77 replies to this topic

[meade4ever]

I have opened a new thread to describe the construction of the 250 hypo refractor.
It is still not finished and I'm looking for comments, advices but, please, do not post free bashing on the hypo concept, that can be done in another thread.
I would like to share here the hypo adventure, get some tips to improve it, get some comments of the images taken, and will try to take pics as requested.

So lets begin with the heart of each refractor, the objective glass (even if, in an Hypo, the OG is not alone).
Mine as been made by John Wall himself, it is a 250mm PCX, made from hard crown and and figured to a very good figure.
With a focal ratio of only F:17, it is not suited for an hypo refractor and should be stopped down to 100mm for the F:40. I think John made it when he worked with the dyalite refractors.
John recommends F/D 30 as the minimum focal ratio for an hypo.

This requirement limits the aperture of hypo refractors to 150mm (or it woulds require a mount which cost tens of time more than the refractor itself).
However this lower focal ratio allowed me to build this giant refractor, I can even lift it alone.

John even made the collimation cell, amazing work of art :

[GlennLeDrew]

It's too bad this big lens is of too short a focal length for full aperture use in a Hypo; it makes for unnecessary weight.

If I were figuring such a lens, I would probably aspherize it so as to eliminate spherical aberration, for a start. Then one would have the option to use it by itself with suitably narrow filters.

But then again, a 10" parabolic mirror of f/5 is so much better all around, and not expensive, and more compact...

No bashing here; just applying a wider perspective.

[Mark Harry]

I found a positive meniscus lens with a concave back, made aspherizing one surface relatively easy to eliminate SA with the dialyte design I played with. It should work here as well, I'd think.
M.

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[Mark Harry]

Here's the graphs associated with this particular 4" dialyte

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[Mark Harry]

and spots. As can be seen, SA is pretty well controlled, with lateral spread still apparent; though not terribly so.
The only drawback, the correcting achro, unlike the HYPO is -NOT- standard. But it's relatively easy to obtain the glass (which is B270 crown, and SF1 flint)

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[Crayfordjon]

The OG was aspherized by autocollimation not test plate. Other than you Mark, could I request on Louis's behalf, not to highjack this thread with yet more discussions about theorectical high end optics, this is not the venue for it. We want to entice those who are not academics, and top optical engineers, to make an easy hands on practical scope, without all the scary stuff about Strehl ratios OSLO spot diagrams, and other stuff that will scare away ordinary enthusiastic telescope makers.

[Crayfordjon]

The Hypo Louis constructed is based on the duplex system, here is the optical diagram. It has a singlet OG and two ex bino OGs, The only part that has to be made by the ATM is the OG, this can be made from Chance Pilkington float glass, cheap and highly transparent, only possible by the magnifient technology used to produce it. Normal mirror making techniques are only required to make one, if the grinding and polishing are carefully carried out the lens need not be figured but used as is. As the focal ratio is very long, the curve can be generated without a lot of hard work, also the curve is very shallow at F:30 to F: 40.

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[Mark Harry]

Yes,gladly! I didn't mean to use too much complex information; rather to show that perhaps aspherizing one surface like I did originally with my 8" lens did help with improving the spherical abberation encountered with a PCX lens.
I think this is the perfect idea for being introduced to optics (hypo) A beginner can learn a lot!
Though i -DO- think this 30" disc might be just a little bit too thin to fool around with! (abt 3/8"-10mm)

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[GlennLeDrew]

Jon,
I see in your sketch that you show a 50mm f.l. eyepiece, which at a working f/3.72 yields a 13.4mm exit pupil. A Coolpix camera's iris in daytime might be open to 1-2mm. A 2mm iris results in the scope being stopped down by a factor of 13.4 / 2 = 6.7, thus reducing the aperture from 180mm to a miniscule 27mm. That makes the OG effectively f/162.

[MKV]

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[meade4ever]

I opened a new thread because the first one was hijacked, and the same thing happen here.
Many threads talking about the hypo have been opened, why not post there ?

All I want is to share my 18 months experience in making a such big refractor and hope that others would follow the road that John discovered.

I remember John warned me many times that it was not a good idea to begin with a big diameter. Of course he was right, making a such monster took me time to find the correct red lenses assembly and how to support it.
But I told him that all I wanted was to make something big enough to catch the audience and to promote the hypo in which I really believe.

