107 replies to this topic

[CosmoSat]

The past few days have been lurking this forum for any initial reports on the new design Houdini coma correcting eyepieces. 

 

Appears that f-4 set to become the new f-5 for newtonians if this design lives up to its claims? And maybe more interest in f-3.3 scopes.

 

Me for one have been avoiding the faster scopes simply 

for not having to go through the hassle of using a coma corrector (ofcourse other than the price tags on those optics😅)

 

Those who bought these, kindly share your thoughts on these.

 

Clear Skies!

Satish

 

[Houdini]

You can find some user reviews at the end of the "Vendor and Group Announcements" forum thread https://www.cloudyni...ing-april-10th/ .

 

I hope the reviews from CloudyNights readers will soon find their way into the normal "Eyepieces" forum.

[ngc7319_20]

There is already a long thread running here...

 

https://www.cloudyni...oming-a-reality

 

... though with few reports.  Some folks report using them in refractors...  Hmmm...

 

It reminds me of the University Optics Pretoria 28mm -- a coma-correcting design from the 1980's.  And the related Brandon Coma Correcting 20mm from the same era.  Neither really caught on.  They were coma-correcting eyepieces with ~50 degree AFOV in an era when 82 degree eyepieces were becoming popular.  Maybe at 86 deg AFOV these will catch on better.

[Houdini]

The thread I linked is the followup of the thread you referred to, with more recent information.

You'll find the first user reviews from European observers near the bottom.

 

Holger reported about his first 10" f/5 binoscope session here on Cloudy Nights, I quote from the thread in the Reflectors forum:

 

I got some patches of clear sky this evening and had the 20mm Houdini eyepieces in my 10" f/5 binoscope. 69mm is probably the lowest IPD at which you can use two of these, it's just working for me.

In my garden I do not have the conditions for serious deep sky testing. All I can say for now is that stars remain stars out to the edge of the field at f/5 - imaging quality is excellent including field curvature correction. And there is no edge of field brightening from distortion (something I know from the Nikon NAV SW). The 20mm Pentax XW in comparison feel like tunnel vision and develop bad stars from ~70% of the field on. So here is a clear winner.

I noticed two similar comments on Dutch and French astronomy forums with quick tests using refractors:

- A Dutch user with a 100 mm F/9 Takahashi refractor: "Compared to a Nagler 20 type 5, the Houdini is much better".

- A French user with a 130 mm F/6 refractor: "The edge of the field is sharper with the Houdini 20 than with the Nagler 22."

 

With a 500 mm F/4 telescope under a SQM 21.6 sky Jean-David wrote a long report on the French Astrosurf forum.

A quote:

 

As far as correction is concerned, this eyepiece lives up to its spec: no coma right up to the edge. It's quite impressive, in fact. I spent a good while comparing it with the Nagler 22 (in the Paracor) and the correction is at least as good as in the corrected 22. The field of view of the Houdini is the same as that of the naked Nag 22; used with the Paracorr, the Nagler 22 therefore gives a smaller field of view.

So, duly noted, it works like a charm!

I didn't see any noticeable difference between the 20 and 22 in terms of sharpness, transparency, sky background, etc.

...

I'm quite impressed with the performance of this eyepiece, with its impeccable coma correction (at F4 in any case), all of which is quite light and for a very, very reasonable price.

Edited by Houdini, 22 April 2025 - 10:10 AM.

[Olimad]

I have high expectations for these eyepieces. Depending on how they perform, I’ll adapt my next scope purchase accordingly.

 

If they offer a focal length around 30mm as well, that would be great. For higher magnification, I know that the APM CC 2.7x Barlow works well at f/4 when paired with good orthos and Plössls.

 

Let see if other manufacturers will follow the Houdini trend.... 

 

Thanks Robert.

Edited by Olimad, 22 April 2025 - 02:04 PM.

[jrmacl]

I have high expectations for these eyepieces. Depending on how they perform, I’ll adapt my next scope purchase accordingly.

