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just when we thought we were safe

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#1 auriga

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Posted 23 July 2013 - 09:35 PM

We thought we were safe in needing a ladder with a large scope.

Then the duo struck:

First it was the jpastrocraft/mikelockwood Sweet Sixteen 16"f/4. Seated viewing in all positions.

Then it was the astrocraft/lockwood 14.5" f/2.5. Seated viewing in all positions.

Then is was the 20" f/3 astrocraft/lockwood, which Bob S has. Seated viewing in all positions.

All were no ladder scopes.

Now we have the jpastrocraft/ lookwood 25" f/3, you might have to stand to observe at the zenith, or maybe not, but in any event no ladder is required,

Where will all this end? Doesn't anyone want to catch falling people at public outreach events?

Bill

#2 frito

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Posted 23 July 2013 - 11:44 PM

ladders are cool and all but if i don't have to climb one then why have to climb one? :)

#3 Jon Isaacs

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Posted 24 July 2013 - 06:21 AM

ladders are cool and all but if i don't have to climb one then why have to climb one? :)


- Because F/4 - F/5 is a much nicer place to be than F/3...

- Because an F/4 - F/5 is affordable when compared to an F/3...

- Because you need the exercise. :)

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#4 tezster

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Posted 24 July 2013 - 06:30 AM

The ladder-making industry must be near a point of crisis, indeed, with the increasing number of large scopes now being designed for observing while seated or standing. I'm surprised that the .001% reduction in ladder sales haven't sent the ladder-making interest groups and lobbyists flooding the astronomy community, decrying the introduction of these new-fangled, short focal-length dobs.

#5 nicknacknock

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Posted 24 July 2013 - 07:00 AM

Ah, diversification is the key! Them ladder manufacturers are offsetting the dramatic fall in ladder sales with the increase in sales of neat little chairs for us astronuts!

#6 csrlice12

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Posted 24 July 2013 - 07:42 AM

They could always go into the eyepiece cap/cup manufacturing business and make up the difference....... :question:

Imagine, caps/cups that actually fit....

#7 FJA

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Posted 24 July 2013 - 08:18 AM

With my knees, doing away with ladders or even steps (my 18" is an f/4.3 but even so the eyepiece is about 6ft 5in off the ground) is an attractive idea but bank account says 'no' to buying a fast big dob.

#8 Cotts

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Posted 24 July 2013 - 08:30 AM

The ladder-making industry must be near a point of crisis, indeed, with the increasing number of large scopes now being designed for observing while seated or standing. I'm surprised that the .001% reduction in ladder sales haven't sent the ladder-making interest groups and lobbyists flooding the astronomy community, decrying the introduction of these new-fangled, short focal-length dobs.


Obviously we need a 65+ page thread to discuss this important issue....

Dave

#9 csrlice12

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Posted 24 July 2013 - 09:34 AM

You do realize that Astro ladders are much more expensive...because it has the word Astro in it.....

#10 Sean Cunneen

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Posted 24 July 2013 - 12:48 PM

I am doing what I can to help, I am building a 16" f7.2. Now won't you please consider a long focus instrument before the industries' bell has rung?

#11 Jeff Morgan

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Posted 24 July 2013 - 01:04 PM

I am doing what I can to help, I am building a 16" f7.2. Now won't you please consider a long focus instrument before the industries' bell has rung?


Well I can't top f/7.2, but would still like to think I am doing my part while we patiently wait for monsoon season to end.

The ladder that goes with this scope is even American made!

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#12 Cotts

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Posted 24 July 2013 - 04:34 PM

Gorgeous scope! But those six-truss jobs always look lopsided and waaaaay out of collimation.

Of course they aren't, really....

My 16" f/5 requires one step on a stepstool for me so I guess I qualify for something or other...

Dave

#13 GOLGO13

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Posted 24 July 2013 - 11:52 PM

I have enjoyed looking through scopes which required a ladder. But that experience also made me really appreciate my XT10i and not needing to deal with that. I've decided that I will never go above a point where I would need a ladder. I've also decided that a 10 inch scope is plenty big enough for me, and I'll just mooch off someone else who has a bigger scope when i can.

