132 replies to this topic

[Houdini]

Hello all,

 

In the past we've discussed coma correcting eyepieces: eyepieces specifically designed for Newtonian telescopes with parabolic mirror that remove the need for a separate coma corrector like the Televue Paracorr.

In November last year we talked about this in this topic https://www.cloudyni...ece/?p=13092088 .

 

I have now completed a number of promising eyepiece designs I already hinted at in November.

I've published a (long-ish) article about the subject on my Cruxis web site, you can start reading https://www.cruxis.c...acorrecting.htm if you're interested.

 

Let me give you some highlights:

• It is very much possible to design coma correcting eyepieces that rival the very best standard eyepieces. Not only in terms of image quality (tightness of stars across the field) but also for aspects that have a big impact on the observing comfort, including eye relief, distortion, curvature, size and weight.
   
• It is possible to design a whole line of 86° or 100° coma correcting eyepieces from 32 mm down to 6 mm, so that one can get rid of a coma corrector altogether.
   
• I've created a dedicated "Eyepiece Designer" ray tracing software to help me design the eyepieces. The tool can automatically optimize the layout and glass types of the eyepiece.
   
• Well designed coma correcting eyepieces are not bulkier than standard eyepieces! The "cost" of the coma correction most of the time is just a single lens inside the eyepiece.

To illustrate the last point, the image below compares the Televue Paracorr + Nagler 31 combo to my 28 mm 86° coma correcting eyepiece design. I superposed the 28 mm 86° lens layout on a photograph of the Televue combo. While the compact 28 mm would weigh less than half of the 1.35 kg of the bigger combo, it produces a 15% larger true field with excellent edge correction, comfortable eye relief, and minimal distortion and curvature.

 

 

At the moment it's all "design" and not "reality", but I sincerely hope that at some point some of these eyepieces will be manufactured. I'll do my best to make this happen, as I truly believe they would be awesome performers in many telescopes.

 

I know some of the forum members at Cloudy Nights are quite skeptical about the idea, that's why I'm interested in your feedback, remarks, suggestions, or help in finding a manufacturer for some of these eyepieces.

 

Thanks for reading!

Robert

[Jethro7]

Hello Robert,

This is interesting for the reasons that you mentioned when dealing with eyepiece choices for Scopes be they Newts and especially Dobs. How ever I would think that the only reason that having eyepieces that have inclusive coma correctors built in are not already on the market is, maybe the lack universality with other types of scope Genres. I don't know how well coma correctors will work with non Newt type scopes? Unless the coma correcting ellement is modular and could be simply removed. I've never bothered to try a Paracorr in one of my refractors to see what the effect are and may try it out on my next session.

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

Edited by Jethro7, 06 August 2024 - 05:51 PM.

[Houdini]

Hello Jethro, the eyepieces will only work as intended in Newtonian telescopes with parabolic mirror.

 

They will still work in all slow telescopes including refractors or SCTs, say, with focal ratio above f/8 or f/10.

For example, the 32 mm 81° eyepiece with 45 mm field stop I showcased, will work quite well in a refractor or SCT at f/8, and very well at f/10.

 

The eyepieces will not work well in fast refractors, I would not recommend them for f/5 to f/7 APOs.

[Jon Isaacs]

My two cents;:

 

I own and use several Dob's as well as refractors ranging from F/5.4 to F/7.  I have three main sets of eyepieces, the Baader Morpheus's, TeleVue Naglers and a mixed set of Ethos and KUO XWAs. These eyepieces work on all my scopes.  I use eyepieces that range in focal length from 41 mm to 3.5 mm with a 2x Barlow.

 

I have a Paracorr 2 and have owned every Paracorr except the 3 inch.  My scopes are balanced with the Paracorr. 

 

The Paracorr adds about 2 inches to the eyepiece height. For me, that's actually a good thing because it provides more room between me and the the telescope. With my largest scope, this is important because that extra space provides more room for the ladder.

 

I do applaud you efforts but I think the flexibility and versatility the Paracorr provides is such a benefit.. All my eyepieces work on all my scopes.

