49 replies to this topic

[Crayfordjon]

A French ATM has built a 250mm (10 inch) Hypochromatic refractor strictly to the specs I laid down, he is Louis Lore. He told me he was impressed that an ordinary set of achromats could correct for colour so well. He used the duplex system, here is the OTA and an image he took afocally through it. Louis will be launching a thread on his Hypo later on in the year, I will make no further comment

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[Crayfordjon]

An image a of roof top, some distance away.

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[philipdo]

That looks impressive. Is this a focal or an afocal image? Any idea of the magnification used for this image?

philip

[philipdo]

Seems like I didn't read your post very well. So this is an afocal image. Magnification?

Philip

[PrestonE]

Very impressive and fantastic image too...

Best Regards,

Preston

[GlennLeDrew]

As I've pointed out in the past, afocal images should be viewed with circumspection. If the exit pupil is large, and the camera lens iris is small, one is effectively stopping down the system. For example, suppose the exit pupil is 6mm. A point 'n shoot camera whose lens is set to a 12mm f.l. at f/11 has an iris of 1.1mm. The telescope aperture is therefore stopped down significantly, to roughly (the camera lens elements ahead of the iris will somewhat modify the scale of the exit pupil) 1.1 / 6 * 10 = 1.8". A 10" system stopped down to 1/5 its aperture will certainly improve aberrations.

Furthermore, does the provided image cover the full field of the eyepiece? Is an especially narrow field eyepiece being employed?

Too many uncertainties to reliably judge performance. A prime focus image would tell us much more.

[Crayfordjon]

Philpdo, the mag is X20.

[meade4ever]

The image John posted is the first result of the JW250 hypo.

The construction began about 18 months ago when I discovered the existence of the hypo in this thread :
http://www.cloudynig...5106270/page...

I was immediately amazed by this new concept because I have always thought that, for the biggest diameters, the refractor's future is linked to single lens with a small corrector behind (something like the chromacor) and not with triplet fluorite lenses.

Further more John described the hypo refractor as a low budget ATM project and very simple to build.
I have more consideration for things simply made and cheap which work rather than high end products at astronomical prices.

I'm going to describe the building of this giant refractor here but I just wanted say first that without the help of John this telescope would not have been made.

During all these months John has always answered all my questions, gave me advices, tips, spent time for testing the OG, predict results with experiments and calculations.
I have never, never been let in the dark.

I always encourage everyone who would like to build it own hypo to contact John who will be more than happy to share his knowledge.

[Mark Harry]

I had good experience (and a lot of educational fun) when I experimented with John's concept. The singlet lens has to be a long focal length, or the aperture of the 8" lens I used had to have a mask in place, since it had a much shorter focal length. Best results I had were when a mask around 2.5-3" was used, and I saw a very sharp image of the underside of spruce needles at 60 yards with the little fine white stripe of the central vane on them.
The resulting focal length was the whole width of the living room for my setup- but it -DID- work as claimed at lowish powers.(IIRC, low powers recommended, or filters for monochrome imaging also a possibility)
My problem, I have limited room for stowing such an instrument, so I never placed everything in a tube of sorts. It could have been made with a couple folding flats to shorten it up.
With the F/Ds recommended for the singlet, a narrow bandpass solar scope for viewing Ha detail with just the front lens itself is readily possible too.
M.

[wiseone]

Many congratulations, John, your Hypochromat design is a real winner! Inexpensive and with surprisingly good performance.
Maybe the era of single objective lens refractors is dawning. Either the Hypochromat design or retrofocally corrected systems are the way to go for large refracting telescopes.

[Crayfordjon]

Thanks Louis and Mark. I admit the Hypo is not high end optics it was never meant to be, it is a good compromise which yields startling results, that is why I started researching it five years ago, when I discovered the optical effect. The Hypo system has strong contraints, these being that it will only work at low powers before serious colour kicks in. There is residual LCA which is reduced by using a very long focus OG at least F;40, but this can be sensibly cut right down by having a reduction lens close to the OG, the criterion being that the OG has just twice the surface area of the red' lens, the diameter being governed by the factor 1.414 the square root of 2. You can work this out for yourself. If you want the OG to have three times the surface area, then the critical factor is the square root of 3. The result of having this arrangement is that the image is very bright. There are other defects which place the Hypo out of the high end optics brigade. The bigadvantage though is the very low cost of making a really large refractor for the price of a six inch Dob, and it is easily built, it has very forgiving tolerances for misalignment and positionng of the red' lens, which can be put any where along the optical axis, however the LCA increases towards the focal plane. This is a scope for beginners, those not skilled in precision optics, and those who have a limited budget. It works well and a lot of astronomy can be done with a Hypo.