It's also one thing for the ladder to be a step stool and another thing for it to be one meant for an orchard.

#14 Peter Natscher

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Posted 25 July 2013 - 10:01 AM

I use a strong 4ft. Werner fiberglass ladder with my 24" f/3.3 dob. I added extra rungs between existing ones so there's now four rungs spaced 6" apart at the lower two feet of the ladder. I only need to use the two or three lower rungs at the most telescope elevation angle. I've added extra rungs to all my past ladders that I've used with my past taller Dobs (5ft. and 6ft. ladders). While observing flat-footed with the dob at 60° or lower angles, I can rest against the 4ft. ladder while observing. Resting against a ladder while observing is comfortable after a few hours of standing at the eyepiece. It steadies my body so that long periods of threshold observing are easier to do and more productive. A 4ft. ladder with extra rungs is relatively small and easy to transport with a larger telescope.

We thought we were safe in needing a ladder with a large scope.

Then the duo struck:

First it was the jpastrocraft/mikelockwood Sweet Sixteen 16"f/4. Seated viewing in all positions.

Then it was the astrocraft/lockwood 14.5" f/2.5. Seated viewing in all positions.

Then is was the 20" f/3 astrocraft/lockwood, which Bob S has. Seated viewing in all positions.

All were no ladder scopes.

Now we have the jpastrocraft/ lookwood 25" f/3, you might have to stand to observe at the zenith, or maybe not, but in any event no ladder is required,

Where will all this end? Doesn't anyone want to catch falling people at public outreach events?

Bill



#15 Jon Isaacs

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Posted 25 July 2013 - 10:40 AM

I have enjoyed looking through scopes which required a ladder. But that experience also made me really appreciate my XT10i and not needing to deal with that. I've decided that I will never go above a point where I would need a ladder. I've also decided that a 10 inch scope is plenty big enough for me, and I'll just mooch off someone else who has a bigger scope when i can.



My 12.5 inch Equatorial requires a ladder but not a tall one, I use it at home on my driveway. In my mind, I never wanted another ladder scope, a short step, yes but a true ladder scope, I didn't want one, too much hassle, too much effort transporting it, setting it up.

But about 4 years ago, my wife and I found the place that was our little dream mountain hideaway and after a while I realized that what with the big garage and everything, I didn't need to be thinking portable so much any more so I began keeping a lazy eye open for something larger than my 16 inch, I was thinking 20 inch, something like that.

Well at some point an 25 inch F/5 Obsession showed up on Astromart at a good price and not far away. I realized that just about any 25 inch would require a ladder, certainly any 25 inch I was willing to pay for would so if I wanted a 25 inch, it was not a question of a ladder or not but rather how tall a ladder...

Anyway, I ended up with the scope and eventually, with the help of Jeff Morgan, eventually purchased a rolling ladder for stability and security. It all works. I would not want to be in the position where this was my only larger scope but I am fine with it as a ladder scope.

Jon

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#16 careysub

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Posted 25 July 2013 - 10:48 AM

We thought we were safe in needing a ladder with a large scope.

Then the duo struck:

First it was the jpastrocraft/mikelockwood Sweet Sixteen 16"f/4. Seated viewing in all positions.

Then it was the astrocraft/lockwood 14.5" f/2.5. Seated viewing in all positions.

Then is was the 20" f/3 astrocraft/lockwood, which Bob S has. Seated viewing in all positions.

All were no ladder scopes.

Now we have the jpastrocraft/ lookwood 25" f/3, you might have to stand to observe at the zenith, or maybe not, but in any event no ladder is required,

Where will all this end? Doesn't anyone want to catch falling people at public outreach events?

Bill


What constitutes "ladder-free viewing" is a matter of opinion due to a 12" or so height difference commonly seen in a mixed sex crowd of adults.

However the consensus seems to be that a "height factor" (FL - 0.5*D) of 64" is about that number (an LB-16 for example).

