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Coma correctors?

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#1 Linked-In-The-Stars

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Posted 08 March 2021 - 05:11 AM

Happy Monday! Just a quick question, I had read a few things about coma correctors and some people are suggesting to use them but I am not sure if they mean more beneficial for photo purposes, or visual too. I am wondering if something like the Baader Mark lll would be worth getting for my setup and if it would benefit me at all, or for that matter, not really be beneficial as well. Pros? Cons? T.I.A!! Clear skies!

 

Dustin



#2 TOMDEY

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Posted 08 March 2021 - 06:00 AM

Coma Correctors add some spherical aberration. So if your target is small center field, the image will be sharper without it. Depending on what you're looking at or imaging, in or out would be optimum. I had an old (Lumicon?) CC that I used on my 12.5-inch F/6 Newtonian, which had spectacular mirrors. Without the CC the spherical was zero. With it in, I got slight but noticeable spherical aberration on-axis, confirmed both with a knife-edge on a star and also image resolution. All CCs introduce some spherical; it's just a question of whether it's a noticeable amount.

 

[Bit more on the technical end: You cannot zero both coma and spherical (of all orders) simultaneously using an entirely dioptric CC that comprises all spherical surfaces. Even if you null both primary Zernikes, the higher-orders remain. And even if you go aspheric to null all orders of spherical, oblique spherical will still be there. BTW, the way the cc works is by shearing the coma of the PM and of the dioptrics in the CC such that they cancel as they shear against each other across the field. It's a high-wire balancing act. This works because sheared Zernike primary spherical is Zernike primary coma! But the higher orders remain. No free lunch.]   Tom


Edited by TOMDEY, 08 March 2021 - 07:14 AM.

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#3 happylimpet

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Posted 08 March 2021 - 06:18 AM

I wish I had Tom's optical understanding but this agrees with my thoughts and experience!!!

 

ie for planetary, no coma corrector, but for deepsky wide field, use it!


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#4 Jon Isaacs

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Posted 08 March 2021 - 07:40 AM

Coma Correctors add some spherical aberration. So if your target is small center field, the image will be sharper without it. Depending on what you're looking at or imaging, in or out would be optimum. I had an old (Lumicon?) CC that I used on my 12.5-inch F/6 Newtonian, which had spectacular mirrors. Without the CC the spherical was zero. With it in, I got slight but noticeable spherical aberration on-axis, confirmed both with a knife-edge on a star and also image resolution. All CCs introduce some spherical; it's just a question of whether it's a noticeable amount.

 

[Bit more on the technical end: You cannot zero both coma and spherical (of all orders) simultaneously using an entirely dioptric CC that comprises all spherical surfaces. Even if you null both primary Zernikes, the higher-orders remain. And even if you go aspheric to null all orders of spherical, oblique spherical will still be there. BTW, the way the cc works is by shearing the coma of the PM and of the dioptrics in the CC such that they cancel as they shear against each other across the field. It's a high-wire balancing act. This works because sheared Zernike primary spherical is Zernike primary coma! But the higher orders remain. No free lunch.]   Tom

 

Not all coma correctors are created equally. Many like the MPCC were designed for photography where spherical aberration is not a big deal. 

 

The Paracorr 2 is a rather different animal, I believe it has 5 elements.  Do you see spherical aberration in your Paracorr 2?  

 

Jon


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#5 TOMDEY

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Posted 08 March 2021 - 07:53 AM

Not all coma correctors are created equally. Many like the MPCC were designed for photography where spherical aberration is not a big deal. 

 

The Paracorr 2 is a rather different animal, I believe it has 5 elements.  Do you see spherical aberration in your Paracorr 2?  

 

Jon

Hi Jon!

 

I'm guessing that the (very old original) Lumicon is inferior to the Paracorr 2. I use the Paracorr for visual and never actually scrutinized it for any hint of spherical. But I think I would have noticed it if it were significantly present. Now that my main scope is F/3.75, I use the Paracorr almost all of the time. But for tiny field high mag --- I often remove the CC, just to get that extra glass out of the train. Like when I'm obsessing over the Crab Pulsar and want every photon I can get.    Tom


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#6 spereira

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Posted 08 March 2021 - 07:57 AM

Folks, let’s please stick to answering the OP’s question, rather than starting a side conversation.

 

Also, let’s please remember there is no Astrophotography discussion here.  If this topic is better discussed in one of the imaging forums, please move there for further discussion.

 

smp



#7 Jon Isaacs

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Posted 08 March 2021 - 08:10 AM

Folks, let’s please stick to answering the OP’s question, rather than starting a side conversation.

 

Also, let’s please remember there is no Astrophotography discussion here.  If this topic is better discussed in one of the imaging forums, please move there for further discussion.

 

smp

 

I am confused.   The topic of the discussion is coma correctors like the Baader MPCC.  In my mind, that means, not just the Baader but also the GSO, the Explore Scientific and the various Paracorrs.

 

I have a 10 inch F/5 Dob, optically it's the scope as Dustins.  I use a coma corrector with it and it does really clean up the field.    

