184 replies to this topic

[TayM57]

Ok, first light report with the 20 Houdini.

 

Excellent report that avoids the mistake others make in their reviews (myself included) in that reviews typically comment on the features of the object being observed, instead of the eyepiece characteristics, which you don't do here. Good job. Whatever you do, do not make the mistake I did last night, which is compare newly purchased EPs to the Morphii. I also came away with the impression that your review was honest. Thank you for that. 

 

M5 is a good target for evaluating an eyepiece as well. I observed M3 and M5 (and to a lesser extent, M44) last night to evaluate my new EPs- and M5 is just a beautiful cluster that makes it easy to evaluate sharpness on and off axis. 

Edited by TayM57, 23 April 2025 - 09:54 AM.

[Starman1]

In my 16" f/3.75, the focal length is 1524mm without Paracorr, and 1753mm with.

My current plans are to use the 22mm Nagler as the low power in the scope, with Paracorr = 80x, 1.02°, 5.12mm exit pupil.

The Houdini 20mm would be 76x, 1.13°, 5.33mm exit pupil.

So, a little larger true field, but nearly equal magnification and exit pupil.

I will look for the secondary shadow of the 21.9% secondary (my pupil is only 4mm), but I don't expect to see it.

 

This would be a fairly low power eyepiece for me.  I usually start at 130x and go up.

I note that the field size is almost identical to a 30mm UFF in my 12.5" at 61x.

[slavicek]

Thank you CrazyPanda for honest review. Very informative and well done. We need info like this to keep ourselves well informed. I promise to be honest with my new N7s once I get the opportunity to test them. 

[CrazyPanda]

Second light report with the Houdini 20

• Scopes: 14.7" F/4.6 dob & A-T 90EDX


• Conditions: Excellent transparency. Very good seeing. SQM 21.05 (average)


• Comparison eyepieces: 21 Ethos (no paracorr)

 

This will be a shorter review. My main goal was to assess transmission and contrast. I don't have a lot of eyepieces at this focal length so the 21 Ethos without a Paracorr was the basis for comparison.

 

14.7" F/4.6 Dob

 

I did a tour of the prominent spring targets: Leo Triplet, Markarian's Chain, M3, M81 & M82, M101, M51, and a few other misc targets. In every single case I felt the 20H had excellent contrast and transmission. In a few cases, maybe a bit better than 21E, but the difference was so subtle it was likely explained by the slightly higher magnification and smaller exit pupil. The relative sky brightness in each eyepieces was exactly what one would expect, with neither appearing unexpectedly darker or brighter. Neither eyepiece was showing me things that the other could not.

 

With better conditions for M3, I tested the central sharpness of each eyepiece. No question that the 20H was every bit as sharp in the center as the 21E. Very, very crisp rendition of M3.

 

I repeated the same test I did yesterday and got the same result - at 50% to the edge, a slight refocus was necessary to bring the stars to critical sharpness. In the non-Paracorr'd 21E, however, there was no way to do that. It just looked bad. At 75% to the edge in the 20H, the view was slightly degraded and no longer critically sharp, and at the very edge only a handful of the brightest member stars were still resolved, but not clean.

 

I also discovered the source of the negative coma that I saw yesterday and it makes perfect sense - it's related to the field curvature I see. In a coma-correcting eyepiece, defocus isn't just defocus, it's also deviation from optimal coma correction. Slightly outside of focus, I was able to induce negative coma. Slightly inside of focus, I was able to induce normal coma. The slight FC I was experiencing requires me to bring the focus inward a tiny bit, which means stars in the ~50-75% zone are slightly outside of focus when the center is in focus, thereby producing slightly negative coma.

 

Tonight my ability to accommodate the FC was better than yesterday (my FC accommodation fluctuates but it's definitely no longer as good as it used to be).