 

If they offer a focal length around 30mm as well, that would be great. For higher magnification, I know that the APM CC 2.7x Barlow works well at f/4 when paired with good orthos and Plössls.

 

Let see if other manufacturers will follow the Houdini trend.... 

 

Thanks Robert.

I don't think a 30mm would sell. These are for fast Newts, a 7.5mm-10mm exit pupil, I think your secondary mirror gonna get in the way.

 

your 30mm is an 11mm with the 2.7x Barlow

Edited by jrmacl, 22 April 2025 - 02:49 PM.

[CrazyPanda]

F/4.5 still needs a coma corrector, and a 30mm is still a 6.7mm exit pupil. I think that would be appropriate, especially if using aggressive line filters. The is likely a much larger market of people who own F/5 and F/4.5 scopes than who own really F/4 and shorter scopes.

[jrmacl]

F/4.5 still needs a coma corrector, and a 30mm is still a 6.7mm exit pupil. I think that would be appropriate, especially if using aggressive line filters. The is likely a much larger market of people who own F/5 and F/4.5 scopes than who own really F/4 and shorter scopes.

I agree there are alot more f4.5 and f5 out there, but even then I don't believe many will be looking for such a large exit pupil. Although I will say this, I recently got my first 32mm Plossl and was a little surprised when I put it in my f4.5 and found out it was just a little darker in the center- not as all as bad as I was led to believe after reading this forum for decades

Edited by jrmacl, 22 April 2025 - 03:01 PM.

[jrmacl]

F/4.5 still needs a coma corrector, and a 30mm is still a 6.7mm exit pupil. I think that would be appropriate, especially if using aggressive line filters. The is likely a much larger market of people who own F/5 and F/4.5 scopes than who own really F/4 and shorter scopes.

and by the way, aren't you one of the guy's that pre-ordered this? I've been just waiting for someone to start a thread on these, was really hoping someone who actually has them would have with their first looks thru.

[sixela]

but even then I don't believe many will be looking for such a large exit pupil.

 

Sure, I mean absolutely no one has either a 31T Nagler or a 28mm Meade 82° or UWAN ;-). And there isn't a thread lamenting the early demise of the 26T5 ;-).

[jrmacl]

Sure, I mean absolutely no one has either a 31T Nagler or a 28mm Meade 82° or UWAN ;-). And there isn't a thread lamenting the early demise of the 26T5 ;-).

I get it, but at the same time there are many who can't. A lot of people have to give up their 31T5 as they get older. Not saying a 30mm wouldn't have some customers, but what percentage of the market would that be? How many ep's does TV market that are greater than 30mm, or even greater than 20mm for that matter, versus how many they market that are under 20mm? I thinking that's why TV pulled the 26mmT5, no?

Edited by jrmacl, 22 April 2025 - 03:19 PM.

[Olimad]

jrmacl, on 22 Apr 2025 - 9:47 PM, said:

I don't think a 30mm would sell. These are for fast Newts, a 7.5mm-10mm exit pupil, I think your secondary mirror gonna get in the way.

your 30mm is an 11mm with the 2.7x Barlow

By 'around 30mm,' I meant an eyepiece with a larger field stop than the 20mm.

The 20mm has a 30mm field stop. If they release a 26-28mm eyepiece with a 36mm field stop, it would be great for achieving a wider TFoV.

They'll soon launch a 1.25'' 12mm eyepiece with an 18mm field stop, so I won't need to Barlow a 30mm eyepiece just to reach 12mm.

 

Edit: If your system is f/4 and you use a Houdini eyepiece, placing a Barlow before the eyepiece will alter the light cone, which would likely affect the performance of the coma-corrected eyepiece, right?

It would mean that the Houdini are sensitive to barlowing.

Edited by Olimad, 22 April 2025 - 03:51 PM.

[TayM57]

Edit: If your system is f/4 and you use a Houdini eyepiece, placing a Barlow before the eyepiece will alter the light cone, which would likely affect the performance of the coma-corrected eyepiece, right?

