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Trying to compensate for something


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22 hours ago, Stu said:

I tend to agree Matthew. I guess that's where the little stubby ones come in to play?

 

Not sure I follow you Stu? 

The lack of stepladder I understand but after that I'm kinda lost. :icon_scratch:

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At what point would you stop and think ...."maybe I might have over done it this time??"   I've never had the luxury of looking through anything this big but even if the views are brea

I tend to agree Matthew. I guess that's where the little stubby ones come in to play?

I still think the the guy is a bloody hero! Whilst the rest of us dream of 18", 20", 22". This man says, "No!!!! Let's do this properly." Paul PS. I defy any red blooded stargazer to lo

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How do those stubbies cope with coma? I assume if you have one your going to have already made the decision to go TV with EP's and no doubt have a CC but even so that one has got to be around the f/3 -3.5 territory. Are coma correctors really that good at cleaning up coma ?? I imagine there is more than your fair share of stars showing up in such a large mirror so there is no escaping stars sat towards the edges, along with any seagulls.

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5 minutes ago, swamp thing said:

Not sure I follow you Stu? 

The lack of stepladder I understand but after that I'm kinda lost. :icon_scratch:

Being able to fit larger DSO in due to a faster focal length.

Edited by spaceboy
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7 minutes ago, spaceboy said:

Being able to fit larger DSO in due to a faster focal length.

How?

All scopes of the same aperture have the same minimum magnification.  They may use different eyepieces to get there but the end result is the same.

Dob's are visual instruments we are not imaging.

 

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13 minutes ago, swamp thing said:

How?

All scopes of the same aperture have the same minimum magnification.  They may use different eyepieces to get there but the end result is the same.

Dob's are visual instruments we are not imaging.

 

Steve, I've obviously not done my sums before commenting. I assumed that the shorter focal length would allow for a wider field of view, but obviously, having considered it more, exit pupils get way too big if you go lower in mag so it's not useable anyway.

So, shorter, more useable scope with no ladder requirement, but plenty of coma to cope with, as ever there is no such thing as a free lunch.

Thanks for pointing it out Steve :) 

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16 minutes ago, swamp thing said:

How?

All scopes of the same aperture have the same minimum magnification.  They may use different eyepieces to get there but the end result is the same.

Dob's are visual instruments we are not imaging.

 

Goes to show what I know :icon_redface:

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This discussion of not being able to fit objects into the FOV reminded me of Stephen O'Meara's story about observing M57 with the 42" scope at Pic du Midi. Apparently he put in an eyepiece that gave 1200x magnification and was surprised to only see two stars and no nebula. Then he realised the FOV was entirely inside the ring!

Minimum magnification = aperture / fully dilated pupil diameter if I've got my sums right.

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10 minutes ago, Stu said:

Steve, I've obviously not done my sums before commenting. I assumed that the shorter focal length would allow for a wider field of view, but obviously, having considered it more, exit pupils get way too big if you go lower in mag so it's not useable anyway.

So, shorter, more useable scope with no ladder requirement, but plenty of coma to cope with, as ever there is no such thing as a free lunch.

Thanks for pointing it out Steve :) 

No worries :) 

It is a shame that we cant have our cake and eat it I know :( 

One day those bigger exit pupil bins will pave the way for similar eyepieces I hope.

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The photos of the extreme dob have lead to an interesting discussion :smiley:

The largest I've observed with is 20 inches and on two occasions. Once was an all black David Lukehurst masterpiece which I believe was an F/4.2. When pointng towards the zenith you needed a very small step to reach the eyepiece - just a low platform really so no problem. The 2nd 20 incher was Steve / Swampthing's own masterpiece and I can't recall using a step at all with that one :icon_scratch:

These occasions have provided long lasting memories for me. The views are astrounding especially of familliar objects. You have to re-think what you are looking at to some extent :icon_biggrin:

Even for the dyed-in-the-wool refractor enthusiast, I'd recommend taking the opportunity to view though a "monster" scope if one comes along.

There is a little thrill that runs up your spine as you move your head and eye to the eyepiece thats hard to describe :icon_biggrin:

Edited by John
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On 9/22/2017 at 19:07, niallk said:

Got to look through a 20" Obsession at the SSP ;) Very sound owner - generously sharing the EP with all!