 

90 EDX

 

I wanted to see how the eyepiece behaved in my 90mm F/6 triplet. I didn't spend much time with this setup, just basically aimed at somewhat dense patch of fairly bright stars and looked to see how they were presented. For the most part, they looked excellent. I could see some mild astigmatism at the edges of the field. The dominant issue was field curvature, no doubt from the refractor. When at best focus, stars in the 50-75% zone had very, very minor negative coma, which is to be expected when pairing a coma correcting eyepiece with a scope that does not have coma.

 

The visibility of this negative coma was negligible and the eyepiece performed very well overall. I didn't put the 21E in the focuser, but I know from experience the field curvature of that refractor combined with the hyper wide field of the 21E is not a great combination.

 

I know that the 31N, however, performs very well, and for whatever reason seems to negate the effects of FC in that refractor for my vision. The 20H wasn't *quite* as nice as the 31N, but it was still good. If I had a choice between the 21E and 20H in that refractor, it would definitely be the 20H. If I had a field flattener in that refractor, maybe I'd have a different opinion.

 

I very much enjoyed the Houdini in tonight's session.

[Houdini]

Thanks again for a second, very detailed report.

 

I would agree with your analysis that the problem you had is related to field curvature and focusing - no other observers have reported this and I haven't experienced it myself.

I will take your experience into account for the eyepiece design software. It's possible that if I can force the field curvature in the other direction, it might be a better fit for people with limited eye accommodation. I will have to look into this.

 

Thanks also for your test with the F/6 refractor. Some CloudyNights users have been very adamant about not even considering using a coma-correcting eyepiece in a refractor .

It is remarkable that a coma-correcting eyepiece performs better than some of the best standard eyepieces in a 90 mm F/6 triplet. I've had a similar report from an observer using a Takahashi 100 mm F/9 refractor, where he preferred the Houdini 20 to the Nagler 20 Type 5.

[Astrojensen]

 

It is remarkable that a coma-correcting eyepiece performs better than some of the best standard eyepieces in a 90 mm F/6 triplet. I've had a similar report from an observer using a Takahashi 100 mm F/9 refractor, where he preferred the Houdini 20 to the Nagler 20 Type 5.

This is not strange, if the H20 has mild field curvature in the same direction as the refractor. It will then be sharper across the field, compared to a completely flat field eyepiece. The coma introduced at f/6, and especially at f/9, is likely smaller than the defocus from (telescope) field curvature in a completely flat-field eyepiece. 

 

The interplay between eyepiece and telescope is complex, and you can often find surprising combinations, that at first thought really shouldn't work, yet do.

 

 

Clear skies!

Thomas, Denmark

[starcanoe]

Isn't a coma correcting eyepiece going to create inverse/reverse coma that is proportional to the f ratio of the optical path  that the eyepieces "sees" optically speaking?

 

So.....a coma correcting eyepiece used on an f9 scope (without any coma) is going to show reverse coma comparable to an f9 system that had uncorrected coma to start with. Not much coma to see.

 

Used on an f6 system.....moderate reverse coma but tolerable.

 

F5 or below....lotsa reversa coma.

[CrazyPanda]

Isn't a coma correcting eyepiece going to create inverse/reverse coma that is proportional to the f ratio of the optical path  that the eyepieces "sees" optically speaking?

 

So.....a coma correcting eyepiece used on an f9 scope (without any coma) is going to show reverse coma comparable to an f9 system that had uncorrected coma to start with. Not much coma to see.

 

Used on an f6 system.....moderate reverse coma but tolerable.

 

F5 or below....lotsa reversa coma.

Yes, that's correct. It would be nice if someone with an NP101 or 127 would do a test to see how the eyepiece behaves at that focal ratio and with a flat field.

[starcanoe]

This is not strange, if the H20 has mild field curvature in the same direction as the refractor. It will then be sharper across the field, compared to a completely flat field eyepiece. The coma introduced at f/6, and especially at f/9, is likely smaller than the defocus from (telescope) field curvature in a completely flat-field eyepiece. 

 

The interplay between eyepiece and telescope is complex, and you can often find surprising combinations, that at first thought really shouldn't work, yet do.

 

 

Clear skies!

Thomas, Denmark

 

Here's something I seem to recall...though I wouldn't bet on the accuracy.