It would mean that the Houdini are sensitive to barlowing.

No different than TV Powermate ----> Paracorr II ----> Eyepiece, which is what TeleVue recommends. How I remember the order is, I know the eyepiece always stays with the Paracorr II. 

 

Unless you meant a regular barlow, in which case, I do not have an answer. 

[SeattleScott]

By 'around 30mm,' I meant an eyepiece with a larger field stop than the 20mm.

The 20mm has a 30mm field stop. If they release a 26-28mm eyepiece with a 36mm field stop, it would be great for achieving a wider TFoV.

They'll soon launch a 1.25'' 12mm eyepiece with an 18mm field stop, so I won't need to Barlow a 30mm eyepiece just to reach 12mm.

Edit: If your system is f/4 and you use a Houdini eyepiece, placing a Barlow before the eyepiece will alter the light cone, which would likely affect the performance of the coma-corrected eyepiece, right?
It would mean that the Houdini are sensitive to barlowing.

Yes it will alter the light cone, but it shouldn’t change coma because coma comes from the mirror. Barlow doesn’t reduce coma or CA of a fast achro, because those are inherent in the telescope itself. It just cleans up astigmatism and field curvature for budget superwides.

And valid point, if one normally barlows before P2, then there shouldn’t be an issue with barlowing a Houdini. Unless perhaps if you use the 2.7x coma correcting barlow, with the coma correcting eyepiece…

[jrmacl]

By 'around 30mm,' I meant an eyepiece with a larger field stop than the 20mm.

The 20mm has a 30mm field stop. If they release a 26-28mm eyepiece with a 36mm field stop, it would be great for achieving a wider TFoV.

They'll soon launch a 1.25'' 12mm eyepiece with an 18mm field stop, so I won't need to Barlow a 30mm eyepiece just to reach 12mm.

 

Edit: If your system is f/4 and you use a Houdini eyepiece, placing a Barlow before the eyepiece will alter the light cone, which would likely affect the performance of the coma-corrected eyepiece, right?

It would mean that the Houdini are sensitive to barlowing.

On the vendor page he says he has plans to release a 28mm Houdini and a Barlow, what's your third wish?

[Olimad]

Was aware of the 2x CC telecentric CC, but not of the 28mm EP.

 

Thanks.

[jrmacl]

Was aware of the 2x CC telecentric CC, but not of the 28mm EP.

 

Thanks.

First post when they were introduced in the Venders Forum (not the newer thread announcing the release date):

 

https://www.cloudyni...ming-a-reality/

 

"Over time, we intend to create a complete line of 28, 20, 12, 9 and 7 mm coma-correcting eyepieces."

 

Then later in post #42 he says:

 

"In the long run I imagine the Houdini series to go from 28 or 30 mm down to 4 mm (e.g. 30 - 20 - 12 - 9 - 7 - 5.3 - 4 mm), which spans a much larger focal length range than the Morpheus. It would not make sense to have a 7 mm 1.25" eyepiece share the same housing diameter as a 28 or 30 mm 2" eyepiece."

Edited by jrmacl, 22 April 2025 - 09:42 PM.

[CrazyPanda]

Ok, first light report with the 20 Houdini.

 

• Scope: 14.7" F/4.6 dob
• Conditions: Sky filled with thin high altitude clouds and generally poor transparency. Good seeing.
• Comparison eyepieces: 21 Ethos, 17 ES92, 28 UWA, and Docter (only for a comfort/immersion test, I did not use it to test anything else)

 

Comfort & Immersion Test

 

To start with, I aimed the 20H at a random spot in the sky, brought it focus, and just looked (no glasses on). The view was extremely big and easy - exactly what I was hoping to get out of the 20H and basically exactly what I wish the eyepiece market had. Plenty of one-hit-wonders, but no complete lines.