 

Jon

[Houdini]

The Paracorr adds about 2 inches to the eyepiece height. For me, that's actually a good thing because it provides more room between me and the the telescope. With my largest scope, this is important because that extra space provides more room for the ladder.

Thanks for your answer!

 

I agree with this point, I use 7 ft and 10 ft ladders, and this is the only "comfort" issue that removing the Paracorr would create.

For me it doesn't counter having 10% or 15% more field of view, with an 1100 mm telescope field of view is at a premium, but YMMV.

 

I do applaud you efforts but I think the flexibility and versatility the Paracorr provides is such a benefit.. All my eyepieces work on all my scopes.

Of course, all of us "old timers" are well equipped with coma corrector and high quality eyepieces, and are used to fiddling with a tunable top. It probably changes our perspective on the relative value of not having to purchase a coma corrector and not having to worry about the exact positioning of all the eyepieces.

But if you had just purchased your first 12 inch f/4 reflector and were in the market for new eyepieces, I guess that coma correcting eyepieces could be a very attractive option.

[jrazz]

I would love to have eyepieces like this but it seems they need to be tailored to a specific focal ratio. I think that might be the real issue with having someone produce this. 

[Houdini]

Eyepieces don't really need to be tailored to the focal ratio. The focal ratio simply decides which focal lengths make sense for your observations with a pupil that is at most 6 to 8 mm diameter.

For example, at f/3 it does not make sense to use a 30 mm eyepiece, this would create an exit pupil of 10 mm, and most of the light would be wasted.

That's why, for example, the 28 mm is shown as "best for f/4 to f/6".

 

Edit: I've now updated the article and removed most of the focal ratio references, as they tend to be confusing.

Thanks, Jordan!

Edited by Houdini, 06 August 2024 - 05:29 PM.

[Starman1]

The Paracorr parfocalizes all my eyepieces. If they were used in the SIPS, they would need 1.2" of focus travel.
I would assume coma correcting eyepieces would all need to be parfocal to have the optimum correction.
That might be difficult with a 10 to 1 range in focal lengths.

[Houdini]

I would assume coma correcting eyepieces would all need to be parfocal to have the optimum correction.

No, that's not the case.

 

The coma correction is built-in and does not require any special positioning. You can focus a coma correcting eyepiece like a normal eyepiece, and you'll get the optimum view. 

 

The parfocality issue you evoke is created by a separate coma corrector, it does not exist if you don't have it in your optical chain. The "tunable top" really is a solution to a problem created by the coma corrector itself . 

Edited by Houdini, 06 August 2024 - 04:58 PM.

[leonard]

 

The coma correction is built-in and does not require any special positioning. You can focus a coma correcting eyepiece like a normal eyepiece, and you'll get the optimum view. 

 

     I for one love the idea , I would like a 12 inch F 4.5 with a set of 6 of these eyepieces . All the eyepieces

are balanced using different materials and combinations thereof to make all 6 within 2 oz of the lowest weight eyepiece . No spinning tops , extra gizmos , no fiddling and fooling , just in ,out , view .

                                                    What’s not to like ?

  Yea ,I know there must be something , there always is .

[luxo II]

Good luck... This path was tried before with the Klee Pretoria eyepieces. They didn't sell well, and died.

 

At the likely cost of a set of these a coma corrector looks attractive, given it can be used with a wide variety of scopes, eyepieces and cameras.

Edited by luxo II, 06 August 2024 - 08:28 PM.

[Houdini]

Thank you for the reactions!

 

I talk about the 28 mm Pretoria in the article, I don't think it's very relevant for today's discussion. It was a 50° eyepiece, which even in the 1980s was not really widefield.

It is not a good reference point for what we want in 2024. The 86° and 100° designs I present are much more "modern" in the sense that they address all the aspects we expect in premium eyepieces. For example, limited distortion across the field. The 28 mm Pretoria was a 28 mm eyepiece in the center of the field, but at the edge a 24 mm; objects would become 20% bigger when moving from the center to the edge.