 

I think the Konig Eyepiece design differs from most other simpler eyepiece designs in that its focal plane  is curved in the opposite direction to most others. And its curvature has the same sign/orientation/magnitude as refractors with a moderate (shorter?) focal length. So it has a built in field flatner so to speak.

[ngc7319_20]

AT72EDII refractor

 

My Houdini 20 arrived today.   I was curious how it would work in my Astro-Tech AT72EDII.  This is a small doublet refractor with FPL-53 and Lanthanum glasses.  The focal length is 430mm making it F/6.  This scope has a little Newtonian-like off-axis coma (i.e. the "comets" point toward the field center) so I thought it was worth a try.   Results were surprisingly good.  Images were sharp over nearly the entire field, other than the expected (large) field curvature of this scope.  In other words, re-focusing for the field edge gave sharp images free from coma or astigmatism.  Adding the TSFLAT2 field-flattener lens to the eyepiece with about 70mm of spacer tubes gave pinpoint images over the central 80% of the field diameter with no re-focusing.  Between 80% and the edge there was a little astigmatism from the flattener. I suppose a younger person with more eye accommodation might not need the TSFLAT2 flattener lens at all.

 

The target was a couple faint light bulbs and street lights 5 miles away that I can see from my porch.  It's cloudy, so that's the best I can do for now.  I use these lights often for testing and I expect the results will be valid for faint to moderately bright stars.

 

I also tried the Nagler 20 Type 2, Nagler 20 Type 5, and Nagler 22 Type 4 eyepieces in the same configuration, but none were as sharp as the Houdini 20.  The Naglers all showed some combination of off-axis coma and / or off-axis astigmatism.  The coma seen in the Naglers was no doubt from the scope, whereas Houdini seemed to correct it.

 

Many refractors have off-axis coma with the little "comets" pointing away from the field center -- opposite the AT72EDII -- so I am not certain how applicable my nice result will be to other refractors.

 

I also tried the Houdini 20 during day time in the same scope -- images were similarly sharp.  The Houdini had some weird orange-colored effects during day time.  If my eye was too far from the eyepiece, the field edges turned bright orange.   These color effects were a little annoying at first, until I got used to the eye position.    At night the color effects all went away -- I presume it is somehow related to eye pupil size.

 

The time from placing the order to delivery in Maryland was 3 days.  I am not aware of any customs charges, though duty / tariff bills sometimes arrive later.  The tariff situation seems to be unpredictable from day to day right now.

Edited by ngc7319_20, 26 April 2025 - 02:33 AM.

[Vostok]

I received the 20 mm Houdini a few days ago and had the chance for short tests on two nights despite us quickly running out of darkness in Finland. More careful testing will have to wait till August.

The concept of the Houdini eyepiece struck me as so interesting that I had to order one immediately when they came available. I then found I had the problem that I don't own any telescopes that produce coma. 

 

That problem was quickly sorted and I now find I own also a GSO 16" f/4.5 and an SW 130PDS 5" f/5. I do have experience with newtonians, big and small, since a few decades, but the availability of the Houdini and my new dark sky location provided enough motivation to pull the trigger on finally getting some decent scopes of my own. Since these particular instruments are also new to me, my observations can hardly provide the level of detail described in the previous reviews in this thread. I also have somewhat limited experience using coma correctors and high end eyepieces (some sporadic views at star parties).

 

In the 16", Houdini 20 provides fantastic views compared to other eyepieces available to me at this time (SW bundled 28mm LET Kellner, a 32mm SWA, and an almost full set of Hyperions). With my spectacles, I did not find it too hard to see the edge of the field. I do have to press the eyeglass to the folded rubber cup, which makes it a bit hard to shift my eye laterally to reach the edge. However, after an hour or two of getting accustomed, I seemed not to notice this problem anymore.

I also very much enjoyed using the Houdini 20mm in the 5" f/5 with no mount, just hugging it hand held sitting down. This configuration is especially suitable for eyeglasses, since I can keep fixating on the center of the field and turning the telescope in my hands happens almost by instinct. Again, compared to other mentioned eyepieces, I found using the Houdini like this endlessly rewarding. I can't wait for the milky way views after summer.