I look at the stars for more that 30 years, but I'm not an optical expert.
I don't care about oslo, I believe in what I see and the first light with my hypo was magic.
I saw venus, saturn and even if the images were far from the ones given by an achro, because of the design, but certainly also because the hypo was not even collimated and w/o mount, these images were still pleasant to see.
Like the ones, with an 14mm UWA eyepiece, in daylight distant from a few meters (yes the hypo can focus as close as you want) to one hundred.
What's wrong with that ?

There are a few members who have build or bought 9, 10 or 11 inches achros (Istar or D&G) and when they say these refractors are "planet killers", everyone is admirative and hopefully nobody hijack their thread.
Where are the olso experts demonstrating with olso diagrams that a 10" f12 achro can't take high powers and is nothing more than a kaleidoscope ?

The thruth is that big refractos attract people, and those who can't spend thousands dollars for a fluorite super hyper ED triplet from Markus are still stastified with their achros which give, at least, nice images at low or medium power from a fraction of an APO price.
What's wrong with that ?

The Hypo is for the others who, like me, are not optical skilled and don't have the money to spend on a D&G refractor, but have never lost their dream to own one day a "giant" refractor.
The JW250Hypo have costed me a fraction of an achro refractor with the same diameter.
It will never be planet killer, but it can be a nice RFT telescope.
What's wrong with that ?

[meade4ever]

Jon,
I see in your sketch that you show a 50mm f.l. eyepiece, which at a working f/3.72 yields a 13.4mm exit pupil. A Coolpix camera's iris in daytime might be open to 1-2mm. A 2mm iris results in the scope being stopped down by a factor of 13.4 / 2 = 6.7, thus reducing the aperture from 180mm to a miniscule 27mm. That makes the OG effectively f/162.

Bingo!

The diagram John posted shows what an hypo with a duplex system is. It is one of the experiments John has made to test the 250mm OG but it is different from my hypo.

Glenn, please wait my results to judge the hypo I have built. I will give you all the data you need to make your own opinion and share with us what you think, but right now w/o any characteristic, I think that nobody here can say something valid.

If you want special shots and are not hurry, I can do them, I have nothing to hide, like I have written, the hypo belongs to its own class of refractor, like achros and apo.

[GlennLeDrew]

Meade4ever,
You did ask for comments and advice. I at least am trying to provide perspective, so that you understand the variables which affect performance.

Have you observed with this scope yet, or is it still in construction? (I'm still unclear about this.) For any Hypo you've observed with, what is the smallest exit pupil you've employed?

Note that if you use an eyepiece which delivers an exit pupil larger than your eye's iris, you are reducing the aperture of the system. For example, if your iris opens to 7mm at night, the 13.4mm exit pupil delivered by a 50mm f.l. eyepiece makes the aperture effectively 94mm, not 180mm. This is just like placing a 94 mm aperture mask over the front lens. A 90-100mm refractor would work at least as well in resolving power and light grasp, and would be a lot more manageable than a 10" behemoth.

At a working f/ratio of f/3.72, the longest f.l. ocular which allows full aperture operation with a 7mm iris is 26mm. For a 6mm iris, 22mm.

Putting together a scope which is not using the full aperture is inefficient, for while the big glass up front *looks* impressive, it's doing absolutely nothing but add needless mass and bulk. Unless an important aspect *is* impressiveness in external appearance.

Again, this is not a slight against the Hypo concept; it's a realistic assessment of an inefficiently dimensioned (and hence utilized) lens. To work efficiently, the OG needs to be suitably long in focal length so that its full aperture can be used (and not stopped down as your short focus 10" must be.) And as Jon makes clear, the Hypo in any event is suited for lower powers/larger exit pupils due to chromatic aberration.

[meade4ever]

All the parts but the optics that have been used to make the hypo were salvage parts, in the hypo's spirit (think Dobson).
This has had an impact, on the hypo performance (see later).

First tube taken from a Meade 856 newtonian reflector (200mm f1200mm) :


Second tube taken from a Meade MTS SN8 shmidt newtonian reflector (200mm f800mm) :


These tubes were used because they have the same diameter as the OG lens cell.