 

Without glasses I could take in the whole field with the lightest bit of contact with the eye cup (folded down). It was *almost* on par with the Docter for comfort, and *almost* on par with the ES92 for immersion, much better than the 21 Ethos for comfort, and maybe tied with it for immersion despite the narrower field. Much, much better than the 28 UWA with Paracorr.

 

Putting my glasses on, and I could just barely take in the field as long as my glasses made full contact with the eye cup. I didn't have to press into the eye cup, but I did have to make contact with it. I could not really roll my head easily to look directly towards the edge. I would say that 20H is not really a glasses-friendly eyepiece.

 

 

Field Illumination Test

 

Zero edge of field brightening, and zero vignetting/edge of field darkening. The field is remarkably uniformly illuminated. The 21 Ethos and 28 UWA (and 31N and 17 NAV-HW) all vignette with the Paracorr. Only the 17ES92 seems to be spared the vignetting caused by the Paracorr.

 

When I use those eyepieces WITHOUT the Paracorr, they all exhibit nicely uniform field as well. Seems the 20H saves you from the ill effects of the Paracorr's tendency to vignette low power ultra and hyperwides - a win for the 20H.

 

 

Star Quality Test

 

I tested against a variety of stars:

 

• Bright star (Regulus)
• Moderate stars (stars in M44)
• Faint stars (stars in M3)

I tested them at four positions:

• Center
• 50% towards the edge
• 75% towards the edge
• At the edge

 

Bright Star: Regulus

 

In the center of the field, Regulus snapped to nice focus in the 20H. Slight defocus on either side showed a very mild amount of chromatic aberration. This is *normal* in most eyepieces I've tested. Only a select few eyepieces are free of CA. The less CA, the tighter the stars tend to be at best focus. The 20H is about on par with the 21E for chromatic aberration. It's better than the 17ES92 and 28 UWA.

 

Moving Regulus towards the 50% field mark showed a very faint hint of a blue/violet separation. This could be controlled with some slight adjustment of eye position.

 

Moving Regulus towards the 75% field mark showed a strong separation into a spectrum. Regulus had dispersed into a spectral line that changed direction as you changed focus.

 

At the very edge, Regulus was a very strongly pronounced line presented as a vibrant spectrum.

 

When comparing Regulus in the other eyepieces WITHOUT a coma corrector, it broke down like this:

 

1. Throughout most of the field, it was obvious the 20H was presenting a cleaner, sharper view. The coma correction was working

2. At the equivalent field position in the 21 Ethos, the coma in the 21 Ethos was arguably a *lesser evil* than the spectral astigmatism being presented at the edge of the 20H. Ditto for the 17 ES92

3. The 28 UWA doesn't have the greatest correction, and it was obvious coma was mixing with astigmatism and the 28 UWA without CC was net worse than the 20H

 

When comparing Regulus in the other eyepieces WITH the Paracorr, it was a different story.

 

1. The 21E presented Regulus in a tighter, more controlled way throughout the entire field. There was some lateral color at the very edge, but significantly better controlled in the 21E + P2 than the 20H.

2. The 17 ES92 was similar to the 21E, but with slightly stronger lateral color than the 21E

3. The 28 UWA + P2 was also slightly better at its edge than the 20H was at its edge (save for the vignetting).

 

Moderate Star Field: M44

 

M44 was a good test because it has a lot of medium, uniform brightness stars spread out across the whole field.

 

My first look through the 20H was.... weird. I feel like I was seeing *negative* coma, but the more I looked and the more I compared it with other eyepieces, the less I noticed it. I need more time with this eyepiece on this or similar targets I think. Generally when I compared the field to the other eyepieces without the P2, it was obvious the 20H was rendering the field much, much better. However, with the P2, the other eyepieces just seemed to provide tighter control of the star shapes throughout the whole field than the 20H was. But as I mentioned, the effect waned as I looked more. Strange.