Do you recall his name?. The guy i know of does live in that part of Ireland and may possibly own a 20" rather than a 30".  Himself and another guy manage to take some stunning images with the scope when they travel to the Burren. One of them at the top of it and the other at the bottom end manually pushing it. 

Edited by LukeSkywatcher
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I hate ladders - I can just about paint the house. I would probably have to risk having one look through it. I love big scopes but don't think I could manage over 20" if it's not in an observatory. 

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Big scopes are very impressive but as others have said, it's the minimum magnification that kills you. Gotta get a light beam 1200mm wide into a hole in a monkey's eyeball 6mm wide. In the UK 200x is going to be over the seeing limit 90% of the time. But looking at a small dim object like the ring nebula under a pristine sky, a scope like that would give one an unforgettable view!

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A rather crucial point! But I still wonder whether (know?) SOME of these
enormous scopes (in wonderful seeing) might have been used visually. :)

As a little thought experiment let's consider a "small" 40 inch refractor:

BigFrac.jpg.bd3fe3765d64b888ed04054a14cfac4b.jpg

http://astro.uchicago.edu/vtour/40inch/

So 1000mm in aperture! It's 63 feet long = 20,000 mm (i.e. f/20!) :cool:

I REMEMBER 40-60mm surplus (ERFLE?) eyepieces advertised in 70's
magazines claiming: "as used in So and So professional observatory"! 
So a BASIC magnification of 300-600x and an exit pupil ~2-3mm? :)

One would / could NOT use a modern GIANT reflector, but I sense
some of these scopes were used *visually* in their time? MIND you,
I'd never manage to FIND much stuff with a FOV only 1/10 deg... :D

Makes ya thing though? (Not being too serious) ;)

Edited by Macavity
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54 minutes ago, Macavity said:

As a little thought experiment let's consider a "small" 40 inch refractor:
So 1000mm in aperture! It's 63 feet long = 20,000 mm (i.e. f/20!) :cool:

Interesting thing to contemplate! 266x minimum useful magnification so one would need an 120mm EP. I guess if you have a naval gun breach as an eyepiece holder you can laugh at 2" EPs so the 46mm field stop's right out the window. I saw a Zeiss EP once that was a 100mm fl and about 5" diameter - so supposing FoV is about 60° one would be looking at about 1/3° of space. Not exactly a comet hunter I'll grant you :D but probably OK for some purposes.  I did look through the Lick 36" with a 55mm plossl but it was Jupiter so the narrow field wasn't an issue.

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I've had a 30" self built Dob since 1985, at the time it was possibly the Worlds largest. I can identify with most of the comments so far. From a logistical point of view, the current setup is one man/woman operational and takes less than 3 minutes from padlock to views through the eyepiece. Even at F4.1 which in 1985 was considered very fast, the height of the step ladder is pretty daunting for most even though it is stable, in the dark when you nudge the telescope to follow an object, is the telescope moving away from you or are you falling backwards!, very similar initial sensation.  The rule of thumb for lowest full aperture magnification is 4X the aperture in inches hence 120x representing about a 25mm eyepiece. At just over 3 metres focal length, as mentioned, many of the popular DSO's don't fit into the field, M31 for instance is really disappointing as just the core fills the field, M42 is still good as there is a wealth of detail, globulars are probably the most impressive sights. The main problem, certainly locally, is wind, anything more than a breeze introduces the shakes. It's probably our least used telescope!.   :icon_biggrin:

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On 9/21/2017 at 20:45, Stu said:

I tend to agree Matthew. I guess that's where the little stubby ones come in to play?

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Yes absolute please.

I know I'd have to shell out on top quality EPs but a short FL dobmonster would be the sweet spot. And I would be more than happy to share the views through those babies!!

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On 9/22/2017 at 20:08, Stu said:

Steve, I've obviously not done my sums before commenting. I assumed that the shorter focal length would allow for a wider field of view, but obviously, having considered it more, exit pupils get way too big if you go lower in mag so it's not useable anyway.

So, shorter, more useable scope with no ladder requirement, but plenty of coma to cope with, as ever there is no such thing as a free lunch.