The upcoming 30 mm is also a very intriguing option especially for this scope. In terms of exit pupil, I wouldn't mind if would be 28 mm instead. Defocused star tests revealed that I'm barely running out of full aperture with a 32 mm eyepiece at f/5, and 30 mm might be a bit close.

I most certainly wouldn't have been bothered with using a separate coma corrector at f/5, so I fully recognize the appeal of these type of eyepieces now.

After these short tests and with my couple new instruments, I'm already very happy with the Houdini. A future scenario, in which I wouldn't own an entire lineup, starts to seem very unlikely.

[Fabricius]

I then found I had the problem that I don't own any telescopes that produce coma. 

[Tucker512]

I just received the 20mm eyepiece yesterday and I love it!  Tried it on my 10" f/3.6 scope and it works beautifully.  No distortion while panning across the sky, sharp to the edge.  Very comfortable to view with.  I think the 86-deg AFOV is perfect.  I'm the optical designer at Starizona and I thought this was such a cool idea and wanted to try it out.  Well done!  Looking forward to using it more any trying it on more telescopes.

 

Scott Tucker

[SandyHouTex]

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

I don't think it's the coma that kills the f/3.3s, it's the collimation.

[Starman1]

I don't think it's the coma that kills the f/3.3s, it's the collimation.

Even with perfect collimation, stars are pretty ugly at f/3.3 without a coma corrector.

But I hear you, the precision of collimation required, and the mechanical structure's ability to hold it through altitude swings means f/3.3 is going to depend on collimation, coma corrector notwithstanding.

One maker, Astrosystems, really beefs up their dob kits for the short f/ratios as a result.

I shudder when I see a lot of these spidery f/3-f/4 scopes in large apertures when such light construction would be iffy on a 10".

[Houdini]

I don't think it's the coma that kills the f/3.3s, it's the collimation.

 

It's a mechanical challenge, but many people have been quite successful with F/2.8 to F/3.6 telescopes.

 

It's not much fun to observe below F/4 without coma correction. Coma increases with the inverse square of the focal ratio, at F/3 you have 4 times more coma than at F/6.

[slavicek]

I should have asked this question earlier but anyway, is there "a design" focal ratio for Houdinis or, for that matter, for any other coma correcting eyepiece? Meaning that if the design ratio is say F/4 then the eyepiece will work best at that F# and will "over-correct" slower scopes and "under-correct" faster scopes? Hence generally giving better views in reflectors but giving "best" views at specific F#?

 

I am sorry but I just cannot justify buying another eyepiece to satisfy my curiosity...  especially when I already have all the Ethos, Parcorr etc... unless it was designed specifically for F/3.3 reflector... 

[Houdini]

Most of the optimizations are done at F/4, but the designs work very well down to F/3 (and maybe below, I've never checked).

 

There is no "under-correction" at F/3, coma is still almost completely removed, but the main problem becomes spherical aberration. SA increases with the inverse cube of the focal ratio, which means that F/3 is more than twice as bad as F/4. And a 20 mm eyepiece at F/3 gives a very large exit pupil (almost 7mm), which increases the difficulty even more.

 

Nevertheless, the performance should be very good. I've posted some graphs in another thread to show the performance at different focal ratios: https://www.cloudyni...ity/?p=13880666

 

In the second graph of that post, you can see that the Houdini 20 at F/3.3 performs better than an uncorrected view at F/8.

Edited by Houdini, 26 April 2025 - 06:59 PM.

[CrazyPanda]

It's a mechanical challenge, but many people have been quite successful with F/2.8 to F/3.6 telescopes.

 

It's not much fun to observe below F/4 without coma correction. Coma increases with the inverse square of the focal ratio, at F/3 you have 4 times more coma than at F/6.

I believe it's actually the *cube*.

 

F/3 has 8x the coma that F/6 does, so it's even worse.