 

What I did notice, however, was mild field curvature when using the 20H. This was unmistakable. A 1/8 to 1/4 turn of the fine focus knob tightened up the stars throughout the 50-75% field range. No refocusing was necessary in the 21E or 17ES92 with P2. Some refocusing helped with the 28 UWA, but only closer to the edges.

 

I don't know if this FC is residual FC in the telescope and I'm used it to being mostly eliminated by the additional field flattening of the P2, or if the FC is in the 20H.

 

Faint Stars: M3

 

M3 (or M13) are great tests for the overall sharpness of an eyepiece's field. In the very center, dozens of perfect little pinpoints of light are visible. But if an eyepiece suffers from inadequate off-axis sharpness, it tends to erase those faint stars.

 

In the center of the 20H, M3 was a perfect rendition. Just as sharp as the 21E with or without the Paracorr. Excellent axial sharpness.

 

At the 50% mark, M3's faint stars disappeared. A slight refocus inward with the fine focus knob and they snapped back into existence.

 

At the 75% mark, M3's faint stars were smeared and much harder to see. Refocusing was not helping. The view was clearly degraded

 

At the field edge, M3 was just an unresolved blur.

 

Repeating this test with the 21E, and 17ES92 with P2, it was clear that M3 was crispy almost right to the edge of those eyepieces. Even at the edge, while it lost critical sharpness, most of the stars were still visible.

 

The 28UWA struggled the most and lost critical sharpness starting around the same point the 20H did. However, these are quite different focal lengths and the lower magnification of the 28UWA (even with P2) makes it harder to pick out the stars in M3, so not a great comparison.

 

First Light Conclusion

 

The 20 Houdini is a wonderfully easy eyepiece to look through without glasses. It's workable with glasses but still not what I would consider glasses-friendly. Field illumination is superb. It's light weight and the safety kerfs on the barrel are much nicer than an undercut.

 

The coma correction clearly works well, and it's strongly preferable to using UWAs and hyperwides without a coma corrector. However, if cost and weight are no concern and you're looking for impeccable sharpness across the whole field, it's not a substitute for a premium ultra or hyperwide + P2. Otherwise, it's a relatively economical way to get a comfortable, coma-corrected, ultra wide field of view.

 

I need to spend more time with it since I did observe some oddness with the coma correction of the eyepiece when looking at M44. I also need to assess its deep sky performance on a night with good transparency.

 

I also have an F/3 mirror on the way. Will be interesting to see how it fares at F/3.

Edited by CrazyPanda, 23 April 2025 - 03:53 PM.

[RLK1]

Nice review, Panda, even if you are crazy! 

 

Hmm, er, ah, I'm guessing you didn't buy the Ror eyepiece cleaning kit on their website....

[astrophile]

Short, informal, mediocre-conditions comparo.

 

The Contendehs:

 

Nighttime weather here on the mid-Atlantic has been lousy ever since receiving my Houdini 20mm last Thursday.  A couple nights with fleeting breaks in a lot of stratus & cirrus, with lousy transparency and meh seeing.  Nevertheless, curiosity got the better of me tonight and I pulled out the backyard dob to get first light on this new piece of kit.

 

My use case: A 10" f/6 Teeter STS with nice Royce conical mirror, under yellow-orange LP and typically mediocre seeing.  Until now, at f/6 I've rarely bothered with a Paracorr--but knowing I should, because coma is still a thing out past 60-70% of the FOV, even if not badly intrusive.  So I figure if the Houdinis are anywhere near comparable in quality of views to the existing high-end widefield offerings, they'll be a good candidate for optimizing f/6 views at home without having to muck with a coma corrector.

 

How does the Houdini fare against the august competition pictured?  Short story: So far, quite well.  I say 'so far' because with the moving clouds and poor transparency--and the season--I didn't have access to objects or a sky that would enable a full assessment.  No large clusters or star clouds; and no way to push contrast comparisons.  However, I was able to view a few objects that started to tell a favorable story: The Castor double; Mizar & Alcor grouping; M81+82; and the fascinating (to me) juxtaposition of M104, the "Jaws" asterism, and the nearby "Stargate" asterism.