[meade4ever]

The 2.7” Focuser is from a Meade APO refractor and is fixed to a Meade 856 mirror cell :






This focuser offers a big internal diameter, which is required with big hypo refractors (fast focal ratio) and the mirror cell offer precise collimation to the focuser and suits perfectly inside the tube.
However this combo has too much lenght and I don't have enough back focus to use a diagonal. A low profile crayford (invented by John Wall !) would have been better.

[Crayfordjon]

Glenn, I have answered your comments on the other thread, I would like to point out to you that we have had this conversation before, on a previous thread, to which a I gave a full and detailed answer, now either you have a short mermory, or you are stirring up trouble, for the benifit of newcomers to this venue, will you please place your comments on the other thread and not use this one! this is for Louis's 250mm Hypo build, so dont muddy the waters here.

[Crayfordjon]

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[Crayfordjon]

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[Crayfordjon]

Mark I like your concept very much, looks good, try and knock one up!.

[nytecam]

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[GlennLeDrew]

Jon,
The OP asked for comments and advice. I'm offering comments and advice. If other than glad-handing and back-slapping bonhomie is wanted here, then it should be so stated. Otherwise a realistic appraisal in the spirit of education surely cannot be taken wrongly?

Not knowing if the OP is aware of the fact of aperture reduction when an iris is coupled to a larger exit pupil, I feel it's only right to point this out. It has nothing to do with discouraging a newcomer.

May I ask you, for the sake of clarity, what is the smallest exit pupil you recommend for the Hypo?

[meade4ever]

I wanted to describe the hypo first, before talking about pics, but if that can reduce the pressure, here some data :
- with the actual OTA length the hypo is used with a 225mm diameter,
- the eyepiece used is a SP32mm,
- the resulting power is 20x,
- the exit pupil is 11mm,
- on the camera is written 33mm 132mm (equiv), I measured the external length diameter and found 15mm.

I know it may be not enough, but I have taken pics with a Meade LPI camera, that I will post later.
Right now I must go with the kids.

[GlennLeDrew]

I wanted to describe the hypo first, before talking about pics, but if that can reduce the pressure, here some data :
- with the actual OTA length the hypo is used with a 225mm diameter,
- the eyepiece used is a SP32mm,
- the resulting power is 20x,
- the exit pupil is 11mm,
- on the camera is written 33mm 132mm (equiv), I measured the external length diameter and found 15mm.

I know it may be not enough, but I have taken pics with a Meade LPI camera, that I will post later.
Right now I must go with the kids.

That's useful information; thanks!

I will assume the "33mm 132mm (equiv)" on the camera refers to the longest focal length, when zoomed fully in, which is 33mm, or an equivalent of 132mm in the standard 35mm camera format. You probably do not zoom all the way in when shooting afocally, but instead set the zoom closer to the middle range, if not toward the wide end. If this is the case, it will be even worse from the standpoint of a mismatch between exit pupil and iris diameter.

The only thing the 15mm clear aperture tells us is that at the maximum zoom setting of 33mm, the lens aperture can be no faster than 33 / 15, or f/2.2. At shorter focal lengths, the f/ratio can be potentially faster than this, the fastest f-stop depending on the design.

At a given f/ratio, the iris diameter varies linearly with the focal lengthLet's assume the zoom range is 3.3-33mm (10X range). For daytime shooting, the f-stop might be set to f/11. This means that over the range if 3-33mm focal length, the iris varies between 0.3 and 3.0mm. In low light conditions, let's assume a constant f/2.2. Therefore the iris range is 1.5-15.0mm.

With an 11mm exit pupil, in order for at least close to the full aperture to be utilized, the camera'a iris must be near to wide open *and* the zoom set to near fully in.

Does your camera have manual settings for aperture and exposure duration? If not, you will need to shoot in sufficiently low light, in conjunction with a lower ISO setting, so that you're getting near the camera's warning on slow shutter speed. This should ensure a wide iris aperture. And of course you will need to zoom a fair ways in as well.

[Crayfordjon]

I have now, perhaps we can have a good thread with Louis presenting his build.

[Crayfordjon]

I have now! Perhaps we can now have a peaceful and constructive thread on Louis's giant refractor, and I have not built a Hypo this size yet. So I am looking forward to how it will perform.