 

The "cost" of the integrated coma correction is usually 1 lens (my 86° designs have 8 elements, my 100° designs 10 elements). I don't think they would be significantly more expensive to manufacture than the current premium eyepieces with 8 or 9 lenses. It's probably all in the batch sizes...

[Houdini]

Somebody sent me a question about how good the coma correction is at different focal ratios.

The designs have mostly been optimized at f/4. They will still work very well at f/3, obviously with larger aberrations because of the faster light cone. At f/5 the eyepieces will perform even better than at f/4.

 

To illustrate this, below the spot diagrams for the 24 mm 86° eyepiece in f/4, f/3 and f/5 telescopes.

In the upper right of the spot diagram you'll find two circles of 2' and 5' diameter that are convenient for evaluating the quality of the design (for reference, the maximum resolution of the human eye is 1'). Star images that fit inside a 2' circle are virtually perfect, inside a 5' circle are very good.

 

 

 

In practice you'll probably not use this 24 mm eyepiece at f/3, it would produce a 8 mm exit pupil. Then again, you might!

Edited by Houdini, 07 August 2024 - 03:26 AM.

[luxo II]

... and what field curvature (radius) are these designed for ? Clearly the target audience is users of large fast dobs...

[Houdini]

They're designed for a flat field.

This is a point I have not really considered - "flat" was kind of the default mode for me. But it might help to design for the moderate curvature produced by a Newtonian with, say, 150 inch focal length.

Thanks for mentioning this, I'll take a closer look!

[Jon Isaacs]

 

But if you had just purchased your first 12 inch f/4 reflector and were in the market for new eyepieces, I guess that coma correcting eyepieces could be a very attractive option.

 

I bought my 12.5 inch F/4.06 twenty-five years ago. At the time, I had a C-8 and not a lot of money. By the time, I had enough money to buy quality eyepieces I had refractors. 

 

Coma correcting eyepieces would seem to have a very limited market, Those who observe with only Newtonians. 

 

As far as the 15% increase in magnification and the 13% narrower field, I rarely use the widest possible field f view. Sometimes I want the brightest view for use with filters, I just remove the Paracorr. Viewing nebulae, coma is not a big deal.

 

A $500 coma corrector that corrects the coma in all my my Newtonians with all my eyepieces is seems too good to be true. It's a one time investment.

 

Many Dob owners also own fast refractors like the TeleVue NP-101(F/5.4) and other F/6 and F/7 Ed/APO's to complement the Dob's with low power wide field views only a refractor can provide. 

 

Jon

[Sarkikos]

I think the concept of incorporating coma correction in an eyepiece is getting things backward.  The eyepiece doesn't create the coma.  The Newtonian telescope does.  Coma correction should be corrected at its source:  the telescope.  Either correct the coma using an add-on coma corrector or install coma correction permanently into the telescope.

 

Should we have eyepieces which correct for the CA in an achromat refractor?  No, we have ED/APO's which have CA correction built in.  

 

Should we have eyepieces which correct for the field curvature in fast refractors or SCTs?  No, we can either install a field flattener/corrector or start out with a Petzval or EdgeHD, which has FC correction built in.

 

So, why should we have coma correction in an eyepiece for coma caused by the parabolic primary of a Newtonian? 

 

Mike

[Houdini]

So, why should we have coma correction in an eyepiece for coma caused by the parabolic primary of a Newtonian? 

Because correcting it in the eyepiece can be the most efficient, economical, and ergonomic solution.

The alternative, using a separate coma corrector inside the focuser, is not a free solution - there's weight, cost, field of view, focusing comfort, focus in-travel. See this page .

Edited by Houdini, 07 August 2024 - 12:59 PM.

[AaronF]

So, why should we have coma correction in an eyepiece for coma caused by the parabolic primary of a Newtonian?

Mike

This makes me wonder whether coma correction could be performed by a specially-shaped secondary mirror, rather than by glass in the eyepiece tube? (Hopefully Don and/or Jon can tell me why this is infeasible )

[Starman1]

This makes me wonder whether coma correction could be performed by a specially-shaped secondary mirror, rather than by glass in the eyepiece tube? (Hopefully Don and/or Jon can tell me why this is infeasible )

No, because the secondary is at an angle relative to the light cone and if it deviates from a flat, aberrations are added.

