www.televue.com/mobile/TV5_page.asp?id=61&Tab=_ttop

Best of luck,

-Niall

[Cumbria Jon]

Thank you all,

I must say I have found the topic of coma correctors, especially their practical "deployment" difficult to understand.  And I think I have it now:

Ideally, fix the position of the corrector in relation to the primary.

This will define exactly where the focal plane of each eyepiece needs to be positioned in relation to the corrector

BUT eyepieces vary a lot in the position of their focal plane.

This can be addressed by parfocalising the eyepieces, by having an adjustable coma corrector (Paracorr), or by racking the focusing in and out to suit each eyepiece.

And the consensus seems to be that the less fiddling doing this in the dark, the safer and the more time spent viewing.

Greatly appreciated - I shall post what I end up with in due course - but  it will be a few months yet.

 

Jon

[cs1cjc]

I am in a definite minority.  Most users of this class of Newtonian are happy without a coma corrector and feel no need for one.  Here, I will put the case for use of coma corrector as strongly and clearly as I can, but do feel free to ignore me!

Pay no attention to those who say you do not need a coma corrector or that their particular Newtonian shows little coma.  All Newtonians of the same focal ratio have the same amount of coma; there is no control available to the designer or maker.  Pay no attention to those who say they are not sensitive to coma.  Their telescope is not sharp outside a small region, whether or not they are sensitive to it. 

Even with a perfect mirror (and not many of those about) the diffraction limited (strehl of 0.8 or better) sweet spot of a Newtonian is small.  Coma robs the image of contrast and indeed brightness long before it becomes visible to the the human eye as a distinguishable aberration.  It is a linear aberration so its impact on sharpness depends only on the eyepiece apparent field of view and not on its focal length.  Diffraction is an issue at high magnification because it dominates coma, however the diffraction limited, Strehl of better than 0.8, sweet spot of a perfect Newtonian is small, dependent only on the focal ratio, with a diameter approximately as follows:

F.4  1.4mm
F/4.5  2mm
F/5  2.8mm
F/6  4.8mm
F/8  11mm
F/10 22mm
For reference the field stop of an 8mm Tele Vue plossl is 6.5mm. 

Outside this region, the view is marred by coma so it can be seen why collimation, getting the sweet spot in the centre of view, is so important.  The telescope still has to be well collimated with a coma corrector, but the sweet spot is enlarged considerably and there is a real difference at the eyepiece, even to my ageing eye.

Somehow Tele Vue has convinced many amateur astronomers that the only way to correct coma is expensive (£460) and complicated; you need to use a turntable.  People do not want to spend such a sum on a corrector even though some will spend it on an eyepiece. 

However the Altair Astro (also known as GSO, Astro-Tech) which I use will do the job for Newtonians down to F/3.5 (according to Astronomics) for £75, with a little set up effort and, for a sensible choice of eyepieces, no turntable.  It has no significant spherical aberration, gets good reviews and the optical design was by Roger Ceragioli who is is a respected telescope builder, amateur astronomer and professional optician.

Strictly, a coma corrector has to be a precise distance from the focal plane, but different eyepieces focus at different heights, hence the turntable supplied with some.  The alternative is to to buy eyepieces that are nearly parfocal or to actively parfocalise them, which Don always recommends.  Parfocalising is a good thing to do even without a coma corrector, because if the focal points are far apart, it can be difficult to recapture an image after changing an eyepiece.  It is a joy to use a scope with eyepieces which are parfocal with eyepiece changing being much less of an issue.

In practice, with careful choice of eyepiece I have found no need to actively parfocalise my eyepieces.  Roger Ceragioli says that the spots (spot size as analysed in optical design software) remain good for a tolerance of +/-10mm of the nominal 75mm which easily covers my eyepiece collection and I would guess that of many others too.

The Altair Astro coma corrector does not come ready for use.  A spacer of the correct size needs to be fitted.  I have tried my best to explain the process of calculating the size here, User_Guide, but despite my best efforts to be clear and concise, this does not read well or easily even to me.  There is a good discussion of it here: Settig_up.

My 2p on this is to budget for a coma corrector and expect to spend a little time setting it up.

[Cumbria Jon]

Thank you Chris, the more expositions I read, the clearer it becomes, hopefully literally.  The two links are useful too, for myself and hopefully anyone else in my position.

Jon