[Houdini]

You can verify that coma increases with the inverse square of the f-number in the second graph of the post I linked above:

https://www.cloudyni...ity/?p=13880666

 

This graph: https://cruxis.com/s...ot Diagrams.png

 

Compare F/8 and F/4 to see approximately the factor of 4.

Or F/6 and F/3.3.

Edited by Houdini, 27 April 2025 - 04:57 AM.

[ngc7319_20]

6" F/4 Imaging Newtonian

 

I did a 4-way shoot out with different coma correctors.  Target was some mag 5 to 7 stars around NGC 4565 and M13.  Scope is an Orion 6" F/4 Imaging Newt, though a ray trace indicates it is closer to 5" F/4.8 due to the secondary being undersized.

 

First place goes to: Nagler 20mm Type 5 with an Explore Scientific Coma Corrector.  Pretty much point-like stars to the edge.

 

Second place is a two-way tie: Houdini 20mm showed a little field curvature at the edge.  This could be minimized by focusing on stars about 1/3 or 1/2 way to the field edge.  I guess field curvature is not surprising since the focal length is pretty short for a Newt.   I note @Vostok also liked Houdini in a similar 5" F/5 Newt.

 

Also in 2nd place: Nagler 20mm Type 5 with the original TeleVue ParaCor / Tunable Top set to the tallest spacing.  Stars at the edge showed a little bit of residual coma.

 

A very close fourth place goes to: Nagler 20mm Type 5 with a GSO Coma Corrector connected together with 28mm length of 48mm extension tubes.  It had a little bit of coma and astigmatism at the edge.

 

Houdini is "plug and play" which is a huge advantage.  With the other 3 coma correctors I probably spent an hour total looking up eyepiece focal plane positions, ParaCorr settings, reading manuals, crunching numbers, converting inches to mm, finding my extension tubes, trying different tubes, etc.  With the Houdini you just drop it in and observe... yea!

 

80mm F/5 Achromat Refractor

 

The field edge was just plain bad with huge coma.  This is expected, of course, since the coma in this refractor and in Houdini have the same sign and add together making even more coma.  Optical theory works...  Scope is one side of an Analog Sky "Heart" bino scope with a cemented 80mm F/5 lens.

Edited by ngc7319_20, 27 April 2025 - 05:13 AM.

[Olimad]

Thanks, NGC, for your reviews.

Have you checked the light transmission—whether the same number of stars could be seen with the different combos you used?

 

I am planning to get  a 6" F4 carbón tube and some Houdinis. The secondary spec is given at 63mm diámeter, which one is yours?

[ngc7319_20]

Thanks, NGC, for your reviews.

Have you checked the light transmission—whether the same number of stars could be seen with the different combos you used?

 

I am planning to get  a 6" F4 carbón tube and some Houdinis. The secondary spec is given at 63mm diámeter, which one is yours?

I measured the relative light transmission of Houdini just with my iPhone camera... it is within 1% or 2% or Nagler 22 T4.  I guess in fairness one should probably compare Houdini vs. eyepiece + coma corrector, but I have not done that.  I did not try to count stars, but I did not notice that it was darker or stars were missing or anything unusual.

 

Secondary in mine has reflective surface about 61mm or 62mm.  I guess it is probably the same as your 63mm with some edge bevel.  The focal plane is very far from the secondary mirror, and 63mm is too small to capture the full F/4 light cone. They allow xetra back focus for camera, filter wheel, off-axis guider, corrector lens, etc., etc.

[Sarkikos]

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?

What does getting older have to do with using a 31T5?

 

Mike

[Sarkikos]

Do the Houdini eyepieces show field curvature in a fast Newt?   This would not be good for me, since my eyes no longer accommodate for focus.  Focusing between center of field and edge of field will not work for me.  If there is FC in the system, I will see it.

 

I can correct for coma in a fast Newt by inserting a coma corrector.  But I can't correct for field curvature from an eyepiece.  

 

Mike

Edited by Sarkikos, 27 April 2025 - 08:05 AM.