 

Bottom line, the Houdini works while presenting a very nice, high-quality field of view.  It maintains sharply defined stars very nearly all the way out to the field stop of its generous FOV.  Granted, f/6 is not an acid test.  However, all of the TeleVues, as sharp as they were near the center (especially the 20T5 and E21), started showing coma around 2/3 the way out.  Not grossly, but it's there--and as Don Pensack has said, once you see it there's no unseeing it.  Even at f/6.

 

Houdini kept up with the 20T5 and the Ethos in terms of sharpness; the latter two did better than the 22T4 as one looked farther off-center, and the Houdini did noticeably better.  I thought the 86deg AFOV of the Houdini presented a nice middle ground between the Naglers' 82deg and the Ethos' 100.  It felt more immersive than the 20T5; about the same as the Ethos and the 22T4.

 

Where the Houdini earned its keep was in viewing multiple objects in the same FOV, where those objects--or parts of them--fell outside ~ the center 50% FOV.  For example, M104 and Jaws.  With M104 nicely framed on one side of the FOV, all of Jaws' stars remained sharp and tight even near-ish to the opposite edge of the Houdini.  Not quite so in the 20T5 and Ethos; and definitely not in the 22T4.  I had to look for the coma in the TeleVues--might not consciously notice in a casual view--but again, no unseeing it once noticed.  I expect this nice characteristic of the Houdini will hold true for other wide or multiple objects, e.g. Markarian's Chain (covered by crud tonight), the Double Cluster, Kemble's Cascade, M31, etc etc.

 

As I said, lousy transparency precluded any decent assessment of contrast and transmission in the Houdini--I'll be looking for that in subsequent, hopefully better sessions.  But already I anticipate the Houdini becoming my go-to 20mm eyepiece for the 10" f/6, and I'll be purchasing the 12mm and 30mm when they come out.

 

*edit: With CrazyPanda's observation I will look more closely next time, and be more conscious, of any refocusing needed in the H20 between on-axis and edge-of-FOV at f/6.

 

A couple side notes.

• This wasn't quite a fair evaluation--the 20T5 is a traveling refractor eyepiece for me, not my "perfect" dob ep anyway--but I had to compare it, as the exact focal-length counterpart to the Houdini 20mm.  (Interestingly, as significant is the size difference between the two, the Houdini and the 20T5 actually weigh very close to the same.)  And I already knew the 22T4 really needs a Paracorr in a reflector to clean up best; and that the Ethos would of course show some coma out at that 100deg AFOV.  I'll keep using the E21 (and my Nag 31, and Nikon NAV-HWs) with Paracorr on the 16" at the dark site, no question.  But it sure is nice now to have a 20mm (and hopefully eventually soon a full eyepiece line) whose presentation I like very much for at-home use without the bother of a Paracorr.
• I normally don't wear glasses at the eyepiece--my viewing eye measures 0.5D in cylinder, so I wear infinity glasses for naked eye views and binoculars, but don't use them at the scope. I did try the Houdini with glasses on, just to see how it worked out.  I'm able to access the full field with eyeguard rolled down and glasses mashed close to my face--so it's doable but for me a near thing.  I have relatively recessed eye sockets, though, and a narrow IPD, neither of which help.

I'm super looking forward to other user reports, to further wringing out the 20mm myself, and to seeing the 12mm and 30mm models come to market.  Thanks Robert Houdart for bringing us this innovation and for executing it so well.

Edited by astrophile, 23 April 2025 - 01:29 AM.

[Houdini]

What I did notice, however, was mild field curvature when using the 20H. This was unmistakable. A 1/8 to 1/4 turn of the fine focus knob tightened up the stars throughout the 50-75% field range. No refocusing was necessary in the 21E or 17ES92 with P2. Some refocusing helped with the 28 UWA, but only closer to the edges.

