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Hi Jason - not knowing much of the mechanics involved, are you saying it would likely be more difficult to collimate, and maintain collimation in a scope like this?

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For larger mirrors like 20"+. This is definitely the future. Maybe someone will soon come up with a homemade tester for fast mirrors that is accurate and simple!

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Is the future for newtonians really a transformation to a CHiefspiegler design?

Shorter,

No obstruction,

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In order to get the maximum TFOV out of them, and still have less than a 6 or 7mm exit pupil, you need überwide eyepieces like the Ethos.

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Yeah, that's something Uncle Al concluded as well, in discussing exactly how to go about coma correction at f/3 and below.

BTW Mike told me last month when he was talking about going to Okie-Tex with the f/2.55 that Al was bringing a prototype Ethos as well. Haven't heard any more. Glad to hear the scope worked well, though!

Best,
Mark

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Dozens of people includiong Al Nagler, Rick of Starmaster scopes and many other very experienced observers were blown away by this scope, not just me. If you want one a bit longer Rick of Starmaster has a 22" f/3.3 here which, with the same prototype paracorr, gives superb views across the field. As for what will be built in the future, check Mike Lockwood's site, visible in the second image I posted upthread. These guys know what they're doing.....
btw I am not connected in any way with this enterprise. Just really impressed.
Dave

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... A telescope is a system. The new emerging system of sub f/4 scopes will likely have thin mirrors of 1.25" or less, even .75", to reduce weight...

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If I'm not mistaken, Mike told me this was indeed a "cellular" mirror in this scope.

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Jim Fly - Manufacturer

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>>>travel in a smaller vehicle <<<

The only thing "smaller" about low f/# scopes is the length of the truss poles, which isn't usually the limitation in putting big scopes in small vehicles.

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There's also eliminating the need for ladders, which take up quite a bit of space. A 16" f/4.5 is around as big as I can go (I'm 6' tall) because I don't want to stand on a ladder, but even if I did want to, there's no room for a ladder in my vehicle. Pushing the design envelope just means more options! - j

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Fight indignorance!

The Green Goblin - 12" of dobsonian excellence!

The PortaBowl-a $100 4.5" f/8 ball-scope YOU can build!

Eero2-a 6" f/5 ball-scope you probably can't.

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What happened to the picture of the scope?

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or larger mirrors like 20"+. This is definitely the future. Maybe someone will soon come up with a homemade tester for fast mirrors that is accurate and simple!

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Addendum - I forget to mention the "accurate and simple" method that Mike's told me he uses - Foucault testing with Couder masks. Doesn't get much simpler than that.

This kind of speed comes in useful in larger sizes, a 28" f/2.5 is a flat-footed dob. Mike said he did the 14.5" f/2.55 just as an experiment, and to see if he could - I don't think anybody should be looking at it as a potential product.

I don't see a 24" f/2.5 (mentioned) as a sit-down scope, unless you have a very tall chair. The zenith position is somewhere around 60", depending on the structure.

Al told me he expected the Ethos design should work very well at f/3, I think that's what being shown now.

The main disadvantage for large very fast scopes that require a new model Paracorr and highest-end EPs as well as large (expensive) diagonals is going to be cost. The mirrors take considerably more work - more testing, more precision, and simply a lot more figuring, to higher tolerances. James Mulherin (OMI) was surprised that Mike's prices on the sub-4 mirrors aren't higher than they are, but I didn't remember to ask him about that when I was chatting.

I don't think this particular branch of the Dob tree is going to take over everything, considering costs, but it is going to become an option for people who want to maximize certain aspects, when coupled with true ultralight design: very thin and/or cellular mirrors utilizing fresh structure designs that support these mirrors efficiently, not just the same-old same-old we're seeing so far - stuffing thick mirrors into a truncated heavy structure.

If you're starting with a very fast large thin mirror, and a lightweight design, the balance equations for the design become a lot more favorable.

Maybe this needs spelling out - that last part is a key reason why building these in large sizes makes sense, from performance, portability, and the general back-saving qualities of ultralights, never mind the lack of a ladder, or even a step. Improved coma correction is the last piece of the puzzle that had to become possible to enable a superior approach for future design. No way is this going to be something everybody will want, but it's certainly something I want.