[Crayfordjon]

Yes Peter it is the way to go when you can achieve an apochromat system for an eight inch retrofocal refractor, where the fluoro glass element is only 30mm dia.

[wiseone]

Yes, John, the 8 inch refractor has a singlet objective, a singlet field lens and two identical doublets further down to give virtually apo performance. Cost is low, and the weight, too. Pity it hasn't taken off yet. Visually, it is absolutely stunning, but you need to use filters for imaging. A Baader fringe killer works very well. The spot diagram tells you a little more. All this was derived from John's amazing 30 inch refractor, all made by himself - an achievement that remains unmatched, and will probably never be bettered.

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[nytecam]

Well done John for recruiting another ATM to the Hypo and from this side of the Pond. It seemed to get 'waterlogged' crossing the Pond :-)

[GlennLeDrew]

Wiseone,
You state "virtually apo performance", the mention that "you need to use fikters for imaging." And that spot diagram is for the relatively restricted spectral range of 560-620nm. What would we see over, say, 450-650nm? Or, for imaging purposes, 400-700nm?

[bremms]

It looks like there is 560, 490 and 620 on the spot diagram.
still 490-620 is a limited range.

[GlennLeDrew]

Oops! I was hasty, in that I overlooked the shorter wavelength being out of sequence in the legend, between two longer wavelengths (an odd practice.)

[Dave O]

Oops! I was hasty, in that I overlooked the shorter wavelength being out of sequence in the legend, between two longer wavelengths (an odd practice.)

Actually, it is not that unusual in OSLO as it aligns the 'frequency' with the 'color' used in the plot ... 1st wavelength is plotted in green, 2nd in blue and 3rd in red.

[MKV]

Wiseone,
You state "virtually apo performance", the mention that "you need to use fikters for imaging." And that spot diagram is for the relatively restricted spectral range of 560-620nm. What would we see over, say, 450-650nm? Or, for imaging purposes, 400-700nm?

That's because his theory is that we don't see unfocused light (?), and imaging devices "see" a much wider spectrum. In fact most ordinary to medium priced digital cameras have a range from 400-700 nm, just like your eyes.

the other thing is, if you can afford a camera with a 390-1100 nm range, you can probably afford a true apochromat. Or, if you're going to use filters for imaging, an oridnary 6-inch f/9 Meade refractor (about $600 for OTA) will do just as well. Heck, for a solar telescope just get a long focus planoconvex lens and a H-alpha filter! yeah, narrow band filters work really well with cheap refractors.

Of course, even a mediocre telescope will make nice clean rooftop images at the prime focus. That's not a real test of quality.

regards,
Mladen

[GlennLeDrew]

What concerns me is that the Hypo advocates, as a demonstration of performance, seem to supply only afocal images, through an eyepiece delivering an exit pupil likely to be larger than the camera's iris, a condition which reduces aberrations by stopping down the aperture.

A better approach is a prime focus image, for it guarantees full aperture useage, and avoids the potential additional aberrations and vignetting at the coupling of eyepiece and camera lens. One might argue that this is not representative of the actual use, where an eyepiece is employed. But I counter with the argument that all telescopes are first assessed by the image delivered at the focus, where a bad image is not usually made better by the addition of an eyepiece.

[MKV]

You make a valid point Glenn, but a prime focus image is simply the lowest possible magnification and therefore it tends to diminish the errors. So, it's Catch 22 no matter what. Afocal images cause vignetting and prime focus images lack in sufficient resolution to show even larger errors. neither method is a true indicator of image qulaity.

Mlden

[GlennLeDrew]

But the prime focus image will well enough reveal the color errors which would afflict a low power view. (I'm not expecting to see those aberrations requiring 1mm exit pupils to detect.) After all, the low power eyepiece field stop typically used and an APS-C sensor are of not too dissimilar size. And 5 micron pixels would provide sufficient resolving power for the purpose. For example, a prime focus image taken with, e.g., an 80mm f/5 or f/6 achro readily reveals CA.

[Crayfordjon]

This thread has drifted away from the French Hypo, we are arguing on how many angels can stand on a pinhead again, however Peter mentioned the thirty inch refractor, so here it is.

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[Crayfordjon]

The Objective singlet lens.

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[wiseone]

I kept the spectral range from 490nm to 620nm for visual use. If a Baader fringe killer filter is used, then anything outside that will be greatly reduced. Of course, you can also use narrow-band filters for imaging.
I do not wish to hijack this thread, so any further questions should be on a new thread.