 

I don't know if this FC is residual FC in the telescope and I'm used it to being mostly eliminated by the additional field flattening of the P2, or if the FC is in the 20H.

Thank you for the detailed report.

I checked the calculated field curvature of the Houdini 20 for your 14.7" f/4.6 configuration (about 1700 mm focal length), and the required accommodation of your eye is 0.6 diopters. This should be achievable for most people.

If focusing in the center leaves the outer field not quite in focus, you can try the opposite: focus midway in the field so that your eye accommodation makes the center sharp.

Our reviews focus mostly on telescopes and eyepieces, but ultimately the human eye is the limiting factor .

[Houdini]

Bottom line, the Houdini works while presenting a very nice, high-quality field of view.  It maintains sharply defined stars very nearly all the way out to the field stop of its generous FOV.  Granted, f/6 is not an acid test.  However, all of the TeleVues, as sharp as they were near the center (especially the 20T5 and E21), started showing coma around 2/3 the way out.  Not grossly, but it's there--and as Don Pensack has said, once you see it there's no unseeing it.  Even at f/6.

 

Houdini kept up with the 20T5 and the Ethos in terms of sharpness; the latter two did better than the 22T4 as one looked farther off-center, and the Houdini did noticeably better.  I thought the 86deg AFOV of the Houdini presented a nice middle ground between the Naglers' 82deg and the Ethos' 100.  It felt more immersive than the 20T5; about the same as the Ethos and the 22T4.

 

Where the Houdini earned its keep was in viewing multiple objects in the same FOV, where those objects--or parts of them--fell outside ~ the center 50% FOV.  For example, M104 and Jaws.  With M104 nicely framed on one side of the FOV, all of Jaws' stars remained sharp and tight even near-ish to the opposite edge of the Houdini.  Not quite so in the 20T5 and Ethos; and definitely not in the 22T4.  I had to look for the coma in the TeleVues--might not consciously notice in a casual view--but again, no unseeing it once noticed.  I expect this nice characteristic of the Houdini will hold true for other wide or multiple objects, e.g. Markarian's Chain (covered by crud tonight), the Double Cluster, Kemble's Cascade, M31, etc etc.

Thanks for the great report.

At f/6 you would probably never bother with a separate coma corrector, but indeed, the coma correction still makes a difference on a well corrected eyepiece.

[Houdini]

A report by user maire on the French Astrosurf forum, comparing with Pentax XW 23 in Paracorr 2 using a 300 mm (12") F/4 telescope.

 

Pentax XW23 85° + Paracorr Type 2 and The Houdini 20 mm 86°. The T300 F/4 single-arm “craftsman-quality optics” instrument.

 

With the Pentax XW23 behind the coma corrector, I get 60x magnification.
With the Houdini 20, I have a magnification of (drum roll...) 60x too !!!!

It's quite amazing, the field of view is exactly the same. You'd have to smoke super-thick carpet to see any difference. The coma correction is identical. Sharpness to match. Perhaps at the extreme edge of the field (like around 5% of the distance from the edge to the center...) the shape of the corrected stars changes a little, but is not at all degraded. But that's just looking for lice on the head of an Ourang-outan.

 

Conclusion: the Houdini 20 does the job superbly. The Houdini is a lot lighter... I'll have to rethink the balance of my instrument, even if it'll do.

 

 

[sixela]

spectral astigmatism

Just to be a stickler for nomenclature: this is usually called "lateral colour" (i.e. the magnification is not exactly the same for all colours, which offsets the images for different colours with respect to each other), and while it is "spectral" it has little to do with astigmatism.

[CrazyPanda]

Just to be a stickler for nomenclature: this is usually called "lateral colour" (i.e. the magnification is not exactly the same for all colours, which offsets the images for different colours with respect to each other), and while it is "spectral" it has little to do with astigmatism.

Fair enough, though there is some astigmatism component because the orientation of the shape does change as you shift focus, though the position of the lateral color does not (red is always to the outside, blue/purple to the inside).