Best,
Mark

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Here's the picture.

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Here's the picture.

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Dozens of people including Al Nagler, Rick of Starmaster scopes and many other very experienced observers were blown away by this scope, not just me. If you want one a bit longer Rick of Starmaster has a 22" f/3.3 here which, with the same prototype paracorr, gives superb views across the field..... These guys know what they're doing.....
btw I am not connected in any way with this enterprise. Just really impressed.
Dave

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Such scopes better have the adequate mechanics to fine tune and maintain collimation.

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FYI Jason, reliable information suggests the "XLK" was involved.

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Why yes, yes it was. And it did maintain collimation once I remembered to tension the spider!

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Achieving "perfect" collimation isn't difficult– it's just being attentive. It's holding it through the range of motion, especially when your secondary size is even larger, that's the problem. Fortunately, faster scopes allow for squatter, more stable triangles in the trusses.- j

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Correct on all points, Jay. Thanks for saying that so I didn't have to.

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A telescope is a system. The new emerging system of sub f/4 scopes will likely have thin mirrors of 1.25" or less, even .75", to reduce weight. They will have Al Nagler's new Paracorr to correct coma and will use the 21 Ethos as the low power eyepiece, to keep the exit pupil in bounds....

The 14.5" f/2.55 I see as a demonstration; f/4 is sufficiently short for a 14.5 for seated observing in all positions, and Starmaster is already selling these.

But the f/2.55 focal ratio would be a big advantage in a larger scope, such as a 24 inch, which wouldn't need a ladder. In fact, seated observing would be possible even at zenith.The gain in convenience would be great, and the advantages for public outreach would be huge: just walk up to a 24 inch scope and sit down and look into the eyepiece. Bill

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I agree with all of that Bill, except that for a 20" mirror I will only go as thin as 1.25" (edge thickness). My 20" F/3 is 1.25"-thick Pyrex, and I look forward to making some 20" F/3.3s from quartz.

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If I'm not mistaken, Mike told me this was indeed a "cellular" mirror in this scope.

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Sorry Jim, you are mistaken. It's a 1.35"-thick Pyrex monolithic blank.

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Addendum - I forget to mention the "accurate and simple" method that Mike's told me he uses - Foucault testing with Couder masks. Doesn't get much simpler than that..... This kind of speed comes in useful in larger sizes, a 28" f/2.5 is a flat-footed dob. Mike said he did the 14.5" f/2.55 just as an experiment, and to see if he could - I don't think anybody should be looking at it as a potential product.

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...and also careful figure of revolution testing.

It is now a *successful* experiment, so who knows where it will go and what people will want. The idea of a large F/2.6-F/2.8 instrument is now very interesting to me.

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if we would have went with f 10 CHief the eyepiece would be around 5.5 ft // here is a pic showing the 20 f8 CHief and my 20inch f5.25 newtonian lower eyepiece on the offaxis chief

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Ed Jones, the creator of the "Chiefspiegler" is a friend of mine. I've been to his house. He is a gentleman, a skilled optical designer, optician, and ATM, and "chiefs" are very fine telescopes, as I'm sure your 20" telescope is.

You have very clearly and profusely stated many statistics/advantages regarding the Chiefspiegler on many internet forums and groups. However, making a fast telescope was the point of my experiment and this thread, not making a Chief. A chief is not a fast telescope. So, I suggest that rather than posting here, you instead start your own thread.

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Mike Lockwood
20" F/3 MX Starmaster, MX = Mike's eXperiment
14.5" F/2.55 self-built Newtonian
Nine other self-built telescopes, 4.25" to 30"
TeleVue Paracorr, eyepieces
http://www.Loptics.com/

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Such scopes better have the adequate mechanics to fine tune and maintain collimation.

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FYI Jason, reliable information suggests the "XLK" was involved.

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Why yes, yes it was. And it did maintain collimation once I remembered to tension the spider!

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Cool!!!!!!!!!