The Lockwood Custom Optics website places the minimum FR for visual use at 2.75 (that Astrocraft seems to be cheating - I wonder how well the EP/eye system is handling that steep visual cone).

So: 64/(2.75 -0.5) = 28.4". If you think your focuser-end optics can handle F/2.5 then it increases to 32".

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)

#17 derangedhermit

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Posted 25 July 2013 - 04:04 PM

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)


I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee

#18 Mike B

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Posted 25 July 2013 - 04:50 PM

I would not want to be in the position where this was my only larger scope but I am fine with it as a ladder scope.


Jon, it's good to see at least some of us moving on up in the telescope realm. :smirk:

Considering the things which go bump in the night, and not wishing to numbered amongst them, i've always been inclined to keep both feet on the ground. So i stopped the fever at 15" F4.5.... however, given the opportunity to keep a larger rig permanently stationed in the boondocks, i think i could manage with a ladder like Jon's.
;)
But until such time, the ladder manuf. crisis will have to be resolved without me; for i feel it's wrong to wring wealth with rungs wielded too widely, so prefer a standing dissent to a decent descent.

#19 Deep13

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Posted 25 July 2013 - 06:04 PM

A friend of mine has a home-built 20" f/6. "I don't like coma," he said. I guess he doesn't like coma-correcting lenses either.

#20 Jon Isaacs

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Posted 26 July 2013 - 04:59 AM

But until such time, the ladder manuf. crisis will have to be resolved without me; for i feel it's wrong to wring wealth with rungs wielded too widely, so prefer a standing dissent to a decent descent.



I still haven't quite figured it out but is sure is clever. :)

Jon

#21 Bob S.

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Posted 26 July 2013 - 05:38 AM

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)


I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee


Lee, Having done a serious "Pepsi Challenge" between my former 28" f/3.5 Lockwood/Starmaster and a 28" f/2.75 Lockwood/Webster, I can tell you for sure that the f/2.75 is not only doable but with a just noticeable difference was beating my f/3.5.

I would feel very comfortable putting my 20" f/3 Lockwood/JP Astrocraft up against anything else of the same aperture size as long as you let me use the SIPS system from Starlight Instruments/TeleVue and Ethos ep's. The SIPS (Starlight Integrated Paracorr System) is able to more adequately correct for coma with different focal length eyepieces than the Paracorr II that I have.

#22 GOLGO13

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Posted 26 July 2013 - 01:56 PM

If you have a ladder scope...getting a ladder like that one Jon is very important...especially for when someone not experienced wants to take a look. I would love to join you out there.

#23 derangedhermit

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Posted 26 July 2013 - 05:04 PM

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)


I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee


Lee, Having done a serious "Pepsi Challenge" between my former 28" f/3.5 Lockwood/Starmaster and a 28" f/2.75 Lockwood/Webster, I can tell you for sure that the f/2.75 is not only doable but with a just noticeable difference was beating my f/3.5.

I would feel very comfortable putting my 20" f/3 Lockwood/JP Astrocraft up against anything else as long as you let me use the SIPS system from Starlight Instruments/TeleVue and Ethos ep's. The SIPS (Starlight Integrated Paracorr System) is able to more adequately correct for coma with different focal length eyepieces than the Paracorr II that I have.

Lockwood is one of the "few people" I mentioned above. It is a remarkable accomplishment that an f/2.75 primary can give better views than an equally well made f/3.5.

I retract the statement about doubting anyone wants to make an f/2 paraboloid, since Lockwood says on his web site that he will make them, although I don't know if he would recommend one for visual use in a Newtonian.

I would very much like to be able to afford a large excellent f/3 such as yours. That's why I have been looking into very fast large mirrors, and attempted to answer the question.

Do you know why the SIPS solution works better for coma correction over a range of eyepiece focal lengths than the Paracorr II? It is the same optical design, is it not? Are the eyepieces parfocal, or close to?