Thanks for pointing it out Steve :) 

When I first read @swamp thing comment, I thought yep I have been a fool and not thought about that properly, just seeing a very long telescope and thinking narrow fields of view.  However, I have now thought about it a bit more and I am not so sure.  Perhaps the practical reality is somewhere in the middle of the two statements made.  But please tell me I am being an idiot if I am way off base here.

 

Laying out a couple of critical formula to use:

 

Magnification = Telescope Focal Length / Eyepiece Focal Length = Telescope Aperture / Exit Pupil

 

Which can be rewritten as:

 

Exit Pupil = Telescope Aperture / Magnification which also = eyepiece focal length / Telescope f/#

 

True Field = Eyepiece Field Stop Diameter / Telescope Focal length x 57.3o

 

True field can be approximated to Eyepiece Apparent Field / Magnification (though aspects like amount of rectilinear distortion can mean it is not quite precise, but field stop diameter is not always a known quantity).

 

Let us consider two telescopes, both with a 50cm mirror (I want!) with one scope a step ladder requiring f/5 focal ratio and the other a fireman’s ladder inducing f/7 (perhaps unreasonable but helps to illustrate the point).

 

Starting with the f/5 scope, using a 55mm Plossl such as the Televue would yield a 45.5x magnification and a true field of 1.1 degrees which is the maximum achievable with a 2” eyepiece in this scope.  However, the exit pupil is an unusable 11mm, so your pupil diameter will stop down the telescope considerably.

 

However, switching to a 31mm Nagler Type 5 (yes I am a disciple of the church of Tele Vue :icon_biggrin:) yields a magnification of 80.6x, true field of 0.96 degree and an exit pupil of 6.2mm.

 

Now turning our attention to the f/7 scope, the plossl yields 63.6x at a 0.75 degree field and a slightly too large but arguably useable exit pupil of 7.9mm.  The 41mm Panoptic yields the same true field but a better exit pupil of 5.9mm.

 

Using the 31mm Nagler results in 112.9x, 0.69 degrees and an exit pupil of 4.4mm.

 

So the maximum available field of view available is greater in the shorter f/# scope with production line available eyepieces.

 

Of course, hypothetically two scopes of the same aperture can always achieve the same max field of view, but to do so in this example would require a custom made eyepiece (but if you can afford that monster, then a custom EP might well be within your means).

 

In my f/7 example scope above, the field stop diameter would need to be approx. 59mm to yield the same field as the 31mm Nagler in the f/5 scope, so a 3” format eyepiece would be needed.  That 30mm, 100 deg Explore Scientific monster 3" EP is not quite there at around 0.86 deg true field.

 

I guess what I have also illustrated is just how important the development of eyepieces with very wide apparent fields such as the Nagler and Ethos are to the giant Dob mob.  But as Steve says, when you start considering all other issues like coma, required collimation precision and fabrication difficulty (cost) you cannot quite have your cake and eat it!

Edited by DirkSteele
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I have had two opportunities to view through large sized scopes. Both these have been at SGL star parties. The first was through a 22" which required a step ladder - the view of the Sombrero Galaxy was fantastic. I have also looked through Steve's (swamp thing) scope and really enjoyed the view of M81 and M82. Of course I have had lots of chances to view through the 16"- 18" Dobs again some wonderful memories.

I would love the opportunity to climb a stable ladder to view some Globs and PN. In the meanwhile I will stay with my 12".

Since writing this post I remember being invited (2015) by the Astronomers at the Lick Observatory to visit one night and view through the little Refractor they have. Unfortunately I was not able to return at that time but I did take a photo.2015.california-107.thumb.jpg.4b14fa0bcd0da1b8469dbe553a6b050f.jpg

 

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1 hour ago, DirkSteele said:

When I first read @swamp thing comment, I thought yep I have been a fool and not thought about that properly, just seeing a very long telescope and thinking narrow fields of view.  However, I have now thought about it a bit more and I am not so sure.  Perhaps the practical reality is somewhere in the middle of the two statements made.  But please tell me I am being an idiot if I am way off base here.