Just ensure the XLK mirror surface is positioned as close as possible to the focal plane for maximum allowed accuracy

Jason

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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs

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Well aside from the issue of extreme coma, it seems to be the question of focal ratio devolves to that of practical use of eyepieces. Assuming a practical range of exit pupils to be 5mm to 0.5mm, we get eyepiece focal lengths of 15mm to 1.5mm - yes 1.5mm! Compare f/8 at 40mm to 4mm - exactly bracketing the typical range of eyepiece focal lengths.

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Without amplification the f/3 scope is going to be limited to very low powers because even short eyepieces will give low powers.

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I don't know about that. Large apertures tend to be atmosphere-limited rather than exit-pupil limited at the high end. A 2mm exit pupil seems to be a sweet spot for DSOs, and that would be a 6mm eyepiece for an f3 (or a 7mm eyepiece with a paracorr). There are 6mm and 8mm Ethoses, 7mm Naglers and Pentax XWs. The Pentaxes go down to 3.5mm, for a 1mm exit pupil, and naglers go down to 2.5mm. That's without even using a barlow or powermate.

How many nights does a 20"+ scope get to a 0.5mm exit pupil? That's over 1000x. I rather suspect that most scopes of that aperture or larger spend most of their time at 1mm or higher exit pupils.

Let's take the example of an f3, with the new paracorr (which I will assume for the moment has the same 1.15x barlow effect the old paracorr). So it's effectively f3.6.
Let's also assume you want to keep to a max of a 5mm exit pupil, based on your example.

17mm Ethos = 4.72mm exit pupil
10mm ethos = 2.78mm exit pupil
8mm ethos = 2.22mm exit pupil
6mm ethos = 1.67mm exit pupil
3.5mm nagler or pentax XW = 0.97mm exit pupil

So we can span from 1mm to 5mm exit pupils nicely, without using a barlow, and with nothing with less than a 82 degree AFOV. If we use a 2.5x powermate with the ethoses, we can easily get down to 0.66mm, all with 100 degree AFOV.

Now, if your observing style is centered on high power, small field observing (i.e. planets, planetary nebs, small globs, etc.), then you should probably look at longer fl designs. But you probably won't be going for really large aperture, either, since the atmosphere will usually keep you under 500x anyway, unless you are lucky enough to live someplace with incredible seeing like Southern Florida.


Jarad

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Edited by Jarad (09/22/09 01:42 PM)

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There is 7.7 times more glass to remove in parabolizing an f/2.55 than an f/5. Did it seem like it took that much longer to parabolize or did you anything to shorten that up?

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This scope provides superb views (with a new Paracorr prototype that Al Nagler is working on specifically for scopes from about f/2.5 to f/3.5). Long lineups at Okie-Tex confirmed the quality. f/2.55!! I wouldn't have believed this.... I think the central obstruction is about 30% which isn't too bad...
I hope the designer (in photo) doesn't mind my posting this.
Dave

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One word: awesome!

Think I better start saving for a 24" f/2.55...

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I'm attaching a photo of the 14.5" F/2.55 OTA to this post. I think it will appear at the bottom of this message. You can just see my reflection in the secondary mirror as I took the photo.

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the 1/3 of the light that is blocked by the secondary what part of the image is less bright because of this // very cool telescope pushing the edge

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A 30% diameter central obstruction does NOT block 30% of the light - if blocks 0.3^2 = 9% of the light. And the light loss is spread evenly over the whole image, as long as the exit pupil is small enough to fit into your eye.

The bigger effect of a large CO is diffraction, which will primarily affect low contrast resolution. So these will not be optimized for planetary viewing on the cloud bands of Jupiter or low-contrast details on Mars. That doesn't mean they won't perform well - aperture can make up for a lot. The "rule of thumb" is to subtract CO diameter from the aperture to get a rough equivalent of an unobstructed scope in terms of low contrast performance. So a 20" with a 30% CO would perform similar to an unobstructed 14" in terms of low-contrast detail, but would be like a 19" in terms of light gathering and resolution for high-contrast detail.