Can you give ballpark relative price differences between a 20" f/4, a 20" f/3.5" and a 20" f/3? I would guess going from f/4 to f/3.5 costs at least 50% more, and going from f/3.5 to f/3 perhaps double.

Lee

#24 Bob S.

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Posted 26 July 2013 - 06:09 PM

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)


I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee


Lee, Having done a serious "Pepsi Challenge" between my former 28" f/3.5 Lockwood/Starmaster and a 28" f/2.75 Lockwood/Webster, I can tell you for sure that the f/2.75 is not only doable but with a just noticeable difference was beating my f/3.5.

I would feel very comfortable putting my 20" f/3 Lockwood/JP Astrocraft up against anything else as long as you let me use the SIPS system from Starlight Instruments/TeleVue and Ethos ep's. The SIPS (Starlight Integrated Paracorr System) is able to more adequately correct for coma with different focal length eyepieces than the Paracorr II that I have.

Lockwood is one of the "few people" I mentioned above. It is a remarkable accomplishment that an f/2.75 primary can give better views than an equally well made f/3.5.

I retract the statement about doubting anyone wants to make an f/2 paraboloid, since Lockwood says on his web site that he will make them, although I don't know if he would recommend one for visual use in a Newtonian.

I would very much like to be able to afford a large excellent f/3 such as yours. That's why I have been looking into very fast large mirrors, and attempted to answer the question.

Do you know why the SIPS solution works better for coma correction over a range of eyepiece focal lengths than the Paracorr II? It is the same optical design, is it not? Are the eyepieces parfocal, or close to?

Can you give ballpark relative price differences between a 20" f/4, a 20" f/3.5" and a 20" f/3? I would guess going from f/4 to f/3.5 costs at least 50% more, and going from f/3.5 to f/3 perhaps double.

Lee


Lee, I cannot speak to the costs with any kind of authority. The Paracorr II has recommended settings that sometimes are kind of ballpark wheras the SIPS allows all of the ep's to be spaced at the correct distance for sharpest/tightest views. The SIPS has a larger range of movements I think than the regular Paracorr but it also only uses a 1.5" focus travel for the focuser itself. In the Pepsi challenge that we did, the one variable that was different between the two scopes was that the f/2.75 was using the SIPS and I was using a Paracorr II. On my recent f/3, I chose the SIPS but sometimes still revert to the Paracorr II for cerain applications. I think Mike Lockwood has suggested that going faster than f/2.75 is not recommended by him. He has a test scope with an f ratio of f/2.55 and so I think he has had enough experience given that he also has a 20" f/3 just like mine that is 1.25" thick and he has come up with those recommendations.

It is just my gut feeling but I would not feel comfortable using anybody but Lockwood for a mirror in the f/2.75-f/3.0 range at this time. I think he has amply demonstrated his ability to nail these ultra-fast mirrors. I would suspect that some other very skilled opticians are honing their skills in this range and it is likely that Lockwood will not occupy this rare space for long? Only time will answer this question. And, as Mel Bartel opined at a lecture I attended a couple of years ago, the speed of the mirrors will be constrained by the correctors and eyepieces built to handle ever faster optics. Bob

#25 Jeff Morgan

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Posted 26 July 2013 - 11:19 PM

Considering the things which go bump in the night, and not wishing to numbered amongst them, i've always been inclined to keep both feet on the ground. So i stopped the fever at 15" F4.5.... however, given the opportunity to keep a larger rig permanently stationed in the boondocks, i think i could manage with a ladder like Jon's.


Actually these types of ladders move quite nicely. Even on rocky ground they are quite stable. You can even lean sharply over the rail and feel safe.

While I suppose a full-sized van or pick-up truck would work, the key is to have some type of trailer at your disposal. My kayak micro-trailer does double duty.

Of course the whole premise of this thread is flawed. Assuming more than a few dozen of the sub f/3's ever get built, even those owners will soon want more aperture (you just can't beat human nature). And then it's back to ladders.

Spend a little more for a ladder you feel secure on and a lot of problems get bypassed from the start.

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