 

Looking at Dustin's eyepiece collection, I think a coma corrector would help some but the wide field eyepieces in the longer focal lengths will show enough off-axis astigmatism so that the gain will be limited.  With the shorter focal length eyepieces, being plossls, with their narrower field of view, the coma won't be so bothersome so again, of limited gain.

 

Jon


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#8 SloMoe

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Posted 08 March 2021 - 08:13 AM

Do you need one,?

 

For visual in an f/5 newt, almost not, with your eye pieces you won't see enough of an improvement to purchase one at this time.

One way to find out is when using your widest fov eyepieces do the stars at the edge of your fov seem to stretch?

 

If a star is a pin point at the edge and remains that way all the way across, then no.

 

Not all wide field eyepieces are created equal either, 

 

When you start to see the star stretch at the edge of your fov then it's time to consider a Paracor-II for visual.


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#9 SloMoe

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Posted 08 March 2021 - 09:05 AM

Dustin, please understand I'm not diss'ing your eyepiece collection, not at all, 

 

You have an excellent scope for visual, easy to collimate, the mirrors will cool quickly, and the two WO eyepieces are nicely corrected.

 

Plossl's get so underrated in the quest for wider fov's and that photo perfect view, people will try and convince you it might be possible, and recommend various eyepieces & accessories, but all in all there's still seeing conditions and light pollution to deal with, just like finger prints on a camera lens,  

 

Your plossl's will give you the best visual detail you can get without the Coma Corrector, just gotta bump the dob more often to keep up with the target, in a tracking mount the plossl is king of the detail.

 

For your DSO's you might consider a 2" Barlow for your WO's to bring you to around 100X  and still retain 72° and 23mm eye relief.

 

Your 33 in the Barlow will yield 0.85° fov and at a magnification level of 85X and your 40 will yield 1.13°fov at 64X.

 

That pretty well will provide you with enough fov to see a large number of DSO's in their entirety with enough light gathering to provide some decent detail in them without the use of a coma corrector.

 

Use your plossl's for solar system targets, no need for a wide fov other than the tracking issue, well, except for Lunar viewing, that thing will drive your fov fever to no end.



#10 Linked-In-The-Stars

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Posted 08 March 2021 - 09:11 AM

I appreciate the feedback everyone, had to re-read a few comments to try and decipher some comments in lamens terms lol however you spell that. I do plan on buying better eyepieces pretty much from maybe 5mm all the way up again, and have different collections for different reasons and try to get better quality ep’s for viewing. The big wfov ep’s I have now I will say 75% of the visual is pinpoint, out toward the edge they become fuzzy or blurry and not pin point anymore which is bothersome to a point. I don’t know if this is because of the eyepiece itself maybe not the very best quality out there or if it’s something I can correct down the road with some other steps, or if the cnc would help me visually. My high power plossls are about useless, as they’re probably as cheap as they come. Hence the other reason why I am looking to upgrade my collection. Thanks everyone for the help, I think maybe down the road I can at the very least see if it’ll help me in observing, any way I can improve my time observing I’m open to trying. Thanks again! Very good responses and helped me understand them a bit more! Ty!
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#11 Jon Isaacs

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Posted 08 March 2021 - 09:35 AM

I appreciate the feedback everyone, had to re-read a few comments to try and decipher some comments in lamens terms lol however you spell that.

<...snip...>

 

Ty:

 

This is how I see it:

 

You have some decent eyepieces, enjoy them despite their issues.  There will be a time to purchase a coma corrector but right now, I think you would be better off, concentrating on upgrading your eyepieces, particularly your shorter focal length eyepieces, a little wider field of view, more eye relief, both these would be very helpful.  

 

The question is, how much are you thinking of spending?  You could buy a set of TeleVue Nagler eyepieces $325+ each or you could buy a set of Astro-Tech Paradigms at $60 each or something in between.  Both would represent a marked improvement.  

 

I will share this, not to brag but just to provide some perspective.  I have a 10 inch F/5 Dob similar to yours as well as a number of other telescopes.  I do have a set of TeleVue eyepieces ranging from the 41mm Panoptic to the 3.5 mm Nagler.  They are very nice eyepieces.  

 

But I also have a set of Astro-Tech Paradigm eyepieces and for observing from my backyard with the 10 inch, very often I will just grab the set of Paradigms and enjoy the views.  I have all of them except the 15mm.  The 12mm, the 8mm and the 5mm are all quite sharp at F/5, I believe the 15mm is also quite sharp.  The 18mm and the 25mm show some off-axis astigmatism, probably similar to your 33mm and 40mm William optics.  I think the Paradigms could be a nice upgrade to your Plossls and at $60 each, very affordable.

 

Of course, it also makes sense to invest more if that feels right.  Eyepieces are a long term investment and may be used with future scopes.. 

 

Jon


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#12 SeattleScott

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Posted 08 March 2021 - 10:08 AM

As a general rule, you want to address the weakest link in the system. Right now coma is a weakness for you but it isn’t the weakest link.

Scott


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