Jarad

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I think his diagonal might be undersized, by my calculations he can only bring the focus out only less than 1/2 inch above the upper rim ray with zero field. Mike is this correct?

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Oh good grief, here we go again! Every time I debut a new fast telescope someone says the secondary is too small.... this happened with the 20" F/3 too.

According to my calculations and testing of the actual telescope, it's just about perfectly sized. The upper cage is a fairly tight fit, and the focuser is down low.

....and if the upper cage blocks light off-axis or the paracorr sticks down a little bit into the optical path, I just don't care!

It's experimental, people! (But it works really, really, really well!)

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Mike Lockwood
20" F/3 MX Starmaster, MX = Mike's eXperiment
14.5" F/2.55 self-built Newtonian
Nine other self-built telescopes, 4.25" to 30"
TeleVue Paracorr, eyepieces
http://www.Loptics.com/

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I for one hope you'll keep experimenting. Being able to enjoy the view through a premium, large aperture Dob while sitting (in contrast to standing on a tall ladder in the dark) is a huge advance in amateur observational astronomy, at least in my book.

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It's experimental, people! (But it works really, really, really well!)

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Mike how short can you go ? congrads on trying something new.LOOKS like you are the guy for ultrafast newtonians . kf

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gparker, I know what you mean. I was talking with someone about my 20" f/4.3 Zambuto/Starmaster and we were discussing if I should throw the slow piece of trash in the garbage Bob Schilling

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BOB...Where the H&%$#! have you been?
And where/when/why did you get such an old-fashioned, slow scope???
Will you be at Chiefland any time soon?

PS....sorry for the 'personal; note.
For Others: If you're at the Chiefland Fall Star Party, stop by and see what a 28" f/3.6 can do with Jupiter, et.al.
Puts a huge hole in the myth that bigger, faster, obstructed scopes can't be planet killers!

Mike Harvey

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Seriously, though. If we can get down to f/3 then it opens up a whole new range of apertures to the amateur crowd.

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then your focal plane would be BEHIND the primary so you wouldn't need a secondary!!

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That's called a refractor, but I can't afford a 14.5" refractor that is perfectly color corrected.

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Mike Lockwood
20" F/3 MX Starmaster, MX = Mike's eXperiment
14.5" F/2.55 self-built Newtonian
Nine other self-built telescopes, 4.25" to 30"
TeleVue Paracorr, eyepieces
http://www.Loptics.com/

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As promised, here's a link to the article about the 14.5": 14.5" F/2.55 Article Link

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Mike how short can you go ? congrads on trying something new.LOOKS like you are the guy for ultrafast newtonians . kf

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I think F/2.5 is a reasonable stopping point for now. I could make an F/2 mirror for dedicated imaging, but I am not sure correctors or eyepieces would work so well there for visual use.

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I for one hope you'll keep experimenting. Being able to enjoy the view through a premium, large aperture Dob while sitting (in contrast to standing on a tall ladder in the dark) is a huge advance in amateur observational astronomy, at least in my book.



Hi,
I agree entirely. This has been my experience.
Bill Meyers
16"f/4 jpastrocraft.com "sweet sixteen"

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Could you imagin a 30" f/2.55? The thing would be about 6 feet high! that would be awesome!

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better yet: negative f ratio!!!

then your focal plane would be BEHIND the primary so you wouldn't need a secondary!!

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This is, to all intents and purposes, a refractor......

Dave

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David Cotterell
14th Floor Observatory
Richmond Hill, Ontario
200 mm Intes f/10 Mak-Cass
80mm WO Zenithstar FD
66mm WO ED
Losmandy G-11 mount with Gemini
Ioptron minitower
Canon XSi

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Thats a unique, and nice scope, but I would think there would be some optical drawbacks besides disaterous coma, without the new paracor

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I think the central obstruction is about 30% which isn't too bad...
I hope the designer (in photo) doesn't mind my posting this.
Dave

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harder mirrors to make= more $$ .Special eyepieces MORE $$ [you afraid to have your feet only 3 ft off the ground]Congrads to MIKE succeeding in making the fast mirrors.

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