 

Laying out a couple of critical formula to use:

 

Magnification = Telescope Focal Length / Eyepiece Focal Length = Telescope Aperture / Exit Pupil

 

Which can be rewritten as:

 

Exit Pupil = Telescope Aperture / Magnification which also = eyepiece focal length / Telescope f/#

 

True Field = Eyepiece Field Stop Diameter / Telescope Focal length x 57.3o

 

True field can be approximated to Eyepiece Apparent Field / Magnification (though aspects like amount of rectilinear distortion can mean it is not quite precise, but field stop diameter is not always a known quantity).

 

Let us consider two telescopes, both with a 50cm mirror (I want!) with one scope a step ladder requiring f/5 focal ratio and the other a fireman’s ladder inducing f/7 (perhaps unreasonable but helps to illustrate the point).

 

Starting with the f/5 scope, using a 55mm Plossl such as the Televue would yield a 45.5x magnification and a true field of 1.1 degrees which is the maximum achievable with a 2” eyepiece in this scope.  However, the exit pupil is an unusable 11mm, so your pupil diameter will stop down the telescope considerably.

 

However, switching to a 31mm Nagler Type 5 (yes I am a disciple of the church of Tele Vue :icon_biggrin:) yields a magnification of 80.6x, true field of 0.96 degree and an exit pupil of 6.2mm.

 

Now turning our attention to the f/7 scope, the plossl yields 63.6x at a 0.75 degree field and a slightly too large but arguably useable exit pupil of 7.9mm.  The 41mm Panoptic yields the same true field but a better exit pupil of 5.9mm.

 

Using the 31mm Nagler results in 112.9x, 0.69 degrees and an exit pupil of 4.4mm.

 

So the maximum available field of view available is greater in the shorter f/# scope with production line available eyepieces.

 

Of course, hypothetically two scopes of the same aperture can always achieve the same max field of view, but to do so in this example would require a custom made eyepiece (but if you can afford that monster, then a custom EP might well be within your means).

 

In my f/7 example scope above, the field stop diameter would need to be approx. 59mm to yield the same field as the 31mm Nagler in the f/5 scope, so a 3” format eyepiece would be needed.  That 30mm, 100 deg Explore Scientific monster 3" EP is not quite there at around 0.86 deg true field.

 

I guess what I have also illustrated is just how important the development of eyepieces with very wide apparent fields such as the Nagler and Ethos are to the giant Dob mob.  But as Steve says, when you start considering all other issues like coma, required collimation precision and fabrication difficulty (cost) you cannot quite have your cake and eat it!

Matthew, I think the only thing wrong with your examples is that you used f5 and f7, rather than f3 or less, which is where the stubby scopes operate.

In this case, a 31mm Nag gives a 10.3mm exit pupil and a 41mm Pan gives 13.6mm.

Realistically, with a 7mm exit pupil in say a 32" f3 scope, you are looking at a 21mm eyepiece (got to be Ethos, right?) giving x116 and a 0.86 degree fov assuming I did my sums correctly.

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1 hour ago, Stu said:

Matthew, I think the only thing wrong with your examples is that you used f5 and f7, rather than f3 or less, which is where the stubby scopes operate.

In this case, a 31mm Nag gives a 10.3mm exit pupil and a 41mm Pan gives 13.6mm.

Realistically, with a 7mm exit pupil in say a 32" f3 scope, you are looking at a 21mm eyepiece (got to be Ethos, right?) giving x116 and a 0.86 degree fov assuming I did my sums correctly.

Yep, a good point.  The Ethos 21mm is definitely the EP for scopes less than f/4.  I lent mine to someone with an f/3.5 Dob once and almost had to wrestle it back from him!  In your example it gives a 7mm exit pupil which works.  If we were to compare this f/3 to say and f/5 which if I recall the big Obsessions were a few years ago, that same 32" scope would be best served by the 31mm Nagler (using the Tele Vue line up) with a 0.59 degree field (131x and 6.2mm exit pupil).  However, nothing in their EP offering gets to the same max field available in the f/3 model without having to large an exit pupil.  But it can obviously be achieved going down the custom route.  Siebert Optics make some huge EPs (up to 110mm in a 4.3" body), just need a focuser that can accommodate them I guess.

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