A coma corrector, or coma correcting Barlow, eyepiece, or SIPS is perpendicular to the light cone.

 

Like many telescope users, I have two scopes, one of which would not be compatible with coma-correcting eyepieces.

So I would not be a customer for such eyepieces.  I suspect there would be a limited market because such eyepieces would also likely be expensive to make.

And in today's market, it is almost becoming mandatory for the eyepieces to have long eye relief, so they're be larger and heavier than standard eyepieces with shorter eye reliefs.

[Sarkikos]

Because correcting it in the eyepiece can be the most efficient, economical, and ergonomic solution.

The alternative, using a separate coma corrector inside the focuser, is not a free solution - there's weight, cost, field of view, focusing comfort, focus in-travel. See this page .

No, not really.  Not efficient or economical if you have other telescopes besides Newtonians.  Many observers have several telescopes, often a mixture of Newtonians, refractors and Cats.  Should they have a separate set of eyepieces for the Newtonians just to avoid having to buy and install a Paracorr or SIPS?

 

For myself, if eyepieces with built-in coma correction were available, I would not buy them.  This might be attractive for observers who have one big Dob and that's it, no refractors or Cats.  On the other hand, many of them have a SIPS installed in their big Dob, and so wouldn't need coma correction in their eyepieces.

 

Mike

Edited by Sarkikos, 07 August 2024 - 02:48 PM.

[AaronF]

No, because the secondary is at an angle relative to the light cone and if it deviates from a flat, aberrations are added.
A coma corrector, or coma correcting Barlow, eyepiece, or SIPS is perpendicular to the light cone.

That makes sense, thanks!

[Houdini]

No, not really.  Not efficient or economical if you have other telescopes besides Newtonians.  Many observers have several telescopes, often a mixture of Newtonians, refractors and Cats.  Should they have a separate set of eyepieces for the Newtonians just to avoid having to buy and install a Paracorr or SIPS?

 

For myself, if eyepieces with built-in coma correction were available, I would not buy them.  This might be attractive for observers who have one big Dob and that's it, no refractors or Cats.  On the other hand, many of them have a SIPS installed in their big Dob, and so wouldn't need coma correction in their eyepieces.

Note that coma correcting eyepieces would work perfectly fine on any telescope from f/8 on, and very well from f/10 on. So most SCTs and long refractors would be well served by the coma correcting designs. In particular the 32 mm 81° design with 45 mm field stop would be wonderful to use in f/10 SCTs.

The only issue is with fast refractors in the range f/5 to f/7, the coma correcting eyepieces would show inverse coma.

 

I'm fine with people not loving the idea, it's OK and stimulating to have different opinions. And more choice on the market is better, to each their own.

[Starman1]

So you can:

--have them manufactured yourself under your own brand (requires large $ to get them made, and lots of time for prototypes).

--sell the designs to an eyepiece manufacturer

--go in with a larger retailer to make and import them

--get an importer/reseller like Baader or Tele Vue or APM to get the designs made under their aegis, where you get a royalty for each one sold

[ausastronomer]

So you can:

--have them manufactured yourself under your own brand (requires large $ to get them made, and lots of time for prototypes).

--sell the designs to an eyepiece manufacturer

--go in with a larger retailer to make and import them

--get an importer/reseller like Baader or Tele Vue or APM to get the designs made under their aegis, where you get a royalty for each one sold

 

I certainly think there is a market for them. As Robert said they aren't for everyone, but things have come long way from the days of the UO Pretoria eyepieces. They came at the wrong time, when everything else was going to a wider AFOV.  These also address and deal with a lot of the ergonomic issues the Pretoria didn't.

 

As you point out there are a number of options to bring these to market. I couple of your options I wouldn't recommend, namely APM and Televue, for different reasons.

 

I would be heading direct to KMO and seeing if they wanted to run with it.   They would be able to keep costs down, where a lot of the others couldn't, due to location and other reasons.

 

Cheers,

Edited by ausastronomer, 07 August 2024 - 08:17 PM.